Interplay between a hydrodynamic instability and a phase transition: the Faraday instability in dispersions of rodlike colloids

TL;DR

This study investigates how Faraday surface waves in suspensions of rodlike colloids induce local nematic ordering and exhibit hysteretic, subcritical behavior due to shear-thinning properties, combining experiments, modeling, and simulations.

Contribution

It reveals the coupling between hydrodynamic instability and microstructural phase transition in colloidal suspensions, supported by experimental and numerical evidence.

Findings

Faraday waves cause local nematic ordering in colloids.

Surface waves exhibit hysteresis and subcritical transition.

Large domains align with flow in near-transition suspensions.

Abstract

Strong effects of the Faraday instability on suspensions of rodlike colloidal particles are reported through measurements of the critical acceleration and of the surface wave amplitude. We show that the transition to parametrically excited surface waves displays discontinuous and hysteretic features. This subcritical behaviour is attributed to the shear-thinning properties of our colloidal suspensions thanks to a phenomenological model based on rheological data under large amplitude oscillatory shear. Birefringence measurements provide direct evidence that Faraday waves induce local nematic ordering of the rodlike colloids. While local alignment simply follows the surface oscillations for dilute, isotropic suspensions, permanent nematic patches are generated by surface waves in samples close to the isotropic-to-nematic transition and above the transition large domains align in the flow direction. This strong coupling between the fluid microstructure and a hydrodynamic instability is confirmed by numerical computations based on the microstructural response of rodlike viruses in shear flow.