Why Does a Drop of Dishwashing Liquid Make Pepper Rush Away?

Sprinkle a little pepper over a dish of water and touch the surface with one drop of dishwashing liquid. The pepper suddenly races outwards. It may look as though the detergent is repelling the grains, but the pepper is mainly a floating tracer: the water surface itself is moving.

At a clean water surface, attraction between water molecules produces surface tension. Dishwashing liquid contains surfactants. When their molecules gather at the air–water interface, they lower the surface tension near the point of contact. The surface is therefore no longer uniform. Regions with higher surface tension pull more strongly, driving surface liquid away from the low-tension patch towards the higher-tension region. This motion, driven by a surface-tension gradient, is a Marangoni flow. The moving water carries the pepper with it and rapidly clears the centre.

The result is not simply that “lower surface tension makes everything spring away”. What matters is a temporary difference from one place to another. As the surfactant spreads and the tension becomes more even, the obvious flow weakens. The amount and shape of the pepper, and traces of oil in the dish, can alter the pattern without changing the central mechanism: uneven surface tension moves the water; there is no mysterious repulsive force between pepper and detergent.

https://www.usgs.gov/special-topics/water-science-school/science/surface-tension-and-water
https://web.mit.edu/1.63/www/Lec-notes/Surfacetension/Lecture4.pdf