Light-switchable deposits from evaporating droplets containing motile microalgae

Deposits from evaporating droplets have shown to take a variety of shapes, depending on the physicochemical properties of both solute and solvent. Classically, the evaporation of droplets of colloidal suspensions leads to the so-called coffee ring effect, a nuisance in many technological applications. Here, we investigate deposits from evaporating droplets of living motile microalgae (Chlamydomonas reinhardtii) and take advantage of their light-sensitivity via phototaxis to control the final pattern: adjusting the incident angle and wavelength of the light makes it possible to force or completely suppress the formation of algal 'coffee rings', and even to control their spatial structure.