Figure 3: The Marangoni flow induced by addition of surfactant overcomes the coffee ring effect.

Capillary flow of eGFP-labelled E. coli bacteria is observed in the absence (a) or presence of 1,25% w/v Triton X-100 (b). Marangoni numbers are calculated to be circa 0 and 10, respectively. In the absence of surfactant the capillary flow results in a narrow band of bacteria, which accumulates at the edge of the droplet, known as the coffee ring effect (a). Addition of Triton X-100, a non-ionic surfactant, induces a Marangoni flow opposing the coffee ring effect thus creating vortices, or swirling of the bacteria in the drop, depicted at the right. As a result, the bacteria are nearly uniformly deposited on the substrate upon drying. The scale bar, 30 μm and time is given in seconds. The schematics at the right are based on numerical integration of the velocity profiles calculated from the model discussed in the results.