The Physics Of Salad Dressing

Contrary to the assumption that coarsening is driven by Brownian motion, scientists have shown that droplet size influences phase separation.

AsianScientist (Jul. 20, 2015) – Oil and water don’t mix, but now we know why, thanks to a paper published in Nature Communications. The researchers, from the University of Tokyo, have shown that phase separation in liquids occurs as a result of the ordered motion of droplets.

Phase separation is a universal phenomenon that can be observed in many common situations, such as when oil and water in a salad dressing separate out after having been well shaken. Called coarsening, the process occurs when a great number of small droplets coalesce into larger (coarse) droplets.

The mechanism by which droplets grow is known to be dependent on the volume ratio between the two liquids. In particular, for an intermediate volume ratio between the two liquids—neither extremely large nor extremely small—coarsening proceeds by droplet collision and coalescence, processes thought to be driven by the random thermal Brownian motion of the droplets.

In the present study, Professor Hajime Tanaka and Postdoctoral fellow Ryotaro Shimizu at the University of Tokyo used simulations to show that the collision and coalescence of liquids is in fact driven by the regular motion of droplets due to local concentration differences that arise as a result of differences in droplet size.

Specifically, when a large droplet is near a small droplet, they approach each other, while droplets of equal size move apart. This mechanism can be understood as a consequence of dynamical coupling between concentration diffusion and fluid flow.

This finding indicates that transport can be due to either diffusion, flow, or a combination of the two, but always takes place from smaller to larger domains. In other words, domain coarsening obeys the rule that “big always wins over small.” This finding, which overturns 30 years of conventional wisdom, also has implications for how systems evolve in non-equilibrium processes.

The article can be found at: Shimizu and Tanaka (2015) A Novel Coarsening Mechanism of Droplets in Immiscible Fluid Mixtures.

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Source: University of Tokyo.
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