Letter pubs.acs.org/Langmuir
Contact Angle Distribution of Particles at Fluid Interfaces Craig Snoeyink, Sourav Barman, and Gordon F. Christopher* Department of Mechanical Engineering, Texas Tech University, Lubbock, Texas 79409-1035, United States ABSTRACT: Recent measurements have implied a distribution of interfacially adsorbed particles’ contact angles; however, it has been impossible to measure statistically significant numbers for these contact angles noninvasively in situ. Using a new microscopy method that allows nanometer-scale resolution of particle’s 3D positions on an interface, we have measured the contact angles for thousands of latex particles at an oil/water interface. Furthermore, these measurements are dynamic, allowing the observation of the particle contact angle with high temporal resolution, resulting in hundreds of thousands of individual contact angle measurements. The contact angle has been found to fit a normal distribution with a standard deviation of 19.3°, which is much larger than previously recorded. Furthermore, the technique used allows the effect of measurement error, constrained interfacial diffusion, and particle property variation on the contact angle distribution to be individually evaluated. Because of the ability to measure the contact angle noninvasively, the results provide previously unobtainable, unique data on the dynamics and distribution of the adsorbed particles’ contact angle.
P
articles larger than a few nanometers permanently attach to liquid interfaces because of surface tension, which results in a large detachment energy, Edetach, Edetach = πr 2γow(1 + cos(θ0w ))2
Particle-laden interfaces exhibit long-term stability because of the difficulty of reducing the interfacial area once particles are permanently adsorbed to the interface. This ensures the longterm stability of drops with these interfaces, which enhances the bulk emulsion stability. Furthermore, a particle-laden interface provides two additional mechanisms that contribute to bulk stability and rheology: mechanical protection from coalescence and interfacial rheology. Mechanical protection from coalescence occurs because of particles forming a protective shell around drops that prevent liquid interfaces from coalescing.1,2 The protection this shell provides and its ability to mitigate coalescence are dependent on θow.21 Once on the interface, particles form a dissipative, viscoelastic surface that affects the bulk viscoelasticity.22 The microstructure of the interface resists deformation, which is dictated by interparticle forces23−25 that are strongly dependent on θow.26−28 Multiple properties of Pickering emulsions are strongly influenced by θow; therefore, understanding the θow of adsorbed particles represents the best way to begin to understand the properties of Pickering emulsions. Accurately characterizing θow, however, is difficult. This is due to two related but different problems: the difficulty in accurately measuring θow of interfacial particles and the large number of variables that affect θow. There exist a number of methods that allow the measurement of θow for particles residing at an interface. There has been an excellent recent review of these methods by Maestro and coworkers;3 our goal is not to review all existing methods but to outline, in general,
(1)
where r is the particle radius, γow is the interfacial fluid tension between the oil and water, and θow is the three-phase contact angle measured through the water phase. This energy is typically orders of magnitude larger than the thermal energy for particles larger than 10 nm.1−3 The adsorption of particles to interfaces has been employed to create novel and useful materials such as colloidal templates for nanocomposites,4 colloidosomes,5,6 liquid marbles,7 and particle-stabilized Pickering emulsions.8,9 Pickering emulsions in particular have been found to have widespread application in food products, cosmetics, wastewater treatment, and oil recovery/treatment because of their long-term stability and novel rheological properties.10−17 Because of their prevalent use, controlling the stability and rheology of Pickering emulsions has been increasingly examined; in particular, the role particle properties play in material functions has been of interest.18−20 There is a basic understanding of the role of the particle contact angle in Pickering emulsion behavior; a particle’s θow determines the ability to form an emulsion as well as the nature of the resulting emulsion. Particles must partially wet each phase for an emulsion to exist; extreme contact angles smaller than 10° or larger than 170° will not form Pickering emulsions. The contact angle regulates whether oil in water (o/w) or water in oil (w/o) emulsions are formed. The empirical condition can be summarized as follows: if θow < 90, then a w/o emulsion forms, but if θow > 90, then an o/w emulsion forms.1−3 © 2014 American Chemical Society
Received: December 15, 2014 Revised: December 15, 2014 Published: December 30, 2014 891
