Diffusion-based height analysis reveals robust microswimmer-wall separation

TL;DR

This study introduces a diffusion-based method to measure microswimmer-wall separation, revealing a new activity-induced behavior called ypsotaxis, which influences microswimmer motion near walls.

Contribution

The paper presents a novel experimental approach to determine microswimmer-wall separation and uncovers the activity-induced ypsotaxis phenomenon.

Findings

Microswimmers maintain a fixed height above walls across various conditions.

Ypsotaxis is driven by swimmer activity, not external factors.

Results impact future modeling of microswimmer propulsion mechanisms.

Abstract

Microswimmers typically move near walls, which can strongly influence their motion. However, direct experimental measurements of swimmer-wall separation remain elusive to date. Here, we determine this separation for model catalytic microswimmers from the height dependence of the passive component of their mean-squared displacement. We find that swimmers exhibit "ypsotaxis", a tendency to assume a fixed height above the wall for a range of salt concentrations, swimmer surface charges, and swimmer sizes. Our findings indicate that ypsotaxis is activity-induced, posing restrictions on future modeling of their still debated propulsion mechanism.