Soft makes it hard to swim: Role of micro-confinement softness in active swimmer dynamics

Biological microswimmers alter their motility in response to external cues like chemical concentration, boundary proximity, and ambient flow. However, the response of self-propelled microswimmers to the stiffness of a confining wall in their vicinity, and their dynamics remain ambiguous. Here, we experimentally investigate the effects of varying softness of the micro-confinement walls on the swimming dynamics of self-propelled microswimmers, considering active droplets as a model system. We note dramatic differences in the swimming dynamics of these droplet microswimmers in a rigid micro-confinement, and in a soft-walled micro-confinement of identical geometry. While for the rigid case the active droplets exhibit steady unidirectional swimming velocity along a confinement wall, in case of soft confinements the microswimmer exhibits unsteady swimming dynamics with intermittent deceleration, stopping, and subsequent acceleration. These changes in the self-propulsion velocity are accompanied by sharp reorientation of the swimming direction. Interestingly, the microswimmer exhibits autonomous changes in its flow field characteristics over such stopping and acceleration events. We use high-resolution microscopy and micro-PIV analysis to characterize such dynamics. Our study will aid the understanding of bio-physical response of swimming micro-organisms to mechanical cues from the confinement walls, and conceptualization of active cargo-delivery in soft, complex environment.