Osmotic compression of droplets of hard rods: A computer simulation study

TL;DR

This study uses computer simulations to explore how hard-rod droplets deform and align under osmotic compression, revealing shape transitions and internal structure changes consistent with theoretical predictions.

Contribution

It demonstrates the spontaneous formation of nematic tactoids with specific director fields and estimates surface anchoring and elastic constants from droplet morphology.

Findings

Small tactoids have uniform director fields.

Large tactoids exhibit bipolar director fields.

Shape and director-field changes allow estimation of elastic properties.

Abstract

By means of computer simulations we study how droplets of hard, rod-like particles optimize their shape and internal structure under the influence of the osmotic compression caused by the presence of spherical particles that act as depletion agents. At sufficiently high osmotic pressures the rods that make up the drops spontaneously align to turn them into uniaxial nematic liquid crystalline droplets. The nematic droplets or "tactoids" that are formed this way are not spherical but elongated, resulting from the competition between the anisotropic surface tension and the elastic deformation of the director field. In agreement with recent theoretical predictions we find that sufficiently small tactoids have a uniform director field, whilst large ones are characterized by a bipolar director field. From the shape and director-field transformation of the droplets we are able to estimate the surface anchoring strength and an average of the elastic constants of the hard-rod nematic.