Motile bacteria crossing liquid-liquid interfaces

Abstract

Real-life bacteria often swim in complex fluids, but our understanding of the interactions between bacteria and complex surroundings is still evolving. In this work, rod-like \(\textit{Bacillus subtilis}\) swims in a quasi-2D environment with aqueous liquid-liquid interfaces, i.e., the isotropic-nematic coexistence phase of an aqueous chromonic liquid crystal. Focusing on the bacteria motion near and at the liquid-liquid interfaces, we collect and quantify bacterial trajectories from the isotropic to the nematic phase. Despite its small magnitude, the interfacial tension of the order of 10 \(\mathrm{\mu N/m}\) at the isotropic-nematic interface justifies our observations that bacteria swimming more perpendicular to the interface have a higher probability of crossing the interface. Our force-balance model, considering the interfacial tension, further predicts how the length and speed of the bacteria affect their crossing behaviors.