A single drop of soap can propel a small boat across water by instantly lowering the surface tension behind it.

The Marangoni Effect describes the mass transfer along an interface between two fluids due to a gradient of surface tension. In this experiment, water molecules at the surface are held together by strong cohesive forces. When a surfactant like liquid soap is introduced, its molecules wedge themselves between the water molecules, significantly reducing the surface tension in that specific area.This creates a tension imbalance. The high surface tension of the pure water at the front of the boat pulls more strongly than the low surface tension created by the soap at the rear. This physical tug-of-war results in a net force that drives the boat forward until the soap spreads across the entire surface and the tension equalizes.This phenomenon was first identified by the Italian physicist Carlo Marangoni, who published his findings in his 1865 doctoral thesis at the University of Pavia. Beyond simple science demonstrations, the Marangoni Effect is critical in modern industrial processes. It influences the drying of silicon wafers in semiconductor manufacturing and explains the formation of 'tears of wine' in a glass.Researchers at institutions like MIT continue to study these fluid dynamics to improve coating technologies and microfluidic devices. The speed of the boat depends on the concentration of the surfactant and the temperature of the water. Typically, a small paper boat can reach speeds of several centimeters per second using just a tiny amount of dish soap.