Auto-production of biosurfactants reverses the coffee ring effect in a bacterial system

The deposition of material at the edge of evaporating droplets, known as the ‘coffee ringeffect’, is caused by a radially outward capillary flow. This phenomenon is common to a widearray of systems including colloidal and bacterial systems. The role of surfactants in counteractingthese coffee ring depositions is related to the occurrence of local vortices known asMarangoni eddies. Here we show that these swirling flows are universal, and not only lead toa uniform deposition of colloids but also occur in living bacterial systems. Experiments onPseudomonas aeruginosa suggest that the auto-production of biosurfactants has an essentialrole in creating a homogeneous deposition of the bacteria upon drying. Moreover, at biologicallyrelevant conditions, intricate time-dependent flows are observed in addition to thevortex regime, which are also effective in reversing the coffee ring effect at even lowersurfactant concentrations.