Collective motion of Escherichia coli bacteria near the contact line of an evaporating drop

The bacterial dynamics and the formation of the deposit at the contact line of an evaporating sessile drop containing Escherichia coli bacteria are studied experimentally. Due to the evaporation, a radially outward flow arises inside the drop which causes an increase in cell density near the contact line. This increase gives rise to the collective motion of cells, which form spatially periodic, radially inward jets. The size of these jets have scales larger than the size of an individual bacteria. We investigate how the wavelength and amplitude of the jets depend on the bacterial motility, density and evaporation rate of the droplet and how the instability evolves over time.