ABSTRACT
The Marangoni-Be´nard effect is induced by thermal fluctuations of the surface tension at the liquid/air interface. Imagine a clean glass container with a flat bottom wall (e.g., a Petri dish), lying horizontally and filled with a thin layer of pure water with the condition that the thickness of the liquid is much smaller than the diameter of the dish. Two main interfaces are involved: liquid/air and liquid/glass. We consider the ideal conditions leading to a flat, smooth, and unperturbed free surface of water. If the bottom of the dish is heated, the liquid/air interface will undergo temperature fluctuations that will lead to gradients in surface tension. The surface tension decreases with increasing temperature (see Figure 2.4) and water will flow from warmer regions to nearby colder regions, so that a net fluid flow will be generated between regions of lower surface tension to regions of higher surface tension. This is the so-called Marangoni effect. Such a flow will be radial (parallel to the water/air interface) and convection lines will be generated because warm bulk water will ascend perpendicular to the water/air interface and the colder liquid will descend building cells. This is the Marangoni-Be´nard effect. When the liquid is confined between two parallel rigid surfaces, cellular patterns are built due to buoyant convection, because of the thermally induced density gradient. This is the so-called Rayleigh-Be´nard effect, which is also present for the Marangoni-Be´nard case. Hence, the case of a free surface is also referred to as the Rayleigh-Be´nard-Marangoni effect.
