SUPER-SOFT AND SUPER-ELASTIC DRY GELS · SUPER-SOFT AND SUPER-ELASTIC DRY GELS ... elastomers made...

Sunday July 10 2016 Talk

SUPER-SOFT AND SUPER-ELASTIC DRY GELS

Michael Rubinstein, Department of Chemistry, University of North Carolina, USA [email protected]

William F. M. Daniel, Department of Chemistry, University of North Carolina, USA Joanna Burdyńska, Department of Chemistry, Carnegie Mellon University, USA

Mohammad V. Vatankhah, Department of Chemistry, University of North Carolina, USA Krzysztof Matyjaszewski, Department of Chemistry, Carnegie Mellon University, USA

Jaroslaw Paturej, Department of Chemistry, University of North Carolina, USA ; Institute of Physics, University of Szczecin, Poland

Sergey Panyukov, P.N. Lebedev Physics Institute, Russian Academy of Sciences, Russia Andrey V. Dobrynin, Polymer Program, Institute of Materials Science, University of Connecticut, USA

Li-Heng Cai, School of Engineering and Applied Sciences, Harvard University, USA Thomas E. Kodger, School of Engineering and Applied Sciences, Harvard University, USA Rodrigo E. Guerra, School of Engineering and Applied Sciences, Harvard University, USA Adrian F. Pegoraro, School of Engineering and Applied Sciences, Harvard University, USA

David A. Weitz, School of Engineering and Applied Sciences, Harvard University, USA Sergei S. Sheiko, Department of Chemistry, University of North Carolina, USA

Molecular combs and bottlebrushes are a new class of polymer architecture allowing for anomalously low density of entanglements in polymer melts. The conformations and rheological properties of melts of these branched macromolecule composed of a flexible backbone and side chains densely tethered to it are investigated theoretically, experimentally and by computer simulations.1,2 We develop the rule for dialing in the desired value of the melt plateau modulus of these molecules as low as 1000 times below the conventional values for linear polymer melts and experimentally verify the validity of our theory. The theory also predicts that elastomers made from these melts should be super-elastic and reversibly stretch up to ten times more than elastomers made from linear polymers. Hybrid networks with both permanent and reversible bonds made with this novel architecture are predicted to be super-tough and self-healing. References 1. W.F.M. Daniel, J. Burdynska, M. Vatankhah-Varnoosfaderani, K. Matyjaszewski, J. Paturej, M. Rubinstein, A.V. Dobrynin and S.S. Sheiko, Nature Materials, 2016, 15, 183-190. 2. L.H Cai, T.E. Kodger, R.E. Guerra, A.F. Pegoraro, M. Rubinstein, and D.A. Weitz, Advanced Materials 2015, 27, 5132–5140.

Sunday July 10 2016 Talk

DESIGNING MESOSTRUCTURES FOR FOOD FUNCTIONALITY

Erik van der Linden, Laboratory of Physics and Physical Chemistry of Foods, Wageningen university, Netherlands

[email protected]

We will discuss a few examples of the mutual couplings that exist between specific mesostructures (also typically present in foods) and their dispersing surrounding, and how these couplings across multiple scales dictate the mesostructural functionalities. Such a multi-scale dynamic approach is required in addressing processing, consumption and digestion of foods. We first consider the elasticity of a system consisting of fibrillar structures and discuss the contribution of a fibril to the elasticity as a function of the interaction with the surrounding fibrils (van der Linden and Parker 2005). Next we review how the surrounding of the fibril by means of its pH can affect the system robustness against phase separation, and how this can be altered by adding a surfactant like SDS (Jung, Savin et al. 2008; Kroes-Nijboer, Sawalha et al. 2012). We secondly consider a system of spherical assemblies of globular proteins and how the system’s structural evolution contributes to the elasticity. We discuss how this structural evolution can be influenced by altering the protein surrounding, yielding novel protein functionality (Sağlam, Venema et al. 2011). Finally we address the structural evolution of a more complex (mixed) system consisting of fibrillar and spherical mesostructures, and how multi-scale interdependences between these mesostructures and their surrounding are responsible for the functionality of each of the ingredients and of the overall complex system. We discuss our recent results on the stability emulsions that contain long semi-flexible structures that have a length equal to the diameter of the emulsions droplets. These recent results form an important extension of earlier work (Blijdenstein, Veerman et al. 2004). We also discuss our extensions to other sphere/fibril systems and point to an intricate issue on interpreting the stability of such systems. References: Blijdenstein, T. B. J., C. Veerman, et al. (2004). "Depletion - flocculation in oil-in-water emulsions using fibrillar protein assemblies." Langmuir 20(12): 4881-4884. Jung, J. M., G. Savin, et al. (2008). "Structure of heat-induced beta-lactoglobulin aggregates and their complexes with sodium-dodecyl sulfate." Biomacromolecules 9(9): 2477-2486. Kroes-Nijboer, A., H. Sawalha, et al. (2012). "Stability of aqueous food grade fibrillar systems against pH change." Faraday Discussions 158: 125-138. Sağlam, D., P. Venema, et al. (2011). "Preparation of high protein micro-particles using two-step emulsification." Food Hydrocolloids 25(5): 1139-1148. van der Linden, E. and A. Parker (2005). "Elasticity due to semiflexible protein assemblies near the critical gel concentration and beyond." Langmuir 21(21): 9792-9794.

Monday July 11 2016 Session 1

COLLOIDAL SWARMS CAN SETTLE FASTER THAN ISOLATED PARTICLES

Roberto Piazza, Politecnico di Milano, Italy [email protected]

Enrico Lattuada, Politecnico di Milano, Italy Stefano Buzzaccaro, Politecnico di Milano, Italy

Key Words: Colloids, sedimentation, ultracentrifugation, protein association Colloid sedimentation has played a seminal role in the development of statistical physics thanks to the celebrated experiments by Perrin, which gave a concrete demonstration of molecular reality. Recently, the investigation of sedimentation equilibrium has provided valuable information on a wide class of systems, ranging from simple colloids to active particles and biological fluids [1]. Yet, many aspects of the sedimentation kinetics deserve to be further investigated. Here we present some rather surprising results concerning the effect of interactions on particle settling [2]. Usually, the settling velocity of a colloidal suspension decreases with concentration: this well-known effect is called “hindered’’ settling. By experimenting on model colloids in which depletion forces can carefully be tuned, we conversely show that attractive interactions consistently “promote" particle settling, so much that, close to a phase-separation line, the sedimentation velocity of a moderately concentrated dispersion can even exceed its single-particle value. At larger particle volume fraction , however, hydrodynamic hindrance eventually takes over. Hence, v( ) actually displays a non-monotonic trend that may threaten the stability of the settling front to thermal perturbations. By discussing a representative case, we show that these results are relevant to the investigation of protein weak association effects by ultracentrifugation. References. [1] R. Piazza, Reports of Progress in Physics, 2014, 77, 056602. [2] E. Lattuada, S. Buzzaccaro, R. Piazza, Phys. Rev. Lett. 2016, 116, 038301


GRAVITATIONAL COLLAPSE OF COLLOIDAL GELS

Roseanna Zia, Cornell University, USA [email protected]

Poornima Padmanabhan, Cornell University, USA Key Words: colloidal gels, rheology, Brownian dynamics, gel collapse, colloidal suspensions. We investigate the phenomenon of gravitational collapse in colloidal gels via dynamic simulation in moderately concentrated gels formed via arrested phase separation. In such gels, rupture and re-formation of bonds of strength O(kT) permit ongoing structural rearrangements that lead to temporal evolution—aging—of structure and rheology [1]. The reversible nature of the bonds permits a transition from solid-like to liquid-like behavior under external forcing, and back to solid-like behavior when forcing is removed. But such gels have also been reported to undergo sudden and catastrophic collapse of the entire structural network, eliminating any intended functionality of the network scaffold. Although the phenomenon is well studied in the experimental literature, the microscopic mechanisms underlying the collapse remain murky [2-18]. Here we conduct large-scale dynamic simulation to model structural and rheological evolution of a gel subjected to gravitational stress. The model

comprises 750,000 Brownian particles interacting via a hard-core repulsion and short-range attractive interactions that lead to formation of a gel, periodically replicated to an infinite system [1]. A body force is applied to the gel, and particle positions, velocities, and pressure are measured throughout simulation, as well as the bulk strain of the gel. Three temporal regimes emerge: slow, pre-collapse evolution; collapse and rapid sedimentation; and long-time compaction producing, to our knowledge, the first large-scale dynamic simulation of gravitational gel collapse. We connect the temporal regimes to distinct phases of structural and rheological evolution. A range of attraction strengths, and their effect on the critical force that triggers collapse, are studied. We find that the initial deformation is slow and linear, and the transition to and scaling of the fast strain rate depends on the strength of gravitational forcing, as is the transition to and rate of the final sedimentation regime, in excellent agreement with experimentally reported behavior [3,9,14]: The detailed microstructural evolution

is reported here, along with the dependence of the delay time and speed with attraction strength and magnitude of the applied stress relative to Brownian forces.

[1] R. Zia, B. Landrum, W. Russel. J. Rheol, 58(5), 2014.

[2] C. Allain, M. Cloitre, and M. Wafra. PRL, 73, 1995.

[3] P. Bartlett, L. J. Teece, and M. A. Faers. PRE, 85, 2012. [4] R. Buscall, T. Choudhury, M. A. Faers, J. W. Goodwin, P. A. Luckham and S. J. Partridge. Soft Matt, 5, 2009.

[5] R. Buscall and L. R. White. J. Chem. Soc., Faraday Trans. 1, 83, 1987.

[6] M. A. Faers. Adv. Colloid Interface Sci., 106, 2003.

