Hi-speed film shows how drops behave on a hot surface.
Teachable Topics:
- Phase Transitions
- Boiling Point
- Vapour Pressure
Theory:
Liquid nitrogen is extremely cold and has a boiling point of approximately -196C. This means that the floor in the physics lab, or in just about any room for that matter, is way hotter. In fact, it is so much hotter that when they come in contact, a phenomenon known as the Leidenfrost effect occurs. With this effect it seems as though the liquid is skating or gliding across the surface with little to no friction.
Since the floor is so much hotter than the boiling point, the droplets of liquid nitrogen boil instantly when they touch it. When this happens a small layer of nitrogen gas forms in between the droplet and the floor resulting in motion similar to that of a hovercraft. The droplet can "skate" around on this cushion of gas for some time before it boils up. This effect only happens when a surface exceeds the boiling point of a liquid by a large amount. It can also be seen when small amounts of water are tossed in a hot frying pan.
Apparatus:
- (Demo 1) Liquid nitrogen and a large surface at room temperature
- (Demo 2) Water Drops and a Hot Plate
Procedure:
- Carefully pour the liquid nitrogen on the surface and observe the Leidenfrost effect in action.
- Drop water slowly onto a hot plate that's been set above 100C.
Safety:
- Be very careful when using liquid nitrogen, wear safety goggles. Don't let it touch you or your clothes.
- The liquid nitrogen will spread out pretty quickly. Make sure nobody is too close.
- Don't pour too much liquid nitrogen in one place or you will risk ruining certain flooring materials.
