pubs.acs.org/Langmuir © 2009 American Chemical Society
Charge Interaction between Particle-Laden Fluid Interfaces Hui Xu, John Kirkwood, Mauricio Lask, and Gerald Fuller* Department of Chemical Engineering, Stanford University, Stanford, California 94305-5025 Received August 20, 2009. Revised Manuscript Received September 26, 2009 Experiments are described where two oil/water interfaces laden with charged particles move at close proximity relative to one another. The particles on one of the interfaces were observed to be attracted toward the point of closest approach, forming a denser particle monolayer, while the particles on the opposite interface were repelled away from this point, forming a particle depletion zone. Such particle attraction/repulsion was observed even if one of the interfaces was free of particles. This phenomenon can be explained by the electrostatic interaction between the two interfaces, which causes surface charges (charged particles and ions) to redistribute in order to satisfy surface electric equipotential at each interface. In a forced particle oscillation experiment, we demonstrated the control of charged particle positions on the interface by manipulating charge interaction between interfaces.
1. Introduction Fluid interfaces laden with particles are of interest in the study of Pickering emulsions, foams, surface wetting, and the interactions between particles.1-12 The particles are held on the interface due to a strong capillary force,13,14 described in a free energy form as Eγ ¼ πR2 γð1 ( cos θÞ2
ð1Þ
where R is the particle radius, γ is the fluid-fluid interfacial tension, and θ is the contact angle between the particle and the fluid-fluid interface as measured through the water. For a micrometer-sized particle and a contact angle close to 90°, this capillary free energy is on the order of 106-108 kT, essentially making the particles irreversibly attached to the interface. For charged particles, there is usually a long-ranged dipole-dipole repulsion between them,15-21 which can lead to the formation of 2D hexagonal particle crystals on interfaces. The observation of long-ranged attractions between like-charged particles *To whom correspondence should be addressed. (1) Pickering, S. U. J. Chem. Soc. 1907, 91, 2001-2021. (2) Aveyard, R.; Clint, J. H. J. Chem. Soc., Faraday Trans. 1995, 91, 2681-2697. (3) Binks, B. P.; Lumsdon, S. O. Phys. Chem. Chem. Phys. 1999, 1, 3007-3016. (4) Binks, B. P. Curr. Opin. Colloid Interface Sci. 2002, 7, 21-41. (5) Fujii, S.; Read, E. S.; Binks, B. P.; Armes, S. P. Adv. Mater. 2005, 17, 1014-1018. (6) Tarimala, S.; Dai, L. L. Langmuir 2004, 20, 3492-3494. (7) Melle, S.; Lask, M.; Fuller, G. G. Langmuir 2005, 21, 2158-2162. (8) Xu, H.; Goedel, W. A. Langmuir 2003, 19, 4950-4952. (9) Vignati, E.; Piazza, R.; Lockhart, T. P. Langmuir 2003, 19, 6650-6656. (10) Tambe, D. E.; Sharma, M. M. Adv. Colloid Interface Sci. 1994, 52, 1-63. (11) Tadros, Th. V.; Vincent, B. Encyclopedia of Emulsion Technology; Dekker: New York, 1983; Vol. 1. (12) Denkov, N. D.; Ivanov, I. B.; Kralchevsky, P. A.; Wasan, D. T. J. Colloid Interface Sci. 1992, 150, 589-593. (13) Levine, S.; Bowen, B. D.; Partridge, S. J. Colloids Surf. 1989, 38, 325-343. (14) Xu, H.; Melle, S.; Golemanov, K.; Fuller, G. G. Langmuir 2005, 21, 10016-10020. (15) Pieranski, P. Phys. Rev. Lett. 1980, 45, 569-572. (16) Hurd, A. J. J. Phys. A 1985, 18, L1055-1060. (17) Aveyard, R.; Clint, J. H.; Nees, D.; Paunov, V. N. Langmuir 2000, 16, 1969-1979. (18) Aveyard, R.; Binks, B. P.; Clint, J. H.; Fletcher, P. D. I.; Horozov, T. S.; Neumann, B.; Paunov, V. N.; Annesley, J.; Botchway, S. W.; Nees, D.; Parker, A. W.; Ward, A. D.; Burgess, A. N. Phys. Rev. Lett. 2002, 88, 246102. (19) Frydel, D.; Dietrich, S.; Oettel, M. Phys. Rev. Lett. 2007, 99, 118302. (20) Oettel, M.; Dietrich, S. Langmuir 2008, 24, 1425-1441. (21) Park, B. J.; Pantina, J. P.; Furst, E. M.; Oettel, M.; Reynaet, S.; Vermant, J. Langmuir 2008, 24, 1686-1694.