[7] V. Gopalakrishnan, K. S. Schweizer, and C. F. Zukoski. J. Phys.: Condens. Matter, 18, 2006.

[8] J. J. Lietor-Santos, C. Kim, P. J. Lu, A. Fern ́andez-Nieves, and D. A. Weitz. Eur. Phys. J. E, 28, 2009.

[9] S. W. Kamp and L. Kilfoil M. Soft Matter, 5, 2009.

[10 M. L. Kilfoil, E. E. Pashovski, J. A. Masters, and D. A. Weitz. Phil. Trans. R. Soc. A, 361, 2003.

[11] C. Kim, Y. Liu, A. Kuhnle, S. Hess, S. Viereck, T. Danner, L. Mahadevan, and D. A. Weitz. PRL, 99, 2007.

[12] S. Manley, J. M. Skotheim, L. Mahadevan, and D. A. Weitz. PRL, 94, 2005.

[13] S. J. Partridge. Rheology of Cohesive Sediments. PhD thesis, Bristol University, 1985. [14] W. Poon, L. Starrs, S. Meeker, A. Moussaid, R. Evans, P.Pusey and M. Robins. Faraday Discuss,112,1999.

[15] R. Seto, R. Botet, M. Meireles, G. K. Auernhammer, and B. Cabane. J. Rheol., 57, 2013.

[16] L. Starrs, W. C. K. Poon, D. J. Gibbered, and M. M. Robins. J. Phys.: Condens. Matter, 14, 2002.

[17] L. J. Teece, M. A. Faers, and P. Bartlett. Soft Matter, 7, 2011.

[18] N. A. M. Verhaegh, D. Asnaghi, H. N. W. Lekkerkerker, M. Giglio, and L. Cipelletti. Physica A, 242, 1997.

[19] M. P. Allen and D. J. Tildesley, Computer simulation of liquids. Gloucestershire: Clarendon Press, 1987.

3

4

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8

9

1

h/h0

10-1

100

101

102

103

104

t/(a2/D)

Pe= (4⇡ / 3)∆⇢ga4/ kT

! =0.2, V0 = 5kT

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Padmanabhan & Zia (2016)

FIGURE 1 – IMAGES: SNAPSHOTS FROM

SIMULATION DURING SLOW COMPACTION AND RAPID

GEL COLLAPSE. PLOT: EVOLUTION OF BULK HEIGHT

OVER TIME.


NON-UNIFORM FLOW OF COLLOIDAL GLASSES AND GELS: THE “SHEAR-GRADIENT CONCENTRATION COUPLING INSTABILITY”

J.K.G. Dhont, Forschungszentrum Juelich, Germany

[email protected]

There are several types of shear-induced instabilities in soft-matter systems, like vorticity- and gradient-banding, that are by now well-understood. There is, however, an instability that can be referred to as “the Shear-gradient Concentration Coupling instability” (the SCC-instability) that has been largely ignored due to the lack of understanding of its microscopic origin, since its phenomenological description a few decades ago. This instability is due to a postulated shear-gradient induced mass flux together with a strong coupling of the stress to concentration. The origin of the shear-induced mass flux resulting from direct interactions is so far not understood, and explicit expressions for the corresponding transport coefficient have therefore not been derived. In this presentation, the origin of this mass flux is discussed, an explicit expression for the transport coefficient is presented, and numerical results are discussed for the stationary non-uniform flow profiles and concentration profiles of an initially SCC-unstable system, which will be compared to experiments on hard-sphere glasses H. Jin, K. Kang, A.K. Hyun, J.K.G. Dhont, Soft Matter 10 (2014) 9470.


ROLE OF HYDRATION LAYER ON RHEOLOGY OF NANO ALUMINA SUSPENSIONS

Mufit Akinc, Iowa State University, USA [email protected]

Technological implication of reduction in viscosity of nanosize ceramic suspensions with environmentally benign and inexpensive additives is not trivial. This presentation will discuss the flow characteristics of concentrated nano-alumina powder suspensions. Unusually high viscosities observed for suspensions of nanoparticles compared to those of micron size powders cannot be explained by current viscosity models. For a given solids content, as the particle size decreases so does the interparticle distance leading to overlapping interparticle forces. Concomitant with the particle size reduction, increase in surface area of the solids requires higher surfactant concentrations for effective steric stabilization. The rheology of nanosize alumina suspensions and its variation with solids content and with saccharide concentration were explored by rheometry. The mechanism of dramatic viscosity reduction by saccharide addition (primarily fructose) is studied by TGA, DSC, and NMR. The interparticle forces between the nanometric alumina particles in water and in fructose solutions were investigated by AFM. The interactions between the nano-alumina particles in water can be explained by the DLVO theory. However, DLVO theory can not adequately describe the interactions between particles for suspensions containing saccharide. The interaction forces (amplitude and range) between nanometric alumina particles decrease with increasing saccharide concentration. Formation of so-called hydration layer on alumina nanoparticles in water was hypothesized for years, but never observed experimentally. The direct visualization of hydration layer over nanosize alumina particles was realized with the fluid cell transmission electron microscopy in situ. The hydration layer over the particle aggregates was observed and it was shown that these hydrated aggregates constitute new particle assemblies which in turn alter the flow behavior of the suspensions. These nanoclusters alter the effective solids content and the viscosity of nanosize alumina suspensions. Our findings elucidate the source of high viscosity observed for nano particle suspensions and are of direct relevance to many industrial sectors including materials, food, cosmetics, pharmaceutical among others employing colloidal slurries with nanosize particles.


LONG-RANGE HYDRODYNAMIC INTERACTIONS ENHANCE COLLOIDAL GELATION

James Swan, Massachusetts Institute of Technology, USA [email protected]

Zsigmond Varga, Massachusetts Institute of Technology, USA

Key Words: Colloidal Gels, Hydrodynamics, Simulation Methods Colloidal gels are formed during arrested phase separation. Sub-micron, mutually attractive particles aggregate to form a system-spanning network with high interfacial area, far from equilibrium. Models for microstructural evolution during colloidal gelation have often struggled to match experimental results with long standing questions regarding the role of hydrodynamic interactions. In the present work, we demonstrate simulations of gelation with and without hydrodynamic interactions between the suspended particles. The disparities between these simulations are striking and mirror the experimental-theoretical mismatch in the literature. The hydrodynamic simulations agree with experimental observations, however. We explore a simple model of the competing transport processes in gelation that anticipates these disparities, and conclude that hydrodynamic forces are essential. We employ a minimal model of the hydrodynamic forces between particles, which emphasizes the most important elements of the fluid physics during gelation. Near the gel boundary, there exists a competition between compaction of individual aggregates, which suppresses gelation and coagulation of aggregates, which enhances it. The time scale for compaction is mildly slowed by hydrodynamic interactions, while the time scale for coagulation is greatly accelerated by collective motion of particles within an aggregate. This enhancement to coagulation leads to a shift in the gel boundary to lower strengths of attraction and lower particle concentrations when compared to models that neglect hydrodynamic interactions. Away from the gel boundary, differences in nearest neighbor distribution persist. This result necessitates a fundamental rethinking of how both microscopic and macroscopic models for gelation kinetics in colloids are developed. Further, when long-range electrostatic repulsion between particles are incorporated into the simulation model, differences between models incorporating hydrodynamic interactions and those neglecting them become exacerbated. When aggregation is reaction limited, as is the case for particles interacting via a short-ranged attraction and a long-ranged repulsion (SALR), the diffusive dynamics of particle clusters are key to establishing the correct aggregation rate. Long-ranged hydrodynamic interactions between particles result in aggregates that diffuse anisotropically, which promote the growth of percolated networks as opposed to condensed domains. The figure below depicts the terminal state of discrete element simulations neglecting and including long-ranged hydrodynamic forces under experimentally relevant conditions.

Fig 1. (Left) meta-stable particle clusters formed in discrete element simulations neglecting

hydrodynamic interactions. (Right) colloidal gel formed in discrete element simulations including long-

ranged hydrodynamic interactions.


RHEOLOGY, MICROSCOPIC DYNAMICS AND MATERIAL FAILURE IN THE CREEP OF A COLLOIDAL GEL

Luca Cipelletti, L2C UMR 5221 Université de Montpellier and CNRS, France

[email protected] Stefano Aime, L2C UMR 5221 Université de Montpellier and CNRS, France

Laurence Ramos, L2C UMR 5221 Université de Montpellier and CNRS, France

Key Words: colloidal gel, creep, rheology, light scattering, failure The mechanical properties of amorphous solids such as glasses or gels are currently a topic of intense research, with implications in material science as well in fundamental condensed matter physics. Traditionally, researchers have investigated the relationship between two macroscopic quantities: the stress applied to the sample and the resulting strain. Recent works aim at gaining a deeper understanding of the origin of the rheological properties, by coupling rheology to structural and dynamical measurements. I’ll discuss experiments probing the relationship between rheology and microscopic dynamics during the creep of a colloidal gel under a constant shear stress. The gel creep consists of three regimes. Deviations from a purely elastic (or affine) deformation are observed in the initial regime. These non-affine dynamics are fully reversible upon removing the applied stress, and are associated to the heterogeneity of the local gel elasticity. In the second regime, non-affine displacements grow much slower with strain, but are associated to irreversible rearrangements. In the third regime, a sharp acceleration of the dynamics at small length scale is observed. These faster rearrangements precede the macroscopic failure of the gel by thousands of seconds: they thus are dynamic precursors of failure that allow one to predict the fate of the gel well before any rheological measurement.


