When a liquid is placed on a surface that is above the boiling point of the liquid, it tends to not boil away immediate and skid around the surface for quite some time. This is known as the Leidenfrost effect. At this temperature point, a thin layer of vapor is formed between the hot plate and the water droplet. This vapor layer acts as an insulator and keeps the water from boiling away at a fast pace. The demo room has brass plates with a ratchet design, which were constructed to propel the water droplet in a specific direction. The set up of the demonstration is shown in Figure 1. The following link shows a video demonstrating the Leidenfrost effect: Leidenfrost_mv-1n7fwj8
- Hot Plate
- 8 Brass Plates, each with a ratchet design on one side. This is shown in Figure 2.
- Cup of Water
- Silicone Tongs. These are used to move the hot brass plates.
- Silicone Sheet. This is used to place the brass plates on when they are hot.
- Video Camera
- Place a brass plate on the hot plate. The ratchet side up will cause the water droplet to travel in one direction. This is depicted in Figure 3. The ratchet side down will let the water droplet freely move around on the brass plate.
- Turn on the hot plane and set it at 350*C. Wait a few minutes for the brass plates to heat up.
- Put water into the pipette, and place water droplets on the brass plate. The water droplet will move around, and demonstrate the Leidenfrost effect.
WARNING: The brass plates get VERY hot. They tend to stay hot for around 30 minutes after use. When the brass plates are hot, use the silicone tongs to move them.
The ratchet design allows for different configurations of the brass plates to be constructed. A couple known configurations are shown below.
The purpose of this set up is to create an oscillatory motion of a droplet. Two brass plates are placed beside each other, such that the propagation of the droplet on each plate points towards each other. When a droplet is placed on one of the plates, the ratchet design will propel the droplet onto the other plate. Then the ratchet design on that plate will slow down the droplet, and eventually propel it back towards the original plate. This oscillatory motion will continue to occur. The configuration of the brass plates for this type of motion is shown in Figure 4.
2. Curved Motion
The purpose of this set up is to make the droplet travel in a curved path, instead of a straight line. Three brass plates are used, due to the limited region of the hot plate where the brass plates are able to be heated up. Place two brass plates on the hot plate that will propel the droplet in the same direction. Place one brass plate in-between those two brass plates, such that the direction of propagation is perpendicular to them. This configuration is shown in Figure 5.
3. Spiral Motion
The purpose of this set up is to make the droplet travel in a spiral pattern. The description of this set up will be confusing in words. To make it easier, arrows were placed on the brass plates to show the direction of propagation for each brass plate in the configuration. This is shown in Figure 6. The brass plate to place the droplet on will be the top-right brass plate that is part of the square.