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Intent: Not looking to make-up something, I seek explanations which are possible to duplicate

Description of Phenomenon: The halved side of the bubble would be horizontal to Earth's surface, and the bowl shape would be closer to the ground.

Image of Shape (not the bubble itself): Here's a steel version of the bubbles shape

Related Physics: Here's the best explanation I've found related to the physics know to relate to the issue:

We consider half of the soap bubble. The forces on the hemisphere will be the surface tensions on the two circles and the net force from the excess pressure between the inside and the outside of the bubble. This net force is the sum of all of the forces perpendicular to the surface of the hemisphere, but must be parallel to the surface tension. Therefore we can find it by finding the force on the circle that is the base of the hemisphere. The total force must be zero, so we have

2(2¹r)g = (¹r2) ÆP, which gives ÆP = 4g/r.

SOURCE and RELATED IMAGE

CURRENT-STATUS: I'm out of possible reasons why this would happen, except for one, that being, that any half-bubbles observed might in fact be a whole free-floating bubble with three walls, two of which are refracting near zero light, while the third is highly refractive. The three walls form a single sphere divided in half by single wall; meaning that there's a top-hemisphere, and lower-hemisphere, and the dividing wall. Would this be possible?

BACKGROUND: This phenomenon is based on a question I saw years ago posted on another physics community forum. I'm unable to locate that page, but I've searched for it. On that page, more than three(3) people had independently observed the phenomenon; which to me gives some validity to it existence in some form; that, or someone was just having fun, and gaming the forum.

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I find this question to be very unclear. Could you elaborate and/or post an image? – dmckee♦ Jun 6 '11 at 17:44
@dmckee: There's no record of an example to publish that I'm aware of, what is unclear? – blunders Jun 6 '11 at 17:57
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@blunders I think he was referring to giving a drawing/sketch of the bubble you are referring too. – Justin L. Jun 6 '11 at 22:37
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-1 @blunders Your question is unclear and the average reader will not have time to decipher it through reading your comments scattered all around (You can see the effect in the answers). Thus, please edit (I strongly support the idea of adding a sketch), or I'll close this. – mbq♦ Jun 7 '11 at 11:34
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The "Related Physics" addendum to this question is a discussion of a spherical soap bubble. For purposes of calculation, the author chooses to "consider" only half of the bubble, but the physical system is the entire spherical bubble. So it's not related to the question you ask. – Ted Bunn Jun 7 '11 at 14:12
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4 Answers

up vote 6 down vote accepted

No. Free-floating bubbles form essentially spherical shapes. In general, they form shapes that minimize surface area, subject to constraints such as the bubble having to enclose a fixed volume.

One way to see why the bowl shape you imagine wouldn't work is to consider a small element of the surface right near the "corner" of the bowl (where the spherical and planar surfaces meet). The surface tension on either side of the corner would both pull that corner in towards the center. The sharp corner would therefore become rounded.

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@Ted Bunn: Bowls don't have tops, as far as I'm aware, bubbles of this type are bowl shaped. – blunders Jun 6 '11 at 18:38
@blunders But bowls are made out of solids. Bubbles are made out of liquids. – kharybdis Jun 6 '11 at 19:10
@Spencer Nelson: Bubbles can be frozen, and bowl is used to describe a shape, not one of the states of matter. If "unenclosed hemisphere" or another term is more clear, please let me know, the use of the word bowl was done so in an effort to use plain-english terms where possible. – blunders Jun 6 '11 at 19:35
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I was assuming you were imagining a bubble that completely enclosed a volume. Do you mean a bubble that consists of just a hemisphere, that doesn't completely surround an enclosed volume? If so, is there something holding the "edge" in place? If not, then the same objection I raised applies: surface tension at the edges will pull them in. – Ted Bunn Jun 6 '11 at 19:37
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If there's nothing holding the edge in place, then it'll contract down to nothing (or to a blob of liquid). If there's something holding the edge, then the bubble will from the shape that has that boundary and minimizes the surface area. – Ted Bunn Jun 6 '11 at 21:28
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up vote 3 down vote

There is no need to ask the question about a half-sphere because a general answer regarding any surface edge of a substance with the desired properties (I'll call "bubble surface") will apply for that case. This is a problem about surface tension (ST).

Of course, there are cases where a non-enclosed bubble surface exists, for instance in the case of a bubble blower apparatus. In this case the edges of the surface are attracted (on contact) to the plastic loop that people blow into. It's attracted for the same reason that water has a meniscus that reaches up on the side of a container, and it is a lower energy state. I mean that the surface attached to a wall, like in this example, has a lower surface tension energy than if that same surface area existed elsewhere.

All of the stable bubble surfaces are local minimums for the ST problem. The shape with the lowest ST energy is just a sphere of solid water, although for things like water and the bubble liquid, there is an energy penalty/advantage to adhesion to an external surface, and there can be other forces as well.

