1. The Marangoni Effect If the surface tension were to vary along an interface, there would be a an imbalance of forces which in turn would cause flow. This flow is called the Marangoni effect Surface tension depends on temperature and concentration of surfactant and so one way to cause flow is to generate temperature or concentration changes. Three examples on this flow are presented

2. Pepper Experiment Add pepper flakes to water in a petri dish or clear plastic cup. The pepper floats on the surface of the water and acts as a tracer to show the “spreading” of surfactant across the air-liquid interface. Next add a drop of liquid soap and observe instant flow. Since flow is created by a difference in surface tension (Marangoni Flow), when a drop of surfactant is added to the water, there is a area of low surface tension created by the soap. This low surface tension drives flow to areas of higher surface tension thus creating a flow of the pepper away from the drop of soap.

3. The soap powered boat In the same vein as the pepper experiment make yourself a template of a boat and add a little soap to the trailing edge Observe the boat move Adding more soap may not get the boat to move further as the interface gets saturated with soap molecules and the surface tension gradient vanishes