Light-Induced Phase Separation with Finite Wavelength Selection in Photophobic Microalgae

As with many motile microalgae, the freshwater species can detect light sources and adapt its motile behavior in response. Here, we show that suspensions of photophobic cells can be unstable to density fluctuations, as a consequence of shading interactions mediated by light absorption. In a circular illumination geometry, this mechanism leads to the complete phase separation of the system into transient branching patterns, providing experimental evidence of finite wavelength selection through a motility-induced phase separation mechanism. The finite wavelength selection is a consequence of a vision-based interaction length scale which is fixed by the illumination geometry. A simple drift-diffusion framework allows us to understand how the wavelength depends on global cell density, light intensity, and medium viscosity. Finally, we show that this active phase separation shields individual cells from the deleterious effects of high light intensity, demonstrating that phototaxis can efficiently contribute to photoprotection through collective behaviors on short timescales.