Optically driven translational and rotational motions ofmicrorod particles in a nematic liquid crystalAlexey Eremina,1, Pemika Hirankittiwonga,b,c, Nattaporn Chatthama,b,c, Hajnalka Nádasia, Ralf Stannariusa,Jumras Limtrakulc,d, Osamu Habae, Koichiro Yonetakee, and Hideo Takezoea,1

aDepartment of Nonlinear Phenomena, Institute for Experimental Physics, Otto von Guericke University Magdeburg, Magdeburg 39016, Germany;Departments of bPhysics and dChemistry, Faculty of Science, Kasetsart University, Bangkok 10900, Thailand; cVidyasirimedhi Institute of Science andTechnology, Rayong 21210, Thailand; and eOrganic Device Engineering, Yamagata University, Yonezawa 992-8510, Japan

Edited by David A. Weitz, Harvard University, Cambridge, MA, and approved January 7, 2015 (received for review October 16, 2014)

A small amount of azo-dendrimer molecules dissolved in a liquidcrystal enables translational and rotational motions of microrodsin a liquid crystal matrix under unpolarized UV light irradiation.This motion is initiated by a light-induced trans-to-cis conforma-tional change of the dendrimer adsorbed at the rod surface andthe associated director reorientation. The bending direction of thecis conformers is not random but is selectively chosen due to thecurved local director field in the vicinity of the dendrimer-coatedsurface. Different types of director distortions occur around therods, depending on their orientations with respect to the nematicdirector field. This leads to different types of motions driven bythe torques exerted on the particles by the director reorientations.

colloidal inclusions | liquid crystals | photoisomerization | elasticity |topological defects

Liquid crystals (LCs) are self-organized mesomorphic materi-als that exhibit various symmetries and structures (1). They

are widely used in flat panel displays for their exceptional elec-trooptical properties and a combination of orientational elasticityand fluidity. For example, nematic LCs (NLCs) are distinguishedby their long-range orientational order, which favors alignment ofthe molecules (mesogens) in a preferred direction denoted as thedirector n. An exceptional feature of NLCs is that, despite theirfluidity, they exhibit anisotropic optical and mechanical properties,and thus can transmit mechanical torque because of directionalelasticity (1). Such torque occurs in response to deformations awayfrom a uniform equilibrium state.Such unique features of LCs can be exploited for designing

smart multifunctional materials. Among these materials, colloi-dal dispersions of microparticles and nanoparticles in LCs havebeen actively studied in research on soft-matter physics (2–7).Tunable anisotropic interactions between microparticles dis-persed in LCs give rise to self-assembled 1D and 2D colloidalstructures (6, 8, 9). Such colloidal dispersions are interesting notonly from a fundamental point of view but also from a techno-logical one. A wide range of self-assembled structures of par-ticles and topological defects stabilized by LC-mediated inter-actions find numerous applications in designing metamaterials(10), photonic devices (5, 11), sensors (12), and microrheology(5, 10–12). Here, we demonstrate a phenomenon that can beused in intelligent devices using colloidal dispersions: controlledlight-driven translational and rotational motions of microrods ina NLC matrix.The orientation of LC molecules at an interface is governed by

anchoring conditions, i.e., whether the director is perpendicular(homeotropic) or parallel (planar) to the interface. The orientationof the director at surfaces can be controlled through interfacialenergy, anisotropy of the surface tension, and surface topography,by the pretreatment of the surfaces using surface agents, such aspolymers and surfactants, together with mechanical or opticaltreatments. In most of the previous experiments, the solid interfacewas fixed. Here we use a so-called command surface (13) for LCalignment and its real-time control; the director manipulation is

achieved by manipulating the anchoring condition through a light-induced isomerization of a photoactive azo-dendrimer adsorbed atthe surface of microrods. As the light-induced isomerization takesplace, the anchoring conditions provided by the cis isomer aredifferent from those of the trans isomer. The initial equilibriumstate of the director is lost. An important point here is that thebending direction in the cis form is not random but determined bythe distorted local director near the surfaces. The torque on theparticle exerted by the liquid crystal director reorientation results inspecific particle motions toward a new equilibrium state. Suchmolecular-assisted manipulation of particles provides a tool forstudying interfacial effects in their interplay with the topology ofthe nematic director field, which is a key concept for smartmicrodevices. Development of such devices requires better un-derstanding of the photoisomerization. Some studies of azo-benzenes bound to a dendrimer core have been conducted andreported in the literature (14, 15). The photoisomerization mech-anism, the dependence of the quantum yield on the phenyl ringsubstituents, the solvent properties, and the irradiation wavelength,are still not fully understood (16). The molecular groups attachedto the dendrimer core with highly regular branching do not en-tangle as in the case of conventional macromolecules and seem toexpress similar properties to the original azobenzene chromo-phores. Thus, this provides another way to study isomerization andphotochemical properties of azobenzene molecules.Active steering of particles in LC hosts is usually achieved by

