  When a drop of mercury or other liquid separates into two smaller droplets, there is a temperature change. Which is warmer, the original big droplet, or the two smaller ones? The total volume doesn't change. The answer to this question has to do with surface tension. The forces between molecules within a liquid attempt to pull the molecules into a spherical shape, in order to minimize the surface area, and hence the energy needed to maintain surface tension. (A sphere has the least surface area for any given volume). You usually don't see this phenomenon on Earth, because the force of gravity will flatten any drop that is resting on a surface, and this force is stronger than the surface tension forces. When a drop is falling under the influence of gravity, as in a raindrop, for example, it assumes an irregular flattened shape because of air resistance (not a teardrop shape!). In neither case does a drop assume a perfectly spherical shape. Nevertheless, a drop will attempt to become spherical, and will become as close to that shape as it can, given the other forces acting on it. If you separate the drop into two smaller droplets, they will each attempt to assume a spherical shape also. In order to simplify our calculations, we will assume that the drops are all perfect spheres. What does this have to do with temperature? The answer lies in the surface areas . . .   Because the big drop has less surface area, the surface tension energy is less. Less energy is needed to maintain a single spherical shape than two of them. The excess energy is released as heat within the liquid. However, the increase in temperature (as the two smaller drops come together to form one big drop) is very small ... about 0.01°C for a 1 cm diameter droplet. But it is measureable. |