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sample sizes were low (less than 100 total counts), θow was seen to range over as much as 50° in these measurements. In fact, in some distributions, θow varied from hydrophobic to hydrophilic.34 This distribution affects the attachment energy of particles and interaction potentials, segregates particles to either side of the interface, makes the 2D assumptions used in understanding these systems invalid, and may affect the emulsion stability. However, the mechanism for these distributions, whether in the measurement error, interfacial motion, constrained diffusion normal to the interface, or variation in particle properties, is unknown because of the inability in measuring these effects separately. Nonetheless, the distribution of θow would result in far-reaching effects on the stability of the interface, the type of emulsions formed, and the interfacial viscoelasticity of the interface. We present a new microscopy technique that allows the evaluation of a large number of distinct particle θow in situ on an interface. This method does not manipulate the particles or the interface through any means and therefore allows the direct measurement of unaltered θow. Furthermore, this technique allows dynamic data of particles to be recorded, which allows the measurement of tens of thousands of distinct particle θow values during a single recorded video and hundreds of thousands over a single measurement session. The resulting measurements provide statistically significant numbers to evaluate the distribution of θow and allow the differentiation of the measurement error, constrained diffusion, and variation of particle properties, all of which affect the distribution.
classes of techniques used to measure θow and their benefits/ limitations. There are three primary methods used to evaluate θow: indirect measurement, direct measurement of multiple particles, and direct measurement of single particles. Indirect methods employ measuring a property associated with an interface to evaluate the average contact angle of all particles sitting on an interface. These methods include drop shape techniques,29,30 surface pressure−area isotherms,31 and capillary rise methods.32 These methods measure a property of a system related to the contact angle, which is then related to the contact angle through theory; they do not provide a direct measurement of the contact angle or the particle interfacial position. These methods typically provide a single value of θow that represents the average contact angle for all particles; there is no way to analyze the distribution of contact angles, and any distribution/error is attributed to a measurement error. By freezing33,34 or gelling35,36 a particle-laden interface to trap the particles at the interface, direct measurement of multiple particles is possible. The suspended system is examined through advanced microscopy techniques such as SEM or AFM, which allow the measurement of particle heights from the interface. Using simple geometry, these heights can be directly related to the three-phase contact angle. These methods allow the analysis of tens to hundreds of particles through individual measurements. However, the measurements require significant manipulation of the interface through freezing/gelling, which may affect particle locations, deform particles, or change the system chemical composition. These affects could create atypical contact angles in comparison to an unmanipulated system. Furthermore, because of the nature of the techniques, the temporal dependence of the contact angle is lost. Finally, using microscopy techniques such as digital holography,37,38 it is possible to find the absolute position of particles. This technique is typically restricted to a single particle, although it can be applied to multiple, widely separated particles.37 Using the particle height, the contact angle can be found as in other methods. This method does allow the absolute measurement of the particle position and does not manipulate the interface, ensuring a clean measurement. Furthermore, it is capable of capturing particle dynamics. However, these systems are often difficult to set up, require specialized illumination sources, and allow only limited numbers of measurements. Using the above techniques, it has been found that the particle composition,34,39,40 surface roughness,41 shape/diameter,42,43 spreading agents,44 which phase a particle travels through to get to the interface, and time on the interface can all alter θow;38 as a result, there can be significant variation in θow even within seemingly monodisperse populations, the measurement of which has been particularly lacking. Because of these measurement difficulties, θow is often omitted or assumed to be an arbitrary monolithic value (typically 90°) in studies of Pickering emulsions/particle-laden interfaces.3 This final ̈ given the fact that assumption seems to be particularly naive particle properties of monodisperse systems have been previously shown to be highly variable,45−47 with error bars on measurements suggesting that distributions of θow have variations of up to 10°. The most explicit study of the θow distribution used the freeze−fracture/SEM technique to look at the distribution of θow for polystyrene particles over a range of sizes at both water−decane and water−hexane interfaces. Although typical
