Fig. 7.

The Bond number calculated using the measured volume and surface tension (Figs. 5 and 6) exhibits a monotonically increase for surfactin-producing strains, with a critical value for the onset of surfing (triangles) at ${\text{Bo}}_c^{\text{bact.}}\approx \mathrm{\hspace{0.17em}0.03}-0.05$. The SN solution, from an $\text{OD}\approx \mathrm{\hspace{0.17em}0.27}$ suspension, and the commercial surfactin droplets (0.1 g L⁻¹) do not reach this value, and no surfing is observed.