Figure 4. Oscillatory movement of the bacterial front.

A low concentration of surfactant induces a weaker Marangoni flow which is comparable in strength to the capillary flow. (a) Images of eGFP-labelled E. coli bacteria showing a repetitive forward and backward movement of the collective front relative to the droplet edge (depicted in yellow). The backward movement is due to a Marangoni flow created by a low Triton X-100 concentration (0.3% w/v, Marangoni number of circa 0.1), which opposes the forward movement. (b) Schematic of the oscillatory back and forth movement (arrow in red) of the bacteria in the collective front (green). The capillary flow (bottom black arrow) results in accumulation of all solutes, both bacteria and surfactants, the last inducing the Marangoni flow (upper black arrow) opposing the coffee ring effect. The scale bar, 30 μm and time is given in seconds.