Since time immemorial human beings have observed the curious phenomena of non-coalescence of drops. This happens when a drop of a liquid comes in contact with a liquid surface and does not merge (coalesce) with the liquid surface right away. Rather the drop may remain as if floating on the liquid surface for periods of time ranging from seconds to milliseconds before finally merging with it. In the video below, I used a straw to pick up a volume of my coffee and gently add drops onto the surface of the coffee. The non-coalescence effect is observed in the drops to various extents, and it can be seen clearly in the part of the video slowed down to 240 frames per second.
Although this phenomenon has been investigated by several scientists spanning a time period of more than 100 years, we still don’t know for certain how it happens. The non-coalescence of drops depends on many variables including the nature of the liquid in the drop and the surface upon which it lands, the chemicals dissolved in them, the temperature gradient between the drop and the liquid, the charge of the drops, and the air pressure.
A current hypothesis is that those areas of the drop or liquid surface in contact with the air phase (interfacial) have a molecular organization that is different from the areas away from the air phase (the bulk phase). Thus the drop and the surface upon which it lands do not tend to mix right away when placed in contact with each other. However, as time goes by, the interfacial layer of the drop and the liquid surface tends to dissipate at the point of contact between them (which is no longer exposed to air), and after it has sufficiently thinned, the water drop coalesces with the liquid surface.