Effect of Soap on Surface Tension
To visualize rushing of pepper powder leaving the central region.
A surface of liquid is in the state of tension. If an object is kept on the surface, the object is pulled from all sides by the liquid surface. It therefore stays in equilibrium. If a part of the surface is modified to have less or more surface tension, the force from that part will change. This may disturb the equilibrium and the object may move.
a beaker or plate, pepper powder, detergent powder
Take some water in the beaker or a plate and spread some pepper powder on its surface. The powder will spread uniformly over the surface area. Prepare soap solution in a separate vessel, may be a plate. Dip your finger in the soap solution and then touch with this finger the center of the water surface. You will see pepper rushing towards the periphery and the central area becomes clean, powder free.
Pepper powder float on water due to surface tension. The water surface in the beaker behaves like a stretched membrane. When you sprinkle pepper powder, which is very light, it is held by the water surface.
Soap solution has the property of reducing the cohesive force of interaction between water molecules and thus weakening the surface tension of water. When you touch the center of the surface with soap solution, the surface tension in this central area reduces. It pulls on the powder with a smaller force. But the water in the outer regions is still pulling the powder with large force like a stretched rubber membrane. As a result the entire powder rushes towards the periphery.
Put several drops of cooking oil on water surface instead of pepper powder. The oil drops come together. What happens when soap is added?