18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016 Non-intrusive wettability characterization on complex surfaces using 3D Laser Scanning Confocal Fluorescence Microscopy Dalila Vieira 1 , Ana S. Moita 1,* , António L. N. Moreira 1 1: IN+ - Dep. of Mech. Eng., Instituto Superior Técnico, Universidade de Lisboa, Lisbon, Portugal * Correspondent author: [email protected]Keywords: Contact angles; Fluorescence Imaging, 3D Laser Scanning Confocal Fluorescence Microscopy; Wettability; Complex Surfaces ABSTRACT The present paper addresses a non-intrusive technique, the Laser Scanning Confocal Fluoresce Microscopy - LSCFM to characterize the wettability on complex surfaces. The technique is firstly validated comparing the equilibrium contact angles obtained by an optical tensiometer with those measured using the LSCFM, for milimetric droplets deposited on smooth glass surfaces. The results show that the contact angles measured by the LSCFM are consistently similar to those measured by the tensiometer, although they are generally lower. A detailed analysis on the accuracy of the measurements shows that the angles obtained with the tensiometer are more likely to be affected by the flattening of the droplet, which is particularly evident in large droplets, where gravitation effects can play already an important role. Substantial differences are however obtained between the apparent angles measured by the tensiometer and those evaluated with the LSCFM technique. Hence, the liquid-solid contact line is visibly distorted by the pillars, which results in high differences in the local contact angles. Nevertheless, the measurements performed show much more stable and consistently constant angles measured with the LSCFM technique, when repeating the measures on surfaces with similar topology. This is possible as the LSCFM technique can provide a detailed reconstruction of the surface topology at the liquid-solid interface region. This feature allows identifying stable Wenzel wetting regimes, in geometries for which the apparent contact angles measured with the tensiometer can be up to 40º higher than those obtained with the LSCFM. Following these results, a few usual approaches to predict the wetting regimes, based on geometric relations are revisited. The analysis suggests that these approaches allow identifying trends with the apparent angles, but fail in predicting the angles measured by the LSCFM, being therefore unable to predict the unstable non-wetting regimes that were observed. 1. Introduction The wettability is a key parameter governing heat, mass and momentum transport at liquid- solid-vapor interfaces, which affects numerous applications. For instance, enhanced surfaces with variable wetting regimes are known to strongly affect nucleation and bubble dynamics, with consequent increase of the heat transfer coefficients (e.g. Betz et al., 2013, Moita et al., 2015, Valente et al., 2015). In the coating industry, the development of particular coatings over oil
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18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016
Non-intrusive wettability characterization on complex surfaces using 3D Laser Scanning Confocal Fluorescence Microscopy
Dalila Vieira1, Ana S. Moita1,*, António L. N. Moreira1 1: IN+ - Dep. of Mech. Eng., Instituto Superior Técnico, Universidade de Lisboa, Lisbon, Portugal
The present paper addresses a non-intrusive technique, the Laser Scanning Confocal Fluoresce Microscopy - LSCFM to characterize the wettability on complex surfaces. The technique is firstly validated comparing the equilibrium contact angles obtained by an optical tensiometer with those measured using the LSCFM, for milimetric droplets deposited on smooth glass surfaces. The results show that the contact angles measured by the LSCFM are consistently similar to those measured by the tensiometer, although they are generally lower. A detailed analysis on the accuracy of the measurements shows that the angles obtained with the tensiometer are more likely to be affected by the flattening of the droplet, which is particularly evident in large droplets, where gravitation effects can play already an important role. Substantial differences are however obtained between the apparent angles measured by the tensiometer and those evaluated with the LSCFM technique. Hence, the liquid-solid contact line is visibly distorted by the pillars, which results in high differences in the local contact angles. Nevertheless, the measurements performed show much more stable and consistently constant angles measured with the LSCFM technique, when repeating the measures on surfaces with similar topology. This is possible as the LSCFM technique can provide a detailed reconstruction of the surface topology at the liquid-solid interface region. This feature allows identifying stable Wenzel wetting regimes, in geometries for which the apparent contact angles measured with the tensiometer can be up to 40º higher than those obtained with the LSCFM. Following these results, a few usual approaches to predict the wetting regimes, based on geometric relations are revisited. The analysis suggests that these approaches allow identifying trends with the apparent angles, but fail in predicting the angles measured by the LSCFM, being therefore unable to predict the unstable non-wetting regimes that were observed.
