At the scale of bacteria moving across surfaces, capillary forces are enormous, many times their flagellar propulsion force. During the internship we will investigate how bacteria collectively overcome this dominating constraint. To start with, we will study the spreading of drops on hydrogels, and study the flows induced by chemical gradients. A just acquired confocal chromatic probe, coupled to stuctured light profilometry, will be used to characterise the non-dispersive swelling front that precedes the bacterial spreading. If time permits we will compare the observations to experiments involving bacteria. The wider aim of the proposed PhD thesis is to quantify the forces at play during mass swarming of Bacillus subtilis, notably by controlling the dominant capillary forces. Our improved understanding of microscopic interactions will then be used, in collaboration with Pr J Tailleur (MIT&MSC), to develop coarse-grained statistical active matter models for the colony morphogenesis.