The galinstan swimmers

06 August 2016 - Making It Move XVIII

GalinstanMakeItMove.PNGMaking-it-move research ordinarily is a pretty obscure branch of chemical physics but this week one exploit caught the attention of mainstream newspaper The Independent here. Something to do with a likeness to a science-fiction fantasy. As reported by Zavabeti et al. in Nature Communications (DOI - Open Access) the actual move-it-move proof of concept (previous episode here ) is not spectacular, the Marangoni effect is known to propel small objects on a surface. In this case the swimmer is a small galinstan droplet. This metal (made of gallium, indium and tin) is commercially available and a replacement for mercury in thermometers. The swimming pool is just one very narrow lane with hydrochloric acid on one side and a sodium hydroxide solution on the other side. Yes, from a chemical availability perspective the experiment can be repeated at home. What happens is that the surface tension gradient pushes the droplet towards the basic end of the pool and movement is created. Maximum velocity: 25 millimeters per second. Notably a second experiment was about what happens if the droplet is unable to move. In a modified setup the droplet was confined and exposed to a basic and an acidic flow on opposite sides. The interesting observation was that the droplet now elongated itself (aspect ration 1.46) moving away from the acidic end in the direction of the basic end. And this observation is what got The Independent journalists in a frenzy , reminding them of metal shape-shifting borgs. All a bit far-fetched of course but they are forgiven. The researchers themselves stay closer to home and envision the droplets edging themselves into microfluidic chips as switches which is a lot more cool.

Movie time!