Fig. 3.

(A) Snapshots of induced colony surfing, where the gray level indicates the local slope. Two drops are deposited side by side. From the right drop, the bacteria were removed by centrifugation and filtration. The bacteria-laden drop (B, 1 $\mathrm{\mu }$L) starts sliding after 8 h, whereas the drop containing only supernatant (SN, 10 $\mathrm{\mu }$L) remains immobile despite its much larger volume. The magnified details at the top right (white rectangle) show how, at $t\approx 10\text{h}$, a dendrite of swarming bacteria contaminates the immobile drop (white arrow marks contact line CL), which starts sliding almost instantaneously. (B) Kymograph of a longitudinal cut through the SN drop, showing the very sudden onset of motion of the drop of supernatant after its contamination; see Movie S2.