PARTICLE-STABILIZED WATER DROPLETS THAT SPROUT MILLIMETER-SCALE TUBES

Paul S. Clegg, University of Edinburgh, United Kingdom [email protected]

Miglė Graužinytė, University of Edinburgh, United Kingdom Joe Forth, University of Edinburgh, United Kingdom

Katherine A. Rumble, University of Edinburgh, United Kingdom

Key Words: Growth, Compositional ripening, Colloids, Interfacial elasticity A layer of colloidal particles will become irreversibly trapped at a fluid–fluid interface if they exhibit partial wettability with both fluid phases. This effect has been exploited to create Pickering emulsions, armored bubbles, and new materials of various kinds. When the interfaces are densely coated with particles, they behave like rigid elastic sheets with moduli that are proportional to the underlying interfacial tension. The interfaces are permeable, a characteristic that can, for example, lead to compositional ripening of Pickering emulsions Here we show that when particle-stabilized water droplets are created in a bath of toluene with ethanol, millimeter-scale tubes are observed to sprout from the top of the droplets. Growth is driven by the ethanol partitioning from the toluene into the water which leads to an internal overpressure. Vertical growth occurs over many minutes; finally the tube buckles when it can no longer support its own weight (Figure 1). There are several different growth modes controlled by the concentration of ethanol and of silica particles.[1] An alternative way to manipulate the system is by using a different alcohol, leading to insight on the role of the underlying three-fluid phase diagram. Our work paves the way for future studies of droplet growth because the liquid droplets and the interfacial properties can be independently studied.

[1] M. Graužinytė, J. Forth, K. A. Rumble, P. S. Clegg, Angew. Chem. Int. Ed. 54, 1456 (2015).

FIGURE 1. SHOWING AN EXAMPLE GROWING WATER DROPLET STABILIZED BY INTERFACIAL PARTICLES. HERE A

50µL DROPLET OF WATER HAS BEEN INJECTED INTO A BATH OF TOLUENE CONTAINING 15 VOL.% ETHANOL AND

0.19 VOL.% SILICA NANOPARTICLES. THE DROPLET IS OBSERVED TO SPROUT A VERTICAL TUBE WHICH GROWS

WITH TIME UNTIL BUCKLING OCCURS (FINAL FRAME). THE SCALE BAR IS 5 MM.


MICROSTRUCTURE AND YIELDING OF MICROFIBER GELS

Patrick T. Spicer, Department of Chemical Engineering, University of New South Wales, Australia [email protected]

Jie Song, Department of Chemical Engineering, University of New South Wales, Australia Marco Caggioni, Complex Fluid Microstructures, P&G, USA

Todd M. Squires, Department of Chemical Engineering, University of California, USA

Large aspect ratio cellulose nanofibers are able to a form poroelastic network at low volume fractions via aggregation and entanglement, forming a gel without significantly modifying viscosity[1]. The gels have a small but useful yield stress and a better ability to suspend particles than non-interacting higher volume fraction glasses[2] because the sparse fiber networks can significantly restructure at small strains. Yielding behavior can thus strongly depend on the fluid microstructure[3]. We study here deformation and yielding of aqueous cellulose fiber gels. Confocal imaging shows how gel yield stress relates to structural deformation rate because of localized network restructuring. Such response is advantageous to applications like surface coatings, nasal sprays, cosmetics, and foods. Understanding the mechanism of rate- and length-scale dependent yielding, and relating microstructure changes to bulk rheology[4], will enhance our ability to formulate, model, and design complex fluids with novel performance. References [1] - Solomon MJ, Spicer PT. Microstructural regimes of colloidal rod suspensions, gels, and glasses. Soft Matter, 6, 1391 (2010). [2] - Emady H, Caggioni M, Spicer P. Colloidal microstructure effects on particle sedimentation in yield stress fluids. J Rheol. 57, 1761 (2013). [3] - Joshi YM. Dynamics of colloidal glasses and gels. Annu Rev Chem Biomol Eng. 5, 181, (2014). [4] - Hsiao L, Newman RS, Glotzer SC, Solomon MJ. Role of isostaticity and load-bearing microstructure in the elasticity of yielded colloidal gels. Proc Natl Acad Sci, 109, 16029, (2012).


COHERENT X-RAY STUDIES OF THE MICROSCOPIC DYNAMICS UNDERLYING THE PHASE BEHAVIOR AND NONLINEAR RHEOLOGY OF GEL-FORMING NANOCOLLOIDAL SUSPENSIONS

Robert Leheny, Johns Hopkins University ,USA

[email protected] Michael Rogers, Johns Hopkins University ,USA

Kui Chen, Johns Hopkins University ,USA Martine Bertrand, Johns Hopkins University ,USA Tyler Shendruk, Johns Hopkins University ,USA

Suresh Narayanan, Johns Hopkins University ,USA Subramanian Ramakrishnan, Johns Hopkins University ,USA

James Harden, Johns Hopkins University ,USA

This talk will describe two related projects exploring the properties of gels formed from nanometer-scale colloids. The first involves the phase behavior and microstructural dynamics of concentrated binary mixtures of spherical colloids with a size ratio near two and with a tunable, intrinsic short-range attraction. In the absence of the attraction, the suspensions behave as well mixed, hard-sphere liquids. For sufficiently strong attraction, the suspensions undergo a gel transition. However, the fluid-gel boundary does not follow an ideal mixing law, but rather the gel state is stable at weaker interparticle attraction in the mixtures than in the corresponding monodisperse suspensions. X-ray photon correlation spectroscopy measurements reveal that, in contrast with depletion-driven gelation at larger size ratio, gel formation in the mixtures coincides with dynamic arrest of the smaller colloids while the larger colloids remain mobile. Complementary molecular dynamics simulations indicate the arrest results from microphase separation that is caused by a subtle interplay of entropic and enthalpic effects and that drives the smaller particles to form gel nuclei in the vicinity of the larger colloids. The second part of the talk will describe coherent x-ray experiments on concentrated (monodisperse) nanocolloidal gels subjected to in situ large-amplitude oscillatory shear, which provide unique information about the spatial character of nanometer-scale particle rearrangements associated with nonlinear rheology and yielding of the gels. The oscillatory strain causes periodic echoes in the x-ray speckle pattern, creating peaks in the intensity autocorrelation function. The peak amplitudes are attenuated above a threshold strain, signaling the onset of irreversible particle rearrangements. The gels display strain softening well below the threshold, indicating a range of strains at which deformations are nonlinear but reversible. The peak amplitudes decay exponentially with the number of shear cycles above the threshold strain, demonstrating that all regions in the sample are equally susceptible to yielding and surprisingly that the probability of a region yielding is independent of previous shear history. The wave-vector dependence of the decay rate reveals a power-law distribution in the size of rearranging regions, suggesting a nonequilibrium critical transition at yielding.


CORE/SHELL CAPSULES FORMED BY SILICA PRECIPITATION IN BIOPOLYMER COACERVATE SCAFFOLDS

Philipp Erni, Firmenich SA, Corporate Research, Geneva, Switzerland

[email protected]

Key Words: Complex Coacervation, Biomineralization, Encapsulation, Wetting. Rheology. Delivery systems with low-permeability barriers and controllable release are crucial for the encapsulation of cells, pharmaceuticals, vitamins, inks, or fragrance and flavor molecules [1-3]. Here, we describe core/shell capsules with dense walls composed of a biopolymer scaffold and interpenetrated by a network of amorphous silica. We first generate a weakly acidic hydrogel shell around an oil drop via interfacial deposition of complex coacervates, formed by phase separation of a protein with a weakly anionic polysaccharide. Following covalent crosslinking of the protein component in the coacervate gel, this shell then serves as a scaffold to induce protein-directed mineralization of silicon dioxide from a liquid-silica precursor. The precipitation process occurring in the hydrogel scaffold of the capsule shell simultaneously consumes water and forms silica, yielding dense shells with a very low permeability for volatile organic compounds and adjustable mechanical characteristics. We use solid state CP/MAS 29Si NMR spectroscopy to characterize the silica/biopolymer hybrid shells and find a high fraction of interfacial silanol groups, associated with a large internal surface area of the inorganic silica phase. Combined with specific surface area measurements, thermogravimetry, SEM imaging and energy-dispersive X-ray spectroscopy (EDS) of the shells, these data reveal a composite shell material wherein the precipitated silica interpenetrates the crosslinked coacervate scaffold [1]. To evaluate the core/shell capsules as delivery systems for low molecular weight volatile payloads, we perform mechanical testing on ensembles of individual capsules to compare the stiffness, modulus and rupture behavior relevant for mechanically-driven release. Furthermore, we investigate the role of three-phase wetting phenomena at the coacervate/oil/water interface during the formation of the core/shell structure using rheology and an analysis of interfacial stresses [2], accounting for both capillary forces and viscoelastic effects. The co-continuous silica/coacervate shells created here have outstanding barrier and mechanical properties, even for encapsulation of relatively small volatile molecules. As a specific example, we encapsulated model volatile fragrance molecules for protection against evaporation and degradation and demonstrate mechanically triggered release upon fracture. More generally, this method is applicable to a wide range of chemical systems provided their oil/water partition coefficients allow to form an initial oil-in-water emulsion. [1] P Erni, G Dardelle, W Fieber et al. (2013), Angew Chem - Int Ed, 52, 10334. [2] G Dardelle, P Erni (2014), Adv Colloid Interface Sci, 206, 79. [3] G Dardelle, M Jacquemond, P Erni (2016), in press.


GRAVITY-DRIVEN INSTABILITIES IN FIBRILLAR COLLOIDAL GELS CONTAINING A SECOND DISPERSE PHASE

Krassimir P. Velikov, Unilever R&D Vlaardingen ; Soft Condensed Matter, Debye Institute for NanoMaterials

Science, Utrecht University, The Netherlands. [email protected]

Anke Kuijk, Unilever R&D Vlaardingen, The Netherlands. [email protected]

Eline M. Hutter, Unilever R&D Vlaardingen, The Netherlands. Sandra Veen, Unilever R&D Vlaardingen, The Netherlands.

