Filling an Emulsion Drop with Motile Bacteria

We have measured the spatial distribution of motile Escherichia coli inside spherical water droplets emulsified in oil. At low cell concentrations, the cell density peaks at the water-oil interface; at increasing concentration, the bulk of each droplet fills up uniformly while the surface peak remains. Simulations and theory show that the bulk density results from a traffic of cells leaving the surface layer, increasingly due to cell-cell scattering as the surface coverage rises above similar to 10%. Our findings show similarities with the physics of a rarefied gas in a spherical cavity with attractive walls.