Phase behaviour and defect structure of soft rods on a sphere

TL;DR

This study uses molecular dynamics simulations to explore the phase behavior and defect structures of soft rods confined on a sphere, revealing how different phases emerge and transition based on aspect ratio and density.

Contribution

It provides the first detailed phase diagram and defect analysis of soft spherocylinders on a spherical surface, highlighting the topological constraints and defect dynamics.

Findings

Nematic phase appears only above aspect ratio 6.0-7.0.

Ordered phases have total +2 defect charge due to topology.

Sequence of phase transitions driven by packing fraction changes.

Abstract

Using particle-resolved molecular-dynamics simulations, we compute the phase diagram for soft repulsive spherocylinders confined on the surface of a sphere. While crystal (K), smectic (Sm), and isotropic (I) phases exhibit a stability region for any aspect ratio of the spherocylinders, a nematic phase emerges only beyond a critical aspect ratio lying between 6.0 and 7.0. As required by the topology of the confining sphere, the ordered phases exhibit a total orientational defect charge of +2. In detail, the crystal and smectic phases exhibit two +1 defects at the poles, whereas the nematic phase features four +1/2 defects which are connected along a great circle. For aspect ratios above the critical value, lowering the packing fraction drives a sequence of transitions: the crystal melts into a smectic phase, which then transforms into a nematic through the splitting of the +1 defects into pairs of +1/2 defects that progressively move apart, thereby increasing their angular separation. Eventually, at very low densities, orientational fluctuations stabilize an isotropic phase. Our simulations data can be experimentally verified in Pickering emulsions and are relevant to understand the morphogenesis in epithelial tissues.