Viscosity ratio across interfaces controls the stability and self-assembly of microrollers

TL;DR

This study explores how the viscosity ratio across fluid interfaces influences the stability, motion, and self-assembly of torque-driven microrollers, revealing key hydrodynamic control mechanisms.

Contribution

It demonstrates the role of viscosity ratio in controlling microroller dynamics and collective behavior through combined theoretical and numerical analysis.

Findings

Viscosity ratio affects particle speed and direction.

Viscosity ratio influences fingering instability growth.

Hydrodynamic interactions govern self-assembled structures.

Abstract

We investigate the individual and collective dynamics of torque-driven particles, called microrollers, near fluid-fluid interfaces. We find that the viscosity ratio across the interface controls the speed and direction of the particles, their relative motion, the growth of a fingering instability, and the self-assembled motile structures that emerge from it. By combining theory and large scale numerical simulations, we show how the viscosity ratio across the interface governs the long-range hydrodynamic interactions between particles and thus their collective behavior.