Shape dependent Laplace vortices in deformed liquid-liquid slug flow

Understanding the hydrodynamics of liquid-liquid slug flow is important in the emerging field of plug-based microfluidics; however, the subtle aspects of the vortex geometry are still not comprehensively understood. This paper discusses the hydrodynamics of deformation dependent vortices that develop inside a water-in-oil slug as it flows through a channel. In contrast to prior studies, our simulations and experiments on slug flow reveal multiple vortices inside the moving slug, caused by the deformation of the hemispherical caps by Laplace pressures. These vortices appear in the front and rear of the plug at capillary number between 10^-4 and 10^-2. A theoretical and simulation model shows the cause of asymmetry in slug deformation and the resulting vortices. Understanding the relevant parameters helps in optimizing slug flow for mixing and particle manipulation, which is important for plug-based microreactors and bead based assays.