1. Introduction The wettability is a key parameter governing heat, mass and momentum transport at liquid-solid-vapor interfaces, which affects numerous applications. For instance, enhanced surfaces with variable wetting regimes are known to strongly affect nucleation and bubble dynamics, with consequent increase of the heat transfer coefficients (e.g. Betz et al., 2013, Moita et al., 2015, Valente et al., 2015). In the coating industry, the development of particular coatings over oil
18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016
surfaces for applications in detergency, oil recovery and lubrication is still a challenging task in which the wetting properties of the surface play a vital role (Farinha and Winnik, 1999, 2000). In Biomedical and Bioengineering fields, the successful implementation of lab-on-chip devices for biochemical analysis offers a significant reduction of samples and reagents, as well as faster analysis, given the smaller diffusion distances, which also allow a more efficient control of the reactions. In this context, the transport and manipulation of biological samples inside liquid droplets under electrostatic actuation offers great potential for the development of these micro-devices (e.g. Pollack et al., 2011), but requires devising biocompatible interfaces with particular wetting characteristics (e.g. Moita et al., 2016). However, wettability is usually characterized by macroscopic quantities, which are often limited by the spatial resolution of the available diagnostic techniques and cannot be accurately related to the multiscale phenomena occurring at the interfaces. Hence, wettability is usually quantified by the apparent contact angle θe, which is obtained at the equilibrium between the interfacial tensions acting as a droplet is gently deposited over the surface. This balance is given by the well-known Young equation (1805): σlvcos θe+σls = σsv, where, σ stands for the interfacial tension between liquid (l), solid (s) and vapor (v) phases which meet at the contact line. Based on this angle, it is widely accepted that a surface is lyophilic (i.e. promotes the liquid spread) for 0<θe<90º and lyophobic (i.e. repels the liquid) for θe>90º. The terms hydrophilic/hydrophobic, derive from the specific attraction/repellency of water. Under extreme scenarios many authors (e.g. Bhushan and Jung, 2011) consider that a surface is superhydrophilic for θe<10º and superhydrophobic for θe>150º. Young apparent contact angle does not account for chemical or geometrical heterogeneities of the surface. To cope with this, the classical theories of Wenzel (1936) and of Cassie and Baxter (1944) propose corrections to the apparent Young angle, to account for these effects. Nonetheless, the applicability of these equations is still currently discussed in the literature, mostly because they are macroscopically good approximations to estimate the contact angle, but do not address the phenomena occurring at the contact line (e.g. Gao McCarthy, 2007, Marmur 2011). Complementary information is obtained from the quasi-static and dynamic contact angles, which allow accounting for the hysteresis associated to these heterogeneities. However they are still macroscopic angles, thus restraining the analysis performed. Also, quasi-static angles do not accurately characterize dynamic wetting situations and dynamic angles are very difficult to obtain with the currently available techniques. Based on these quantities, several authors argue that superhydrophobic surfaces require not only high static contact angles (larger than 150º), but also low hysteresis (usually smaller than 10º), so that the wetting regime is stable enough and an activation energy
18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016
barrier is not transposed leading the contact line to move (e.g. He et al., 2003). Several criteria have been proposed to assure stable heterogeneous or homogeneous wetting regimes, but doubts persist in the absence of accurate experimental validation. Given that contact angles above 120º usually require surface structuring, several micro-and-nano patterning strategies have been addressed to devise superhydrophobic surfaces (e.g. Bhushan and Her, 2010, Bhushan and Jung, 2011, Gao et al., 2011, Liu et al., 2013, Pereira et al., 2014). These strategies rely on estimating a stable Cassie state, based on geometrical relations between the topographical patterns and the apparent contact angles (e.g. Jung and Bhushan, 2009, Lee et al., 