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Name: Jessica
Status: Educator
Grade: 9-12
Location: MD
Country: United States
Date: November 2007


Question:
What happens if you put two liquids of the same densities into the same container if they are immiscible? Will it float, sink, or be suspended midway?



Replies:
Not having gravity to help sort the globs and pop the bubbles, the messed-up mix is slower to separate, and might never separate completely into only two regions. The regions or globs will be packed together side-by-side or whatever, with smaller ones taking up the corners, somewhat like multisized balloons randomly crowded into a box.

I suppose very slow forcible circulation might help complete the separation process, by forcing every glob to face every other for a while, so they have time to merge if they are the same substance.

Infinite time usually has a similar effect. Each liquid is always very slightly soluble in the other, and molecules can migrate from small globs to larger ones. Eventually the small ones are gone, and in the long run there can be only two. The perfect lowest-energy situation has that last dividing membrane cutting the container across its smallest two dimensions, like a 100ml graduated cylinder cut at the 50ml mark. But that's not because of gravity, that's because the cylinder is long and thin and surface tension energy likes to be minimized.

I wonder if sometimes the one that wets the container best ends up surrounding the other. That too is about surface energy.

It can be pretty hard to get most liquid pairs to do these ideal things. The membranes between globs get stuck in various places quite well. A de-foaming agent (opposite of an emulsifier) helps some. Try a little silicone oil.

It is tough to get and maintain "exactly" equal densities. If the membranes are lubricious enough not to get stuck, density differences smaller than 0.1% may incline the liquids to settle out with the heavier one on the bottom.

Jim Swenson



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