Colloids in Cholesterics: Size-Dependent Defects and Non-Stokesian Microrheology

TL;DR

This study investigates how colloidal particles in cholesteric liquid crystals exhibit size-dependent defect structures and non-Stokesian microrheology, revealing complex defect evolution and unusual drag force scaling.

Contribution

It introduces a simulation of colloids in cholesterics showing defect structure transitions and demonstrates non-Stokesian drag behavior in microrheology.

Findings

Defect structures evolve from dipolar to twisted disclinations with size.

Drag force scales superlinearly with particle radius, violating Stokes' law.

Hydrodynamic interactions are complex and size-dependent.

Abstract

We simulate a colloidal particle (radius R) in a cholesteric liquid crystal (pitch p) with tangential order parameter alignment at the particle surface. The local defect structure evolves from a dipolar pair of surface defects (boojums) at small R/p to a pair of twisted disclination lines wrapping around the particle at larger values. On dragging the colloid with small velocity v through the medium along the cholesteric helix axis (an active microrheology measurement), we find a hydrodynamic drag force that scales linearly with v but superlinearly with R-in striking violation of Stokes' law, as generally used to interpret such measurements.