Fig. S4.

Monitoring the bacterial concentration in sessile droplets (initially: $V=5\mathrm{\mu }\text{L}$, $\text{OD}=\mathrm{\hspace{0.17em}0.27}$) on agar (0.7%, B medium, $T=303\text{K}$, RH = 90%) before colony surfing shows that the suspension remains dilute over the course of the experiment. (A–C) Microscope images at 40× magnification, focused at midheight in the drop ($z=\mathrm{\hspace{0.17em}0.5}{h}_{\text{max}},h_{\text{max}}\simeq 0.3\text{mm}$). The depth of field is of the order of 1 $\mathrm{\mu }$m. The bacterial concentration within the droplet remains very dilute even at $t=300\text{min}$ for both strains 168 and SSB 1071. (D) Mean density of bacteria in a slab such as shown in A–C, averaged across the droplet’s height, i.e., over ≃15 to ≃20 slabs—depending on the droplet growth—recorded every 14 $\mathrm{\mu }$m, from top to bottom. The thickness of a slab is set by the depth of field, $\simeq 1\mathrm{\mu }\text{m}$. No major increase in bacterial density is observed. Note that only strain SSB 1071 undergoes colony surfing (at $t\simeq 350\text{min}$).