Buckling instability causes inertial thrust for spherical swimmers at all scales

TL;DR

This paper introduces a novel swimming mechanism using buckling instability in spherical shells, enabling propulsion across all Reynolds number regimes, with potential applications in microrobotics and drug delivery.

Contribution

It demonstrates a new propulsion method based on buckling instability triggered by pressure waves, effective at all scales including microscopic shells.

Findings

Net displacement occurs at all Re regimes.

Optimal displacement at intermediate Re due to history effects.

Microscopic shells can achieve high-frequency propulsion.

Abstract

Microswimmers, and among them aspirant microrobots, generally have to cope with flows where viscous forces are dominant, characterized by a low Reynolds number ($Re$). This implies constraints on the possible sequences of body motion, which have to be nonreciprocal. Furthermore, the presence of a strong drag limits the range of resulting velocities. Here, we propose a swimming mechanism, which uses the buckling instability triggered by pressure waves to propel a spherical, hollow shell. With a macroscopic experimental model, we show that a net displacement is produced at all $Re$ regimes. An optimal displacement caused by non-trivial history effects is reached at intermediate $Re$. We show that, due to the fast activation induced by the instability, this regime is reachable by microscopic shells. The rapid dynamics would also allow high frequency excitation with standard traveling ultrasonic waves. Scale considerations predict a swimming velocity of order 1 cm/s for a remote-controlled microrobot, a suitable value for biological applications such as drug delivery.