Active particles in geometrically confined viscoelastic fluids

TL;DR

This study experimentally investigates how active particles behave in confined viscoelastic fluids, revealing effects like effective repulsion, asymmetric movement, and phase transitions from liquid-like to ordered states.

Contribution

It provides new insights into active particle dynamics in viscoelastic environments under various geometrical constraints, highlighting the role of viscoelasticity in particle interactions and collective behavior.

Findings

Viscoelastic fluids induce effective repulsion near surfaces.

Confinement causes asymmetry and hydrodynamic torques on particles.

Active particles exhibit phase transitions between liquid-like and ordered states.

Abstract

We experimentally study the dynamics of active particles (APs) in a viscoelastic fluid under various geometrical constraints such as flat walls, spherical obstacles and cylindrical cavities. We observe that the main effect of the confined viscoelastic fluid is to induce an effective repulsion on the APs when moving close to a rigid surface, which depends on the incident angle, the surface curvature and the particle activity. Additionally, the geometrical confinement imposes an asymmetry to their movement, which leads to strong hydrodynamic torques, thus resulting in detention times on the wall surface orders of magnitude shorter than suggested by thermal diffusion. We show that such viscoelasticity-mediated interactions have striking consequences on the behavior of multi-AP systems strongly confined in a circular pore. In particular, these systems exhibit a transition from liquid-like behavior to a highly ordered state upon increasing their activity. A further increase in activity melts the order, thus leading to a re-entrant liquid-like behavior.