Extreme congestion of microswimmers at a bottleneck constriction

TL;DR

This study investigates how microswimmers, specifically extit{Chlamydomonas Reinhardtii}, densely clog at bottlenecks under light attraction, revealing power-law blockage times and increased evacuation times with higher swimming speeds, linking to crowd dynamics phenomena.

Contribution

It provides experimental insights into microswimmer clogging behavior at constrictions, highlighting the role of swimming speed and crowd dynamics, and discusses the influence of hydrodynamics and friction.

Findings

Blockage time distribution follows a power law.

Evacuation time increases with swimming velocity.

Clogging events are intermittent with bursts of algae.

Abstract

When attracted by a stimulus (e. g. light), microswimmers can build up very densely at a constriction and thus cause clogging. The micro-alga \textit{Chlamydomonas Reinhardtii} is used here as a model system to study this phenomenon. Its negative phototaxis makes the algae swim away from a light source and go through a microfabricated bottleneck-shaped constriction. Successive clogging events interspersed with bursts of algae are observed. A power law decrease is found to describe well the distribution of time lapses of blockages. Moreover, the evacuation time is found to increase when increasing the swimming velocity. These results might be related to the phenomenology of crowd dynamics and in particular what has been called the Faster is Slower effect in the dedicated literature. It also raises the question of the presence of tangential solid friction between motile cells densely packed that may accompany arches formation. Using the framework of crowd dynamics we analyze the microswimmers behavior and in particular question the role of hydrodynamics.