Colloids in active fluids: Anomalous micro-rheology and negative drag

TL;DR

This study uses simulations to explore how colloidal particles behave in active nematic fluids, revealing unusual phenomena like negative drag and non-Stokesian resistance due to active stresses.

Contribution

It demonstrates for the first time that active nematic fluids can produce negative viscous drag on colloids, expanding understanding of micro-rheology in active matter.

Findings

Negative viscous drag observed in contractile active fluids.

Drag force is non-Stokesian and depends on particle size.

Disruption of orientational order explains anomalous behavior.

Abstract

We simulate an experiment in which a colloidal probe is pulled through an active nematic fluid. We find that the drag on the particle is non-Stokesian (not proportional to its radius). Strikingly, a large enough particle in contractile fluid (such as an actomyosin gel) can show negative viscous drag in steady state: the particle moves in the opposite direction to the externally applied force. We explain this, and the qualitative trends seen in our simulations, in terms of the disruption of orientational order around the probe particle and the resulting modifications to the active stress.