droplet coalescence

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Water droplets don’t always immediately disappear into a pool they’re dropped onto. If the droplet is small and doesn’t have much momentum, it will join the pool gradually through a process known as the coalescence cascade, seen here in high speed video. The droplet bounces off the surface, then settles. A thin layer of air is caught between it and the pool. Slowly the weight of the drop pushes that air out until there is contact between the drop and pool. Before the drop can merge completely, though, surface tension pinches it off, creating a smaller daughter droplet. Ripples caused by the merger help bounce the little droplet, which repeats the same process until the tiniest droplet merges completely. (Video credit: B. ter Huume)

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Worlds in a Drop

This is what happens when you float a thin film of oil on water and magnify the interaction with a special microscopy technique. John Hart, a professor at the University of Colorado Boulder who studies fluid dynamics and planetary atmospheres, trained his microscope on the two fluids as they interacted. What he captured by magnifying the interactions by just 12.5 times is as much art as it is science. 

For his efforts, he earned third place in Nikon’s Small World in Motion competition last year. He used a technique called reflected light differential interference contrast microscopy to see this hidden world. 

The explanation that accompanies the video explains what’s going on: “Thin film of volatile oil floating on a water surface. Evaporation occurs in isolated pools a few microns deep. Flow instabilities at the edges bring oil into the centers of the pools, forming droplets. Coalescence leads to bigger drops that have more volume per unit of surface area, and slower evaporation.  Interference colors dramatize the motions. The complex microscopic fluid dynamics are relevant to the longevity of fuel spills.”

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