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I notice this phenomenon typically when mixing hot or warm water with cold water. Basically, tiny droplets of hot water travel inside the body of cooler water and persist. I have included a photo of when I noticed this happening (in a bathtub with hot water coming out of a showerhead) and those little white spheres are what I'm asking about. I can also provide a short video if it would be helpful.

Specifically, the conditions to generate these droplets are as follows: Take a US consumer detachable (<2.5 gallon per minute flow rate) shower-head with hot water (near the temperature of my home's hot water heater which is a typical US residential gas-powered water heater with a standard temperature set-point), and angle the water towards the far end of the bathtub. This shower-head has different nozzle settings and the one that I have chosen produces a fine mist of droplets similar to that produced when using a typical plastic spray bottle. This is not a uniform effect as the water is emitted from nozzles that are placed in an annulus. I have noticed that from this shower-head, water is emitted as a cone; droplets and partial streams in the middle of the cone are much hotter than those on the outside of the cone. This middle stream of water is maybe 50-60 degrees Celsius, which I estimate because I can touch the water stream for ~3 seconds before feeling it is too hot. The outer water cone is warm to the touch, so presumably it is between 36 degrees and 45 degrees Celsius.

I believe the majority of the water (especially that on the outside), through a combination of evaporative cooling and conductive cooling (due to the fiberglass body of the bathtub) forms a lower temperature body of water. Some droplets, however, retain a much higher temperature (as the shower-head is not able to produce a completely uniform droplet spray) and join the body of water. For some reason, these water droplets do not coalesce with the cooler body of water.

Is there a name for this phenomenon, and a description for the conditions to reproduce it?enter image description here

I understand there are complaints about image quality. I cannot fix these as I do not have access to photography equipment to take higher quality images. (This photo was taken with a flagship 2023 smartphone).

The best text description that I can give is that the droplets appear as oil droplets do in water. However, presumably because the index of refraction difference is not as great (between warmer and cooler water?), it is more difficult to make out the droplets (which might be why it's hard to take a good photo with a smartphone camera).

The video here contains a link to a short clip, which might be more helpful than a static image. Note the droplets in the top left, they are liquid water of the same composition as that of which they are contained in. When I pan to the right in the video, one can observe fast moving droplets that are further upstream. Clearly they have managed to travel from the shower head all the way through the higher velocity stream of the middle of the tub before settling near the drain where the water is moving more slowly. The lifetime of these droplets is on the order of seconds.

It is difficult for me to reproduce this effect as I must angle the shower head (it is detachable) and position it specifically to generate this phenomenon (and getting a good camera angle whilst doing this is very hard for me).

Additionally, I notice this phenomenon occasionally when urinating (as a male), and when pouring hot cooking water down a sink that already has a small amount of water.

My assumption is that this phenomenon is similar to that of liquid water droplets floating on the surface of a larger body of water (this can happen when smaller droplets formed from the collision of a drop of water with a water body reach the surface of the body). There is air in the case, and I don't immediately see air in this case, which is why I assume it might be a different phenomenon and warrants asking a question.

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    $\begingroup$ I am not seeing much in your photo. Some places have a small water heater in the bathroom near the shower. Those may produce hot water that doesn't have a uniform temperature because of poor mixing. Is that what you are asking about? $\endgroup$
    – mmesser314
    Dec 23, 2023 at 14:32
  • $\begingroup$ could it be that your hot water supply has impurities that could form a bubble? $\endgroup$
    – anna v
    Dec 23, 2023 at 15:07
  • $\begingroup$ These aren't bubbles. I will upload a short clip indicating such: bubbles don't disappear in water. These are droplets of hot water that don't coalesce with the body of cooler moving water they are part of. It looks like oil in water but it's not oil or impurities: all water is coming from the same showerhead, so it's homogenous in composition. $\endgroup$
    – hedgepig
    Dec 23, 2023 at 15:32
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    $\begingroup$ Please improve the quality of question, a clearer illustration and description is required to answer. Btw my guess is difference in surface tension of hot and cold water, but it may also be liquid liquid equilibrium.. $\endgroup$
    – Qwerty
    Dec 24, 2023 at 8:56
  • $\begingroup$ I have added a video since a static image does not provide enough detail. I really can't provide better visual clarity as the lighting conditions and my photography "equipment" do not suffice. I have also done my best to improve the description and condition that I generated this effect. $\endgroup$
    – hedgepig
    Dec 25, 2023 at 8:19

2 Answers 2

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Great question! I have observed this effect too while watering plants in my garden. When I direct the water spray to a water puddle at an angle, the water droplets seem to bounce up and down! (I didn't have to use warm water though, but I guess the effect will be stronger when you have more vapors) There is a Smarter Every Day video on this exact phenomenon.

Image credits: Journal of unsolved questions

This paper describes what you observed in detail. Why does this happen? If you have two bodies of water touching, they should immediately coalesce due to their cohesive properties. The key here is that the water droplet and water surface do not come in contact. When you throw water droplets of just the right size to a water surface at just the right angle, a cushion of air gets trapped between their interfaces. This air eventually then gets channeled out of the gap due to the weight of the water drop.

Image credits: Smarter Every Day.

To test this hypothesis, the authors measured the residence times (the time water droplets stay intact before coalescing on the water surface) with different air pressures. When you have thicker air, it should channel out slower, and the residence times should increase. And that's exactly what they observed.

Image credits: NCBI

There's more. The bouncing water drops also keep splitting into smaller and smaller droplets until they become invisible to the human eye. Check out this video.

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Its a variant of the Leidenfrost phenomenon: The hot droplet is isolated from the cold water by its surface gas shell of damp.

Leidenfrost

There is a video of a hot metal ball in water, the gas shell provided by the medium, the energy by the hot ball.

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    $\begingroup$ The temperature of the hotter water droplets is set to a value within the range of a residential water heater, so I'm having trouble understanding how it could generate so much vapor to prevent mixing. The Leidenfrost effect requires a much greater temperature differential than the one that is present here. $\endgroup$
    – hedgepig
    Dec 25, 2023 at 8:09

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