2009, Jeong et al. 2012), which again are limited on their measurement accuracy. Additional complexity arrives when analyzing sub-millimetric droplets, as in this case the effect of line tension of the three-phase contact line becomes large when compared to the effect of surface tension, thus influencing the contact angles. These small droplets however are argued to be important to truly characterize the wettability of patterned surfaces, as they can be geometrically related to the topographical patterns (e.g. Sundberg et al., 2007, Wu et al., 2011). These issues highlight the need for diagnostic techniques with higher spatial and temporal resolution. The main methods to measure the contact angles are based on visualization and post-processing of images taken to sessile droplets, using optical tensiometers and goniometers, which, despite considering observations through a microscope, are limited to millimetric droplets. The angles are taken from algorithms to adjust the profile of the droplet to the Young-Laplace equation, being almost impossible to obtain a good description of the liquid-solid interface region. For very small droplets, of the order of a few microns and contact angles bellow 30º Sundberg et al. (2007) suggest the use of a method based on the interference pattern of the reflected excitation light along a cross-section through the center of the droplet. An alternative solution, pointed by few authors to provide a good compromise for both low and high contact angles is Laser Scanning Confocal Microscopy with fluorescent droplets (e.g. Farinha and Winnik, 1999, 2000, Sundberg et al., 2007, Salim et al., 2008, Wu et al. 2011, Papadopoulos et al., 2013). Confocal Laser Scanning Fluorescence Microscopy is based on the filtering of emission light (fluorescence) by confocal pinholes. The pinhole selects the light emitted by the sample (in this case a liquid fluorescent droplet) in the plane of the focused incident laser spot. Out-of-focus fluorescence is imaged before or after the pinhole and therefore does not reach the detector. 2D images of the droplet in the focal plane are obtained by a raster sweep of the droplet. During the laser scanning across the specimen, the analog light emission signal is detected by the photomultiplier and converted into a digital signal. Hence, the relative intensity of the fluorescent light emitted from the laser hit point corresponds to the intensity of the resulting
18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016
pixel in the image. 3D reconstruction of the droplet is possible by stacking 2D optical sections collected in series of well-defined z-steps. This method allows enhanced improved lateral and in-depth resolution, when compared to conventional microscopy, but has still many issues to solve. For instance, the aforementioned authors limit their measurements to small droplets of the order of tens of microns, to be able to reconstruct the droplet profile. Then the angle is determined from geometrical relations, often based on symmetry assumptions for the shape of the droplet. Optical distortion, reflection and even diffraction in large droplets (with hundreds of microns or millimeters) are considered an obstacle, so that detailed description of the liquid-solid interfacial region is hardly studied. Consequently, the application of this technique to the characterization of the wettability in complex surfaces has not been yet well explored, exception made to few authors (e.g. Wu et al., 2011, Papadopoulos et al., 2013). Within this scope, the present work explores the use of Laser Scanning Confocal Fluorescence Microscopy to characterize the wettability of smooth and rough surfaces, within extreme wetting regimes. The importance of the scale for the measurements is evaluated based on a comparison with measurements performed using an optical tensiometer. The possible effect of the droplet size is also evaluated. Particular emphasis is given to the evaluation of the measures performed near the liquid-solid interface region, given its relevance in the description of the wettability in complex surfaces. Based on these detailed measurements, Cassie to Wenzel transitions are explored, correlating the material angle obtained with the 3D-LSCFM at the liquid-solid interface with the geometrical parameters of the patterns. 