Fig. 5.

Relative volume change of deposited drops of bacteria, SN, solutions of commercial surfactin, and pure water, with respect to their deposited volume $V_i=2\mathrm{\mu }\text{L}$. Higher bacterial optical densities at deposition result in stronger volume increase, likely because the surfactant production rate is higher. Removing the bacteria through centrifugation from an $\text{OD}=\mathrm{\hspace{0.17em}0.27}$ suspension and filtration just before deposition (SN) does not affect the initial volume growth rate, but the subsequent evolution differs from the bacterial suspension (green vs. black curves). Pure surfactin–water drops also cause a volume increase, whereas no volume increase is measured for drops of pure water, nor for the non-surfactin-producing strain 168. The onset of surfing—if it occurs—is marked by double triangles. Solid lines stem from averaging over several experiments; the corresponding SD is indicated by shaded areas.