How systems are endowed with migration capacity is a fascinating question
with implications ranging from the design of novel active systems to the
control of microbial populations. Bacteria, which can be found in a variety of
environments, have developed among the richest set of locomotion mechanisms
both at the microscopic and collective levels. Here, we uncover experimentally
a new mode of collective bacterial motility in humid environment through the
depinning of bacterial droplets. While capillary forces are notoriously
enormous at the bacterial scale, even capable of pinning water droplets of
millimetric size on inclined surfaces, we show that bacteria are able to
harness a variety of mechanisms to unpin contact lines, hence inducing a
collective slipping of the colony across the surface. Contrary to
flagella-dependent migration modes like swarming we show that this much faster
`colony surfing' still occurs in mutant strains of \textit{Bacillus subtilis}
lacking flagella. The active unpinning seen in our experiments relies on a
variety of microscopic mechanisms which could each play an important role in
the migration of microorganisms in humid environment.Comment: 6 pages, 7 figures, SI: 5 movies, 10 figures, 1 tabl
