Shear-induced ordering of nano-pores and instabilities in concentrated surfactant mesh phases

TL;DR

This study investigates how shear stress induces a transition from a disordered to an ordered nano-pore mesh phase in concentrated surfactant systems, revealing complex non-equilibrium phase behaviors.

Contribution

It provides the first detailed experimental evidence of shear-induced ordering and structural symmetry in surfactant mesh phases using in-situ synchrotron X-ray diffraction.

Findings

Shear stress causes a transition to a rhombohedral ordered phase.

The ordered phase exhibits isomorphic twinning and buckling.

Rich non-equilibrium phase transitions are observed in surfactant systems.

Abstract

Mixed surfactant systems with strongly bound counterions show many interesting phases such as the random mesh phase consisting of a disordered array of defects (water-filled nano-pores in the bilayers). The present study addresses the non-equilibrium phase transition of the random mesh phase under shear to an ordered mesh phase with a high degree of coherence between nano-pores in three-dimension. In-situ small-angle synchrotron X-ray study under different shear stress conditions shows sharp Bragg peaks in the X-ray diffraction, successfully indexed to the rhombohedral lattice with R$\bar{3}$m space group symmetry. The ordered mesh phase shows isomorphic twinning and buckling at higher shear stress. Our experimental studies bring out rich non-equilibrium phase transitions in concentrated cationic surfactant systems with strongly bound counterions hitherto not well-explored and provide motivation for a quantitative understanding.