Here is a picture of a bubble trapped inside a thick hand sanitiser.
These bubbles never rise despite not sticking to the walls! They just remain stationary, in the middle of the liquid, despite the buoyancy forces.
What is keeping them stationary?
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$\begingroup$ words like surface tension, non-newtonian fluids are thrown at me, but I haven't seen/heard a coherent satisfactory answer. $\endgroup$– karthikeyanJul 17, 2018 at 17:35
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$\begingroup$ Why should they rise? $\endgroup$– Jon CusterJul 17, 2018 at 17:50
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$\begingroup$ @JonCuster Bubble is lighter(less denser) than the surrounding liquid $\endgroup$– karthikeyanJul 17, 2018 at 17:56
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$\begingroup$ Are they in the same position when you look again 1 day later? $\endgroup$– sammy gerbilJul 17, 2018 at 17:58
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$\begingroup$ @sammygerbil yeah! After few days too. $\endgroup$– karthikeyanJul 17, 2018 at 18:00
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1 Answer
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Since you mention in comments that words like "surface tension and non-newtonian fluids" are thrown at you, I'll avoid using those without explanation in the answer.
The simple way to explain it, without breaking into these dynamics right away (which I don't think is necessary anyways) is to consider the viscous drag, and the buoyant force.
Viscous drag is due to the viscosity of the fluid causing resistance to movement (if you're unfamiliar with viscosity and don't want to try and figure it out on Wikipedia, it's basically how "thick" the fluid feels, how much it resists flowing). In a hand sanitizer mixture, the fluid has a fairly high viscosity, especially compared to water (to see what I mean, try to pour out hand sanitizer without squeezing, it will take awhile if it even wants to come out at all). This relatively high viscosity creates large drag forces which will resist movement of the bubble (it's a bit more complicated than simple drag models, especially for a gel like this; but that's higher level fluid dynamics and perhaps material science).
This viscous drag force would oppose the buoyancy's attempts to move it. Using a simple drag equation we would see a small but gradual movement upwards; as the drag equation only gives a drag force when there is a relative velocity between them. It is possible for the bubbles to flow at a hard to notice crawl. See for example pitch drop experiments where viscous effects can slow things so far that they barely appear to behave as fluids.
I'm not an expert in non-Newtonian fluids, so it's possible that the bubble is able to stop completely due to some higher order effects which the drag equation wouldn't consider. I don't know enough about that to say either way, but it might be beyond the scope of what you wanted to know anyways. Doing a simple search, I believe the effect of completely stopping could be explained by considering the sanitizer as a Bingham Plastic; which can resist a certain amount of stress before yielding; as a Newtonian fluid would to any amount of stress.
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$\begingroup$ Thanks. I am aware of Viscocity. Fortunately I am an Mechanical engineering student. But the thought that there are viscous forces in action with no motion is really non-intuitive. Thanks for the link to the pitch drop experiment! Really interesting. Also, want to know if there are any surface tension forces that might be influecing this? Or are those trivial in this case? $\endgroup$ Jul 17, 2018 at 18:04
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1$\begingroup$ @karthikeyan Think of it like a static friction. I believe hand sanitizer is fairly non-newtonian, and behaves more like a Bingham plastic; where in it reacts elastically to the fluid stress initially, and then after a certain yield threshold is reached, it can begin to flow as a fluid. This would mean if the buoyant force isn't strong enough, the fluid acts as a spring holding it back, instead of only providing a drag force when there is movement. (I took mechanical engineering as well, hopefully this makes sense) $\endgroup$– JMacJul 17, 2018 at 18:13
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$\begingroup$ yeah! The spring analogy is really nice! I guess you took Mechanical engineering more rigorously! Thanks $\endgroup$ Jul 17, 2018 at 18:15
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$\begingroup$ @karthikeyan I was a pretty lackluster student, so I wouldn't say that. I found mechanical engineering gives you a good basis for understanding mechanics; but to find mechanics of anything specific you have to look it up yourself. In this case, the whole "buoyant object force balance" topic was just something that interested me. Not so much in school; but there were a couple questions on physics.SE that got me thinking about it more. $\endgroup$– JMacJul 17, 2018 at 18:19
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$\begingroup$ despite a lackluster period in school, you definitely have acquired a knack for explaining stuff in simpler language! Thanks a lot. $\endgroup$ Jul 17, 2018 at 18:22