  When a drop dries, it often leaves a ring-shaped stain through a ubiquitous
phenomenon known as the coffee-ring effect. This also occurs when the liquid
contains suspended microbes; evaporation leaves cells at higher concentrations
in the ring than the drop interior. Using biofilm experiments and cellular
automata simulations, we show that the physical structure created by the
coffee-ring effect can dramatically alter the outcome of inter-bacterial
competition. We experimentally study this effect using two strains of
$\textit{Vibrio cholerae}$ that compete using a contact-dependent killing
mechanism termed the type 6 secretion system. By creating a heterogeneous
density profile, the coffee-ring effect changes the outcome of competition: the
bacterial strain that wins in the low-density interior loses in the far denser
ring. Through simulations parameterized with experimentally-determined density
profiles, we recapitulate our experimental findings. We examine the role of a
$\textit{V. cholerae}$ strain's frequency, its relative efficacy at killing
competitors, and the initial concentration of cells in the droplet in
determining the outcome of competition. By scaling from individual cellular
interactions to overall changes in strain frequency, our work demonstrates how
the coffee-ring effect plays a powerful role in structuring microbial
communities, indirectly driving ecological changes in community composition.

Soft Condensed Matter;Biological Physics;Cell Behavior