# Hydrodynamics of flagellated microswimmers near free-slip interfaces

## TL;DR

This study uses numerical simulations to explore how flagellated microorganisms swim near free-slip interfaces, revealing different behaviors compared to no-slip walls and implications for biofilm formation.

## Contribution

It provides new insights into microorganism hydrodynamics near free-slip surfaces, highlighting differences from no-slip boundaries and their effects on swimming trajectories.

## Key findings

- Microorganisms can escape or collide with free-slip surfaces depending on initial conditions.
- Swimmers follow a counter-clockwise trajectory near free-slip interfaces.
- Hydrodynamics near free-slip surfaces differ significantly from no-slip walls.

## Abstract

The hydrodynamics of a flagellated microorganism is investigated when swimming close to a planar free-slip surface by means of numerical solu- tions of the Stokes equations obtained via a Boundary Element Method. Depending on the initial condition, the swimmer can either escape from the free-slip surface or collide with the boundary. Interestingly, the mi- croorganism does not exhibit a stable orbit. Independently of escape or attraction to the interface, close to a free-slip surface, the swimmer fol- lows a counter-clockwise trajectory, in agreement with experimental find- ings, [15]. The hydrodynamics is indeed modified by the free-surface. In fact, when the same swimmer moves close to a no-slip wall, a set of initial conditions exists which result in stable orbits. Moreover when moving close to a free-slip or a no-slip boundary the swimmer assumes a different orientation with respect to its trajectory. Taken together, these results contribute to shed light on the hydrodynamical behaviour of microorgan- isms close to liquid-air interfaces which are relevant for the formation of interfacial biofilms of aerobic bacteria.