Two drops on a surface coalesce

Simple acts like the coalescence of two droplets sitting on a surface can be beautiful and complex. As the droplets come together, they form a thin neck between them, and the curvature of that surface causes capillary forces that drive fluid into the neck. For two dissimilar droplets, like the ones above, there can be additional forces. Here, the upper drop is pure water, but the lower one has added surfactants, which reduce its surface tension. That difference in surface tension creates a Marangoni flow that tends to pull fluid away from the neck. The result is that full coalescence takes longer. Depending on other factors in this tug-of-war between capillary action and Marangoni flow, the process of coalescence can look very different. In this example, there’s a fingering instability that occurs as the neck spreads. Change the circumstances slightly and the drops may chase each other instead of merging or will merge with a perfectly smooth contact front. (Image and research credit: M. Bruning et al.)

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