convection mechanisms, such as electrophoresis, or using high-power laser radiation in optical tweezers (17). Manipulation of themesogen orientations is another way to control colloidal particles in

Significance

This paper addresses light-driven dynamic motions of micro-rods suspended in a liquid crystal (LC) host. The results pre-sented in this paper advance molecular-assisted manipulationsof colloidal particles and their assemblies, which is a key con-cept for smart microdevices, using their interplay with the to-pology of the nematic director field. Almost all previous studieson colloids in LC (except some work by Yamamoto et al.) havedealt with the observation and analysis of defect structuresaround colloid particle(s) with fixed anchoring conditions.Herein we use a photoactive surfactant, which enables controlof the LC anchoring by light irradiation. The induced reversibleanchoring transition is accompanied by a reorientation of therods and/or their translational motion.

Author contributions: A.E., R.S., and H.T. designed research; A.E., P.H., and H.N. per-formed research; O.H. and K.Y. contributed new reagents/analytic tools; A.E., P.H., N.C.,and H.N. analyzed data; and A.E., R.S., J.L., and H.T. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.1To whom correspondence may be addressed. Email: alexey.eremin@ovgu.de or takezoe.h.aa@m.titech.ac.jp.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1419850112/-/DCSupplemental.

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an LC host. This approach mimics molecular motors, which useconformational changes of molecules (18). Several types of artificialmolecular motors and actuators were designed whereby the energyof light is transformed into the mechanical energy (19–21). In thecase of LCs, a change of the anchoring conditions can be achievedby photosensitive functionalizations of the surface with mesogen-like moieties connected via light-sensitive azo linkages. Yamamotoet al. used a photosensitive surfactant and succeeded in manipu-lating the anchoring condition of colloidal particles in NLCs (22).There is, of course, a large number of papers dealing with light-driven motion and deformation in (soft) solids, e.g., liquid crystalelastomers that make use of cis−trans isomerization (for instance,refs. 23–29).

Yonetake et al. synthesized a poly (propyleneimine) liquid crys-talline dendrimer, which spontaneously adsorbs at LC−glass inter-faces and favors homeotropic alignment of NLC molecules (30).This material was used to fabricate in-plane-switching-mode LCdisplays without pretreatment of substrate surfaces (31). Li et al.recently synthesized a photosensitive dendrimer with azo linkages(azo-dendrimer) in the mesogenic end chains (Fig. S1) and dem-onstrated the adsorption of the dendrimer at a glass interface (32).By UV irradiation, an orientational change of the mesogenic moi-eties occurs associated with the trans−cis photoisomerization, asillustrated in Fig. 1 A and B. Such asymmetric adsorption of den-drimers at surfaces has already been proved with a surface second-harmonic generation experiment (33). Hence, the director field wasdistorted under suitable illumination. The azo-dendrimer has al-ready been used for controlling ordering transitions in mobile LCmicrodroplets in a polymer matrix (34) and defect structures inmicroparticles in LCs (35). In contrast, the situation is different inour study: Because the embedded particles are mobile and aniso-metric (rod-shaped), light irradiation brings about a dynamicmotion of the enclosed colloids. We exploit the spontaneous ad-sorption of the photoactive dendrimer on various interfaces tofunctionalize the surfaces of rod-shaped microparticles suspendedin a nematic host. As a result, we not only can control themolecularorientation at the immobile rods by light but can also mechanicallyrotate and translate them.Now we describe our present experimental results: All of the

experiments were performed using 4′-n-pentyl-4-cyanobiphenylliquid crystal (5CB) mixed with 0.1 wt% azo-dendrimer and glassrods of 10- to 20-μm length and 1.5-μm diameter. For details, thereader is referred to Experimental Procedures. First, we describethe photo-induced director field change around immobilemicrorods. The changes of the director configuration can beeasily studied when the rods are immobile, i.e., attached to theglass substrate. The case of cells with homeotropic anchoring is

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Fig. 1. Conformations of the photoactive dendrimer: (A) trans-conforma-tion favoring homeotropic alignment of the mesogens and (B) cis-confor-mation favoring planar alignment obtained under UV light irradiation.

