The biological term ‘chemotaxis’ refers to the movement of a living organism (bacteria etc.) in the direction of a gradient in concentration of any substance. For example, a bacterium can move towards a direction along which the concentration of its food increases, or it can move away from a poisonous substance. The presence of these substances in the medium in which these organisms swim will generate chemical stimuli in the chemotactic receptors in their body, helping them to reorient in the desired direction and continue their journey. As our understanding stands today, chemotaxis has helped early single and multi-cellular organisms during evolution, as a fundamental way to reach up to their food to continue living and reproduction. It also plays a crucial role in directing sperms towards an unfertilised egg to form a zygote.
A movement that results from a concentration gradient is super-familiar to fluid dynamicists. In several cases, concentration gradient can cause gradients in surface tension and this will result in ‘Marangoni flows’. In our childhood days, we used to ‘utilise’ this phenomena to drive special boats over a tank of water. We would cut out a piece of plastic that would float on water and put some dishwashing liquid along one of its edges. You can also do it by placing a refill tube of a ball pen on water after removing its nib. Since soap/ink reduced the local surface tension, an unbalanced force would drive this boat in a preferred direction. Of course, we didn’t call them ‘the Marangoni boats’; we called them ‘let’s-make-a-soapy/inky-mess-on-the-floor-after-playing-with-boats’.
A funny thing happens when you combine Marangoni flows with evaporation. When you have two volatile droplets sitting next to each other, the evaporation from their surfaces can bring them closer so that they will merge. But what will happen if the droplets have concentrations of a second liquid such that one of them drives itself away from the other due to Marangoni effect and the other droplet tries to catch up? Apparently the resulting motion of the droplets looks very similar to chemotaxis exhibited by living organisms. Recently I came across this video on YouTube where Derek, who runs a cool channel – Veritasium, has demonstrated and explained this very well.