When a drop settles gently against a pool of the same liquid, it will coalesce. The process is not always a complete one, though; sometimes a smaller droplet breaks away and remains behind (to eventually do its own settling and coalescence). When this happens, it’s known as partial coalescence.
Here, researchers investigate ways to tune partial coalescence, specifically to produce more than a single droplet. To do so, they add surfactants to the oil layer surrounding their water droplet. The surfactants make the rebounding column of water skinnier, which triggers the Rayleigh-Plateau instability that’s necessary to break the column into more than one droplet. (Image and video credit: T. Dong and P. Angeli)
Pour the Greek liquor ouzo into water, and your glass will billow with a milky, white cloud, formed from tiny oil droplets. The drink’s unusual dynamics come from the interactions of three ingredients: water, oil, and ethanol. Ethanol is able to dissolve in both water and oil, but water and oil themselves do not mix.
In this video, researchers explore the turbulent effects of pouring ouzo into water. In particular, pouring from the top creates a fountain-like effect, due to a tug-of-war between the ouzo’s momentum and its buoyancy. Momentum wants the ouzo to push down into the water, and buoyancy tries to lift it back up. For an extra neat effect, they also show what happens when the ouzo is confined to a 2D plane and what happens when momentum and buoyancy act together instead of oppositely. (Image and video credit: Y. Lee et al.)
In recent years, Arctic permafrost has thawed at a surprisingly fast pace. Much of that is, of course, due to the rapid warming caused by climate change. But some of that phenomenon lives underground, where water’s unusual properties cause convection in gaps between rocks, sediment, and soil.
Water is densest not as ice but as water. This is why ice cubes float in your glass. Water’s densest form is actually a liquid at 4 degrees Celsius. For water-logged Arctic soils, this means that the densest layer is not at the frozen depth but at a higher, shallower depth. This places a dense liquid-infused layer over a lighter one, a recipe for unstable convection.
Illustration of underground convection and permafrost thaw. On the left: temperature and density of the water in Arctic soil varies with depth. The temperature gets colder the deeper you go, but because water is densest at 4 degrees Celsius, the density is greatest at a shallower depth than the freezing interface. As a result of this unstable configuration (dense water over less dense water), convection can occur (right).
In a recent numerical simulation, researchers found that this underground convection caused permafrost to thaw much more quickly than it would due to heat conduction alone. In fact, the effects appeared in as little as one month, so in a single summer, this convection could have a big effect on the thaw depth. (Image credit: top – Florence D., figure – M. Magnani et al.; research credit: M. Magnani et al.)
