Dynamics of Dissolving Surface Droplets
Figure 1 shows a glass containing both oil and water. General knowledge tells us that oil and water do not mix, and therefore the oil and water will stay separated in two layers. Even if we take a needle and blow a droplet of water inside the oil, or vice-versa, water and oil will not mix. Like many things, the reality is not that simple and it will depend on the type of oil, the amount of oil and many more other parameters if and how exactly the oil mixes with water. So even though Figure 1 represents an everyday and seemingly simple situation, it is a showcase for a complex system of intriguing physical phenomena: droplets and droplet dissolution processes. The beauty of this topic lies in the fact that a droplet of one liquid inside another liquid (for example oil in water) is comparable to a droplet of water drying in air or a bubble of gas in your fizzy drink. Therefore, if we understand the mixing of oil and water, we better understand the behavior of spilled coffee, we can predict how long it will take for paint to dry, four our glass of beer to loose its 'fizz', and we can understand why the olive oil and vinegar in out 'vinaigrette' indeed refuses to mix.
Erik Dietrich, Enschede 2016