Active depinning of bacterial droplets due to gravity

A bacterial culture on a nutrient rich agar plate exhibits swarming motility. Many species of swarming bacteria collectively extract water from the underlying agar and produce surfactants to enhance the spreading of the bacterial droplet to colonise the entire plate. When the agar plate is placed on an incline, this bacterial droplet de-pins to slide down the surface. This de-pinning has so far been presumed to be due to the reduction in surface tension of the droplet by the surfactants produced by the bacteria. Contrary to this belief, we present experimental evidence that motility of the individual bacteria and the stiffness of the agar dominates the de-pinning dynamics to an extent that de-pinning can happen even in mutant strains incapable of producing surfactants. We also present a fluid dynamical model to describe this active de-pinning mechanism of droplets containing surfactant producing bacteria such as Pseudomonas aeruginosa and Bacillus subtilis.