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on fluid interfaces has been puzzling and is still a subject of controversy.22-31 In most studies, a single, isolated interface (e.g., air-water or oil-water interface in a Langmuir trough) has been used to analyze the behavior of particles on interfaces.15,17,18,30 However, in cases where coalescence must be considered (e.g., in the stability of Pickering emulsions and foams), particle-laden interfaces will necessarily interact. In other words, the interfaces will be in the proximity of another interface, and the separation between the interfaces can be at distances where the interactions between interfaces is no longer negligible. Recent studies on drop coalescence between particle-laden fluid interfaces offer insight toward this understanding. By bringing two particle laden interfaces together, the particles on the interfaces may aggregate to form structures of rings, disks (including double- and multilayer disks), or even holes (particle depletion zones), depending on whether one is squeezing out an oil layer between two bodies of water or squeezing out water between two bodies of oil.32-36 The surface concentration of the particles also has a strong influence on the outcome. For particles having a large contact angle with the oil-water interface, the bridging of two interfaces by a densely packed particle monolayer has been observed, which can prevent the two interfaces from coalescing.11,12,33-35 The formation of intricate particle structures, as well as the occurrence of particle bridging across interfaces, indicates that the two approaching (22) Stamou, D.; Duschl, C. Phys. Rev. E 2000, 62, 5263-5272. (23) Nikolaides, M. G.; Bausch, A. R.; Hsu, M. F.; Dinsmore, A. D.; Brenner, M. P.; Gay, C.; Weitz, D. A. Nature 2002, 420, 299-301. (24) Foret, L.; Wuerger, A. Phys. Rev. Lett. 2004, 92, 058302. (25) Megens, M.; Aizenberg, J. Nature 2003, 424, 1014. (26) Oettel, M.; Domı´ nguez, A.; Dietrich, S. Phys. Rev. E 2005, 71, 051401. (27) Kralchevsky, P. A.; Denkov, N. D.; Danov, K. D. Langmuir 2001, 17, 7694-7705. (28) Danov, K. D.; Kralchevsky, P. A.; Boneva, M. P. Langmuir 2004, 20, 6139-6151. (29) Loudet, J. C.; Alsayed, A. M.; Zhang, J.; Yodh, A. G. Phys. Rev. Lett. 2005, 94, 018301. (30) Chen, W.; Tan, S.; Ng, T.; Ford, W. T.; Tong, P. Phys. Rev. Lett. 2005, 95, 218301. (31) Oettel, M.; Domı´ nguez, A.; Dietrich, S. Langmuir 2006, 22, 846-847. (32) Stancik, E. J.; Kouhkan, M.; Fuller, G. G. Langmuir 2004, 20, 90-94. (33) Stancik, E. J.; Fuller, G. G. Langmuir 2004, 20, 4805-4808. (34) Ashby, N. P.; Binks, B. P.; Paunov, V. N. Chem. Commun. 2004, 436-437. (35) Horozov, T. S.; Aveyard, R.; Clint, J. H.; Neumann, B. Langmuir 2005, 21, 2330-2341. (36) Horozov, T. S.; Binks, B. P. Angew. Chem., Int. Ed. 2006, 45, 773-776.
Published on Web 10/23/2009
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interfaces do interact with each other, though the nature of the interactions is complex, which may involve of one or more of the following interactions: electrostatic, hydrodynamic, or capillary interactions. Among all the interactions, the long-range electrostatic (or charge) interaction between interfaces deserves particular attention, as one can imagine that a charged particle on one interface can be influenced by an electrostatic interaction from the opposing interface. In a previous study of particle bridging,37 we made a preliminary observation that, when a water drop coated with charged particles was brought close to a flat oil-water surface laden with similarly charged particles, the particles on both interfaces began to migrate, but in opposite directions. The particles on one interface (either the drop or the flat surface) were attracted toward the point of closest approach while the particles on the opposite interface were repelled away from this location.37 This particle migration occurs only when the two interfaces approach each other down to very small distances, suggesting that the interaction between the interfaces plays a major role in driving such motion. However, the details of the interaction and the manner in which the interaction acts on the particles remains unclear. In this paper, we investigate this phenomenon by carefully analyzing and interpreting the data obtained through experiments on the particle migration. We also demonstrate how to utilize such interactions to control and manipulate the motion of particles on the interfaces.
Figure 1. Schematic description of the experimental setup. The line is the axis of rotational symmetry of the capillary tube and the drop. The intersecting point of the line with the interface is considered to be the center of that interface. Note that the water drop is separated from the flat water phase below by an oil phase decane.
