Yashodhan Hatwalne

Rheology of active-particle suspensions.

We study the interplay of activity, order, and flow through a set of coarse-grained equations governing the hydrodynamic velocity, concentration, and stress fields in a suspension of active, energy-dissipating particles. We make several predictions for the rheology of such systems, which can be tested on bacterial suspensions, cell extracts with motors and filaments, or artificial machines in a fluid. The phenomena of cytoplasmic streaming, elastotaxis, and active mechanosensing find natural explanations within our model.

A phenomenological model for the undulating twist grain boundary-C* phase

We propose a simple phenomenological model which is able to account for the various twist grain boundary (TGB) phases, including the recently discovered undulating twist grain boundary-C* (UTGBC*) phase. In the UTGBC* phase, the smectic C* (SmC*)-like blocks and the grain boundaries separating them undulate to form a two-dimensional square lattice perpendicular to the TGB helix axis. We treat the grain boundaries separating adjacent smectic blocks as interfaces with an anisotropic interfacial tension. At moderate chiral strengths we find a TGBA-TGBC-SmC* sequence. As the chiral strength is increased this goes to the sequence TGBA-UTGBC*-SmC*. Such sequences have been observed experimentally.

Laser induced rotation of trapped chiral and achiral nematic droplets

We study the response of optically trapped achiral and chiralized nematic liquid crystal droplets to linear as well as circular polarized light. We find that there is internal dissipation in rotating achiral nematic droplets trapped in glycerine. We also demonstrate that some chiralized droplets rotate under linearly polarized light. The best fit to our data on chiralized droplets indicates that rotational frequency of these droplets with radius R is approximately proportional to 1/R 2, rather than to 1/R 3.

Escape configuration lattice near the nematic-isotropic transition: tilt analogue of blue phases.

We predict the possible existence of a new phase of liquid crystals near the nematic-isotropic transition. This phase is an achiral, tilt analogue of the blue phase and is composed of a lattice of double-tilt, escape-configuration cylinders. We discuss the structure and the stability of this phase and provide an estimate of the lattice parameter.

Equilibrium of fluid membranes endowed with orientational order.

Minimization of the low-temperature elastic free-energy functional of orientationlly ordered membranes involves independent variation of the membrane-shape, while keeping the orientational order on it (its texture) fixed. We propose an operational, coordinate-independent method for implementing such a variation. Using the Nelson-Peliti formulation of elasticity that emphasizes the interplay between geometry, topology, and thermal fluctuations of orientationally ordered membranes, we minimize the elastic free energy to obtain equations governing their equilibrium shape, together with associated free boundary conditions. Our results are essential for understanding and predicting equilibrium shapes as well as textures of membranes and vesicles; particularly under conditions in which shape deformations are large.

Elasticity of smectic liquid crystals with in-plane orientational order and dispiration asymmetry

The Nelson-Peliti formulation of the elasticity theory of isolated fluid membranes with orientational order emphasizes the interplay between geometry, topology, and thermal fluctuations. Fluid layers of lamellar liquid crystals such as smectic-C, hexatic smectics, and smectic-C^{*} are endowed with in-plane orientational order. We extend the Nelson-Peliti formulation so as to bring these smectics within its ambit. Using the elasticity theory of smectics-C^{*}, we show that positive and negative dispirations (topological defects in Smectic-C^{*} liquid crystals) with strengths of equal magnitude have disparate energies-a result that is amenable to experimental tests.

The cone phase of liquid crystals: Triangular lattice of double-tilt cylinders

We predict the existence of a new defect-lattice phase near the nematic-smectic-C(NC) transition. This tilt-analogue of the blue phase is a lattice of double-tilt cylinders which are disclination lines in the smectic layer normal as well as the c-field. We discuss the structure and stability of the cone phase. We suggest that many ‘nematics’ exhibiting short range layering and tilt order may in fact be in the molten cone phase, which is a line liquid.