Interaction of colloids with a nematic-isotropic interface

TL;DR

This paper investigates the complex interactions between cylindrical colloids and the nematic-isotropic interface using Landau-de Gennes theory, revealing defect dynamics, interface bending, and scaling laws for the forces involved.

Contribution

It provides a detailed numerical analysis of colloid-interface interactions, highlighting defect behavior and deriving simple force scaling laws in liquid crystal systems.

Findings

Defect annihilation occurs as colloids approach the interface.

The interface bends towards the colloid to minimize elastic energy.

A well-defined minimum in interaction force exists near the interface.

Abstract

The Landau-de Gennes free energy is used to calculate the interaction between long cylindrical colloids and the nematic-isotropic (NI) interface. This interaction has two contributions: one is specific of liquid crystals and results from the deformation of the director field close to the particles or to the interface, while the other is generic and results from wetting and surface tension effects. Deep in the nematic phase the director field of long cylindrical colloids, with strong homeotropic anchoring, exhibits two half-integer defect lines. As the colloid moves towards the interface, the director configuration changes through a series of discontinuous transitions, where one or two of the defects are annihilated. In addition, the NI interface bends towards the colloid in order to minimize the elastic free energy in the nematic. In the isotropic phase, the colloid is surrounded by a thin nematic layer that reduces the surface free energy under favorable wetting conditions. The interaction has a well-defined minimum near the interface. In this region the director and interfacial structures are complex and cannot be described analytically. Using the numerical results for the Landau-de Gennes free energy in the harmonic region, we obtained simple scaling laws for the (linear) force on the colloid.