2. Experimental Section Spherical polystyrene particles with a mean diameter of 3.0 μm and a surface charge density of 7.5 μC/cm2 were acquired in the form of a surfactant-free, aqueous dispersion from Interfacial Dynamics Corp. Through dilution with deionized water (specific resistance 18.2 MΩ 3 cm, Millipore) and isopropyl alcohol (Mallinckrodt), a working dispersion consisting of (5.0-10.0) 108 particles/mL in a 20% isopropyl alcohol solution was created. Note that isopropyl alcohol is included as an aid to spreading the particles on interfaces. Sonication of this dispersion for 5 min prior to injection on interfaces helped to ensure the absence of aggregates. The interfaces themselves were created between deionized water (Millipore) and decane (Fisher). Prior to use in experiments, the decane was passed three times through a column of aluminum oxide (Fisher) to remove any polar contaminants that might have been present. The experimental setup is shown in Figure 1, which is similar to that used in previous experiments on drop coalescence and particle bridging.32,33,37 A glass capillary was used to create a water droplet in the oil phase near the flat interface. A volume of the working dispersion was spread onto the flat interface and/or the surface of the droplet to deposit the particles at a desired interfacial particle concentrations. After the system was allowed to come to equilibrium, the droplet was brought down to the flat interface via a xyz stage with motion controlled by micrometerdriven actuators. Particles at both interfaces were observed from the angle perpendicular to the flat interface using a Nikon Eclipse TE300 inverted microscope. Images were recorded via CCD cameras (Hamamatsu) attached to the microscopes. Metamorph software from Universal Imaging Corp. was used to perform the image analysis.38
3. Results and Discussion 3.1. Observation of Particle Attraction/Repulsion at Interfaces. As mentioned in the Experimental Section, a (37) Xu, H.; Lask, M.; Kirkwood, J.; Fuller, G. Langmuir 2007, 23, 4837-4841. (38) Monteux, C.; Kirkwood, J.; Xu, H.; Jung, E.; Fuller, G. G. Phys. Chem. Chem. Phys. 2007, 9, 6344-6350.
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Figure 2. (A, B) Schematic description of a particle-coated water drop approaching a flat oil-water interface laden with particles, resulting in the concentration of particles on the drop surface while the depletion of particles on the flat interface. (C, D) Experimental observation that corresponds to the description. Note that the particles appearing darker in color are on the flat interface.
millimeter-sized (in diameter) water droplet was generated at the end of a capillary tube, which was connected to a syringe pump. Part of the capillary tube and the entire water drop were immersed in decane that covered the top of a bulk water phase in a Petri dish. The decane and the bulk water form a nearly flat oil-water interface while the decane and the drop formed the curved oil-water interface. After depositing particles onto both interfaces (with a surface concentration of (0.2-1) 106 particles/ cm2), the drop was brought down toward the flat interface using the motor-driven xyz motion stage. When the drop approached sufficiently close to the flat interface, but without contacting it (estimated to be several micrometers apart based on the focusing contrast of the particles on the two interfaces), the particles on both interfaces began to migrate. An example is shown in Figure 2. In this case, particles on the flat interface were repelled out of the center region forming a particle depletion zone at the center while the particles on the drop surface were attracted toward the center forming a denser particle monolayer. This countermigration is evident in Figure 2D where the particles on the drop surface, being less in focus, appear as larger spheres than the particles on the flat interface, which are seen as dark, smaller spheres. The opposite case of particle depletion on drop interfaces and particle DOI: 10.1021/la903099a
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Figure 4. In this experiment, the water drop coated with very few particles was brought close to a flat interface laden with particles. The particles on the drop suface were attracted to the center forming particle cluster while the particles on the flat interface were depleted out of the center. (A) The formation of a threeparticle cluster. (B) The formation of a large particle cluster consisting of about 25 particles.
Figure 3. A clean water drop approaching the particle-laden oilwater interface. Two scenarios, either particle depletion or particle concentration, were observed. (A-C) Sequential images of particle depletion; (D) schematic description of (A-C); (E-G) sequential images of particle concentration; (H) schematic description of (E-G).
