Surfing on chemical waves: a simple yet dynamically rich two-sphere responsive gel swimmer

Abstract: Self-oscillating gels are chemically-responsive hydrogels coupled to an oscillating chemical reaction of a stimulus solute. In response to the oscillating solute concentration field, responsive gels periodically swell and deswell, expelling their adsorbed water as they transition to a drier state, and reswelling once they return to a hydrophilic state. This volume phase transition occurs when the local stimulus concentration crosses a critical value, about which the hydrophilicity of the polymer chains changes abruptly. These gels have been used to make surface crawlers or other pulsatile machines, but here we show that a very simple system comprising two oscillating gel spheres linked by a rigid rod can also swim in the inertialess Stokes regime - albeit rather slowly. Developing a full continuum-mechanical model for a gel that employs a reaction described by the Brusselator model to generate oscillating chemical concentrations that couple to gel and fluid dynamics, we quantify the rate of shrinkage and swelling and associated flows as the gels pump out or draw in water. Herein, we derive analytical results for the swimming velocity of these swimmers, and upon placing them in a solute bath, identify two modes of behaviour upon encountering propagating reaction-diffusion waves: 'bobbing' and 'surfing'. Though somewhat slower than flagellated swimmers, the relative simplicity of the system, with no hinges or moving components, lends itself well to large scale production.