Numerical Simulation of Droplet Breakup when Impacting a Narrow Gap

When a droplet impacts a pore with sufficiently high velocity the droplet breakups into liquid patterns both above the surface and inside the pore. In the present work, Computational Fluid Dynamics (CFD) simulations are carried out, considering the results obtained by an experimental analysis of droplets impacting on a single narrow gap, to study the factors that control the resulting droplet breakup. The single pore has the form of a slit with a width of either 100 or 150 microns across, and is several times longer than the impacting drop diameter. A droplet with a diameter of 2 mm impacts the gap at either 0.5 or 1.5 m/s. Both the experiments and the numerical simulations show that the droplet remains intact at 0.5 m/s but on the contrary cleaves into two halves at 1.5 m/s. A VOF-based numerical simulation framework that has been previously implemented in OpenFOAM and has been validated against droplet impacts on surfaces with different wettabilities, is utilised to reproduce these experimental runs. Experimental measurements are unable to capture the pressure and velocity fields that develop within the drop at the various stages of impact, however detailed pressure and velocity fields are predicted by the numerical simulation. From the overall analysis of the numerical predictions, characteristic pressure gradients within the droplet are revealed. Furthermore, the volume of the droplet that penetrates into the gaps with respect to time is quantified in detail utilising the numerical simulation results, revealing that the impact velocity does not significantly affect the early stages of the droplet penetration into the considered narrow gaps, while the gap width has a considerable effect in the droplet penetration rate from the early stages of the considered droplet impacts.