Colloidal aggregation and dynamics in anisotropic fluids

Abstract

We present experiments and numerical simulations to investigate the collective behavior of submicrometer-sized particles immersed in a nematic micellar solution. We use latex spheres with diameters ranging from 190 to 780 nm and study their aggregation properties due to the interplay of the various colloidal forces at work in the system. We found that the morphology of aggregates strongly depends on the particle size, with evidence for two distinct regimes: the biggest inclusions clump together within minutes into either compact clusters or V-like structures that are completely consistent with attractive elastic interactions. On the contrary, the smallest particles form chains elongated along the nematic axis, within comparable timescales. In this regime, Monte Carlo simulations, based on a modified diffusion-limited cluster aggregation model, strongly suggest that the anisotropic rotational Brownian motion of the clusters combined with short-range depletion interactions dominate the system coarsening; elastic interactions no longer prevail. The simulations reproduce the sharp transition between the two regimes on increasing the particle size. We provide reasonable estimates to interpret our data and propose a likely scenario for colloidal aggregation. These results emphasize the growing importance of the diffusion of species at suboptical-wavelength scales and raise a number of fundamental issues.colloidal dispersion | liquid crystal | elasticity C olloidal dispersions are metastable systems consisting of small solid particles (size < 1 μm) immersed in a liquid phase. Compared with homogeneous liquids, they are characterized by a huge amount of interfaces, and their physical properties are mainly controlled by the aggregation state of the constitutive particles, which depends on their mutual interactions. In general, van der Waals attractions compete with the osmotic pressure of liquid films surrounding the particles, due to the presence of ions or molecules (electrostatic and steric effects, respectively) (1). Depletion interactions may also be invoked when several populations of objects, differing in size, coexist and diffuse in the same medium (2, 3). Hence, depending on the global balance of forces, the suspended particles may either remain isolated from each other or cluster into various kinds of structures ranging from compact aggregates to interconnected networks. A whole variety of different states are achievable, i.e., from solid opaque pastes to transparent newtonian fluids, which makes colloidal dispersions very attractive from a practical viewpoint as we can easily switch from one state to the other through chemical and/or physical means (pH, salt, additives, shear flow, etc.). A good example is provided by electrorheological fluids (and their magnetic counterparts), whose viscosity can be tuned by the application of an external field (4, 5). Needless to say, colloidal dispersions are ubiquitous in a broad spectrum of applications including food, cosmetics, and coating industries. Given their paramo...

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