The Olympic green swimming pool mystery has been solved. See media coverage from The Independent and Buzzfeed For days one of the pools was changing into an ever darker color of green in sharp contrast to the pool next to it. Swimmers had good reason to fear for their health. The green was of a distinct algea color but the pool was supposed to have chlorine in it? Turns out the people in charge of the pool had been dumping hydrogen peroxide into it, depleting the chlorine. According to Wikipedia's swimming pool sanitation page, basic disinfectants in pools are elemental chlorine gas (professional use) and sodium hypochlorite (NaOCl, home use). Chlorine reacts directly with hydrogen peroxide to form oxygen and HCl. With NaOCl the reaction products are NaCl, oxygen and water.
Question remaining unanswered: why was the hydrogen peroxide dumped in the first place. It is used in general to dechlorinate waste water before it can be safely discarded into the environment. So the presence of hydrogen peroxide at the site is reasonable. Must have been a valve switched in the wrong the direction? Perhaps the amount of chlorine in the pool was miscalculated and too high? And how to quickly fix that error with H2O2? Or some genius decided to disinfect the pool with chlorine AND hydrogen peroxide? Twice the result?
Making-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.