Analog studies of mantle convection: the curse of surface tension (or not) ?

Whenever you study a phenomenon of mm to a few cm-scale in the laboratory which involves an interface, the question of surface tension arises. Surface tension is due to the fact that molecules prefer to stay with their own kind. Therefore, the creation of an interface between two fluids requires energy, and this influences the dynamics around the interface.

Surface tension can be a blessing: it produces the round shape of rain drops or the nice bubble shapes of colorful liquid in a lava lamp. It allows objects with a higher density to float on a liquid (such as an insect on water, or a silicone plate on sugar syrup). It can generate flow up a capillary.

However, it can also be a curse in the case of thermal convection. Purely thermal convection  develops when a plane layer of fluid is heated from below and cooled from above. The engine of motion is the thermal buoyancy of the fluid. This is what is happening in a planetary mantle on scales of hundreds to thousands kilometers. This is also what is happening in a closed box in the laboratory. But as soon as an interface exists, either between an upper and a lower experimental mantle, or in the case of a free surface at the top of the fluid layer, surface tension effects can become important. For exemple, the variation of surface tension with temperature was responsible for the beautiful honey-comb patterns imaged by Benard (1901) in the first systematic study of thermal convection with a free-surface. Surface tension is also going to act against the initiation of subduction (which acts to break the surface). 

We shall review in this presentation the signatures of surface tension in a convective context, and the different ways to minimize and/or remove the effects of surface tension in convection experiments, such as using miscible liquids, or a layer of experimental « sticky air ».