Microbes crossing liquid-liquid interfaces

Rod-like Bacillus subtilis swim by rotating their flagella bundles in the viscous liquid, balancing the propulsion force with the viscous drag force at a low Reynolds number environment. We investigate their motion in a quasi-two-dimensional environment with aqueous liquid-liquid interfaces, i.e., the isotropic-nematic coexistence phases of a water-based liquid crystal (LC) called lyotropic chromonic LC. Focusing on B. subtilis' interaction with the liquid-liquid interfaces, we observe that the incident angle to the interface dominantly decides if a bacterium crosses the interface or gets trapped at the interface. A bacterium with a lower incident angle, i.e., more normal to the interface, has a higher probability of crossing the interface. We observe no strong correlation between the crossing probability and the incident speed and body length. Assuming an interfacial deformation and rupture at a critical deformation, we propose a force-balance model where the propulsion force, viscous drag force, and interfacial tension are intertwined. The crossing criterion from the model supports that the incident angle may play a vital role in the crossing behavior.