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MATERIALS
Surfactant-free sulfate polystyrene particles (charge density 7.8 mC/ cm2, diameter 1 μm ± 30 nm) were obtained from Interfacial Dynamics Corporation as aqueous dispersions containing 8 wt % particles. These particles are hydrophobic with a nominal contact angle of 116°; this value is obtained on the basis of a review of the literature in which identical systems have been studied and represents the nominal measured contact angle of the studied system.3,48 Because our goal is to study the distribution of these particles, we believe that this value is reasonable to use, and as shown later, it does not impact analysis. Particles were cleaned of any residual surfactant through repeated centrifugation.47 To avoid any centrifugation-induced aggregation in the bulk, particle solutions were sonicated between each run in the centrifuge until they were well dispersed. No aggregation of particles in the bulk was observed under microscopic observation. The particle solution was diluted with isopropyl alcohol and DI water such that the volumetric ratio was 1:10:5, respectively. The water subphase was a mixture of deionized water and 30% glycerin by weight. This mixture has a density of 1075 kg/m3 at 25 °C, thus avoiding sedimentation of the polystyrene particles, with a density of 1055 kg/m3. The water−glycerol mixture was introduced into the bottom of the measurement cell depicted in Figure 1, which is similar to those used by Park and coworkers.45−47 This measurement cell consists of an outer PEEK wall epoxied to a glass slide. Once water is introduced onto the glass slide, the inner wall, consisting of a PEEK cylinder on top of a stainless steel ring with silicon spacers to allow the water− glycerol mixture to flow between the two chambers, is lowered in. The stainless steel forms a surface energy discontinuity with the PEEK, pinning the contact line at this location. Decane (Sigma-Aldrich) is introduced into the middle cylinder, and then approximately 30 μL of the diluted particle solution is introduced above the decane, quickly settling down and populating the oil−water interface with particles. On the basis of our observations, the majority of particles end up adsorbed to the interface. 892
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particles on an Olympus IX-70 microscope. Attached to the microscope is the BBM system consisting of a refocusing lens ( f RF = 100 mm) located on its focal length from the microscope image plane, immediately followed by a fluidic axicon with an effective surface angle of 0.114° and a modifying lens ( f M = 150 mm, LM = 140 mm) with a total distance between the axicon and camera (Ltotal) of 278.5 mm.53 It is important to note that the resolution of this and all particlelocating measurements is heavily dependent on the image signal-to-noise ratio (SNR), and as such, it is anticipated that the resolution will vary between particles as a result of variations in lighting and image quality.54 The configuration used in this study produced a depth resolution of approximately 45 nm for SNRs encountered in this measurement. The images were recorded using a monochrome CMOS camera (PL-E531 from PixeLINK) and analyzed using the image analysis algorithms described by Snoeyink and Wereley.55 The results of these image analysis algorithms are the x, y, z coordinates of every particle in an image relative to the center of the focal plane; this measurement does not require any knowledge of the particle size to determine the position. Measurement of Interface Location and Particle Dynamic Position. The first step toward determining the particle population’s distribution of contact angles from the raw particle positions is to subtract the mean particle height from each individual particle location. This has the benefit of removing any bulk motion from the particle position as well as removing any drift in the focal plane location. To obtain the distribution of contact angles from the relative particle positions, it is first necessary to correlate particles between frames. A simple nearest-neighbor particle tracking algorithm was used to build particle paths from the individual locations, excluding from consideration particles that were greater than 2 μm apart between frames. With these trails, it is possible to start gaining information on the time-varying properties of the particle position. Because at this stage in the analysis the location of the oil−water interface is uncertain, a line is fit to the path of each particle using least-squares optimization to serve as a reference. A marked difference in the variation in height as measured from this line was found between particles adsorbed on the interface and those not adsorbed (Figure 2). Figure 3 shows the distribution of particle height standard deviations for all particles observed as measured from their
Figure 1. Schematic of the experimental setup showing the construction of the sample, the optical setup for the BBM system, and an example of particle images acquired by the BBM system. Note that the image has been inverted to assist in visibility.
