Frightened Runaway Pepper!

The Scientist of the Week sprinkles a pinch of pepper in a bowl of water. The pepper floats in the middle of the bowl. When he adds a drop of soap, the pepper runs away to the edge of the bowl.

At first, the pepper scattered all about the water because the water is pulling on the pepper evenly from all directions. When you drop the detergent into the water, it reduces the cohesiveness between the water and the pepper. In other words, it reduces the pulling action on the pepper and the pepper appears to run away from the detergent. But the water around the edges (untouched by the detergent) still has its full pulling strength.

Cohesion is the force that holds a material together. It results from the attraction that atoms and molecules have for one another. This attraction decreases as the distance between particles increases. Thus, with few exceptions, cohesion is highest in solids. Liquids are less cohesive than solids, and gases are practically noncohesive. Powders can also exhibit cohesion, especially if they consist of fine particles packed together. Packed dirt, for example, can be a solid driving surface because of its cohesive properties.