Peter van der Weg, Unilever R&D Vlaardingen, The Netherlands. Fibrillar networks are of great importance for biological systems and many industrial applications. We investigate gravity-driven instabilities in fibrilalr colloidal gels containing a second disperse phase. We use a model system containing a surfactant-stabilized oil-in-water emulsion dispersed in a gel of cellulose microfibrils in the presence of carboxymethyl cellulose. Optical scanning of the creaming emulsion containing gels along the height was used to quantify the network evolution over time. We find a remarkable correlation between the concentration of microfibrils and creaming behaviour such as initial creaming speed and final gel height. We compare this behaviour to the theoretical model of gravitational stability of poroelastic gels, which was extended to account for particle shape anisotropy and the presence of a second disperse phase.


CUBOSOMES AS POTENTIAL THERANOSTIC TOOLS

Sergio Murgia, University of Cagliari, Italy [email protected]

Key Words: Liquid crystalline nanoparticles, nanomedicine. Application of nanotechnology to medicine offers a versatile approach for the design and the development of new multi-functional drug delivery systems where imaging probes, drugs, and targeting agents can be combined and applied as therapeutic/diagnostic (theranostic) tools. Furthermore, this application requires the development of biocompatible and non-toxic systems with tunable properties. Here, monoolein-based nanoparticles known as cubosomes are proposed as theranostic platform in nanomedicine. These nanoparticles show high structural stability and, because of their intrinsic nanostructure, can be loaded with hydrophobic cargos. It will be shown that cubosomes can effectively be loaded with anticancer drugs, UV-visible or NIR emitting fluorophores, while simultaneously conjugated with cells-targeting ligands. Their living cells imaging skills, addressing abilities toward HeLa cells, as well as biodistribution in vivo will be also presented.

Figure 1 – Cryo-TEM image of a cubosomes (left), and fluorescence microscopy image of HeLa cells incubated with monoolein-based cubosomes (right).

Tuesday July 12 2016 Session 3

MODELING THE SHEAR AND EXTENSIONAL RHEOLOGY OF SALIVA AND MUCIN HYDROGELS USING A STICKY GEL NETWORK MODEL

Gareth h. Mckinley, Massachusetts Institute of Technology, USA

[email protected] Caroline wagner, Massachusetts Institute of Technology, USA

Colloidal, Macromolecular and Biological Cells: Formulation, Properties & Applications There is increasing interest in using rheological measurements of saliva and other bodily secretions such as cervical and respiratory mucus as non-invasive diagnostics for pathology and disease. However, there is only limited literature available on the shear and extensional rheology of saliva, and nearly no consideration of its temporal stability in the face of biological degradation. Indeed, capillary breakup extensional rheometry (CaBER) data of saliva samples at various ages shows that both the time to breakup and relaxation time of these highly elastic but low viscosity aqueous solutions decrease as a function of age. The viscoelastic properties of saliva can primarily be attributed to the presence of large glycoproteins (MUC5B mucins) in solution. It is well known that these ‘MUCmers’ physically associate and interact with each other and their surroundings via ion-mediated crosslinking and hydrogen bonding interactions to form a weak hydrogel or ‘pre-gel’. This motivates the development of a Sticky Network model for mucin-containing solutions, building on earlier work of Tripathi et al for synthetic HEUR associative polymer systems. The mucin macromolecules are modeled as a semi-dilute and semi-flexible network of physically-associated finitely extensible elastic segments with a stretch-dependent ‘stickiness’ energy parameter that must be overcome in order for the chains to be able to reversibly dissociate from the rest of the network under imposed deformation. We show that this model is able to accurately capture capillary thinning and filament rupture behaviour of saliva using biologically-derived parameters, and can systematically account for temporal changes in the rheology through a progressive decrease in the molecular weight of the MUC5B chains. To probe the role of the different association mechanisms in the network we construct a series of model mucopolysaccharide hydrogels using purified MUC2 and MUC5 mucins in aqueous solution, We characterize the linear and nonlinear rheology of these physically-associated networks and show that their linear viscoelastic properties are characterized by broad power-law relaxation characteristics. To demonstrate the very wide range of time-scales and length-scales that characterize the different network interactions present in these hydrogels we use a series of additives (low-molecular weight surfactants, salt and reducing agents such as acetylcysteine) to respectively disrupt the hydrophobic, ionic and disulphide interactions between the MUCmers that form the hydrogel network.


ULTRALIGHT, REUSABLE BIOPOLYMER AEROGELS: FORMATION MECHANISMS TO APPLICATIONS IN SELECTIVE FLUID SORPTION AND OIL SPILL REMEDIATION

Anurodh Tripathi, Department of Chemical & Biomolecular Engineering, North Carolina State University, USA

[email protected] Orlando J. Rojas, Department of Chemical & Biomolecular Engineering ; Department of Forest Biomaterials,

North Carolina State University, USA Saad A. Khan, Department of Chemical & Biomolecular Engineering, North Carolina State University, USA

Highly porous (99.7 %), ultra-light (4.3 mg/ml) and mechanically robust cellulose ester aerogels with tailored hydrophobicity are synthesized. The aerogels achieve maximum compression strain of 92 % without failure and reach a compressive stress of 350 kPa, which is 100 times higher than that reported for cellulosic aerogels. In its native, unmodified state, the aerogels are hydrophilic and display unprecedented water uptake (45-90 g/g) while affording wet strength. Further adjustment of the aerogels towards hydrophobicity and oleophilicity via chemical vapor deposition with an organo-silane species reveal them to exhibit high oil retention (20-30 g/g aerogel) while maintaining mechanical integrity for fast oil cleanup from aqueous media under marine conditions. The modified aerogels are reusable and durable as they retain their hydrophobicity for months under ambient conditions. The Zisman and Fowkes theoretical frameworks are used to identify the selectiveness of the aerogel and establish a criterion for separation of various non-polar fluids from water media.


POROSITY GOVERNS NORMAL STRESSES IN POLYMER GELS

Daniel Bonn, nstitute of Physics, University of Amsterdam, Amsterdam, The Netherlands [email protected]

Henri C. G. de Cagny, Institute of Physics, University of Amsterdam, Amsterdam, The Netherlands Bart E. Vos, FOM-Institute AMOLF, Amsterdam, The Netherlands

Mahsa Vahabi, Department of Physics and Astronomy, VU Universiteit Amsterdam, The Netherlands Nicholas A. Kurniawan, FOM-Institute AMOLF, Amsterdam, The Netherlands

MasaoDoi, Center of Soft Matter Physics and its Applications, Beihang University, Beijing, China Gijsje H. Koenderink, FOM-Institute AMOLF, Amsterdam, The Netherlands

Fred C. MacKintosh, Department of Physics and Astronomy, VU Universiteit Amsterdam, The Netherlands When sheared, most elastic solids such as metals, rubbers and polymer hydrogels dilate in the direction perpendicular to the shear plane. This well-known behaviour known as the Poynting effect is characterized by a positive normal stress [1]. Surprisingly, biopolymer gels made of fibrous proteins such as fibrin and collagen and many tissues exhibit the opposite effect, contracting under shear and displaying a negative normal stress [2, 3]. Here we show that this anomalous behaviour originates from the open network structure of biopolymer gels, which facilitates interstitial fluid flow during shear. Using fibrin networks with a controllable pore size as a model system, we show that the normal stress response to an applied shear is positive at short times, but decreases to negative values with a characteristic time scale set by pore size. Using a two-fluid model, we develop a quantitative theory that unifies the opposite behaviours encountered in synthetic and biopolymer gels. Synthetic polymer gels are impermeable to solvent flow and thus effectively incompressible at typical experimental time scales, whereas biopolymer gels are effectively compressible. Our findings suggest a new route to tailor elastic instabilities such as the die swell effect that often hamper processing of polymer materials and furthermore show that poroelastic effects play a much more important role in the mechanical properties of cells and tissues than previously anticipated.


MATERIALS CONSTRUCTION THROUGH PEPTIDE DESIGN AND SOLUTION ASSEMBLY

Darrin J. Pochan, University of Delaware, USA [email protected]

Key Words: hydrogel, peptide, injectable solid, biomaterials, nanofibril Self-assembly of molecules is an attractive materials construction strategy due to its simplicity in application. By considering peptidic molecules in the bottom-up materials self-assembly design process, one can take advantage of inherently biomolecular attributes; intramolecular folding events, secondary structure, and electrostatic interactions; in addition to more traditional self-assembling molecular attributes such as amphiphilicty, to define hierarchical material structure and consequent properties. These self-assembled materials range from hydrogels for biomaterials to nanostructures with defined morphology and chemistry display for inorganic materials templating. The local nano- and overall network structure, and resultant viscoelastic and cell-level biological properties, of hydrogels that are formed via beta-hairpin self-assembly will be presented. Importantly, the hydrogels do not form until individual peptide molecules intramolecularly fold into a beta-hairpin conformation. Subsequently, specific, intermolecular assembly occurs into a branched nanofibrillar network. These peptide hydrogels are potentially excellent scaffolds for tissue repair and regeneration due to inherent cytocompatibility, porous morphology, and shear-thinning but instant recovery viscoelastic properties. Slight design variations of the peptide sequence allow for tunability of the self-assembly/hydrogelation kinetics as well as the tunability of the local peptide nanostructure and hierarchical network structure. In turn, by controlling hydrogel self-assembly kinetics, one dictates the ultimate stiffness of the resultant network and the kinetics through which gelation occurs. Examples of peptide primary structure alteration and the alteration of bulk network properties will be discussed. During assembly and gelation, desired components can be encapsulated within the hydrogel network such as drug compounds and/or living cells. The system can shear thin but immediately reheal to preshear stiffness on the cessation of the shear stress. Additionally, a new system comprised of coiled coil motifs designed theoretically to assemble into two-dimensional nanostructures not observed in nature will be introduced. The molecules and nanostructures are not natural sequences and provide opportunity for arbitrary nanostructure creation with peptides.


