Electric-field-induced capillary attraction between like-charged particles at liquid interfaces
by M. G. Nikolaides, A. R. Bausch, M. F. Hsu, A. D. Dinsmore, M. P. Brenner, C. Gay & D. A. Weitz
In the recent years, scientists are usually interested in nano and micro scale experiments. In this article, the authors examed nanometer and micrometer sized charged particles at aqueous interfaces and showed that they are stabilized by a repulsive Coulomb interaction. Importantly, the authors indicated that when one of the phases forming the interface is a nonpolar substance that cannot sustain a charge, the particles will exhibit long-ranged dipolar repulsion. Moreover, if the interface area is confined, mutual repulsion between the particles can induce ordering and even crystallization. As for capillary forces, the authors discussed that they are typically caused by interface deformations. In the article, the authors did extensive experiments and presented quantitative measurements of attractive interactions between colloidal particles at oil–water interface and showed that the attraction can be explained by capillary forces that arise from a distortion of the interface shape. The author's explanation also suggested that the attractive interactions might be controllable. The mechanism is that by tuning the polarity of one of the interfacial fluids, it should be possible to adjust the electrostatic stresses of the system and hence the interparticle attractions.
The authors presented much information in the article. Those important experimental results and procedures are well illustrated by these four figures. First and foremost, figure 1 shows a fluorescence microscopy image of an ordered structure of colloidal particles at an oil–water interface. The authors use this to demonstrate the long range of the repulsive interaction. Moreover, in the experiment, the authors used poly(methyl methacrylate) (PMMA) particles, sterically stabilized with poly(hydroxystearic acid). The particles are suspended in decahydronaphthalene at a volume fraction of 0.01%. The particle cores are labelled with fluorescent dye. An emulsion of water drops in oil is produced by gently shaking around 2 ml of deionized water in 1ml of the decalin–PMMA mixture. The particles are strongly bound to the interface because of the surface energies. The authors used this to determine the contact angle. All in all, the particles are never observed to escape, which implies that the particles are trapped by a barrier of several tens of kBT. The mean squared displacement measured at short delay times provides a measure of the particle diffusion coefficient, which is intermediate between that for a particle in the water and in the oil. This confirms that the particles are at the interface, partially extending into each of the fluids.
Figure 2 is a scatter plot showing positions of a seven-particle hexagonal crystallite on a water droplet of 24 mm radius. Figure 3 shows secondary interparticle potential minimum derived from experimental observations. The data points are obtained from the particle distribution function of the crystallite shown in Fig. 2. The inset is a sketch of the full interparticle potential and includes the repulsive barrier that stabilizes the particles, and a deep primary minimum at short range that is due to van der Waals attraction. According to the authors, these crystallites were observed to collapse and form a gel after several hours, confirming the presence of the primary minimum and the large repulsive barrier. Figure 4 provides a sketch of the equipotential lines at the fluid interface and the resulting distortion of the oil–water interface.
In this article, the authors believes that the experimental results are useful to convey the essential physics, which is that, first, dipolar electric fields induce surface charges that distort the interface, and second, the dipolar interaction causes repulsion, while the interfacial distortion causes capillary attraction. According to the authors, this finding has broad implication because adsorption of charged particles at fluid interfaces is a common phenomenon in foods, drugs, oil recovery, and even biology.