Stress response and structural transitions in sheared gyroidal and lamellar amphiphilic mesophases: lattice-Boltzmann simulations

TL;DR

This study uses lattice-Boltzmann simulations to explore how gyroidal and lamellar amphiphilic mesophases respond to steady shear, revealing shear-thinning behavior, structural transformations, and pattern formations influenced by amphiphile presence.

Contribution

It provides detailed insights into the stress response and structural transitions of gyroidal and lamellar mesophases under shear using a bottom-up lattice-Boltzmann model.

Findings

Both mesophases exhibit shear-thinning behavior.

Crystalline gyroid transforms into interconnected irregular structures under shear.

Amphiphile presence increases late-time shear stress and induces interface patterning.

Abstract

We report on the stress response of gyroidal and lamellar amphiphilic mesophases to steady shear simulated using a bottom-up lattice-Boltzmann model for amphiphilic fluids and sliding periodic (Lees-Edwards) boundary conditions. We study the gyroid per se (above the sponge-gyroid transition, of high crystallinity) and the molten gyroid (within such a transition, of shorter-range order). We find that both mesophases exhibit shear-thinning, more pronounced and at lower strain rates for the molten gyroid. At late times after the onset of shear, the skeleton of the crystalline gyroid becomes a structure of interconnected irregular tubes and toroidal rings, mostly oriented along the velocity ramp imposed by the shear, in contradistinction with free-energy Langevin-diffusion studies which yield a much simpler structure of disentangled tubes. We also compare the shear stress and deformation of lamellar mesophases with and without amphiphile when subjected to the same shear flow applied normal to the lamellae. We find that the presence of amphiphile allows (a) the shear stress at late times to be higher than in the case without amphiphile, and (b) the formation of rich patterns on the sheared interface, characterised by alternating regions of high and low curvature.