What is the change if a droplet of water is added to another droplet of water on a flat surface There is a tiny droplet of water lying on a flat clean surface and I add a droplet of water to the tiny droplet on the surface. How can I explain the change that might occur?
 A: you explain it in terms of the surface energy of the flat clean surface and the surface tension of the water in the droplet: you make a vector sum of the surface tensions at the point where the water, the air, and the solid surface meet and solve for the wetting angle at that point.
Next you notice that if you add more droplets of water to the existing droplet, the contact point advances with one angle- and if you remove a little bit of water from it, the contact point retreats with a different angle. The difference between the advancing and retreating contact angle is called wettability hysteresis and reflects the fact that the water in the droplet has had enough time to adsorb onto the wetted surface, making it "friendlier" with the rest of the water in the droplet.
There is zero wettability hysteresis if the solid surface is something like teflon or if it is coated with a film of oil.
There are more useful and interesting experiments you can do with this basic setup. Write back if you want more and I'll provide it.
