Novel Colloidal Interactions in Anisotropic Fluids

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.

Transport of particles in liquid crystals

“…The matching between the local radial director 0 /  n r r near the sphere and the uniform far-field is provided by a disclination ring of strength (-1/2). The Saturn ring structure, however, is usually unstable, as the disclination ring can shrink into a point defect, the so-called "hyperbolic" hedgehog 33 , Fig.1(c), thus reducing the elastic energy. The resulting dipolar structure with a point defect-hedgehog is energetically stable in most cases of normal anchoring.…”

“…Figure 2 shows the optical microscopy textures of a sphere with normal anchoring and accompanying hyperbolic hedgehog and its modification by a strong alternating current (AC) electric field. represents the most dramatic departure of the LC-colloid system from its isotropic counterparts, as the director distortions around the particles bring a number of new facets in the behaviour of colloids, such as anisotropic inter-particle forces 33 , levitation 44 and nonlinear electrophoresis 45 . A rough estimate of the order of magnitude of the elastic energy associated with the director distortions around a strongly anchored micron-size particle placed in an otherwise uniform nematic cell, is 56 .…”

“…For example, the cores of line defects that frame focal conic domains in smectic LC can lift spherical particles above the base of the domain to minimize the surface anchoring energy 59 . Colloidal particles have been shown to be attracted by the cores of point defects 33 and disclination lines 58,104 . An example of accumulation of polymer particles at the core of a singular disclination of strength 1/2 is illustrated in Fig.…”

“…Cell thickness d, m As follows from the consideration above, location of a particle in a LC is dictated not only by gravity and Brownian motion, as in an isotropic fluid, but also by the director field, its gradients, balance of elastic and surface anchoring forces, as in Fig.7, by geometry and properties of bounding walls and interfaces, Fig.4,5 63,64 , by presence of other particles 33 , etc. Spectacular effects of linking colloidal particles into clusters by disclination lines have been demonstrated by Ljubljana group 49,54 .…”

Colloidal caterpillars for cargo transportation

“…S1A). The resulting configuration of the director field around the particle has a dipolar symmetry and leads to anisotropic interactions [16][17][18][19][20][21] reminiscent of the interaction between electric dipoles. The pair interaction is of the order of several thousand k B T , leading to the formation of stable colloidal structures such as chains or two-and three-dimensional crystals.…”