How bacteria in evaporating drops escape the coffee-ring effect

When a droplet containing E. coli bacteria evaporates, the bacteria get transported to the contact line and form a deposit, similar to the classic coffee-ring effect. However, here we show that when the number density of the motile bacteria is large enough, entirely new types of structures may form: radially inward-pointing fingers that consist of bacteria that move collectively inward thereby escaping the deposit. At high number density these fingers destabilize and undergo a flapping motion. We investigate the physical mechanism behind the formation of these bacterial structures. By performing systematic experiments we quantify the effect of the evaporation rate, bacterial number density and motility on the formation, evolution and dimensions of the bacterial deposit. We show that the fingers are caused by an instability in number density; due to their collective motion the bacteria induce a flow that transports additional bacteria to regions that are already dense and allows them to escape from the contact line.