BIOPOLYMERS, NANOPARTICLES AND SURFACTANTS: SHORT STORIES IN BUILDING-UP GELS FROM SELF-ASSEMBLY

Cécile A. Dreiss, King’s College London, United Kingdom

[email protected]

Key Words: biopolymers, nanoparticles, cyclodextrins, wormlike micelles Hydrogels obtained from the chemical and physical association of macromolecules, surfactants and nanoparticles, are a huge area of materials science and have found numerous applications in food, personal care products and biomedicine. The macroscopic properties of hydrogels are a complex interplay between the microscopic and mesoscopic supramolecular organization; thus, both their dynamics and structure are dictated by the interactions between the constituents, the fabrication pathway and resulting spatial organization over different length scales. Work in our group has explored various approaches to make gels from non-covalent interactions, spanning biopolymers1-4, wormlike micelles5,6 or host-guest interactions with cyclodextrins7-9. Biopolymers offer a number of advantages over their synthetic counterparts, but suffer from a lack of characterization. This talk will describe our approach to make cheap, functional materials based on widely available biopolymers obtained from natural sources, such as gelatin, or polysaccharides.1-4,10 We will describe the impact of using a hybrid gelation process, combining physical gelling and chemical cross-linking, as well as gels made from hydrophobic interactions between modified biopolymers (dextran or gellan gum) with surfactant micelles.10 Time allowing, I will report on some recent work involving surfactants and laponite architectures, leading to pH- and temperature-responsive gels. The nanoscale morphology of these gels is characterized by small-angle neutron scattering, which is correlated to the rheology of the gels to extract useful structure-function relationships. [1] M.A. da Silva et al. Biomacromolecules (2015) 16, 1401-1409 [2] M.A. da Silva et al. Macromolecular Bioscience (2014) 14, 817-830. [3] F. Bode et al. Soft Matter (2013) 9, 6986-6999 [4] F. Bode et al. Biomacromolecules (2011) 12, 3741-3752 [5] C. A. Dreiss Soft Matter (2007) 2, 956-970 [6] C. Zonglin et al. Chem. Soc. Rev. (2013) 42, 7174-7203 [7] G. González-Gaitano et al. Langmuir (2015) 31, 5645-5655 [8] A.G. Peréz et al. Langmuir (2014) 30, 11552-11562 [9] M.A. da Silva et al. Langmuir (2013) 29, 7697-7708 [10] H. Afifi et al. Soft Matter (2011) 7, 4888-4899


ON CELLULOSE DISSOLUTION AND GELATION

Ulf Olsson, Physical Chemistry, Lund University, Sweden [email protected]

Key Words: Cellulose, solubility, gelation, fibers. Cellulose is the World’s most abundant biopolymer and an important renewable raw material for many materials. For applications like textile fibers it is necessary to first dissolve the cellulose pulp. However, dissolving cellulose turns out to be a challenge. This crystalline polymer is insoluble in most classical polar and non-polar solvents, but soluble in certain ionic liquids and also (partly) in concentrated NaOH. Replacing sodium with the larger and more hydrophobic/amphiphilic tetrabutyl ammonium cation, increases the solubility. Cellulose solutions can turn into gels. The reason for this appears to involve the issue of crystal polymorphism. While natural (wood) cellulose, so called Cellulose I dissolves, solutions may become supersaturated with respect to the more stable Cellulose II. Recrystallizing polymers from semi-dilute solutions may lead to gelation as chains can participate in more than one nucleus.

FIGURE 1 – (TOP) SMALL ANGLE X-RAY SCATTERING PATTERS OBTAINED FROM CELLULOSE

DISSOLVED IN 40 WT.% TBAH(AQ) AT 2 AND 10 G/CM3, RESPECTIVELY. (BOTTOM)

ILLUSTRATING THE DIFFERENT SOLUBILITY OF CELLULOSE I AND CELLULOSE II AND ITS

CONSEQUENCE.

(M. A. Behrens, J. A. Holdaway, P. Nosrati, U. Olsson RSC Adv., 2016, 6, 30199)

(


STRUCTURAL AND DYNAMIC ASPECTS OF PLASTICIZATION AND ANTIPLASTICIZATION IN CARBOHYDRATE GLASSES

Job Ubbink, Food Concept & Physical Design "The Mill", Switzerland ; H.H. Wills Physics Laboratory, University

of Bristol, United Kingdom [email protected]

Key Words: Encapsulation, biostabilization, neutron scattering, dielectric spectroscopy, positron annihilation lifetime spectroscopy Carbohydrate glasses are widely used in the protection of active ingredients in pharmaceutics and in foods. In the pharmaceutical domain, bioactive proteins and peptides are commonly stabilized by amorphous matrices based on the discaccharide trehalose [1], whereas in the food domain, oxidation-sensitive active ingredients, such as polyunsaturated fatty acids and essential oils are often encapsulated in matrices based on starch hydrolysates and sucrose [2]. In recent years, it has become clear that, whereas the glass transition of the glassy matrix is relevant for the protective properties of the amorphous matrix, the glassy-state structure and dynamics excert a controlling role as well, specifically in relation to the antiplasticization of the main constituent of the glassy matrix by low-molecular weight diluents [2]. In this lecture, I am reviewing experimental evidence for the antiplasticization and plasticization in carbohydrate glasses. I will first be discussing recent insights in impact of low-moelcular weight diluents on the dynamics of the glassy matrix as probed by dielectric spectroscopy, neutron scattering and solid-state NMR [1, 3]. I will then turn to our recent insights in the dependence of the molecular packing of carbohydrate glasses on composition, pressure and temperature as determined by positron annihilation life time spectroscopy and volumetric measurements [2, 4]. Finally, I will attempt to link the regimes as identified via the structural and dynamic properties in order to formulate a general hypothesis for the mechanism of plasticization and antiplasticization of carbohydrate glasses by low molecular weight diluents. [1] M.T. Cicerone, M.J. Pikal, K.K. Qian, Stabilization of proteins in solid form, Advanced Drug Delivery Reviews, 93 (2015) 14-24. [2] J. Ubbink, Structural and thermodynamic aspects of plasticization and antiplasticization in glassy encapsulation and biostabilization matrices, Advanced Drug Delivery Reviews (in press), doi:10.1016/j.addr.2015.12.019 (2016). [3] M.T. Cicerone, J.F. Douglas, β-Relaxation governs protein stability in sugar-glass matrices, Soft Matter, 8 (2012) 2983-2991. [4] S. Townrow, M. Roussenova, M.I. Giardiello, A. Alam, J. Ubbink, Specific Volume-Hole Volume Correlations in Amorphous Carbohydrates: Effect of Temperature, Molecular Weight, and Water Content, J Phys Chem B, 114 (2010) 1568-1578.


NATURE-INSPIRED HYDROGELS THAT CHANGE SHAPE IN RESPONSE TO EXTERNAL STIMULI OR TO SPECIFIC BIOMOLECULES

Srini Raghavan, University of Maryland, USA

[email protected]

Materials found in nature frequently show an ability to change their shape or external morphology under different conditions. For example, the leaves of plants like the Venus fly trap transform from an open to a closed shape when an insect sits on it. Inspired by these natural systems, we have attempted to create hydrogels that undergo a change in their shape in response to external stimuli such as pH, temperature, light, or solvent quality. Towards this end, we have developed a method for creating hybrid hydrogels that comprise different gel types juxtaposed in predetermined zones or patterns. When such a hybrid gel is exposed to the stimulus of interest, only the portions of the hybrid that are responsive to the stimulus are affected. This introduces differential stresses in the material, which ultimately results in radical changes in shape. For example, we have made self-folding hydrogel sheets, where an initial flat sheet curls up into a folded tube under the action of the appropriate stimulus. An additional challenge in this area is to design gels that are responsive to specific solutes (e.g., proteins or enzymes, or small molecules such as glucose or fructose). In this regard, we have designed a hybrid gel containing specific regions of a degradable biopolymer. When exposed to a specific enzyme, the biopolymer portions get degraded causing the overall gel to change its shape. This transformation in the gel is highly specific and occurs with very low (millimolar) Sconcentrations of the biomolecule. Such shape-changing gels could have potential applications in biosensing, drug delivery and tissue engineering.