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Fig. 2. Immobilized microrods attached to a surface in a planarly aligned LC cell. The rubbing direction is the horizontal direction of the images. (A) A rod lyingalong the rubbing direction without UV irradiation. The director map with dipolar point defects is shown, together with a microscope image in the Inset. With theinserted λ wave plate diagonal to the crossed polarizers, different birefringence colors are seen at both sides along the rod, and they are consistent with thedirector map shown. (B) The same rod under UV irradiation. In the vicinity of the particle, each of the director fields in A and B is mirror symmetric in the cell planeand, in first approximation, axially symmetric about the rod axis. (C) A rod lying approximately diagonal to the rubbing direction without UV irradiation. Thedirector map with quadrupolar point defects is shown, together with a microscope image in the Inset. (D) The same rod under UV irradiation. Both director fieldsin C and D have C2 symmetry about the cell normal in the cell midplane. The disclination loop shown in C, that is present at normal anchoring, vanishes under UVirradiation when the director anchors tangentially, as shown in D. It leaves two point defects. The director orientation and its change by UV irradiation are clearlyvisible in the images with a wave plate (see Insets). (E) Image of a rod between crossed polarizers without UV irradiation. (F) A bright field image of the same rodwithout polarizers. The arrows indicate the position of topological defects at the ends of the rod. The cell thickness is 10 μm.

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described in Fig. S2. The situation is more complicated in cellswith a planar anchoring condition. Several configurations arepossible: rods aligned parallel (i), perpendicular (ii), and di-agonal (iii) to the rubbing direction (Fig. S3). Experimentally,many rods are initially aligned at an angle Θ of 60°–70° to thedirector n, although normal anchoring conditions seem to favorΘ = 90°. This discrepancy may be attributed to an asymmetry of thedirector field at the ends of the rod and in the vertical dimension,and the different energies of the associated defect lines. Two kindsof director field configurations were found: dipolar with a singlehyperbolic defect (Fig. 2A) in case i and quadrupolar with a dis-clination loop surrounding the rod (Fig. 2C) in case iii. Theseconfigurations have been established earlier in thin 2-μm cells byTkalec et al. (6). In our case, to study mobile rods, thicker cells arepreferred, which, however, makes the optical characterizationdifficult. Without UV irradiation, the director is normal to thesurface of the rod (Fig. 2 A and C). This corresponds to a radialhedgehog configuration of the director field with the effectivetopological strength +1. To comply with the homogeneous far-field director outside the rod, a disclination loop is expected toencircle the rod in case iii. Evidence of that is shown in Fig. 2 Eand F, and the structure of the disclination loop is schematicallysketched in Fig. 2C. The loop is attracted to the diagonal edges ofthe rod to relive the mechanical strain on the director field.

Under UV irradiation, the director configuration changes: Thenormal configuration of the director transforms into a tangentialone. This is evident from the changes of the interference colors tothe complementary ones observed with a wave plate (see sche-matics in Fig. 2 A−D, and microscope images in Fig. 2 A−D,Insets). The loop collapses and transforms into two separated

surface boojums with1/2 topological charge on the surface of therod to satisfy planar anchoring conditions. This motion results ina pair of defects attached to the corners (see Fig. 2F). Disclinationloops (Saturn ring) were confirmed by Tkalec et al. (6) for rodsaligned perpendicular to the rubbing direction (case ii). Thoseloops were also tilted with respect to the rods axes.Free rods exhibit opto-mechanical responses: (i) rotation of

