Self-propulsion of an elliptic phoretic disk emitting solute uniformly

Abstract

Self-propulsion of chemically active droplet and phoretic disk has been widely studied; however, most research overlooks the influence of disk shape on swimming dynamics. Inspired by the experimentally observed prolate composite droplets and elliptic camphor disks, we employ simulations to investigate the phoretic dynamics of an elliptic disk that uniformly emits solutes in the creeping flow regime. By varying the disk's eccentricity $e$ and the P'eclet number $\Pe$, we distinguish five disk behaviors: stationary, steady, orbiting, periodic, and chaotic. We perform a global linear stability analysis (LSA) to predict the onset of instability and the most unstable eigenmode when a stationary disk spontaneously transitions to steady self-propulsion. In addition to the LSA, we use an alternative approach to determine the perturbation growth rate, offering valuable insights into the competing roles of advection and diffusion. The steady motion features a transition from a puller-type to a neutral-type swimmer as $\Pe$ increases, which occurs as a bimodal concentration profile at the disk surface shifts to a polarized solute distribution, driven by convective solute transport. An elliptic disk achieves an orbiting motion through a chiral symmetry-breaking instability, wherein it repeatedly follows a circular path while simultaneously rotating. The swinging periodic motion, emerging from a steady motion via a supercritical Hopf bifurcation, is characterized by a wave-like trajectory. We uncover a transition from normal diffusion to superdiffusion as eccentricity $e$ increases, corresponding to a random-walking circular disk and a ballistically swimming elliptic counterpart, respectively.