Rectification of translational bacterial motors

A dense suspension of swimming bacteria encapsulated in an aqueous droplet immersed in oil can set the droplet in motion, thus turning the droplet into a “translational bacterial motor”. The droplet movement is caused by the bacterial collective motion, through the generation of viscous forces in the thin lubrication film between the drop and the substrate. Thus, at short time scales the drop motion is ballistic, driven by the coherent bacterial movement, but at long time scales it is random, due to the fluctuating nature of the bacterial turbulent motion. To rectify the movement of the translational bacterial motors, we break the spatial symmetry by an anisotropic substrate. We use standard and greyscale UV lithography to produce and explore different geometries and spatial patterning of the substrate. By using a substrate with an array of triangular pillars, the x-y symmetry is broken, which reflects on different diffusion rates in each direction. This is contrasted with round pillars, for which no asymmetry is observed. On the other hand, the use of ratchet-like substrates produces a net rectification of the translational bacterial motors.