Physics

Prof. Surajit Dhara
University of Hyderabad

Abstract:

Topological defects have been objects of intense studies in various disciplines starting from cosmology to condensed matter, optics and more recently in active matter. In liquid crystals (LCs) they are produced during the symmetry breaking phase transition. Such defects can be induced by dispersing foreign nano- and micro-particles in LCs. The embedded particles create elastic distortions in the LC medium inducing topological defects, and interact via long-range anisotropic elastic forces so generated. These forces obviously have no analogues in regular colloidal systems in an isotropic dispersive medium. An interesting manifestation of such novel forces is the ability of the colloidal system to self-assemble. In an experiment, such a process can be conveniently guided to create 2D and 3D colloidal crystals, with complex architectures.

In this talk, I will present some of our recent studies on particle induced defects and transformations of such defects across the phase transitions in liquid crystals. We show that the elastic properties and the emergence of smectic layering have profound effects on these defects, in terms of the colloidal pair-interactions and their resulting two-dimensional assemblies. Finally, I will present some recent results on the electric field driven transport properties of Janus particles.  In a striking departure from conventional electrophoresis, we show that metal-dielectric Janus particles can be piloted at will through a nematic liquid crystal film, in the plane perpendicular to an imposed AC electric field. We achieve complete command over particle trajectories by varying field amplitude and frequency, exploiting the sensitivity of electro-osmotic flow to the asymmetries of particle and defect structure. We propose a new method for measuring the induced electrostatic dipole moment of the Janus particles, through competition between elastic and electrostatic interactions. These findings open unexplored directions for the use of colloids and liquid crystals in controlled transport, assembly and dynamical topology.