concentration on flat interfaces was also observed, as was mentioned in a previous paper.37 It is interesting to note that the particles on the two interfaces always move in opposite directions, i.e., countermigrate. It is worth noting that the gaps between the interfaces were nominally 3 orders of magnitude smaller than the drop radius, suggesting that curvature of the droplet does not play a substantial role in affecting this phenomenon. These particle attraction (or repulsion) events were observed even if one of the interfaces was free of particles. As shown in Figure 3, when a clean water drop was brought down to a particlecovered flat interface, the particles on the flat interface were either attracted to the center or repelled away from the center. Interestingly, the occurrence of particle repulsion (Figure 3A-D) or attraction (Figure 3E-H) seems to be random in this case. Empirically, we observed that increasing the initial surface concentration of the particles increased the probability of particle attraction while aging of the water drop (e.g., by keeping a water drop in decane for more than 5 min before descending down to the flat interface) increased the probability of particle repulsion. In some experiments, only sparse collections of particles were introduced onto the drop surface, either by carefully controlling the particle deposition process or simply by accident. These particles, which were initially distributed randomly across the drop surface, could eventually be attracted to the center of the drop surface forming a 2D particle cluster as the drop approached the flat interface (Figure 4), while the particles on the flat interface were evacuated from the center region. Figure 4A shows a triangular particle cluster with only three particles present on the drop surface, while Figure 4B shows a larger particle cluster with more particles attracted to the center. It should be noted that the particle cluster was disbanded with particles diffusing away from the center when the drop was lifted away from the flat interface, suggesting that interaction between the two interfaces drove the formation of such particle clusters. The dynamics of particle cluster formation is shown in Figure 5 with sequential images depicting the process of two particles joining a cluster. The size of the cluster grows over time as more particles migrate toward and join the cluster. The formation of such particle clusters is a phenomenon reminiscent of long-ranged 3162 DOI: 10.1021/la903099a
Figure 5. (A-D) Images capture the process of two particles joining the particle cluster forming at the center of the drop surface while the particles on the flat oil-water interface were driven out of the center.
particle attraction on an oil-water interface as reported by Nikolaides et al.23 In that work, the existence of long-ranged particle attraction on an interface was evident. However, the origin of such interactions is still a subject of ongoing debate.24-26,28,30,31 One argument raises concern about the experimental conditions under which the observations were made.26 It has been argued that the long-ranged particle attraction at an interface could be induced by external sources of perturbations (e.g., electrical fields) if the interface under study was not properly isolated. In the following section, we offer an explanation for the particle attraction/repulsion observed in the present experiments investigating two approaching interfaces. 3.2. Mechanism of the Observed Particle Attraction/ Repulsion. There are three possible interactions that may be relevant to the observed particle attraction/repulsion in these experiments: hydrodynamic, capillary, and electrostatic interactions. Hydrodynamic interaction plays an important role in drop coalescence processes.39-43 As two drops are approaching one another, the fluid sandwiched between the drops thins out, creating an outward hydrodynamic draining flow. A dimple on (39) Menchaca-Rocha, A.; Martı´ nez-Davalos, A.; Nun~ez, R.; Popinet, S.; Zaleski, S. Phys. Rev. E 2001, 63, 046309. (40) Wu, M.; Cubaud, T.; Ho, C.-M. Phys. Fluids 2004, 16, L51. (41) Aarts, D. G. A. L.; Lekkerkerker, H. N. W.; Guo, H.; Wegdam, G. H.; Bonn, D. Phys. Rev. Lett. 2005, 95, 164503. (42) Fezzaa, K.; Wang, Y. Phys. Rev. Lett. 2008, 100, 104501. (43) Hu, Y. T.; Pine, D. J.; Gary Leal, L. Phys. Fluids. 2000, 12, 484-489.
