Metallic paint flows like silver lava in this macro video from Chemical Bouillon. The paint has been mixed with an unknown fluid (my guess is alcohol) to produce the flows we see here. My suspicion is that we’re seeing solutal convection where variations in surface tension create convective flow within the liquid. What do you think? (Video and image credit: Chemical Bouillon)
We typically think of convection in terms of temperature differences, but the real driver is density. In the animations above, cream sitting atop a liqueur is undergoing solutal convection – no temperature difference needed! The alcohol in the liqueur mixes with the cream to form a lighter mixture that rises to the surface. The lower surface tension of the alcohol is also good at breaking up the cream, forming little cells. As the alcohol in those cells evaporates, the cream gets heavier and sinks down into the liqueur, where it can pick up more alcohol, rise back to the surface, and begin the cycle again. (Image credit: J. Monahan et al., source)
Tea is a popular beverage around the world, and nearly everyone has their own method for making the perfect cup. Perhaps unsurprisingly, scientists have studied tea physics as well. One such study used both experiments and numerical simulations to study tea infusion from teabags. The authors looked at round, two-dimensional teabags in two configurations – one in which the bag was left still during infusion and one in which the bag was dunked up and down in the water.
In the static case, as the hot water leeches solutes out of the tea leaves, it forms a buoyant convection current. In this case, the convection is driven by solute concentration, not temperature. The convection creates a re-circulation in the cup that helps slowly distribute the tea solutes.
The dunking method, unsurprisingly, distributes tea solutes much faster. In addition to stirring the cup’s contents, dunking helps drive flow through the tea leaves, releasing solutes faster. Although the authors study the two methods in detail, they decline to pass judgement on what method is “the best”. (Photo credit: T. Foster, source; research credit: G. Lian and C. Astill; submitted by Marc A.)
Solutal convection, rather than relying on temperature gradients, can occur due to gradients in concentration or in surface tension. While less spectacular than this previously posted video, this video contains a nice simplified explanation of the mechanism. And, as noted in the video, this is a demo you can do yourself at home.
In this video, the HouseholdHacker heads to the kitchen and uses milk, food coloring, and dish soap to create some solutal convection much like this one with cream and liqueur. The food coloring serves as a tracer for the fluid motion; it’s really the interaction of the milk and the dish soap that drives the motion. The dish soap lowers the surface tension of the milk, causing motion via the Marangoni effect. That motion redistributes some of the dish soap as well, causing further motion in the form of a surface tension instability. As with the cream and liqueur experiment, the fat in the milk is important; you won’t see much (if any) effect with fat free milk because its surface tension properties aren’t as dissimilar to dish soap. (via misterhonk.de)
We are used to associating convection with differences in temperature, but what’s actually necessary for a Rayleigh-Taylor-type instability is a density variation (and a gravitational field). The solutal convection seen above when mixing liqueur with cream is caused by the interaction of density and surface tension. When the alcohol of the liqueur mixes with the cream, it forms a less dense alcohol-cream that tries to rise to the surface. The alcohol also breaks the surface tension of the cream, causing it to contract and open cells where the alcohol surfaces. As the alcohol evaporates, the alcohol-cream mixture gets denser and sinks back down where it can pick up more alcohol and start the process again. (via jshoer and io9)