2. Experimental procedure Water droplets with diameters (of the deposited droplet) ranging between tens of microns and 2.6mm are deposited on the test surfaces. The millimetric droplets are generated from the tip of a needle fed by a syringe pump with a flow rate of 0.03ml/min are deposited on the test surfaces, inside a small chamber, pre-saturated with water, to minimize evaporative effects by mass diffusion. Micrometric droplets result from a polydispersed spray applied to the surfaces. The experiments are performed at room temperature (20±3ºC). With the concentrations used here (0.0007936mg/ml<Concentration<0.496mg/ml) the thermophysical properties of the water-dye solutions are very close to those of water: density ρ=998kg/m3, within an accuracy of ±5%, dynamic viscosity µ=8.9x10-4 Pa.s, within an accuracy of ±5% and liquid surface tension σlv=73.8mNm-1 with a standard error of the mean of 0.04. Density is measured with a pycnometer for liquids and the dynamic viscosity is obtained with an ATS RheoSystems (a division of
18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016
CANNON® Instruments, Co) under controlled temperature conditions. The surface tension is measured under controlled temperature conditions (20±3ºC) with an optical tensiometer THETA (Attention), using the pendant drop method. The value taken for the surface tension of each solution tested is averaged from 15 measurements. Detailed description of the measurement procedures is provided in Moita et al. (2016). Quantitative measurements of the diameter and of the equilibrium angle are taken from optical tensiometry and by post-processing the images taken by the camera of the tensiometer, following the procedure further described in 2.2. These measurements are then compared to those obtained by Laser Scanning Confocal Fluorescence Microscopy, according to the procedure explained in 2.3. 2.1. Preparation and characterization of the surfaces
Glass (borosilicate), aluminium and silicon wafer surfaces are used as test substrates. Prior to the measurements, the surfaces are rinsed with distilled water, washed with acetone in ultrasonic bath, rinsed again with distilled water and dried with air. The primary control of surface ageing and contamination is made by apparent contact angle measurements, using the optical tensiometer. The aluminium surfaces are further treated with a chemical coating (a commercial compound called Glaco Mirror Coat Zero, from Soft99 Co – Kato et al., 2008, which is mainly a perfluoroalkyltrichlorosilane combined with perfluoropolyether carboxylic acid and a fluorinated solvent) to turn them superhydrophobic. Before applying the coating, the surfaces are washed for 30 min in an ultrasonic bath in water at 40oC. Then they are dried with air and finally are washed again during 30 min in an ultrasonic bath in acetone at 40oC. The homogeneity of surface topography and morphology is first checked by Laser Scanning Confocal Microscopy (Leica SP8 Confocal Microscope) using the reflection mode. Then, the stochastic roughness profiles are measured using a Dektak 3 profile meter (Veeco) with a vertical resolution of 20nm. The data obtained from the profilemeter are further processed to determine the mean roughness Ra (according to standard BS1134) and the mean peak-to-valley roughness Rz (following standard DIN4768). Average representative values of Ra and Rz are taken from 10 measurements distributed along the entire surface. For the glass surfaces (microscope glass slides from Marienfeld) Ra=Rz=0µm (within the vertical resolution of the profile meter). For the aluminum surfaces, Ra=1.0µm and Rz=1.58µm. A silicon wafer is used to produce the surfaces with regular micro-patterns. The wafers are coated with aluminum (to allow a deeper etching) and afterwards with photoresist. The regular
18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016
patterns are transferred by high resolution printing (performed at INESC-MN) and photolithography and are then submitted to plasma etching for 5-7hours. Wet etching is used to remove the aluminum coating. The surfaces are micro-structured with regular arrays of squared pillars and cavities, with characteristic dimensions a – length of the square size, h – height of the pillar/depth of the cavity and S – pitch, i.e. distance between consecutive pillars/cavities. These quantities, which were measured with the Dektak 3 profile meter (Veeco) are identified in Fig. 1 and summarized in Table 1. Although the etching tends to round the squares’ corners, they seem rounder due to the position of the camera on the profilemeter. This also gives the impression that the squares are rectangles.
