Geometry-driven trapping and escaping of bacteria in arrays of micropillars

Interactions between bacteria and solid structures often involve rich classes of forces, including hydrodynamic, electrostatic, and steric forces. Here, we investigate a potential geometry-based abstraction of such interactions by considering bacteria of varying sizes in pillar arrays with given geometries. Using the smooth-swimming Escherichia coli strain as an archetype, we observed that bacteria tended to orbit a single pillar when their sizes were sufficiently short while they crossed the lattice through those gaps when otherwise. We argue that such demarcated “trapping” and “escaping” states can be explained by the geometric constraints of the finite gaps between adjacent pillars, which prevent those long cells from orbiting any pillars. To validate this geometry-based argument, we performed the same measurements with enlarged pillar gaps. We show that those previously escaping long cells switch to a trapping state, consistent with the vanishing geometric constraints at large pillar gaps. We also explore these geometric effects on wild-type E. coli that are free to tumble, as advanced complicacies to the bacteria-structure interactions.