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RESULTS Multiple Particle Contact Angle Measurements Using Bessel Beam Microscopy. As discussed earlier, the contact angle of a small particle adsorbed onto an interface is particularly difficult to access experimentally. As a proxy for this value, we measure the absolute height of the particles using Bessel beam microscopy (BBM) and relate the height to the contact angle using the following relationship ⎛ −z ⎞ π θ = −sin−1⎜ ⎟ + ⎝ r ⎠ 2
(2)
where z is the height of the particle center of mass above the interface and r is the radius of the particle. The Bessel beam microscopy system is particularly well suited for this measurement. In contrast to other methods capable of locating fluorescent particles in three dimensions, the BBM system produces the absolute location of particles ranging in size from 10 nm to a few micrometers without calibration and is capable of uniquely high spatial resolutions when locating fluorescent particles.49−52 A schematic of the experimental setup is shown in Figure 1. For this experiment, a depth resolution on the order of 100 nm or better was required. To obtain this resolution, a 20× 0.45 NA objective from Olympus was used to image the
Figure 2. Plots showing a marked difference in the z diffusion behavior between particles entrained on interface (red) and particles floating freely in aqueous solution (blue). Note that the amount of apparent thermal motion in the z direction varies by over an order of magnitude. The black line is the smoothed spline fit of particle motion, and the measured particle positions are plotted as translucent 1 μm spheres. 893
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Figure 4. Distribution of particle heights relative to the estimated oil− water interface (green histogram). This distribution (thin black line) is a convolution of both the true distribution of particle heights (solid red line) and both the particle diffusion and measurement uncertainty (thin dashed black line).
Figure 3. Plot of particle height standard deviation measured from the line fitted to the particle path.
fitted line. As can be seen in the figure, the vast majority of particles viewed have a height variation of less than 120 nm with a long tail of particles with height variations greater than this value. The reason for this variation can be seen in Figure 2, where the 3D paths for two particles, one on the interface (top red) the other off of the interface (bottom blue), are plotted alongside each other. In this figure, the particle’s smoothed path is shown, and each recorded particle position is plotted as a translucent 1 μm sphere. As is evident, the z motion of the two particles varies by more than an order of magnitude, making this a natural filter for determining which particles are absorbed on the oil−water interface. For this study, we chose a threshold in height variation of 120 nm for those particles on the interface. Once the population of particles on the interface was determined, all of the interface particle positions for a given measurement were collected. For each measurement, a region of particles was observed for 500 frames with typically around 200−300 particles in view. After analysis, this results in over 100 000 particle locations to which a paraboloid is fit using least-squares optimization. A paraboloid was chosen as opposed to a flat plane in order to account for any slight curvature that might exist in the interface. This paraboloid represents a best estimate of the plane intersecting all of the particle’s center of mass; only under the specific conditions of a 90° contact angle is this the best estimate of the oil−water interface. For the polystyrene−decane−water system used here, the multiphase contact angle is 116°.3,48 However, provided the contact angle does not vary as a function of lateral particle location, the fitted plane is parallel to the oil−water interface and offset by a fixed amount that can be estimated given the properties of the system (220 nm for polystyrene−water−decane). As such, it provides a reference point by which to evaluate the variation in contact angle for the particles. Determination of Mechanisms Causing the Particle Height Distribution. The height of each interfacial particle in each frame was recalculated relative to the fitted and offset interfacial plane for each experiment. The distribution of these recalculated particle heights, over 950 000 in all, is plotted in Figure 4. The particle height distribution is modeled by a normal distribution, centered at 0.05 μm and with a standard deviation of 176 nm, shown as a thin black line in Figure 4. Because of measurement error, the distribution extends beyond the actual diameter of the particles. This distribution of measured particle heights is a convolution of several factors:
thermal motion, measurement uncertainty, and base variation in the population contact angle. Provided the thermal motion and measurement uncertainty are independent of the contact angle, the following relationship for population variation holds σtotal =
σt 2 + σu 2 + σα 2
(3)
