Capturing self-propelled particles in a moving microwedge

TL;DR

This study uses computer simulations to explore how a moving wedge-shaped obstacle can trap self-propelled particles, revealing different trapping states influenced by obstacle geometry, particle density, and drag speed.

Contribution

It introduces a novel simulation approach to analyze trapping behavior of active particles by a moving wedge, identifying conditions for various trapping states and transitions.

Findings

Complete trapping occurs when drag along the inner wedge is sufficiently fast.

Reentrant transition from no trapping to trapping and back occurs with increasing drag speed.

Self-assembled polar smectic structures form at the transition to complete trapping.

Abstract

Catching fish with a fishing net is typically done either by dragging a fishing net through quiescent water or by placing a stationary basket trap into a stream. We transfer these general concepts to micron-sized self-motile particles moving in a solvent at low Reynolds number and study their collective trapping behaviour by means of computer simulations of a two-dimensional system of self-propelled rods. A chevron-shaped obstacle is dragged through the active suspension with a constant speed $v$ and acts as a trapping "net". Three trapping states can be identified corresponding to no trapping, partial trapping and complete trapping and their relative stability is studied as a function of the apex angle of the wedge, the swimmer density and the drag speed $v$. When the net is dragged along the inner wedge, complete trapping is facilitated and a partially trapped state changes into a complete trapping state if the drag speed exceeds a certain value. Reversing the drag direction leads to a reentrant transition from no trapping, complete trapping, back to no trapping upon increasing the drag speed along the outer wedge contour. The transition to complete trapping is marked by a templated self-assembly of rods forming polar smectic structures anchored onto the inner contour of the wedge. Our predictions can be verified in experiments of artificial or microbial swimmers confined in microfluidic trapping devices.