Wednesday July 13 2016 Session 4

SUPERRESOLUTION MICROSCOPY OF INDIVIDUAL AND DENSELY PACKED PNIPAM MICROGELS

Frank Scheffold, Department of Physics, University of Fribourg, Switzerland [email protected]

Gaurasundar M. Conley, Department of Physics, University of Fribourg, Switzerland Sofi Nöjd, Physical Chemistry, Department of Chemistry, Lund University, Sweden

Marco Braibanti, Department of Physics, University of Fribourg, Switzerland Peter Schurtenberger, Physical Chemistry, Department of Chemistry, Lund University, Sweden

Key Words: dSTORM, polymers, pNIPAM microgels, nanoscale materials Responsive microgels are among the most studied polymeric systems of the last decades. The N-isopropylacrylamide (NIPAM) monomer can be readily cross-linked during synthesis to obtain polymer microgel particles with a size that can be controlled in the range 100-1000nm. The vast interest stems from the fact that the polymer is thermosensitive with a lower-critical solution temperature of approximately 32°C, which is close to physiological conditions. The well-defined shape and size of a colloid provides control over the microstructural length scales and response times while the polymeric nature offers physico-chemical control parameters that

can be sensitive to external stimuli. The present work reports on groundbreaking advances in the study of these nanoscale materials. We employ, to our knowledge for the first time, dSTORM superresolution microscopy to study the internal density profile and shape of microgels 'in-situ' - meaning in their natural state in solution. As we demonstrate this allows us to monitor 'live' the change in microstructure across the volume phase transition when particles are swelling and deswelling. Moreover we can observe shape changes, such as faceting, in dense microgel systems where particles are compressed by the presence of its neighbours. This is all but impossible with other nanoscale imaging techniques

used to date (AFM, Cryo-TEM). We demonstrate that the idensity profiles obtained for individual microgels are in quantitative agreement with static and dynamic light scattering results [1]. We believe our work is pioneering and will set the stage for many more applications of superresolution microscopy for the study and characterization of nanoscale polymeric systems including microgels with functional subunits and a more complex architecture. [1] G.M. Conley, S. Nöjd, M. Braibanti, P. Schurtenberger and F. Scheffold, Superresolution Microscopy of the Volume Phase Transition of pNIPAM Microgels, Colloids and Surfaces A, in press

Figure 1 Nanoscale image of microgels. The cut

reveals the internal density distri-bution. dSTORM

resolution: lateral 30nm and axial 60nm.


SPATIAL AND TEMPORAL CRYOEM OF MOLECULAR GELS AND 1-DIMENSIONAL STRUCTURES

Danino, Dganit, Technion – Israel Institute of Technology, Israel [email protected]

Understanding the structure and structure-property-function relationship is key for the development of new functional materials. Structural analysis of multiscale soft systems may, however, be limited due to the ‘invisible’ complexity of the structures. Cryo-electron microscopy (CryoEM) techniques which comprises cryo-TEM and cryo-SEM are non-invasive methods that enable direct detection of soft suprastructures in solution at their hydrated state, at multiple length scales, and at high resolution. Additionally , analysis is done directly, i.e., without the need for a pre-determined model or post-imaging analysis. Cryo-TEM, for example, is highly effective for resolving the coexistence of multiple nanostructures and short-lived intermediates [1], thus providing particle-specific unique data that cannot be obtained from techniques such as scattering or rheology that probe bulk properties. Cryo-SEM covers a wide scale of structures and can readily be applied to highly visocus systems. Combined with another CryoEM method, Cryo-Tomography, one can resolve the detailed spatial organization in 3 dimensions. This talk will focus on characterization of soft molecular matter systems by CryoEM techniques, and will emphasize analysis of molecular gels and 1-dimensional sturtcures, with examples from our recent works with surfactants, lipids, peptides and proteins.

Figure 1. Cryo-TEM analysis of entangled and branched micellar netwworks

References: 1. Danino D, Cryo-TEM of Soft Molecular Assemblies, Curr Opin Colloid In 17(6), 316–329 (2012). 2. Danino et al., From Discs to Ribbons Networks: The Second Critical Micelle Concentration in Nonionic Sterol Solutions J Phys Chem Lett 7, 1434–1439 (2016).


PROBING YIELD STRESS FLUIDS WITH A VIBRATIONAL RHEOMETER

Stuart Prescott, UNSW Australia [email protected]

Nicholas Best, UNSW Australia Hao Ji, UNSW Australia

Patrick Spicer, UNSW Australia Key Words: yield stress measurement, instrumentation. Low volume fraction colloidal gels can possess small yield stresses that are able to trap particles or bubbles within the matrix indefinitely. At rest, the stress applied to the network by a probe particle is limited by the density difference between the probe and continuous phase materials. However, vibration of the sample is an acceleration that causes the inertial particles to impart a stress on the fluid; the stress that results from a vibration is also a function of the frequency and amplitude of the vibration. The microscale fluid properties around the probe particles can be elucidated by studying the effects of vibration on the sample. While applying a vertical mechanical vibration to the sample (1 to 5 mm amplitude, 10 to 100 Hz), we make use of high speed particle tracking to record particle trajectories and measure strain, yielding, flow, and recovery of various complex fluid networks. The measurements enable comparison of the suspension and yielding behaviour of complex fluids with similar rheology but greatly varying microstructures, allowing determination of the optimal approaches to stabilisation of various formulations. Dispersions of colloidal microgels, nanofibres, and wormlike micelles are used in different combinations to explore the robustness of disparate structures to repeated perturbations. Measurements are made of local strain, elasticity, yield stress, and sedimentation rate and compared to continuum predictions for yield stress fluids with more homogeneous microstructures.

Figure 1 – Time slice images of an individual tracer particle within the oscillating sample. The centre

of motion does not change over time for this particle since it is trapped within the network gel.

Figure 2 – Stroboscopic time slice images of a tracer particle that is trapped within the network

gel at the lowest frequencies but is able to sediment as the fluid yields under the larger

stress applied at higher frequencies.

Figure 3 – Digitised trajectory of a sedimenting tracer particle after yielding, along with the trajectory of the

external reference marker used as part of the analysis.


RELATIVE HUMIDITY AS A NEW PARAMETER IN RHEOLOGICAL TESTING

Jörg Läuger, Anton Paar Germany [email protected]

Gunther Arnold, Anton Paar Germany

Key Words: Relative humidity, Moisture content, Gelling, Curing reaction Besides temperature and pressure the water content of a sample as well as the relative humidity of the ambient air are important parameter influencing the rheological behavior of many complex fluids such as for example gels, biomaterials, polymeric systems, food products, and adhesives. The aim of this contribution is to introduce a newly designed environmental control chamber for the use with a rotational rheometer. A combination of a modified convection oven and an external humidity generator enables to work under defined relative humidity (RH) and temperature (T) in the ranges of RH = 5 to 95 % and T = 5 to 120°C. Traditional convection ovens are mainly equipped with electrical heaters. For lower temperatures a cold gas (e.g. LN2) is brought into the chamber and the oven heats against the cold gas. In the new humidity system the convection oven is based on Peltier elements allowing to set temperatures below ambient without the need of a cold gas as input to the oven chamber. In order to control the relative humidity a humidity sensor is located in the oven and the external humidity generator provides the needed moisture of the gas flowing into the chamber. The humidity sensor and the humidity generator are fully integrated into the operating software for the rheometer, allowing the programing of combinations of T and RH including ramps in RH at constant T or ramps in T at constant RH, respectively. Various standard geometries like parallel-plate, cone-and-plate, solid bar for torsional DMTA, extensional tools for extensional DMTA and steady extensional rheological testing, a ball on three plate geometry for tribological investigations, as well as a newly designed modified ring geometry can be used in the humidity system. The later consists of two hollow cylinders, which build a ring geometry. Because the upper cylinder has three rectangular notches only the sample below these notches is used to characterize the sample properties. Hence the characterized sample has a large surface to volume ratio, enabling a fast and homogeneous diffusion of moisture in the sample. Applications examples in the different geometries are presented and show the importance of the relative humidity on a broad variety of samples. The new humidity system allows an easy control of the relative humidity and makes this parameter readily available in advanced rheological testing.


PASSIVE MICRORHEOLOGY AS A USEFUL TOOL FOR MILK GEL ANALYSES

Roland Ramsch, Formulaction, 10 impasse de Bordé Basse, France [email protected]

Giovanni Brambilla, 10 impasse de Bordé Basse, France Mathias Fleury, 10 impasse de Bordé Basse, France

Pascal Bru, 10 impasse de Bordé Basse, France Gérard Meunier, 10 impasse de Bordé Basse, France

Key Words: milk, gel, proteins, gel point, viscoelasticity Passive microrheology based on Diffusing Wave Spectroscopy (DWS) [1,2] is presented as a straightforward tool for the analysis of milk gel preparation. Diffusing Wave Spectroscopy consists of analysing the interferential images of light, which is backscattered by the sample. This so called speckles images, which are detected by a CCD camera, change in time due to the Brownian motion of the particles that scatter the light. The variation of the images as a function of time can be directly correlated to the viscoelastic properties of the sample. As it is an optical method, it is perfectly adapted to study the weak gels of milk products. Nowadays, milk gels such as yogurts or chees have attracted lots of interest due to its growing market. The milk properties, such as pH, calcium content and protein content are very important and change significantly the cheese properties. This work shows how passive microrheology can be used to follow up the milk gel formation with exact gel time determination. Gel time was determined by a new rescaling method, namely Time-Cure Superposition (TCS) [3,4]. This data processing determines the gel point according to the Winter-Chambon criterion [5]. Moreover, the viscoelastic properties of the preparation can be compared according to parameters, such as the protein enrichment, calcium ion addition or others. Results were compared to other instruments (texturometers, rheometer, Optigraph®, etc.). References: [1] D. A. Weitz et al., in Dynamic Light Scattering, W. Brown (Ed.) (Oxford Univ. Press, New York (1993), Chap. 16. [2] D. J. Pine et al., Phys. Rev. Lett. 1988, 60, 1434. [3] T. H. Larsen, E. M. Furst, Phys. Rev. Letters, 2008, 100, 14600 [4] K. M. Schultz, E. M. Furst, Soft Matter, 2012, 8, 6198 [5] H. H. Winter, F. Chambon, J. Rheology 1986, 30, 364-382


SOFT MATTER FILMS INTERFACED TO ELECTRONIC DEVICES: CAPACITANCE-MODULATED FIELD EFFECT TRANSISTORS INTEGRATING PROTEIN LAYERS

Gerardo Palazzo, Dept. of Chemistry and CSGI, University of Bari, Italy

[email protected] Antonia Mallardi, CNR-IPCF, Bari, Italy

Maria Magliulo, Dept. of Chemistry and CSGI, University of Bari, Italy Luisa Torsi, Dept. of Chemistry and CSGI, University of Bari, Italy