rods in the cell plane, (ii) rotation in the vertical plane, and(iii) translation in the cell plane. In the first case, the rods initiallyappear at an angle of ∼70° to the nematic director (Fig. 3 A andB). Under applied UV irradiation, the rods align nearly along thedirector (Fig. 3 A and C). In the second case (Fig. S4), rotationoccurs about an axis perpendicular to the rubbing direction and thecell normal. In both cases, the rotations are fully reversible. Therods return to the original state when UV irradiation is removed(Fig. 3A). The angular variation and the switching rate depend onthe intensity of the UV irradiation (Fig. 3F). The rotational motioninvolves several stages. In the first stage, the reorientation of thedendrimer moieties takes place resulting in a change of the an-choring condition from nearly orthogonal to nearly planar. Thenext (fast, characteristic time scale is 0.01–0.05 s) stage is accom-panied by a rearrangement of the topological defects and a con-tinuous reorientation of the director field. This triggers the last(slow, characteristic time scale is 0.1–2 s) stage: motion of the rod.The time dependence of the rod reorientation is shown in Fig. 3F.The solid curve is a best fit to one of the experimental data based ontheoretical consideration (SI Text). Even at low light intensities, therotation occurs, but the rotation angle is reduced (Fig. 3G). Bothangular variations and the switching rates show a saturation-type

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Fig. 3. Dynamic motion of rods under the action of UV irradiation. (A) Time dependence of the angle between the rod and the rubbing direction. (B) Image of theinitial state of a rod without UV, and (C) the final state under UV irradiation. The length of the rod is 20 μm. The rubbing direction is marked by a white arrow(parallel to n). B and C reveal the rotational motion. (D) Image of a rod without UV irradiation. (E) Image of a rod under UV irradiation at texp = 43 s. D and E revealthe translational motion. A white arrow marks the rubbing direction. A dashed line indicates the initial position of the rod. Images D and E were taken betweenpolarizers parallel to the sides of the frame. The length of the rods in D and Ewas 15 μm. (F) Time dependence of the rod inclination at various intensities of the UVlight. A solid line is the theoretical fit of the experimental data (see SI Text for details). (G and H) UV intensity dependences of the deflection angle and the switchingrates, respectively. The saturation behaviors are theoretically explained (compare H and Fig. S5B).

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dependence on the UV intensity (Fig. 3 G and H), which is alsoexplained theoretically (Fig. S5B).Translational motion was observed in some rods, and the di-

rection was more or less parallel to the long axis of the rods. Fig.3 D and E shows the initial and intermediate stages during UVirradiation. The translational movement was extended to dis-placements of the order of one rod length. We could observe aslight backlash motion when the UV irradiation was stopped.Since UV irradiation affects the conformational state of the

dendrimer adsorbed at the rod surface, it is reasonable to considerthat the director reorientation direction is directly related to thecis conformation. Straight (trans) to bent (cis) conformational

change occurs upon photoisomerization and results in a changeof the anchoring condition. The rotational motion of the rodsoriginates from the torques exerted by the director. How do thosetorques develop? The initial state depicted in Fig. 3B is stabilizedby the director deformation and the configuration of the topo-logical defects around the rod as well as the anchoring on the rodsurface. The torque by the director in the volume is counteractedby the (anchoring) torque at the rod/LC boundary. Under UVlight irradiation, the orthogonal anchoring condition changes tothe planar one and the torque in the volume is not in balance withthe boundary any more. This leads to the rotation of the rod to thenew equilibrium state. In the first approximation, one may de-scribe this anchoring by a potential as Wasin2ðθÞ, where θ is thepolar anchoring angle with respect to the surface, and Wa is thestrength of the anchoring energy (1). The anchoring energy isdetermined by the ratio of cis and trans isomers at a given lightintensity and spectral composition. Planar anchoring correspondsto positive Wa, while the orthogonal anchoring is achieved withWa < 0. Strong anchoring is given if the penetration lengthξ= jK=Waj is short respective to typical geometrical sizes of theexperiment; for weak anchoring, ξ is comparable or larger thansystem sizes. Obviously, strong illumination (large fraction of cismolecules) will correspond to large positive Wa, while no illumi-nation (mainly trans molecules) corresponds to large negative Wa.The experiments suggest a monotonous functional form of thetransition between both states with increasing/decreasing illumi-nation intensity. One can demonstrate that intermediate sta-tionary Wa   situations can be reached at certain moderate UVintensities. Then, the director field is not influenced by the rod;the microscopy image has a uniform color around the inclusions.This corresponds to an infinite ξ. These arguments enable us toroughly estimate the switching rates and qualitatively describetheir dependence on the light intensity, as described in SI Text. Atstrong anchoring, the torque acting on the director is proportionalto KL, where L is a characteristic length (of the order of magni-tude of the rod length) and K is the mean Frank elastic constant.The viscous torque is proportional to ηL3, where η is a meanviscosity of the liquid crystal. This results in a switching timeτ= ηL2=K ≈ 0:5  s. In case of weak anchoring, the switching rate Γdepends on the light intensity through the penetration length  ξ :Γ=K=½ηLðL+ ξÞ�. Since short ξ correspond to high UV in-tensities, whereas low UV intensities yield long  ξ, the switchingrate increases with UV intensity and is expected to reach somesaturation Γ = 1/τ. A qualitative agreement of the estimated