where σtotal is the total variation in height observed for the population, σt is the variation due to thermal motion, σu is the variation due to measurement uncertainty, and σα is the variation in the contact angle.56 To estimate the variation in the population contact angle, we must first obtain an estimate of the particle height variation due to thermal motion and measurement uncertainty. For any single particle, the measured height is expected to change over time because of the thermal motion, measurement uncertainty and, possibly, contact line aging.39 However, the change in particle height due to aging has been shown to be logarithmic, meaning that, for the relatively short time period of our measurements, unless we capture the particle in the first few seconds that it is absorbed onto the interface this variation in particle height over the course of our individual measurement (15 s) is increasingly small and on the order of nanometers. To ensure that particles were well past the point of adsorption, the first recorded observations of the interface were started 2 h after the interface was formed. Further evidence that the contact line aging is minimal can be found in Figure 4, where the variation in all measurements and the variation due to uncertainty and thermal motion are plotted over time. Note that whereas there is fluctuation of these values, there are no significant trends. In addition, the variation due to measurement uncertainty and thermal motion remains relatively steady around the average value of 86 nm. Therefore, we are confident that we are well past any adsorption phenomenon. Because the measurement of the particle depth has an estimated average uncertainty of 45 nm (σu), by characterizing the time variation in height of a single particle relative to the interface we can obtain an estimate of position variation due to thermal motion (σt). To get this estimate of σt, the distribution of heights of individual particles on the oil−water interface measured relative to the line fitted to their path was calculated and plotted as a dotted black line in Figure 4. This distribution, when combined 894
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variations in the quantity z/r will create large variations in the perceived contact angle as z/r approaches ±1. As a result, the shape of the distribution depicted in Figure 6 likely differs from the actual distribution as the contact angle approaches 180°, though this affects a relatively small number of particles and as a result the polydispersity does not affect the distribution significantly. When the analysis is run using the maximum possible particle size of 1.03 μm, the resultant distribution of contact angles changes to 18.5°. Therefore, although the polydispersity does affect the measurement, it does not affect our conclusions. The large variation in the measured particle contact angle presents challenges to some basic notions of particle-laden interfaces. In particular, treating the systems as 2D given the known distribution of particle heights no longer seems appropriate. This adjustment makes the interaction of individual particles on an interface more complicated because the 2D assumptions used in estimating interaction potentials are no longer valid. This has implication on the ability of particles to arrange on the interface into complicated structures and the resulting interfacial rheology of these systems. The range of contact angles observed also indicates that a particle-laden interface does not provide monolithic protection of the interface. In fact, although the measured particle system is primarily hydrophobic, contact angles are clearly less than 90°. However, on the basis of the distribution, it is clear that the bulk particles are in fact hydrophobic, which would enable this system to make a water-in-oil emulsion because of the plurality of particles exhibiting hydrophobicity. Therefore, although we expect such system to make an o/w emulsion, a small subset of particles would prefer to make a reverse emulsion; on an interface between two oil drops, this would likely result in increased chances of coalescence and reduced emulsion stability.
with the measurement uncertainty, is well modeled by a normal distribution with a mean of 6 nm and with a standard deviation of 86 nm. With this estimate of σt+u, we can now estimate the variation in particle height due to the variation in contact angle using eq 2. This estimate of particle height variation due to differences in contact angle is plotted as a solid red line in Figure 4. Inherent Contact Angle Distribution Due to Particle Property Variation. One can use eq 2 to translate the particle height distribution to a distribution of contact angles, the result of which is shown in Figure 6. This distribution has a variation
Figure 5. Plot of total population height variation (blue circles) and thermal motion + uncertainty as a function of time. Both the population height variation and the variation due to thermal motion and uncertainty remain relatively constant, indicating that no significant contact line aging is occurring over the course of the measurement.
