Purely viscous acoustic propulsion of bimetallic rods

TL;DR

This paper demonstrates that bimetallic micro-rods can be acoustically propelled at high speeds through a viscous, non-reciprocal mechanism, providing new insights into microswimmer propulsion beyond traditional streaming models.

Contribution

It reveals a viscous, shape-dependent propulsion mechanism for microswimmers that explains observed speeds without adjustable parameters, surpassing previous streaming-based predictions.

Findings

Micro-rods swim up to 300 microns per second under acoustic excitation.

Traditional streaming models underestimate propulsion speeds by over an order of magnitude.

A shape-dependent viscous mechanism accurately explains the propulsion data.

Abstract

Synthetic microswimmers offer models for cell motility and their tunability makes them promising candidates for biomedical applications. Here we measure the acoustic propulsion of bimetallic micro-rods that, when trapped at the nodal plane of a MHz acoustic resonator, swim with speeds of up to 300 microns per second. While past acoustic streaming models predict speeds that are more than one order of magnitude smaller than our measurements, we demonstrate that the acoustic locomotion of the rods is driven by a viscous, non-reciprocal mechanism relying on shape anisotropy akin to that used by swimming cells and that reproduces our data with no adjustable parameters.