Novel Colloidal Interactions in Anisotropic Fluids
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Abstract:
Small water droplets dispersed in a nematic liquid crystal exhibit a novel class of colloidal interactions, arising from the orientational elastic energy of the anisotropic host fluid. These interactions include a short-range repulsion and a long-range dipolar attraction, and they lead to the formation of anisotropic chainlike structures by the colloidal particles. The repulsive interaction can lead to novel mechanisms for colloid stabilization.Keywords:
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The distance-resolved effective force between two spherical highly charged colloidal macroions is calculated within the primitive model of strongly asymmetric electrolytes using computer simulations. For parameters corresponding to typical experimental samples, a repulsive force is obtained. Possibilities for an effective attraction induced by very strong coupling between the macroions or by a geometric confinement of the macroions between glass plates are briefly discussed.
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Depletion attraction induced by non-adsorbing polymers or small particles in colloidal solutions has been widely used as a model colloidal interaction to understand aggregation behavior and phase diagrams, such as glass transitions and gelation. However, much less attention has been paid to study the effective colloidal interaction when small particles/molecules can be reversibly attracted to large colloidal particles. At the strong attraction limit, small particles can introduce bridging attraction as it can simultaneously attach to neighbouring large colloidal particles. We use Baxter’s multi-component method for sticky hard sphere systems with the Percus-Yevick approximation to study the bridging attraction and its consequence to phase diagrams, which are controlled by the concentration of small particles and their interaction with large particles. When the concentration of small particles is very low, the bridging attraction strength increases very fast with the increase of small particle concentration. The attraction strength eventually reaches a maximum bridging attraction (MBA). Adding more small particles after the MBA concentration keeps decreasing the attraction strength until reaching a concentration above which the net effect of small particles only introduces an effective repulsion between large colloidal particles. These behaviors are qualitatively different from the concentration dependence of the depletion attraction on small particles and make phase diagrams very rich for bridging attraction systems. We calculate the spinodal and binodal regions, the percolation lines, the MBA lines, and the equivalent hard sphere interaction line for bridging attraction systems and have proposed a simple analytic solution to calculate the effective attraction strength using the concentrations of large and small particles. Our theoretical results are found to be consistent with experimental results reported recently.
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The magnetic fields around submicroscopical iron particles were tested with iron oxide colloid in an electron microscope. The colloid particles usually spread evenly by diffusion. Magnetic fields of several hundred oersteds are required to concentrate the colloid particles visibly at certain spots. We found colloid around iron particles concentrated to various degrees. The colloid was attracted less by smaller particles. The smallest particles did not attract any colloid. The particles were measured and grouped into three following categories: (a) Particles with strong, all around colloid attraction, indicating many domains. Stem volume of the dendritic particles much bigger than 109 A3, (b) Particles with colloid attractions at spots, indicating one or a few domains. (c) Particles without colloid attraction, indicating no well-developed domains. Stem volume much less than 109 A3.
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The possibility of long–ranged attractions between colloids trapped at fluid interfaces is a topic of current interest. Ref. 1 proposes an intriguing mechanism of geometrical nature for the appearance of a logarithmic capillary attraction between colloids on a droplet. In this respect we note that such an attraction was already proposed in Ref. 2 with essentially the same physical interpretation as the one given in Ref. 1, i.e., an unbalance between the forces on the particle and on the curved interface, respectively. However, following a different approach it was shown that such an unbalance does not arise [3]. Without being exhaustive, here we correct only those mistakes in Ref. 1 which affect the main conclusion. We demonstrate that there is no logarithmically varying interfacial deformation and consequently no logarithmic capillary attraction. Incidentally, this Comment provides an independent confirmation of the conclusions obtained in Ref. 3. (a) The definition of Π1: For an isolated droplet in equilibrium, the electrical force K = KnQ acting on the particle is compensated by tension exerted by the interface at the contact line θ = θ0. In Ref. 1, the effect of this line tension is modelled by a pressure term Π1 acting along the normal n0 of the undeformed interface. The value Π1 of this model must be determined by Eq. (9), i.e., Newton’s law of action–reaction. Exploiting rotational symmetry, Eq. (9) can be projected onto the unit vector ez(= nQ) and evaluated with Eq. (11):
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