# Engineering passive swimmers by shaking liquids

## TL;DR

This paper demonstrates a novel passive propulsion mechanism for soft, asymmetric microswimmers in shaken liquids, enabling controlled movement and potential applications in medical sorting and targeted delivery.

## Contribution

It introduces a passive swimming mechanism based on symmetry breaking in shaken liquids, applicable to realistic non-buoyant, soft particles like Janus capsules and Lambda-shaped particles.

## Key findings

- Passive locomotion occurs due to asymmetry in Stokes drags during oscillations.
- Swimmers can be directed by adjusting liquid shaking parameters.
- Mechanism can lift or concentrate particles against gravity.

## Abstract

The locomotion and design of microswimmers are topical issues of current fundamental and applied research. In addition to numerous living and artificial active microswimmers, a passive microswimmer was identified only recently: a soft, Lambda-shaped, non-buoyant particle propagates in a shaken liquid of zero-mean velocity [Jo et al. Phys. Rev. E 94, 063116 (2016)]. We show that this novel passive locomotion mechanism works for realistic non-buoyant, asymmetric Janus microcapsules as well. According to our analytical approximation, this locomotion requires a symmetry breaking caused by different Stokes drags of soft particles during the two half periods of the oscillatory liquid motion. It is the intrinsic anisotropy of Janus capsules and Lambda-shaped particles that break this symmetry for sinusoidal liquid motion. Further, we show that this passive locomotion mechanism also works for the wider class of symmetric soft particles, e.g., capsules, by breaking the symmetry via an appropriate liquid shaking. The swimming direction can be uniquely selected by a suitable choice of the liquid motion. Numerical studies, including lattice Boltzmann simulations, also show that this locomotion can outweigh gravity, i.e., non-buoyant particles may be either elevated in shaken liquids or concentrated at the bottom of a container. This novel propulsion mechanism is relevant to many applications, including the sorting of soft particles like healthy and malignant (cancer) cells, which serves medical purposes, or the use of non-buoyant soft particles as directed microswimmers .