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CONCLUSIONS Using our methodology, we are capable of resolving the primary mechanisms that may cause a distribution in contact angle, which has been previously impossible using other techniques. Using this method, we see that PS particles have a contact angle variation of ±19°, which is much larger than estimated in previous measurements, and fit a typical bell curve distribution with a much wider total range of contact angles sampled. Although the measurements provided are specific to 1 μm PS particles at water/decane interfaces, we believe that measurements of a wider array of systems will indicate similar distributions. The degrees of chemical and physical heterogeneity will vary between systems, which will affect the measured distribution, requiring further experimentation. However, the physics that determine the three-phase contact angle do not change with changing systems; therefore, some form of distribution will exist in all systems. This distribution reflects a much broader and more significant value than previously observed and has serious ramifications for the behavior of particle-laden interfaces and Pickering emulsions. Particles on interfaces are typically considered to be 2D in their interactions, but given that the height variation is as much as 20% of the particle diameter, this assumption would seem to be invalid and would require a fundamental reconsideration of the nature of forces between particles at interfaces and would force us to consider forces normal to the interface. Furthermore, for Pickering emulsions, this range of distributions would affect not only the interfacial
Figure 6. Estimated distribution of particle contact angles. The distribution has a mean of 116° and a standard deviation of 19.3°.
in contact angle of 19.3°. It is important to note that this distribution of contact angles is not strongly dependent on the estimated median angle. Varying the median angle by 10° in either direction causes the standard deviation of contact angles to vary by less than 0.5°. It should also be noted that the polydispersity of the particle’s radii, although small, will have an effect on the contact angle distribution at the extreme ends of the distribution in particular. As can be seen in eq 2, small 895
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(10) Frelichowska, J.; Bolzinger, M. A.; Valour, J. P.; Mouaziz, H.; Pelletier, J.; Chevalier, Y. Pickering w/o emulsions: Drug release and topical delivery. Int. J. Pharm. 2009, 368, 7−15. (11) Simovic, S.; Ghouchi-Eskandar, N.; Prestidge, C. A. Pickering emulsions for dermal delivery. J. Drug Delivery Sci. Technol. 2011, 21, 123−133. (12) Dickinson, E. Food emulsions and foams: Stabilization by particles. Curr. Opin. Colloid Interface Sci. 2010, 15, 40−49. (13) Yi, C. L.; Yang, Y. Q.; Jiang, J. Q.; Liu, X. Y.; Jiang, M. Research and Application of Particle Emulsifiers. Prog. Chem. 2011, 23, 65−79. (14) Crossley, S.; Faria, J.; Shen, M.; Resasco, D. E. Solid Nanoparticles that Catalyze Biofuel Upgrade Reactions at the Water/Oil Interface. Science 2010, 327, 68−72. (15) de Oliveira, M. C. K.; Carvalho, R. M.; Carvalho, A. B.; Couto, B. C.; Faria, F. R. D.; Cardoso, R. L. P. Waxy Crude Oil Emulsion Gel: Impact on Flow Assurance. Energy Fuels 2010, 24, 2287−2293. (16) Faria, J.; Ruiz, M. P.; Resasco, D. E. Phase-Selective Catalysis in Emulsions Stabilized by Janus Silica-Nanoparticles. Adv. Synth. Catal. 2010, 352, 2359−2364. (17) Sullivan, A. P.; Zaki, N. N.; Sjoblom, J.; Kilpatrick, P. K. The stability of water-in-crude and model oil emulsions. Can. J. Chem. Eng. 2007, 85, 793−807. (18) Chevalier, Y.; Bolzinger, M.-A. Emulsions stabilized with solid nanoparticles: Pickering emulsions. Colloids Surf., A 2013, 439, 23−34. (19) Dickinson, E. Use of nanoparticles and microparticles in the formation and stabilization of food emulsions. Trends Food Sci. Technol. 2012, 24, 4−12. (20) Dugyala, V. R.; Daware, S. V.; Basavaraj, M. G. Shape anisotropic colloids: synthesis, packing behavior, evaporation driven assembly, and their application in emulsion stabilization. Soft Matter 2013, 9, 6711−6725. (21) Stancik, E. J.; Kouhkan, M.; Fuller, G. G. Coalescence of particle-laden fluid interfaces. Langmuir 2004, 20, 90−94. (22) Oldroyd, J. G. The Effect of Interfacial Stabilizing Films on the Elastic and Viscous Properties of Emulsions. Proc. R. Soc. London, Ser. A 1955, 232, 567−577. (23) Park, B. J.; Furst, E. M. Micromechanics of colloidal aggregates at the oil-water interface. Soft Matter 2011, 7, 7683−7688. (24) Luu, X.