Spreading By Island

Interference fringes beneath a settling drop show bright islands where material is nucleating.

How does a droplet sinking through an immiscible liquid settle onto a surface? Conventional wisdom suggests that the settling drop will slowly squeeze the ambient fluid film out of the way, form a liquid bridge to the solid beneath, and spread onto the surface. But for some droplets, that’s not how it goes.

While watching a glycerol droplet settle through silicone oil, researchers discovered a new mechanism for wetting. Initially, the silicone oil drained from beneath the drop, as expected. But then the thinning of the film stalled. Tiny bright spots (above) appeared beneath the light and dark interference fringes of the parent drop. These are spots of glycerol, formed when material from the main drop dissolved into the oil and then nucleated onto the solid surface below. Over time, the island-like spots of glycerol grew. Eventually one grew large enough to coalesce with its parent drop (below), causing the glycerol to quickly spread over the solid surface!

Islands nucleate and grow beneath a droplet until they're able to coalesce with the parent droplet above.
Islands of liquid (darker rings) grow beneath a parent drop (brighter rings) until reaching a size where they coalesce, causing the interference fringes to disappear.

The key to this phenomenon seems to be that immiscibility isn’t perfect. Even trace amounts of solubility between the drop and surrounding fluid are enough to allow these islands to form. And once formed, the islands will grow as long as the drop fluid and the solid surface are chemically attractive. (Image, research, and submission credit: S. Borkar and A. Ramachandran; see also Nature Behind the Paper)

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