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Fig 1. Image of the surfaces micro-patterned with a) square pillars and b) square cavities. The Figure identifies the main geometric parameters characterizing the surface topography: a - width
of the squares, h - height of the pillars/depth of the cavities and S distance between pillars/cavities.
Table 1. Main topographic characteristics of the micro-patterned silicon wafers.
2.2. Optical tensiometry and image post-processing procedures Equilibrium apparent angles θe are measured with the optical tensiometer THETA from Attention. Four to five consistent measures are taken for each case study, using the sessile drop method. Images of the deposited droplet are taken using a monochrome video-camera coupled with a microscope. The images size is 640×480 pixels and the spatial resolution of the system for the current optical configuration is 156µm/pixel. The images are post-processed by a drop detection algorithm based on Young-Laplace equation (One Attention software). The contact diameter is evaluated based on a home-made post-processing routine developed in Matlab. Average values of the contact diameter are taken from nearly 80 images, for each case study. The main source of uncertainty is the definition of the droplet profile, which is in the worst case scenario of the order of ±2pixels. 2.3. Laser Scanning Confocal Fluorescence Microscopy - LSCFM Rhodamine B (Sigma Aldrich) is used as the fluorescent dye, which was chosen taken into account its excitation and emission wavelengths, to be compatible with the wavelengths available in the laser scanning confocal microscope, but also due to particular characteristics of the experimental conditions, in the present study. Hence, Rhodamine B was found to be aggregated at the solid-liquid interfaces for hydrophobic surfaces by Wu et al. (2011), which is useful to distinguish this interface at the superhydrophobic rough surfaces used here. The fluorescence intensity (emitted per unit of volume) is known to be dependent on the light incident flux, quantum yield, absorption coefficient and dye concentration (e.g. Chamarty et al., 2007). From these parameters, the concentration is the parameter more likely to affect the fluorescence intensity, in the present work. Hence, five different concentrations were considered and a sensitivity study was performed to infer on variations of the fluorescence intensity. Also, a
18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016
specrofluorometer (Cary Eclipse) was used to determine the excitation and emission wavelengths of the various concentrations. The analysis was performed for 100µl of each solution and the final spectra are averaged from three assays. Any significant variation was observed in the excitation wavelengths, which are all detected to be in between 535nm and 560nm Fig. 2). The intensity variation is not high and cannot be correlated with the concentration except for the highest concentration. The peak for the emission wavelength is also similar for the various concentrations, ranging between 574nm and 630nm (Fig. 3). The intensify variation is also not significant, but tends to be attenuated at higher concentrations. Although higher concentrations are required for large (millimetric) droplets and lower optical magnifications (and numerical apertures) due to the low depth resolution, bleaching is more likely to occur. Hence based, the experiments were performed for a concentration of 0.003968mg/ml, for which the emission intensity was the highest.
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18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016
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Fig. 3. Emission wavelength of the Rhodamine B solutions used in the present work. The measurements of the material contact angles and of the contact diameters of the droplets are performed with a Laser Scanning Confocal Microscope (Leica SP8). The laser is used at most at 3.56-3.58% of the maximum power, to minimize photo bleaching. First scans are performed with a low magnification objective (4x, with numerical aperture of 0.10) to have a full reconstruction of the droplets. Then, detailed scans near the liquid-solid contact region are further taken using objectives with higher magnifications (10x, with numerical aperture of 0.4, 20x, with numerical aperture of 0.75 and 63x, with numerical aperture of 0.7). They are all dry objectives from Leica. The z step is optimized depending on the optical configuration. For the measurements performed here, the z step was varied between 50nm and 0.5µm. The pixel size, for the worse resolution is 5.42µm, but can be as low as 500nm. An accurate evaluation of the liquid-solid contact line region requires determining the position for z=0µm. To cope with this, a fluorescent marker, with absorption and emission wavelengths similar to those of Rhodamine B is used to mark a dot on each sample surface used. Given that this dot is marked on a specific plane, z=0 is determined for the z position for which the intensity of the emitted fluorescence signal is maximum. For illustrative purposes, the intensity variation along z is depicted in Fig. 4 for 3 glass slides. The slight variation of the maximum intensity is attributed to bleaching and to the fast deterioration of the marker. The deviation of the peaks in z is associated to the differences in the thickness of the glass slides, inherent to their fabrication process.
