# Active Matter Alters the Growth Dynamics of Coffee Rings

## TL;DR

This paper investigates how active matter, specifically bacterial mobility, influences the coffee ring effect during droplet evaporation, revealing new dynamics relevant to biology and technology.

## Contribution

It demonstrates that bacterial motility significantly alters the growth dynamics of evaporating droplet edges, especially at slower evaporation rates, expanding understanding of active matter in drying phenomena.

## Key findings

- Bacterial mobility impacts coffee ring formation.
- Slower evaporation enhances motility effects.
- Implications for biofilm formation and pathogen spread.

## Abstract

How particles are deposited at the edge of evaporating droplets, i.e. the {\em coffee ring} effect, plays a crucial role in phenomena as diverse as thin-film deposition, self-assembly, and biofilm formation. Recently, microorganisms have been shown to passively exploit and alter these deposition dynamics to increase their survival chances under harshening conditions. Here, we show that, as the droplet evaporation rate slows down, bacterial mobility starts playing a major role in determining the growth dynamics of the edge of drying droplets. Such motility-induced dynamics can influence several biophysical phenomena, from the formation of biofilms to the spreading of pathogens in humid environments and on surfaces subject to periodic drying. Analogous dynamics in other active matter systems can be exploited for technological applications in printing, coating, and self-assembly, where the standard coffee-ring effect is often a nuisance.