Why Do Ice Trees Grow on Frozen Droplets?

Today, another beauty from this year’s Gallery of Fluid Motion, which examines an interesting puzzle associated with frozen water droplets on a flat surface.

Oscar Enriquez and pals at the University of Twente in the Netherlands placed a liquid water dropleton a plate cooled to -20 degrees C and watched as the drop froze.

In liquid form, the drops have a spherical shape. But the frozen drops are angular, with a point at the top, like Mount Fuji. How come?

The video clearly shows what’s going on. “The process of solidification can be observed very clearly due to the change in refraction when water turns into ice,” say Enriquez and co.

This shows that the drop freezes from the bottom up, where it is in contact with the cold plate. As the freezing occurs, the drop remains more or less spherical while the water at the top is still liquid.

But as the final part of the liquid freezes, the surface is pushed up by the water as it expands to form ice. It is this expansion that causes a point to form at the top of the ‘mountain’.

But then something unexpected happens: a tiny ‘tree’ of ice begins to grow from the tip of the ice mountain.

That’s a little more difficult to explain but Enriquez and co eventually settle on an explanation. I won’t spoil the fun. If you want to find out why the tree grows, take a look at the video (link below). It’s a gem.

Ref: arxiv.org/abs/1110.3698: Freezing Singularities In Water Drops
Hi-res video
Lo-res video