Think about broth in the soup, usually it has circles on its surface. What are their properties? Why there are many of them (not a few big blobs)? Are they depended on liquid's temperature? What needs to be added to water so these kind of circles appear?

I can only guess that it is because of oil in the broth, and shape is due to tensions on the surface; is that correct?

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  • $\begingroup$ Yeah, oil (not miscible with water) and surface tension. Instead of spreading out evenly over the surface it can minimize energy by forming blobs like that. $\endgroup$ Mar 4 '14 at 9:05
  • $\begingroup$ @BrandonEnright, what about their numbers? If we assume total area of blobs is constant, then why is it very unlikely that there will be big blobs but few? Rather than many and small. $\endgroup$
    – ile
    Mar 4 '14 at 12:53
  • $\begingroup$ Also, I think I never noticed these kind of circles in regular puddles in streets (when oils from car spill there)? $\endgroup$
    – ile
    Mar 4 '14 at 12:56
  • $\begingroup$ I assume the size of the blobs depends on the kind of fat/ oil and the properties of the rest of the broth. I have seen/eaten broths with mainly large blobs as well as broths with small blobs only. $\endgroup$
    – Dohn Joe
    Mar 4 '14 at 13:51
  • $\begingroup$ When I was young I used to play with the droplets of fat on my soup trying to get them to merge into a single circle. It takes some work but can be done with a careful touch at bridging the impurities separating the drops. $\endgroup$
    – arp
    Jul 13 '19 at 2:22

The circles are droplets of fat (triglycerides: triesters of glycerol and fatty acids). The two phases (water and fat) are immiscible because the water molecules are more attracted (hydrogen bonded) to each other than to the fat molecules.

Absent gravity, energy would be minimized by the droplets being spherical, but gravity flattens the droplets. The surface tensions of each pair of the three phases (air-water, air-fat, and water-fat) would be needed to quantitatively describe the situation.

The droplets being seperate is not the lowest energy state. However, surface tension must be overcome for the droplets to coalesce.

If the soup is refrigerated, the fat droplets usually solidify.

The fat droplets usually come from meat in the soup.

  • $\begingroup$ Do you know of any exposition / analysis what the most likely number and size of the droplets is? I'm assuming this kind of question is very like the question of magnetic domain size (although the mechanisms involved are wholly different). $\endgroup$ Mar 5 '14 at 4:06
  • $\begingroup$ Since it is an unstable state to have multiple droplets, the number and size of the droplets depend upon how the state was formed. Studying that would involve the science of making and breaking emulsions. For example, high-shear would make more and smaller droplets and centrifuging would force the droplets to coalesce. $\endgroup$
    – DavePhD
    Mar 5 '14 at 11:47

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