Just before Christmas I wrote a blog post about Baron Rayleigh’s work on convection, he showed how a layer of colder fluid (eg air or water) on top of warmer fluid, would lead to convection — flow of the colder fluid downwards to be replaced by warmer fluid from the bottom. Rayleigh’s work is a classic, and has been built on to help us understand, amongst many other things, Earth’s weather/climate. Rayleigh assumed that the flow was driven by gravity. The colder fluid on top falls because it is denser. Convection is everywhere in Earth’s atmosphere and oceans. It is sometimes called Rayleigh-Bénard convection, to honour Baron Rayleigh who developed the theory, and Henri Bénard, who did the experiments that inspired the theory.
But I have just learnt that the flow Bénard observed was probably not driven by gravity. Rayleigh’s elegant model was not the right model for what Bénard observed. Bénard did not study convection in air, he studied it in thin liquid layers. A liquid layer open to the air has a surface at the top, that separates the liquid from the air. Associated with this surface is a surface tension, that pulls on the surface – it is the surface tension that causes water droplets to tend to roll up into balls.
When you heat a liquid from below, as Bénard did, then you do indeed make the liquid at the bottom a little lighter, which will drive convection, as Rayleigh pointed out. But there is another way to create convection. This is where a part of the surface of a liquid becomes a little hotter than another part. The surface tension of a liquid decreases as temperature increases, so in the hotter region of the surface, the surface tension is weaker than in the colder region. The stronger surface tension in the colder part then pulls harder than in the hotter region, and this unbalanced force creates convection, just as gravity does.
Forty years after Rayleigh developed his theory, JRA Pearson, a scientist working for ICI (a now taken-over UK paint company) developed the equivalent of Rayleigh’s theory, but where the driving force is surface tension, not gravity. Finally we had a theory to describe Bénard’s experiments. As often in science, progress was more zig then zag, than smooth progress in a straight line.