Differences in surface tension can create Marangoni flow along an interface. Imagine a shallow bowl filled with a liquid. In the middle of the fluid, every molecule is surrounded on all sides by like molecules, which push and pull it equally in all directions. But at the surface, the fluid molecules are only acted on by similar molecules in some directions. This imbalance in molecular forces is what creates surface tension. When the surface tension is constant, the fluid surface is like a taut rubber sheet. Poke a hole in that sheet, and everything pulls away from the hole. Likewise, when the surface tension varies, fluid will move from areas of low surface tension toward areas of higher surface tension. This effect is easily demonstrated at home in a setup like the animation above. Pour milk (higher fat content is better) and food coloring in a shallow container. Then lower the local surface tension using dish soap or rubbing alcohol and watch the colors run away! (Image credit: Flow Visualization at UC Boulder, source video)
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