The case of a bubble surface with the edges unbounded and exposed to air is not a local minimum at all. An infinitesimally small withdrawal of the edge closer to the rest of the fluid results in a lower energy state, as such there exists a force on that part of the fluid and there is no other force balancing it. This is a bit more general than surface problems, but a slope (synonymous with gradient) in an energy field generally results in a force, $\vec{F}=\nabla E$. So whenever such an unbounded surface edge exists, that ST force is actively destroying the shape. If you pop a bubble this will occur. Only a small break in a part of the bubble causes the edge all around that point to recede faster than what our eyes can observe.

The shape in your question has edges, so everything I've said about such surfaces applies. It doesn't exist except for a fraction of a second during which it's destroying itself.

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+1 @Zassounotsukushi: (Note: This comment was also post to @Ted Bunn answer just now.) I'm out of possible reasons why this would happen, except for one, that being, that any half-bubbles observed might in fact be a whole free-floating bubble with three walls, two of which are refracting near zero light, while the third is highly refractive. The three walls form a single sphere divided in half by single wall; meaning that there's a top-hemisphere, and lower-hemisphere, and the dividing wall. Would this be possible? – blunders Jun 6 '11 at 23:13
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@blunders If you are trying to concoct a situation where the shape itself could exist, then fine, by all means, my answer doesn't prohibit that. All I said is that there is a ST force pulling inward that can destroy the bubble. If there are other forces that balance the ST force and create stability then yes it can exist. We can just assuming someone has finely tuned laser beams maintaining this shape, then yes, it can exist in free space. – AlanSE Jun 6 '11 at 23:27
+1 @Zassounotsukushi: Not looking to make-up something, I seek explanations which are possible to duplicate; I say "make-up", since one definition of "to concoct" might be "to make-up"; not saying that's what you're saying, but I just want to be clear my intent is not to do that, nor is that what I'm looking for in an answer. Is the non-half-bubble which visually appears to be a half-bubble possible? I ask because it's not clear to me from your response if it is, or how one might reproduced this; even without the refractive qualities described. Thanks! – blunders Jun 7 '11 at 0:11
@Zassounotsukushi: As a side note, your reference to a laser threw me for a curve, and don't understand means, or might imply. – blunders Jun 7 '11 at 0:12
@Zassounotsukushi: Additional information posted to the body of the question. – blunders Jun 7 '11 at 12:28
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up vote 0 down vote

You ask:

Can a “bowl” shaped liquid half-bubble be free-floating in the air?

and detail:

The halved side of the bubble would be horizontal to Earth's surface, and the bowl shape would be closer to the ground.

I have never seen such a bubble. The bubbles floating in air I have made are soap bubbles, i.e. an elastic surface enclosing air. The shape you propose is non-physical even for these physically attainable bubbles for the reasons given by Ted in the previous answer: the surface tension at the circle of the horizontal cut would be large and destroy any cohesion in shape of the soap bubble, at best turning it into a sphere at worst bursting it.

Now a liquid half bubble free floating in air would be even more non-physical than a soap bubble, as liquids are generally heavier than air and cannot float whether half sphere or whole, as rain shows. I am not aware of a liquid lighter than air.

Edit: From you insistence in the comments I think you must have observed something that you interpreted in the words you use, i.e you thought you saw a floating bowl of liquid.

If so, this would be a mirage, similar to seeing water on hot pavement: the refraction index different in the volume of the bubble than in the surrounding air will give the impression of liquid.

Now on hot days, heated bubbles of air can be created and start to rise ( in large scale that is the way inversion layers happen) , giving the possibility that a mirage effect of looking like liquid might be seen. This bubble will be spherical as per the other two answers, as the shapes of self sustained systems are constrained by the equations involved. BUT it may be that the bubble as it rises it finds a cooler air surface layer against which it is compressed and ends up looking like the bowl you describe.

It will be very rare, but if I saw a "bowl of liquid in the air" that is the interpretation in physics. A mirage detail.

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@anna v: Reading your answer, have the following questions: Your answer seems to infer that if the bubble is made of soap[water], or not, makes a difference, why? Also, your answer appears to describe two-case, one of which is a state where a bubble that is not free-floating, but still a half-bubble, is that correct, and if so, why? – blunders Jun 6 '11 at 19:39
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@blunders There cannot exist a free floating bubble of liquid in air, of whatever shape, because there does not exist a liquid that is lighter than air. So I first examined bubbles that do float in air, and soap bubbles are an example of such bubbles that can float because they are air filled. I will edit to clear this up. – anna v Jun 7 '11 at 4:28
@anna v: I see, by liquid, I mean the surface is not solid, for example, it's not a frozen liquid. Liquid is not meant to mean that it's a full ball of liquid, just that the surface of the bubble is liquid. I've also posted a number of other comments on the page since you first posted your answer that might be of use to you. +1 for following up. – blunders Jun 7 '11 at 5:17
@Anna v: your statements are becoming more, and more confusing. Not sure how to explain it more clearly. Thank you for your interest though! – blunders Jun 7 '11 at 5:48
Right, Anna adapts to Your "thinking". @Anna: I suppose he has seen the "holy grail". – Georg Jun 7 '11 at 9:41
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If I am understanding your question correctly, while I can not explain the physics of the situation, I liken it to a ship floating on the water, and I can give you a way to reproduce the phenomena consistently. The trick lies in freezing the bubbles.

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