Droplet impact and the dynamics of rapidly moving contact lines

When a~liquid drop approaches a flat solid surface, the air~beneath it is compressed,~flattening the bottom of the drop and~forcing initial contact to occur in a~ring-shape, trapping a pocket~of air in its center as two wetting fronts~rapidly expand both~outward and inwards to completely wet the surface. We combine total internal~reflection (TIR) microscopy with a~novel virtual frame technique~(VFT) to directly observe the sub-micron length~scales above a~solid surface as the drop approaches, impacts and then spreads~over it.