Key Words: Biosensors, Field Effect Transistors, ligand binding, Donnan’s equilibria. Soft matter systems interfaced to an electronic device are presently one of the most challenging research activity that has relevance not only for fundamental studies but also for the development of highly performing bio-sensors. Layers of proteins anchored on solid surfaces have small capacitance that undergoes to only minute changes as the ligand–protein complex is formed. For properly designed systems, the protein layer represents smallest capacitance in a series of capacitors and as such dominates the overall capacitance. When such a protein layer is integrated in a Field Effect Transistor (FET) transduction is remarkably sensitive as the transistor output current is governed by the small changes due to ligand binding. These devices operate in aqueous solutions and are promising as portable sensors for point-of-care applications Two recent achievements will be illustrated: A) the sensitive and quantitative measurement of the weak interactions associated with the binding of neutral enantiomers to Odorant Binding Proteins (OBPs) [1]. immobilized to the gate of a bio-FET. Here the minute change in protein layer capacitance upon binding of S(-)-carvone and R(+)-carvone modulate the response of a water-gated OFET, allowing for chiral differential detection. The FET binding curves modelling provide information on the electrochemical free energies derived from the FET dissociation constants while the electrostatic component is isolated from the threshold voltage shifts. These can be combined with the chemical free energies gathered from the complex formation in solution, overall providing a comprehensive picture of the energy balances for a surface-bound pOBP-carvone complex undergoing chiral interactions. B) Hierarchically organized layers of phospholipids and proteins anchored on the surface of the semiconductor and acting as selective recognition elements independently form the solution ionic strength [2-3]. The charged moieties of the bound proteins along with the counter-ions form a layer that is analogous to an ionic gel. The fixed polyelectrolyte ions generate an electric field that confines the mobile counter-ions in the region of the fixed charges. Eventually a Donnan’s equilibrium is reached and the smallest capacitance in series is associated to the Donnan’s electrical double layer. The molecular recognition process (antigen/antibody in the present case) modify the charge density of the outermost layer and thus its capacitance. This capacitive tuning of the bio-FET response is virtually insensitive to the Debye’s length value and therefore is compatible with use of the transistor as sensor directly in biological fluids at high ionic strength . [1] M.Y. Mulla, E. Tuccori, M. Magliulo, G. Lattanzi, G. Palazzo, K. Persaud, L Torsi Capacitance-modulated transistor detects odorant binding protein chiral interactions Nat. Commun. 2015, 6, 6010 doi: 10.1038/ncomms7010 [2] M. Magliulo, A. Mallardi, M. Yusuf Mulla, S. Cotrone, B.R. Pistillo, P. Favia, I. Vikholm-Lundin, G. Palazzo, L Torsi Electrolyte-Gated Organic Field-Effect Transistor Sensors Based on Supported Biotinylated Phospholipid Bilayer Adv. Mater. 2013, 25, 2090–2094 DOI: 10.1002/adma.201203587 [3] G. Palazzo, D. De Tullio, M. Magliulo, A. Mallardi, F. Intranuovo, M.Y. Mulla, P. Favia, I. Vikholm-Lundin, L. Torsi Detection beyond the Debye’s length with an electrolyte gated organic field-effect transistor Adv. Mater. 2015, 27, 911-916. DOI: 10.1002/adma.201403541


SURFACTANT GELS WITH VESICULAR STRUCTURE

Claudia Schmidt, Department of Chemistry, Paderborn University, Germany [email protected]

Felix Grewe, Department of Chemistry, Paderborn University, Germany Stefanie Eriksson, Division of Physical Chemistry, Lund University, Sweden Daniel Topgaard, Division of Physical Chemistry, Lund University, Sweden

Frank Polzer, Institute of Physics, Humboldt University, Berlin, Germany Günter Goerigk, Helmholtz-Zentrum, Berlin, Germany

Key Words: Surfactant gel, vesicle, NMR diffusometry, cryo-TEM. Surfactants are an important class of chemicals, which are used not only as detergents but also in many other areas, such as cosmetics, pharmacy, and the food industry. In aqueous solution, surfactants form aggregates due to their amphiphilic character. This self-assembly leads to micelles and liquid crystalline phases with a variety of structures. Furthermore, non-equilibrium structures, such as vesicles, can be formed, for example, due to shear forces applied when mixing formulations. The structure of a surfactant solution has a large influence on its rheological properties. In particular, vesicular structures often show gel-like properties. We have investigated mixtures of surfactant, fatty alcohol, and water, which show gel-like properties even at high dilution. A system containing sodium dodecyl sulfate (SDS) and cetyl alcohol (CA) [1] with a combined mass fraction of 3 % at varying SDS/CA ratio was studied in detail in order to explore the origin of the gel-like behavior observed at low SDS/CA ratios. The chain melting temperatures of the surfactant/cosurfactant aggregates were determined by proton NMR and differential scanning calorimetry, while the phase structure was investigated by neutron and x-ray scattering as well as by cryo-transmission electron microscopy. Using pulsed field gradient NMR the diffusivity of water in the different samples could be obtained. Gel-like samples were found to show a large fraction of water that has an apparent diffusion coefficient up to three orders of magnitude lower than the one of free water. Combining the results obtained by the different methods a structural model which can explain the gel-like properties could be developed [2]. The gel character is due to large jammed uni- and multilamellar vesicles. As the SDS/CA ratio increases the stacked bilayer membranes cannot be swollen by water any more and a dispersion with a low volume fraction of small compact aggregates results.

[1] R. J. Goetz, M. S. El-Aaser, Langmuir 1990, 6, 132. [2] F. Grewe, J. Ortmeyer, R. Haase, C. Schmidt, "Colloidal gels formed by dilute aqueous dispersions of surfactant and fatty alcohol", in "Colloid Process Engineering", ed. by M. Kind, W. Peukert, H. Rehage, H. Schuchmann, Springer, Heidelberg, 2015, pp. 21-43.

FIGURE 1 – VESICULAR STRUCTURES OF A 3 % SDS/CA/D2O

MIXTURE, RECONSTRUCTED FROM TRANSMISSION ELECTRON

MICROSCOPY


RHEOLOGY AND NONLINEAR MECHANICS OF TRANSIENTLY CROSS LINKED SEMIFLEXIBLE NETWORKS: BUNDLING, RIPPING, HEALING, AND MECHNOMEMORY

Alex Levine, UCLA, USA [email protected]

Transiently cross linked networks of semiflexible filaments make up the principal structural component of the cell — the cytoskeleton. This intracellular network, along with molecular motors, forms the basis for cellular control of morphology and force generation. In this talk, I report on investigations of the effect of transiently bound cross linkers on the structure and mechanics of semiflexible networks. Specifically, I address the role of Casimir or fluctuation-induced interactions between cross linkers in the formation of filament bundles. I report on the linear viscoelasticity of transiently cross-linked networks of bundles. Finally, I discuss the nonlinear mechanical response of such networks, where applied stress induces a persistent structural rearrangement of the network that can dramatically alter its nonlinear response to stresses subsequently applied.


BIO-INSPIRED PROTEIN-BASED BIOMATERIALS

Ulyana Shimanovich, Department of Chemistry, University of Cambridge, UK [email protected]

Weizmann Institute of Science, Israel

Key Words: microgel, protein aggregation, amyloid, protein capsule

Protein functions are as diverse as protein structures. The tunability and biocompatibility of proteins make them attractive candidates for use as building blocks for biomaterials engineering. This strategy provides molecular-level material design, enabling straightforward and independent control over an array of biomaterial properties. A key challenge in our research is to unveil the mechanisms of formation of micro and nano-scale protein-based capsules-gels and shells, as well as to achieve their functionalization for uses in targeted delivery of bioactive materials. The aim to test protein microgels for their ability to act as drug carrier agents, and for the controlled release of different drug-like small molecules as well as the release of the component proteins. Advantages of these systems include compositional definition, control over topology and nanostructure, and the ability to combine multiple different functional components in a modular way.

Thursday July 14 2016 Session 6

CORE-SHELL COMPOSITE HYDROGELS FOR THE CONTROLLED FORMATION AND RELEASE OF NANOCRYSTALS OF POORLY SOLUBLE ACTIVE PHARMACEUTICAL INGREDIENTS

Abu Zayed Md Badruddoza, Massachusetts Institute of Technology

[email protected] P. Douglas Godfrin, Massachusetts Institute of Technology

[email protected] Allan S. Myerson, Massachusetts Institute of Technology Bernhardt L. Trout, Massachusetts Institute of Technology Patrick S. Doyle, Massachusetts Institute of Technology

Key Words: core-shell hydrogels, nanocrystals, crystallization, formulation, confinement Although roughly 40% of pharmaceuticals being developed are poorly water-soluble, this major class of drugs still lacks a formulation strategy capable of producing high loads, fast release kinetics, and low energy input. The development of such innovative biocompatible materials has been a major focus of pharmaceutical materials research. In this work, we develop a novel bottom-up approach for producing and formulating nanocrystals of poorly water-soluble active pharmaceutical ingredients (APIs) using core-shell composite hydrogel beads. We show that the API dissolution profile can be modulated by accurately controlling crystal size and loading and shell thickness. Organic phase nanoemulsions stabilized by polyvinyl alcohol (PVA) and containing a model hydrophobic API (fenofibrate) are embedded in the alginate hydrogel matrix and subsequently act as crystallization reactors. Controlled evaporation of this composite material produces core-shell structured alginate-PVA hydrogels with drug nanocrystals ranging from 500 nm to 650 nm embedded within the core. Adjustable loading of API nanocrystals up to 83% by weight is achieved. Our drug nanocrystal-formulated hydrogels exhibit improved solubility and dissolution rates comparable to commercial dissolution. We also demonstrate that the drug release patterns of the fenofibrate nanocrystals contained in the core can be modulated by altering the thickness of PVA shell of the composite hydrogels. The thickness of the polymer shell of the composite hydrogels can be engineered either by varying the volume fraction of organic phase or by changing the overall core-shell particle size. Thus, these composite materials offer a ‘designer’ drug delivery system by offering a controlled dissolution rate and lag time. Overall, our approach enables a novel means of simultaneous controlled crystallization and formulation of poorly soluble drugs that circumvents energy intensive top-down processes in traditional manufacturing.

