Inertialess jet propulsion of hydrogels*

Small devices designed for biomimetic locomotion need to exploit flows that are not symmetric in time (non-reciprocal) to escape the constraints of the scallop theorem and undergo net motion. In this work, we consider the dynamics of asymmetrically-coated thermoresponsive hydrogel ribbons under periodic heating and cooling in the confined space between two planar surfaces. As the result of the temperature changes, the volume and thus the shape of the slender hydrogel change, which lead to repeated cycles of bending and elastic relaxation, and to net locomotion. Unlike biological slender swimmers, the non-reciprocal bending of the gel centreline is not sufficient to explain for the overall swimming motion. We show instead that the swimming of the gel results from the flux of water periodically emanating from (or entering) the gel itself due to its shrinking (or swelling). The associated flows induce viscous stresses that lead to a net propulsive force on the gel. We derive a theoretical model for this hypothesis of jet-driven propulsion, which leads to excellent agreement with our experiments.