Rebound and scattering of motile Chlamydomonas algae in confined chambers

TL;DR

This study investigates how motile Chlamydomonas algae tend to accumulate in corners within confined microfluidic chambers, combining experiments and geometrical modeling to explain the trapping mechanism.

Contribution

It introduces a combined experimental and geometrical modeling approach to explain corner accumulation of motile algae in confined geometries, incorporating scattering laws and hydrodynamic interactions.

Findings

Most trajectories end in corners, indicating trapping.

The geometrical model reproduces experimental distributions.

Motility causes trapping in complex geometries.

Abstract

Motivated by recent experiments demonstrating that motile algae get trapped in draining foams, we study the trajectories of microorganisms confined in model foam channels (section of a Plateau border). We track single Chlamydomonas reinhardtii cells confined in a thin three-circle microfluidic chamber and show that their spatial distribution exhibits strong corner accumulation. Using empirical scattering laws observed in previous experiments (scattering with a constant scattering angle), we next develop a two-dimension geometrical model and compute the phase space of trapped and periodic trajectories of swimmers inside a three-circles billiard. We find that the majority of cell trajectories end up in a corner, providing a geometrical mechanism for corner accumulation. Incorporating the distribution of scattering angles observed in our experiments and including hydrodynamic interactions between the cells and the surfaces into the geometrical model enables us to reproduce the experimental probability density function of micro-swimmers in microfluidic chambers. Both our experiments and models demonstrate therefore that motility leads generically to trapping in complex geometries.