18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016
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Afterwards, 2D scanning is performed for various z steps in both ascending and descending z directions. The diameter of the droplets is measured from the 2D images as illustrated in Fig. 4.
Fig. 4. Example of 2D images obtained from LSCFM, for evaluation of the diameter of the
droplets.
Finally, XZ and YZ profiles are taken and used for the 3D reconstruction of part of the profile of the droplet, as exemplified in Fig. 5. Both XZ and YZ planes are considered, to further check for the reproducibility of the measurements and to detect asymmetries in the droplet. The reconstructed profiles are then re-processed in a home-made Matlab routine to determine the
18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016
boundary of the droplet and the contact angle. Also here, the main source of uncertainty is the definition of the droplet profile, which is in the worst case scenario of the order of ±2pixels.
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18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016
c)
Fig. 5. a) Reconstructed view of a millimetric droplet in the various planes, b) 3D reconstruction of the droplet, c) XZ and YZ reconstructed profiles.
3. Results and discussion 3.1 Contact angles on smooth surfaces To validate and explore the technique, preliminary results were obtained by measuring the equilibrium contact angles on smooth glass slides. The equilibrium angles are measured for millimetric and micrometric droplets to infer on their dependence on droplet size. Fig. 6 compares the equilibrium angles obtained with the optical tensiometer and with the Laser Scanning Confocal Fluorescence Microscopy-LSCFM technique. Only millimetric droplets are considered here, given the limited spatial resolution of the optical tensiometer. Overall the results show consistently similar measurements provided by both techniques, although the values obtained from the LSCFM tend to be lower, when compared to those given by the optical tensiometer. Any significant asymmetry is observed for the macrometric droplets, which is evident from the image of the deposited droplet, shown in Fig. 6 Consistently, the angles measured in plane XZ are similar to those measured in plane in YZ. Considering the spatial resolution, even taking into account similar uncertainties in determining the boundary of the droplet in both methods, the error associated to the worse resolution used in the LSCFM is of 10.84µm (with negligible propagation to the angle measurement) against 312µm obtained with the optical tensiometer. Furthermore, it is worth mentioning that in the present work, only a part of the droplet, near the contact line region is considered for the reconstruction of the XZ and YZ profiles, from tens up to hundreds of microns. Under these conditions, the contact angles can differ for more than 20º depending on the size and positioning of the section, i.e. on how far the angle they are actually measured from the contact line. This difference is associated to a flattening of the droplet, which is particularly evident in large droplets, where gravitation effects can play already an important role. This difference seems larger for higher angles and is in
XZprofile YZprofile
18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016
agreement with the observations reported by Salim et al. (2008). Given their resolution, the measures obtained with the optical tensiometer will be influenced by this flattening effect, so they tend to be higher.
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Fig. 6. Comparison between the equilibrium angles measured with the optical tensiometer and with the LSCFM technique, for millimetric droplets.
