What governs droplet spreading on a wetted wall

Understanding the flow dynamics within a thin wall film during droplet impact spreading is crucial. A key open question concerns the dominant forces governing the temporal decay of stagnation point flow strength. This study employs direct numerical simulations to investigate the flow field across a range of viscosities and different wall boundary conditions. Results demonstrate that viscous forces, particularly wall friction, significantly influence the temporal evolution of the stagnation point flow, even at high Reynolds numbers [O(105)], causing deviation from inviscid theory and adherence to an exponential decay. Furthermore, a comparison with an analytical boundary layer model reveals that pressure, gravity, and surface tension forces are negligible during spreading.