Phenomena

The Mystery of the Binary Droplet

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What goes on inside an evaporating droplet made up of more than one fluid? This is a perennially fascinating question with lots of permutations. In this one, researchers observed water-poor spots forming around the edges of an evaporating drop, almost as if the two chemicals within the drop are physically separating from one another (scientifically speaking, “undergoing phase separation“). To find out if this was really the case, they put particles into the drop and observed their behavior as the drop evaporated. What they found is that this is a flow behavior, not a phase one. The high concentration of hexanediol near the edge of the drop changes the value of surface tension between the center and edge of the drop. And that change is non-monotonic, meaning that there’s a minimum in the surface tension partway along the drop’s radius. That surface tension minimum is what creates the separated regions of flow. (Video and image credit: P. Dekker et al.; research pre-print: C. Diddens et al.)

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