Tiny Internal Tornadoes Bring Drops to Life
Sunday, April 5th, 2015Tiny Internal Tornadoes Bring Drops to Life
http://www.nytimes.com/2015/03/24/science/tiny-internal-tornadoes-bring-drops-to-life.html Results from evaporation & surf. tension differences betw #water & PEG
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“The coloring turns out to be incidental. What is important is that food coloring contains propylene glycol. The combination of that fluid with water holds the droplets together and makes them move.
Water evaporates more quickly than propylene glycol and has greater surface tension. These differences result in continual movement inside a droplet. But the “little tornado inside” the drop reaches a balance that actually holds the droplet together, Dr. Prakash said.
The droplet moves when a change in relative humidity alters the tornado. Evaporated water from one droplet is a subtle but powerful signal, because it increases the humidity near another drop. That changes the second drop’s rate of evaporation, which disturbs the internal balance, and the dance begins.
By varying the percentages of the two fluids, the researchers were able to get droplets to move in ways that seemed mysterious — they sorted themselves according to their internal composition, formed a straight line and even climbed vertically.
The moving droplets could be useful. For instance, a mist might be used for cleaning surfaces, because the drops don’t leave any bit of themselves behind when they move.”
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