# Guiding microscale swimmers using teardrop-shaped posts

## TL;DR

This paper demonstrates a geometric method using teardrop-shaped posts to bias and direct the motion of artificial microscale swimmers, enabling concentration, sorting, and targeted transport of cargo.

## Contribution

It introduces a novel geometric approach to bias artificial swimmer motion using teardrop-shaped posts, enhancing control over their directionality.

## Key findings

- Swimmers are hydrodynamically attracted to teardrop-shaped posts.
- Bias in motion increases with swimming speed.
- Aligned arrays of posts can produce macroscopic directional motion.

## Abstract

The swimming direction of biological or artificial microscale swimmers tends to be randomised over long time-scales by thermal fluctuations. Bacteria use various strategies to bias swimming behaviour and achieve directed motion against a flow, maintain alignment with gravity or travel up a chemical gradient. Herein, we explore a purely geometric means of biasing the motion of artificial nanorod swimmers. These artificial swimmers are bimetallic rods, powered by a chemical fuel, which swim on a substrate printed with teardrop-shaped posts. The artificial swimmers are hydrodynamically attracted to the posts, swimming alongside the post perimeter for long times before leaving. The rods experience a higher rate of departure from the higher curvature end of the teardrop shape, thereby introducing a bias into their motion. This bias increases with swimming speed and can be translated into a macroscopic directional motion over long times by using arrays of teardrop-shaped posts aligned along a single direction. This method provides a protocol for concentrating swimmers, sorting swimmers according to different speeds, and could enable artificial swimmers to transport cargo to desired locations.