Spiky contacts: The impact of an emulsion drop on a solid surface

We use ultra-high-speed video, at frame-rates up to 7 million fps, to capture the first contact of an emulsion drop impacting on a smooth solid surface. The lubricating air layer causes rapid deceleration before contact, forming a dimple at the bottom tip of the drop. When the disperse-phase emulsion droplets are of higher density than the continuous phase of the main drop, the deceleration produces local spikes extruding out of the free surface. These spikes form when the impact Weber number exceeds a critical value of about 15. Time-resolved interferometry, shows the emergence and shape of these spikes, which are 10-20 microns wide. When the spike Weber number exceeds 2, the spikes make local contacts with the solid, before the main outer kink makes a ring contact, which entraps the central air disc. The presence of the multiple spikes breaks up the entrapped air layer into random air patches which could adversely affect printed coatings.