# A fine balance of chemotactic and hydrodynamic torques: when microswimmers orbit a pillar just once

## TL;DR

This study investigates how self-propelling droplet microswimmers interact with microfluidic pillars, revealing that chemical and hydrodynamic forces cause them to orbit once before detaching, with implications for understanding microswimmer behavior.

## Contribution

The paper introduces a combined experimental and modeling approach to quantify chemical and hydrodynamic torques influencing microswimmer detachment from pillars.

## Key findings

- Droplets tend to orbit once before detaching due to chemical and hydrodynamic effects.
- The Langevin dynamics model accurately describes swimmer behavior near pillars.
- Chemical and hydrodynamic parameters depend on boundary conditions like wall curvature.

## Abstract

We study the detention statistics of self-propelling droplet microswimmers attaching to microfluidic pillars. These droplets show negative autochemotaxis: they shed a persistent repulsive trail of spent fuel that biases them to detach from pillars of a certain size after orbiting them just once. We have designed a microfluidic assay recording swimmers in pillar arrays of varying diameter, derived detention statistics via digital image analysis and interpreted these statistics via the Langevin dynamics of an active Brownian particle (ABP) model. By comparing data from orbits with and without residual chemical field, we can independently estimate quantities like hydrodynamic and chemorepulsive torques, chemical coupling constants and diffusion coefficients, as well as their dependence on boundary conditions like wall curvature.