Gas density influences the transition from capillary collapse to surface seal in microfluidic jet impacts on deep pools

Abstract

Studies of liquid jet impacts onto a deep liquid pool are of great significance for a multitude of engineering and environmental applications. During jet impact, the free surface of the pool deforms and a cavity is generated. Simultaneously, the free surface of the cavity extends radially outward and forms a rim. Eventually the cavity collapses by means of gas inertia and surface tension. In this work we study numerically such cavity collapse, under different impact velocities and ambient gas density conditions. An axisymmetric numerical model, based on the volume of fluid method is constructed in Basilisk C. This model is validated by qualitative and quantitative comparison with theory and experiments, in a parameter range that has not been previously explored. Our results show two distinct regimes in the cavity collapse mechanism. By considering forces pulling along the interface, we derive scaling arguments for the time of closure and maximum radius of the cavity, based on the Weber number. For jets with uniform constant velocity from tip to tail and $We \leq 150$ the cavity closure is capillary dominated and happens below the surface (deep seal). In contrast, for $We \geq 200$ the cavity closure happens above the surface (surface seal) and is dominated by the gas entrainment and the pressure gradient that it causes. Our results provide information for understanding pollutant transport during droplet impacts on large bodies of water, and other engineering application, like additive manufacturing, lithography and needle-free injections.