Non-Coalescence of Fluid Jets and Drops

2 fluid jets come together but do not blend

From two streams forming a river to carbonated water and flavor syrup creating a soda fountain drink, our experience and intuition tell us that two volumes of the same fluid readily merge with one another when brought close together.

Contrary to our expectations, in the photos to the right we see fluid jets and drops rebound off each other—why don't they blend into one liquid?

Even though they are all the same fluid, these jet streams do not coalesce because they are lubricated by a thin film of air. The motion of the jets keeps replenishing the air film, sustaining it indefinitely.

In the image below, because of the air film, as drops collide with the center jet, they bend the jet stream when they cannot merge into it.

fluid jet drops not blending
Image credit: Navish Wadhwa and Sunghwan Jung; Department of Engineering Science and Mechanics, Virginia Polytechnic Institute and State University, Blacksburg, VirginiaThe liquid in these photos is silicone oil (viscosity 10 cSt at 25 °C) with a diameter roughly equal to 500 µm.

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