Pulsating air bubbles can swim in anisotropic fluids*

Small-scale locomotion in fluids has been of great interest, from microorganisms to active matter and spermbots. Here we explore the swimming of a spherical bubble with a periodic change in its radius. Its spatial symmetry and the scallop theorem [1] tell us that the bubble with the reciprocal motion cannot achieve propulsion. In anisotropic fluids, however, the bubbles can swim. The spherical bubble dispersed in homogeneously aligned nematic liquid crystals (LCs) accompanies either a hyperbolic point defect or a disclination ring called a Saturn-ring. The pulsating bubble generates LC flow of which spatial and time-reversal symmetry are broken because of LC’s director configuration and viscoelastic response, respectively. The bubble with the point defect exhibits the net propulsion, while the swimming of the other one depends on the shape of the ring defect. We also introduce our theoretical understanding of this propulsion mechanism, with ideas to maximize swimming efficiency.

[1] E. M. Percell, Am. J. Phys. 45, 3-11 (1977)