Hydrodynamic attraction of bacteria to gas and liquid interfaces*

Motile microorganisms often encounter solid and fluid boundaries. Understanding the underlying physics of their accumulation near these boundaries can impact a wide range of applications from biofilm formation to micro-robot design. We combine experiments and numerical simulations to investigate bacteria (Escherichia coli) attractions to various fluid-fluid interfaces. We show that the strongest cell accumulation occurs near the lowest viscosity ratio (gas interfaces). We develop a new theory based on Brownian dynamics including hydrodynamics and short-range physio-chemical interactions between bacteria and surfactant-laden interfaces and explain our experimental findings. We include higher order singularities in our hydrodynamic model and study their effects on bacterial orientations, trajectories, and accumulation.