Numerical Simulation of the Passage of Small Liquid Droplets Through a Thin Liquid Film

We simulate numerically a novel method for dispensing, mixing and ejecting of picolitre liquid samples in a single step. The system consists of a free liquid film, suspended in a frame and positioned in front of a droplet dispenser. On impact, a picolitre droplet merges with the film, but due to its momentum, passes through and forms a droplet that separates on the other side and continues its flight. Through this process the liquid in the droplet and that in the film is mixed in a controlled way. We model the flow using the Navier-Stokes together with the Cahn-Hilliard equations. This system allows us to simulate the motion of a free surface in the presence of surface tension during merging, mixing and ejection of droplets. The influence of dispensing conditions was studied and it was found that the residual velocity of droplets after passage through the thin liquid film matches the measured velocity from the experiment well.