 Thus if due to a small fluctuation temperature, one part of the surface is hotter than another, there will be flow from the hotter part to the colder part, driven by this difference in surface tension, this flow is called the Marangoni effect.This flow will transport thermal energy, and the Marangoni number compares the rate at which thermal energy is transported by this flow to the rate at which thermal energy diffuses.For a liquid layer of thickness formula_4, viscosity formula_2 and thermal diffusivity formula_16, with a surface tension formula_17 which changes with temperature at a rate formula_18, the Marangoni number can be calculated using the following formula