Despite providing accurate measurements, several difficulties were found in the preliminary measurements obtained with the LSCFM for the droplets over the rough superhydrophobic surfaces. Besides the aforementioned flattening effect, which is overcome by considering the proper optical configuration, the spherical shape of the droplet promotes reflective and even diffractive effects which affect the incident light and the emitted signal, producing erroneous reconstructions of the droplet, particularly near the contact line, the most sensitive region to scan. The use of a single dye also turns difficult to detect particular defects of the surface, which may affect the contact line. To cope with this, ongoing work considers the use of different dies for the liquid and for the substrate. Looking now for a wider range of droplet sizes, there is a clear dependence of the contact angle with the droplet radius, for droplets smaller than 100-120microns, as depicted in Fig. 7, for the measurements obtained with the LSCFM technique. The results are qualitatively in good agreement with those reported by Sundberg et al. (2007) for water droplets on glass and on silanized glass with trimethylchlorosilane (TMCS), using a similar technique. Dispersion of data is associated to the uncertainty of the post-processing methods, which require further optimization, but can also be attributed to asymmetries of the droplets, which may also contribute to the different values obtained from the measures taken from the XZ and with the YZ profiles.
18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016
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>220µm). Such asymmetries, which are quite clear from the 2D and 3D reconstructed images (a global view capturing various droplets is provided in Fig. 8, although the quantitative analysis always considers isolated droplets) are very likely to occur in the present work and in the study of Sundberg et al. (2007), given that the micro-droplets are obtained from applying a polydisperse spray over the solid surface.
Fig. 8. 3D reconstruction of several microdroplets obtained by deposition of the polydisperse
spray over a glass slide.
18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016
3.2 Contact angles on complex surfaces A significantly different scenario is however observed for droplets deposited on complex surfaces. Fig. 9 illustrates the XZ and YZ plane views of the 3D reconstruction of the droplet profiles on surfaces micro-patterned with pillars and cavities. The LSCFM technique allows the detailed visualization of the surface topology, as the liquid penetrates into the cavities. The 3D reconstructions of the droplet on both XZ and YZ planes show an evident distortion of the contact line, which assumes an octagonal shape, very similar to that reported by Papodopoulos et al. (2013). Consequently, the contact angle can differ more than 40º, depending where it is measured, being this probably one of the main reasons for the significant difference that can be observed between the angle measured with the LSCFM and the apparent angle given by the tensiometer (Fig. 10). The detailed measurement of the local contact angle, as a function of the surface topology, following the work of Wu et al. (2011) is an ongoing process, to be presented in a future paper.
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Fig. 9. 3D reconstructions of the droplet on micro-patterned surfaces with a) micro-pillars (surface S1) and b) micro-cavities (surface C2).
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XYplane XZplane
18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016
It is worth mentioning that there are apparent angles of 120º, as measured by the tensiometer, thus evidencing a hydrophobic behaviour, which are in fact up to 40º lower, when measured with the LSDFM technique. This lower angle is in agreement with the observations of the contact line region, which confirm a homogeneous wetting. This may be so since on surfaces S1 and S4, the contact angle decreases 10º during the measure (within 2s after deposition) and for surfaces S9 and S10 this decrease in the contact angle reaches 20º after 7s, which clearly evidences an unstable non-wetting regime. Unlike the apparent angle measurements, which can vary for more than 20º due to hysteresis, the micro angle measured on the surface by the LSCFM technique on similar typologies is quite stable, generally varying by less than 10º (considering left and right angles measured from the droplet profiles reconstructed in both XY and YZ planes). Such similar topologies are possible to identify given that the LSCFM technique provides, as aforementioned, a detailed reconstruction of the surface topology at the liquid-solid interface region.
Fig. 10. Comparison between the equilibrium angles measured with the optical tensiometer and with the LSCFM technique, for millimetric droplets on micro-patterned surfaces. a) XZ plane, b)
YZ plane. It is now worth exploring some of the most usual approaches to predict the wetting regimes and determine their stability. Despite the various discussions on the applicability of the Cassie and Baxter and of the Wenzel equations, most of the criteria establishing the wetting regimes on complex surfaces are based on these simple relations. When observing the liquid-solid interface region of the droplet on the complex surfaces used in the present work, the LSCFM images show, within the resolution used at this stage of the work (50nm), an homogeneous wetting regime for all the surfaces micro-patterned with pillars, exception made to surface S8, which as
18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016
an hierarchical complex pattern of larger and smaller pillars. Following simple geometric analysis based on a semi-empirical approach, and on Cassie and Baxter and Wenzel equations, several authors try to predict the wetting regime based on geometrical relations such as the pitch S (Jung and Bhushan, 2009), the ratio S/a (Lee et al., 2010) and the ratio h/S (Moita and Moreira 2012). These relations depicted in Fig. 11, result in interesting trends when dealing with the apparent angle. For instance, as the pitch S increases (Fig 11a), the apparent angle qualitatively tends to decrease, as the liquid sags and impales into the grooves.
