Light dependent motility of microalgae induces pattern formation in confinement

A collection of self-propelled particles can undergo complex dynamics due to hydrodynamic and steric interactions. In highly concentrated suspensions, it is possible for such particles to form large-scale concentration patterns, where the active suspension separates into regions of high and low particle concentrations. Here we present that suspensions of Chlamydomonas reinhardtii cells, a unicellular soil-dwelling microalgae and a model organism of puller-type microswimmers, may form patterns of high and low cell density regions in confinement under specific light conditions. We find that there are significant deviations in the motility of the cells under different light intensities and cell densities, which regulate patern formation in such active suspensions. Finally, by performing active Brownian simulations of such active particles with the observed motility characteristics, we show that we can re-create the pattern observed in our experiments.