Why do raisins bob up and down in champagne?

The effect you're describing is called the "Dancing raisin effect" and is caused by the different densities of the raisin and the champagne. Here is a more detailed explanation:

1. Surface tension: The surface of the champagne is a thin, elastic layer formed by the cohesive forces between water molecules. This surface tension creates a slight pressure difference between the air above the surface and the liquid below, leading to what's known as the surface tension force.

2. Raisin density: Fresh grapes have a higher density than water. However, as grapes dry and become raisins, they lose water and become less dense than the surrounding champagne liquid.

3. Buoyancy: When a raisin is placed in champagne, it experiences two opposing forces: gravity pulling it down and buoyancy pushing it up. Since the raisin is less dense than the champagne, the upward buoyancy force is greater than the downward gravitational force. This causes the raisin to rise.

4. Adhesion: As the raisin rises, it encounters surface tension forces from the liquid's surface. The surface tension tends to hold the raisin near the surface, preventing it from fully sinking. This effect is known as adhesion.

5. Periodic sinking: As the raisin rises close to the surface, the adhesion force acting on it becomes more significant. Eventually, the upward buoyant force becomes insufficient to counteract the surface adhesion and gravity. This causes the raisin to sink back to the bottom.

6. Repeated cycle: The raisin will continue to experience this cyclic motion of rising due to buoyancy, being held near the surface by surface tension, and then sinking back down. This process results in the raisin's characteristic "dancing" motion.

The "dancing raisin effect" demonstrates how the interplay between density, surface tension, and buoyancy can lead to fascinating physical phenomena in everyday situations.