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Fig. 4. Schematic illustrations of the microrod motions and the acting tor-ques. (A) Translational and (B) rotational motions. Azo-dendrimers and theirbending direction are marked by brown lines and arrows, respectively. Redarrows show the director rotation from 1 (normal to the rod) to 4 (parallel tothe rod) and the torque direction. Green arrows designate the rod motion.

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Fig. 5. Switchable agglomerates of particles stabilized by the director-mediated interactions: doublets of rods (crossed polarizers with a wave-plate) withoutUV (A) and under UV irradiation (B). There is no rotation of the doublet since the mechanical torques on two arms cancel out. The switching of the director isclearly seen from the interference colors. A dumbbell of a rod with two beads without UV (C) and under UV irradiation (D) in unpolarized light. The directororients along the vertical direction in the images.

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switching rate with the experiment can be found by comparing Fig.3H and Fig. S5B.How is the reorientation direction chosen? For rods with

a surrounding director field similar to that shown in Fig. 2A, thedirector has an opposite sense of bending on two sides of the rod,as recognized by different birefringence colors under a waveplate. UV light irradiation under this condition induces the op-posite director rotation at both sides of the rod (Fig. 4A), leadingto lateral translational motion. In contrast, for rods similar tothose shown in Fig. 2C, the director bending direction is thesame at both sides (Fig. 4B), as shown by the same birefringencecolor. Hence, the same director rotation at both sides of the rodupon UV light irradiation exerts torques with the same sign,causing the rod to reorient (Fig. 4).Interactions between microparticles dispersed in an LC matrix

lead to a formation of complex ordered structures. Such agglom-erates of particles may have a form of chains or even more in-tricate structures. In this case, optomechanical effect manifests inthe motion of either the whole agglomerate or its separate parts.Different examples of such structures are shown in Fig. 5.In conclusion, we demonstrated the optomechanical effect of

light-induced rotation and translation of micrometer-sized rod par-ticles in a nematic host. This system represents an optically drivenmolecular microactuator, which exploits molecular reorientation ona particle surface and transforms it into a mechanical torque.

Experimental ProceduresWe used a commercially available 5CB liquid crystal (Sigma-Aldrich) mixedwith 0.1 wt% of poly(propyleneimine)-based azo-dendrimers (32). Glassmicrorods of a diameter 1.49 ± 0.07 μm and lengths between 10 μm and25 μm (PF-15S, Nippon Electric Glass) were mixed with the LC. Both weredissolved in chloroform solvent, and the solution was heated to removechloroform at 70°C.

The experiments were performed using commercial sandwich cells(E.H.C.) with thicknesses 6 μm, 10 μm, and 25 μm. Three types of surfacetreatment were used: (i) nontreated glass, (ii) glass treated for homeotropicanchoring condition, and (iii) polyimide-treated rubbed substrate. In thethird case, the mesogenic director aligns along the rubbing direction of thepolyimide layer.

The microscope observations were made using a polarizing microscopeAxioImager 1.A Pol (Carl Zeiss GmbH) equipped with an AxioCam CCDcamera. Optical characterization of the birefringence was made usingstandard compensation techniques (wave compensator, Berek compensator).A 100-W tungsten lamp in transmissionmodewas used for illumination of thesample. UV irradiation was made simultaneously through a reflection beampath of the microscope. As a UV source, we used the 365-nm line of a 100-WMercury lamp. The filtering was achieved by a G365 interference filter fol-lowed by a dichroic mirror FT395 (Carl Zeiss GmbH).

ACKNOWLEDGMENTS. We acknowledge the support of the Alexander vonHumboldt Foundation for N.C. and H.T.’s stay in Magdeburg, as well as theGerman Research Foundation for support within Project STA 425/28.

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