What it shows:

By providing a cold boundary, you can get water to crystalize as advancing needles of ice.

How it works:

This cold boundary can be provided by a petri dish of alcohol. Adding dry ice to this produces an endothermic reaction that lowers the temperature below 0°C. By placing a smaller petri dish containing distilled water within the alcohol dish (figure 1), the water freezes from the outer edge inwards. In front of a "thick" wall of ice shoots a monolayer of needles. The advance or recession can be controlled by the rate of addition of the dry ice.

Setting it up:

Viewing this demo requires a microscope with TV camera . The petri dishes are placed on the microscope table. Use about 4mm depth of alcohol and 1mm of distilled water. 200ml dry ice is sufficient. Ten power magnification gives a nice image of individual crystals, and the whole thing looks much better through crossed polarizers.

Comments:

Practice with this one. Getting the ice wall is easy, but the needle crystals are temperamental. Keep the ice wall steady and narrow and in the field of view by topping up with dry ice, and when you're ready to demonstrate, give it a hefty dose of the ice. You need only work from one side of the dish.