Vinaigrette dressing. When you shake it, little vinegar droplets scatter through the oil. But when you put it down, the droplets merge and the dressing separates. That’s probably not a big inconvenience to you, but it is to industries that use lots of vinaigrette-like mixtures. So to study this un-mixing, French researchers planned to float water droplets one at a time through a channel of flowing oil. The channel widened in the middle, so the oil would slow down and bump the water droplets together. Then the passage narrowed again, so the oil would speed up and pull the droplets apart.
 
The researchers expected that the droplets would merge when they bumped together.  But that’s not what happened.  In fact, it was when the droplets started to pull apart from each other that they suddenly mixed. In Physical Review Letters, the researchers say that the pulling apart may make the pressure between the droplets fall.  That would make the droplets burst open, and then they could merge into one big water droplet. What the researchers don’t say is whether this finding could lead to a true breakthrough: stay-mixed salad dressing.

—Chelsea Wald

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