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the droplet surfaces can be formed due to increased hydrodynamic pressure at the point of approach, as can be understood from lubrication theory. The outward hydrodynamic draining flow creates a viscous stress on the interfaces and, if strong enough, can drag interfacial materials away with it. In this case, the particles on the interfaces would be driven out of the center region, leading to particle depletion. However, this mechanism cannot explain the simultaneous particle attraction on the opposite interface. In addition, there are other observations that work against hydrodynamic interaction being the source of the observed particle dynamics: (1) the particle countermigration was not affected by substantially reducing the approaching speed of the drop to the flat interface (i.e., independent of the hydrodynamic flow velocity); (2) the particle concentration/depeletion, once achieved, would continue even if the motion of the drop was arrested; (3) the particle countermigration was not observed for particles with less or no charge, indicating that electrostatics rather than hydrodynamics are responsible for the particle dynamics. Another possible explanation is that capillary forces, arising from the local deformation of fluid interfaces containing the particles,44 are responsible for the particle motions. These capillary interactions might be able to explain the particle attraction but cannot explain the particle depletion. We are left with a consideration of electrostatic interactions. Since the particles are charged, we anticipate they will interact via electrostatic interactions, and these interactions may occur between particles within each interface and between particles on the two opposing interfaces. The low dielectric constant (2.0) of the oil phase (decane) will enhance the latter interaction because of a reduction in charge screening. While the interaction between particles on individual interfaces is mostly repulsive, the interaction between the two opposing interfaces may be attractive. To help understand this possibility, we must not limit ourselves to the interaction between individual particles. It is useful to consider the particles as negative charges residing on interfaces of electric conductors, since water is an electrically conducting fluid;the dissociation of water molecules into protons (Hþ) and hydroxyls (OH-) renders it conductive. For a conductor, excess charges will accumulate on interfaces and distribute in such a way that the surface electric potential remains equal everywhere at equilibrium.45 For an isolated conductor with uniform surface curvature, the charge distribution on the surface is uniform. However, when two conductors approach each other (i.e., the drop approaches the flat interface), the electric fields emitted from the two interfaces start to interfere with each other. This interference can lead to a charge redistribution on both interfaces to re-establish the surface equipotential. Typically, a charge separation (polarization), where the charges with opposite signs accumulate on the opposing interfaces, will occur as a result. It is important to note that on the water-decane interface, in addition to the charges offered by the particles, charges will also arise from dissociated Hþ, OH-, and other ions.46 The charge separation is believed to be responsible for the observed particle attraction/repulsion, with the particles (negative charges) concentrating on one interface and with protons (or other positive ions) accumulating on the opposing interface, which excludes the particles from the region of closest approach leading to particle depletion. The different (44) Kralchevsky, P. A.; Nagayama, K. Adv. Colloid Interface Sci. 2000, 85, 145-192. (45) Miller, F. College Physics, 2nd ed.; Harcourt, Brace & World, Inc.: New York, 1967; p 371. (46) Marinova, K. G.; Alargova, R. G.; Denkov, N. D.; Velev, O. D.; Petsev, D. N.; Ivanov, I. B.; Borwankar, R. P. Langmuir 1996, 12, 2045-2051.
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Figure 6. Schematic description of the possible mechanisms associated with the particle depletion and concentration. (A, B) Particle-coated water drop approaching a flat particle-laden oil-water interface, in case (A) the particles on the drop surface are depleted while in case (B) the particles on the flat interface are depleted whereas the particles on the opposite interface are concentrated. On the depleted interfaces, positive charges (e.g., Hþ) are accumulated because of charge separation (polarization). (C, D) A clean drop approaches a flat particle-laden oil-water interface; the particles on the flat interface were either concentrated (C) or depleted (D). In either case, the charge separation results in the accumulation of either positive charges (e.g., Hþ) or negative charges (e.g., OH-) on the drop surface.
Figure 7. (A-C) Images showed that two particles were undergoing oscillative movement when the water drop was made to move up and down (the two particles reside on the drop surface). (D) The interparticle distance oscillates over time.
scenarios encountered in our experiments are schematically represented in Figure 6. 3.3. Controlling the Motion of Particles on Interfaces. In this section, we discuss the control of particle motion on an interface by manipulating the charge interaction between interfaces. As the interaction is dependent on the separation distance between interfaces, the motion of the particles on an interface can be controlled by tuning the separation distance. The protocol for these experiments had the flat interface laden with a particle monolayer while the drop surface contained only two particles. By moving the drop down to the flat interface, one observes particle depletion on the flat interface while a two-particle cluster DOI: 10.1021/la903099a
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is formed at the center of drop surface (Figure 7). It is interesting to note that the interparticle distance of the two-particle cluster increases when the drop is moving slightly away from the flat interface while the opposite process;moving the drop toward the flat interface;results in a decrease of the interparticle distance. By repeating the whole process, i.e., moving the drop up and down, we observed a periodic change of the interparticle distance between the two particles resulting in the oscillatory motion of the particles shown in Figure 7. The ability to control particle motion on interfaces may find use in studies such as particle dynamics on interfaces and interfacial rheology.
4. Conclusions Both particle attraction and particle repulsion were observed on interfaces when a water drop (immersed in an oil phase) was
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brought close to a flat oil-water interface. This occurs when one or both interfaces were laden with charged particles. The observed particle attraction/repulsion can be explained by electrostatic interactions between the two interfaces, causing the surface charge, including the charged particles, to redistribute in order to satisfy a surface electric equipotential on each interface. This experiment indicates that the interaction between particles is not only important on individual interfaces but also important across different interfaces if the interfaces are sufficiently close to one another. In a particle oscillation experiment we demonstrated that the motion of the particles on the interface can be controlled by tuning the separation distance between the two interfaces. Acknowledgment. We acknowledge the National Science Foundation (NSF) and Unilever Co. for funding.
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