-C.; Yu, J.; Striolo, A. Nanoparticles Adsorbed at the Water/Oil Interface: Coverage and Composition Effects on Structure and Diffusion. Langmuir 2013, 29, 7221−7228. (25) Brooks, C. F. Interfacial Rheology. Rheol. Bull. 2011, 11−15. Brooks, C. F. Interfacial Rheology. Rheol. Bull. 2011, 25−30. (26) Oettel, M.; Dietrich, S. Colloidal interactions at fluid interfaces. Langmuir 2008, 24, 1425−1441. (27) Stamou, D.; Duschl, C.; Johannsmann, D. Long-range attraction between colloidal spheres at the air-water interface: The consequence of an irregular meniscus. Phys. Rev. E 2000, 62, 5263−5272. (28) Horozov, T. S.; Aveyard, R.; Binks, B. P.; Clint, J. H. Structure and Stability of Silica Particle Monolayers at Horizontal and Vertical Octane−Water Interfaces. Langmuir 2005, 21, 7405−7412. (29) Guo, Y.; Tang, D.; Du, Y.; Liu, B. Controlled fabrication of hexagonally close-packed langmuir-blodgett silica particulate monolayers from binary surfactant and solvent systems. Langmuir 2013, 29, 2849−2858. (30) Grigoriev, D. O.; Kraegel, J.; Dutschk, V.; Miller, R.; Moehwald, H. Contact angle determination of micro- and nanoparticles at fluid/ fluid interfaces: the excluded area concept. Phys. Chem. Chem. Phys. 2007, 9, 6447−6454. (31) Clint, J. H.; Taylor, S. E. Particle size and interparticle forces of overbased detergents: A Langmuir trough study. Colloids Surf. 1992, 65, 61−67. (32) Galet, L.; Patry, S.; Dodds, J. Determination of the wettability of powders by the Washburn capillary rise method with bed preparation by a centrifugal packing technique. J. Colloid Interface Sci. 2010, 346, 470−475. (33) Isa, L. Freeze-fracture Shadow-casting (FreSCa) Cryo-SEM as a Tool to Investigate the Wetting of Micro- and Nanoparticies at LiquidLiquid Interfaces. Chimia 2013, 67, 231−235.
rheology but also the mechanical protection of the interfaces because this degree of variability may affect how particles pack as interfaces approach each other and may allow regions where liquid interfaces could coalesce more easily. This distribution is the result of monitoring the position of over 2000 particles observed for 500 frames, providing a level of statistical significance necessary to characterize this complex phenomenon. The noninvasive nature of the optical measurements also ensures that the measured distribution is realistic, free of interface manipulation, and captures the full dynamic nature of the particle−interface interactions. This methodology is also capable of observing a wide range of particle diameters, from tens of nanometers to several micrometers. The only requirement for particle size is that it is not so large as to obscure the point spread function of the microscope. In practice, this is rarely an issue as one can simply use a lower magnification with larger-diameter particles. In the future, we will use this feature to expand this technique to allow the determination of the absolute contact angle and its variation by using nanoparticles to locate the interface precisely. The work provided indicates that studying the contact angle of particles on suspended interfaces does not accurately capture the dynamic behavior of particles on interfaces and may result in an erroneous understanding of particle behavior at an interface. We believe that these results indicate that the normal motion of particles at an interface needs to be further considered in order to understand the true “dynamic” contact angle that the measurements shown here suggest exists. Whether the observed results depend on the preparation of the interface, particles, and system used requires further study. Nonetheless, the results indicate that the particle-laden interfaces are much more complicated than previously considered because of the nonmonotonic contact angle that creates a 3D system on the interface rather than a trivial 2D system.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
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DOI: 10.1021/la5040195 Langmuir 2015, 31, 891−897
