what’s going on?
The pepper doesn’t contribute to the motion you saw but makes surface motion clearly visible. The motion results from the reduction in the water’s surface tension when detergent is added.
Surface tension is the result of the strong attraction between molecules in a liquid. Water has an unusually high surface tension compared with most other liquids because water molecules are very strongly attracted to each other. This strong attraction allows you to slightly overfill a glass with water and some insects to skate on its surface.
Detergents are members of an amazing chemical family called surfactants (short for surface active agents). Every detergent molecule has two distinct ends which chemists call the head and the tail. The tail strongly repels water while the head is strongly attracted to it. As a result, detergent molecules prefer being on the surface of water with their water repelling tails sticking up and out into the air.
When you first add detergent to water, the molecules scurry across the surface with their heads down and tails sticking up. Once the surface is full, the remaining detergent molecules begin forming small droplets called micelles by joining their tails. This effectively hides the hydrophobic tails from the surrounding water – but that’s another story.
Now, detergent heads are attracted to water, but not nearly as strongly as water molecules are attracted to each other. This is why detergents reduce the surface tension of water. Imagine a long line of people all holding hands and pulling each other together. The line is under tension. If a person near the middle lets go of both hands, everybody falls away from that person to either side. The tension has been broken. In a similar way, water molecules on the surface pull away from where you add detergent.
This trick only works well once because the detergent molecules that cover the surface of water stay there. But try sprinkling on more pepper – you’ll see cool surface motion as the dry pepper absorbs some of the detergent…