Shear Alignment and Instability of Smectic Phases

TL;DR

This paper investigates the stability of shear-aligned smectic phases, revealing that layer fluctuations destabilize parallel alignment while perpendicular alignment remains stable, with stability also depending on anisotropic viscosity.

Contribution

It demonstrates the generic instability of parallel alignment and the stability of perpendicular alignment in shear flows of smectic phases, considering thermal fluctuations and viscosity effects.

Findings

Parallel alignment is generically unstable due to thermal fluctuations.

Perpendicular alignment remains stable against long-wavelength undulations.

Both alignments can be stable within a narrow viscosity range.

Abstract

We consider the shear flow of well-aligned one-component smectic phases, such as thermotropic smectics and lamellar diblock copolymers, below the critical region. We show that, as a result of thermal fluctuations of the layers, parallel ($c$) alignment is generically unstable and perpendicular ($a$) alignment is stable against long-wavelength undulations. We also find, surprisingly, that both $a$ and $c$ are stable for a narrow window of values for the anisotropic viscosity.