Coalescence of two liquid drops falling in a liquid pool: Interaction of capillary waves

When a small liquid drop comes into contact with a free surface of a liquid pool, the drop undergoes either spreading as it drains completely inside the pool or undergoes partial coalescence dynamics as a satellite drop pinches off depending on its impacting velocity and size, as well as the fluid properties of the liquid. At low impact velocity, the primary drop injected from a needle floats on the free surface while the trapped air between the drop and the free surface drains out. Subsequently, the drop comes in contact with the free surface at a point, and due to high capillary pressure, it undergoes rapid spreading. This in turn generates upward-moving capillary waves that form a liquid column. At this stage, the bottom of the liquid column is squeezed due to the competition between the vertical and horizontal collapses resulting from the upward moving capillary waves and the surface tension force. When the horizontal collapse overcomes the vertical collapse, a liquid column is detached from the free surface as a daughter drop. Subsequently, the daughter drop completely drains in the liquid pool via a coalescence cascading cycle. As the generation of capillary waves plays an important role, we investigate the interaction of the capillary waves generated due to the simultaneous impact of multiple drops on the free surface of a liquid pool. The Navier-Stokes equations are solved in the finite-difference framework and a coupled-level-set-volume-of-fluid (CLSVOF) method is used to track the interface and the interaction of capillary waves generated due to the simultaneous impact of the droplets on the liquid pool. The influence of various parameters, such as the separation distance between the droplets, impact velocity and the size of the droplets has been investigated.