c) Fig. 11. Criteria to establish the wetting regimes for apparent vs micro-contact angles. a) contact angle vs S (Jung and Bhushan, 2009), b) contact angle vs S/a (Lee et al., 2010), c) contact angle vs
h/S (Moita and Moreira, 2012). However, at the contact line region the angle is still strongly affected by the surface topography so, the contact angle does not vary so linearly with S. A similar reasoning can be performed for the other criteria, except for the criteria of Moita and Moreira (2012), since in this case, increasing
18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016
h/S should theoretically preclude the droplet to sag. Also, these criteria can predict the apparent hydrophobic behaviour, which according for instance by Lee et al. (2010) should occur for S/a<1.81, but fail to predict the true wetting regimes given by the micro-angles (Fig. 11b). Also, they do not address the unstable wetting behaviour observed for surfaces S1, S4, S9 and S10. Hence, these preliminary results suggest that a less empirical approach should be performed, based on energy minimization principles, but considering the micro angle, obtained by this more accurate technique. The contact angle decrease due to the wetting transition, as observed on surfaces S1, S4, S9 and S10 must now be investigated with the LSCFM technique under dynamic conditions, to evaluate hysteresis and determine more specific transition characteristics under static conditions (e.g. following the approach of Long et al., 2005) and dynamic conditions, i.e. considering the balance between pressure and surface tension forces (e.g. Zheng et al. 2005), while inferring on the possible influence of the capillary bridges, as suggested for instance by Chandra and Yang (2009). Final remarks The present paper presents the preliminary results of a non-intrusive technique, the Laser Scanning Confocal Fluoresce Microscopy - LSCFM applied to the characterization of the wettability on complex surfaces. The technique is firstly validated by measuring the equilibrium contact angles on smooth glass slides, for a wide range of droplet sizes, from tens of microns to 2.6mm. There is a clear dependence of the contact angle with the droplet radius, for droplets smaller than 100-120microns, which is clearly shown by the LCFM measurements. On the other hand, for milimetric droplets, the contact angles measured by the LSCFM are consistently similar to those measured by the tensiometer, although generally lower. A detailed analysis on the accuracy of the measurements further shows that the angles obtained with the tensiometer are more likely to be affected by the flattening of the droplet, which is particularly evident in large droplets, where gravitation effects can play already an important role. A different scenario is nevertheless observed when characterizing micro-patterned surfaces. The LSCFM technique can provide a detailed reconstruction of the surface topology at the liquid-solid interface region, thus allowing identifying stable Wenzel wetting regimes, in geometries for which the apparent contact angles measured from the tensiometer can be up to 40º higher than those obtained with this non-intrusive technique LSCFM. Following these results, a few usual approaches to predict the wetting regimes, based on geometric relations are revisited. The results obtained suggest that these approaches allow identifying trends with the apparent angles, but
18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016
fail in predicting the angles measured by the LSCFM, being therefore unable to predict the unstable non-wetting regimes that were observed. Acknowledgements The authors are grateful to Fundação para a Ciência e a Tecnologia (FCT) for partially financing this research through the project RECI/EMS-SIS/0147/2012, which also supports Dalila Vieira with a fellowship. A.S. Moita also acknowledges the contribution of FCT for her Post-Doc Fellowship (Ref.:SFRH/BPD/109260/2015). Finally, the authors acknowledge the contribution of Joana Pereira in the data acquisition and post-processing. References
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18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016
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