Photo-gravitaxis in Synthetic Microswimmers

We study the dynamics of active Janus particles that self-propel in aqueous solution by light-activated catalytic decomposition of chemical “fuel.” In experiments, the particles, initially sedimented at a bottom wall, exhibit wall-bound states of motion, dependent on the size of the particle, when illuminated from underneath the wall. Upon increasing the intensity of the light above a threshold value, which is also dependent on the size of the particle, the particles lift off the wall and move way from it, i.e., they exhibit a photo-gravitactic behavior similar to some planktonic microorganisms. The dependencies on the particle size are rationalized by using a theoretical model of self-phoresis that explicitly accounts for the “shadowing” effect of the opaque catalytic face of the particle. Our model allows us to unequivocally identify the photochemical activity and phototactic response as the key mechanisms beyond the observed phenomenology. Consequently, one has the means to design photo-gravitatic particles that can reversibly switch between operating near a boundary or in the volume away from the boundary by judiciously adjusting the light intensity, i.e., simply by “turning a knob."