We study the dynamics of prolate microswimmers traversing clean liquid-liquid interfaces. Using large-scale lattice Boltzmann simulations, we observe that the swimmers can be either trapped or cross the interface, depending on their initial angle, swimming speed, and the interfacial tension between the two fluids. The simulation results are rationalized by considering a competition between interfacial (thermodynamic) and self-propulsion (hydrodynamic) forces. The swimmers get trapped at the interface due to a thermodynamic trapping force, similar to trapping of passive colloidal particles at interfaces, when the forces from the interfacial tension dominate over the swimming forces. The trapping behavior can be captured by calculating a critical capillary number, which involves balancing the interfacial and self-propulsion forces. This prediction agrees remarkably well with the numerical simulations. Finally, our results demonstrate that the torque resulting in a reorientation of the swimmers parallel to the interface has both hydro and thermodynamic components.

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• Fluid-particle interactions
• Wetting