FIGURE 1 –SCHEMATIC ILLUSTRATION OF OUR BOTTOM-UP APPROACH TO CORE-SHELL COMPOSITE

HYDROGEL BEAD FORMATION. NANOEMULSION DROPLETS CARRYING HYDROPHOBIC API ARE DISPERSED

IN AQUEOUS SOLVENT CONTAINING POLYMER ALGINATE. CROSS-LINKING THE ALGINATE ENTRAPS THE

NANOEMULSION DROPLETS IN A POLYMER MATRIX. SUBSEQUENT EVAPORATION FORMS CORE-SHELL

HYDROGELS CONTAINING API NANOCRYSTALS EMBEDDED IN THE CORE.


STRUCTURE-PROPERTIES RELATIONSHIPS OF MULTICOMPONENT POLYSACCHARIDE-PEPTIDE HYDROGELS

Ronit Bitton, Department of Chemical Engineering and The Ilse Katz Institute for Nanoscale Science

&Technology, Ben-Gurion University of the Negev, Israel [email protected]

Key Words: Hydrogels, Polysaccharides, Peptides, SAXS, Rheology Over the past two decades, the potential of hydrogels as scaffolds for tissue regeneration has been widely explored due to their similarity to native extra cellular matrices (ECMs) and the ability to easily adjust both their physicochemical properties as well as their mechanical properties to meet the demands for tissue scaffolds and cell encapsulation. To better emulate the functionality of the natural ECM, current effort in the engineering of synthetic extracellular matrixes has focused on installing molecular features (proteins and bio-interactive polymers) within insoluble scaffolds, either by self-assembly or through covalent modifications of polymer or biopolymer networks. Polysaccharides (e.g hyaluronic acid, alginate, and chitosan) – being non-toxic, hemocompatible, and relatively cheap– possess many of the favorable properties required for biomaterials. Combining polysaccharides and peptides for creating hydrogels for tissue engineering is of particular interest, due to the complementary properties of both molecules: Bio-functionality of the peptides on one hand and similarity to the natural ECM of the polysaccharides on the other. Apart from their direct role in cell interaction, peptide sequences may affect the hierarchical structural organization and mechanical properties of the resulting hydrogel, thus indirectly affecting the cellular response. The overall aim of our study is to develop a fundamental understanding of the structure-mechanical properties relations of multicomponent polysaccharide hydrogels used in tissue engineering applications Here we present a systematic investigation of the effect of RGD- containing peptides on the hierarchical structure of polysaccharide-peptide hybrids (solutions and gels). Polysaccharide type, ligand incorporation method (covalently attached or self-assembled) as well as peptide nanostructure and amount were tested using advanced tools including small angle X-ray scattering (SAXS), electron microscopy and rheology. Our results show that the fraction of the covalently bounded peptide, determines the behavior of a polysaccharide-peptide conjugates in solution, regardless of the specific nature of the polysaccharide. And that the peptides' ability to self-assemble in aqueous solution affects the spatial organization of the polysaccharide and the mechanical properties of the polysaccharide /peptide hybrid hydrogel, both when the peptide is covalently attached to the polysaccharide backbone and when peptide and polymer solutions are simply mixed together. These results indicate the importance of possible intermolecular interactions between the peptide and the polymer in determining the hydrogel's properties. Our findings suggest that elucidating key factors involved in the structure-property relationships of these systems will improve our ability to design and prepare tailor-made scaffolds for a variety of applications.


FROM DILUTE POLYELECTROLYTE SOLUTIONS TO ENTANGLED POLYELECTROLYTE NETWORKS: A STUDY OF SODIUM CARBOXYMETHYL CELLULOSE IN WATER BY LIGHT SCATTERING AND

RHEOLOGY

Juliette S. Behra, School of Chemical and Process Engineering, University of Leeds, UK [email protected]

Timothy N. Hunter, School of Chemical and Process Engineering, University of Leeds, UK Olivier J. Cayre, School of Chemical and Process Engineering, University of Leeds, UK

Johan Mattsson, School of Physics and Astronomy, University of Leeds, UK

Key Words: polyelectrolyte, biopolymer, light scattering, rheology, sodium carboxymethyl cellulose Sodium carboxymethyl cellulose (Na CMC) is widely used in industry for its thickening and swelling properties. Applications are very broad and include pharmaceutical, food, home and personal care products as well as the paper industry, water treatment and mineral processing. Na CMC is a linear negatively charged water-soluble polymer derived from cellulose. Its behaviour in water is known to be very complex and a function of several parameters including the characteristics of the polymer itself [1] such as molecular weight and degree of substitution as well as the solution concentration and dissolution conditions [2] (e.g. addition order of the system components) [3]. While Dynamic Light Scattering (DLS) has been widely used to study the behaviour of polyelectrolytes, relatively few DLS studies have been conducted on Na CMC and, to our knowledge, none in pure water; this is most likely due to the difficulty of preparing salt-free Na CMC solutions of DLS grade. Indeed, the presence of even a few poorly substituted Na CMC fibres suffices to prevent proper DLS data from being collected. The aim of the present study was to investigate the behaviour of Na CMC (Mw = 700,000 g/mol; DS = 0.9) in pure water using both DLS and rheology measurements. A method was developed to prepare samples of appropriate quality for DLS measurements, which could then be successfully run over a wide range of concentrations. Rheology measurements were run in parallel to identify the different concentration regimes, facilitating comparisons to the behaviour typically found for polyelectrolytes (see Figure 1). Both DLS and rheology measurements were combined to look at the relationships between the structure of the Na CMC solutions and their rheological properties.

FIGURE 1 – SPECIFIC VISCOSITY (ΗSP) AS A FUNCTION OF NA CMC CONCENTRATION (COMPLETION IN PROGRESS) η0: zero-shear viscosity (obtained from the Carreau model); ηs: solvent viscosity (experimental value); the values given below the names of the different concentration regimes are the expected exponents of the power laws of the specific viscosity as a function of the polyelectrolyte concentration. Though the experimental exponents are

slightly higher than the theoretical ones, they are in agreement with the literature about Na CMC [3,4].

1. W.-M. Kulicke et al. Polymer, 1996. 37(13): p. 2723-2731. 2. X. Yang and W. Zhu. Cellulose, 2007. 14(5): p. 409-417. 3. D. Truzzolillo et al. Physical Chemistry Chemical Physics, 2009. 11(11): p. 1780-1786. 4. C.G. Lopez et al. Journal of Polymer Science, Part B: Polymer Physics, 2015. 53: p. 492-501.

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ON THE CREEP RINGING BEHAVIOR OF SEMI-DILUTE POLYACRYLAMIDE AND POLYETHYLENE OXIDE SOLUTIONS

Thomas B. Goudoulas, Fluid Dynamics of Complex Biosystems, School of Life Science Engineering

Weihenstephan, Technische Universität München, Freising, Germany [email protected]

Natalie Germann, Fluid Dynamics of Complex Biosystems, School of Life Science Engineering Weihenstephan, Technische Universität München, Freising, Germany

Key Words: Polyacrylamide solutions, Polyethylene oxide solutions, Creep ringing method, Jeffreys model. Aqueous polyacrylamide solutions (PAAm) and polyethylene oxide solutions (PE) of 5, 10, and 15 wt% were characterized under creep measurements. To the best of our knowledge it is the first time that the creep ringing method is used to study these types of polymeric materials. The rheometric measurements were carried out using an Anton Paar MCR 502 rheometer, equipped with a parallel plate measuring geometry with sandblasted surfaces. By performing a stepwise adjustment of the gap, it was possible to keep the maximum normal force during the loading procedure below 5 N. In addition, after each step the time evolution of the relaxation was recorded. In this way, the measurements could be carried out on fully relaxed samples. Systematic creep measurements showed that the initial response correlates to the moment of inertia of the instrument and the geometry. All samples exhibited dumping oscillations. For both PAAm and PE, a profound effect of the polymer concentration on the characteristic ringing frequency and amplitude of oscillations was found. Although, the results are qualitatively comparable, PE exhibits much higher storage and loss moduli. Independent of the magnitude of the applied stress, the initial ringing data coincide until the end of the ringing behavior. However, the long-term creep behavior is significantly affected by the magnitude of the applied stress. In addition, we studied the impact of NaCl and the molecular weight of the polymer on the viscoelasticity of the PAAm solutions. We found that the ionic strength affects both the frequency and duration of ringing. In addition, we found that decreasing the molecular weight of PAAm decreases the amplitude of oscillation and ringing frequency. If the instrument inertia is taken into account, the Jeffreys model provides a satisfying fit to the creep data. All values of the viscoelastic parameters presented here were obtained by fitting this model to the creep compliance curve. The present study shows that the creep ringing method is an extremely helpful since the short- and long-term creep compliance can be simultaneously obtained.

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FIGURE 1 – EFFECT OF PAAM MOLECULAR

WEIGHT ON THE CREEP RINGING BEHAVIOR

FIGURE 2 – EFFECT OF THE POLYMER TYPE

ON THE CREEP RINGING BEHAVIOR

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