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Topic: Water Bubbles -- Fiction or Reality? (Read 5051 times) |
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James Fingas
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Water Bubbles -- Fiction or Reality?
« on: Jan 30th, 2003, 1:28pm » |
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Water Bubbles
I am sure that you are familiar with the concept of a bubble. Bubbles of air can form underwater, and then when they reach the surface of the water, they may persist for a period of time before bursting.
Looking at the phenomenon in a more general sense, we have a two-fluid environment, in which one fluids forms a membrane, dividing the other fluid into two pieces. Using that definition, we can think about water bubbles. Picture a drop of water landing on top of a surface of water, and sitting there, for a short period of time, until bursting and joining the rest of the water. Between the water bubble and the surface of the liquid is a thin membrane of air.
My question is: have you ever seen a water bubble (in air)? If so, under what circumstances? If not, why do you suppose you have never seen one? Certainly, if they exist, they are more rare than air bubbles (in water)--why is this?
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towr
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Re: Water Bubbles -- Fiction or Reality?
« Reply #1 on: Jan 30th, 2003, 1:50pm » |
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I have often seen distinct drops of water 'float' over a water surface, some seconds before it merges with the rest of the water.. if that's what you mean..
And in a sense rain is a 'waterbubble' in air.. rather common.. And when such a rain drop drops in a pool, I on occasion observe above behaviour..
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Phil
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Re: Water Bubbles -- Fiction or Reality?
« Reply #2 on: Jan 30th, 2003, 1:59pm » |
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Yes, I've seen them, but they're small, they only last a second or two at most, and the only way to tell it's not an airbubble is that it makes a slight indentation in the water surface. The way they usually form is when a large drop of water hits the surface, it sends up a few very small drops that are light enough and falling slowly enough (since they only drop a fraction of an inch) to retain their surface tension. I know I actually read a science article about them once, but have no idea where, when, or what it concluded.
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prince
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Re: Water Bubbles -- Fiction or Reality?
« Reply #3 on: Jan 30th, 2003, 3:52pm » |
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I agree with towr, water "bubbles" in air are common. Raindrops are water bubbles in air that are falling, and often, they actually look like bubbles on a raincoat or other water-repellant surface. If you've ever seen footage of astronatauts goofing around in low gravity, you may remember them "drinking" bubbles of liquid floating around them. Mercury looks like bubbles on almost any surface.
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SWF
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Re: Water Bubbles -- Fiction or Reality?
« Reply #4 on: Jan 30th, 2003, 7:38pm » |
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Phil, I think James Fingas was referring to when a thin membrane of air forms between two regions of water, like when a drop of water sits on the surface of a larger body of water. These water bubbles are easy to make if you splash some water while wading in the ocean.
For a drop of water to stay floating on a thin air layer, the air has to support the weight of the whole drop, and the weight of the drop quickly squeezes out the air from the gap. On the other hand, when a drop of air floats below a thin skin of water, the thin film doesn't need to support nearly as much weight. Also due to higher viscosity it is more difficult to squeeze the water from a thin layer than to squeeze out air, so the bubble lasts longer. Polar molecules which align relative to the surface are also more likely to be in a water solution than in air, and these can affect how easy it is to squeeze fluid from the film as well as surface tension.
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BNC
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Re: Water Bubbles -- Fiction or Reality?
« Reply #5 on: Jan 31st, 2003, 12:49am » |
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Take a cup of water near boiling (say a very hot cup of tea). Pick a small amount of the liquid, and drop it down. You’ll se many “mini-bubbles” bouncing on the surface until hitting the cup’s edge.
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James Fingas
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Re: Water Bubbles -- Fiction or Reality?
« Reply #6 on: Jan 31st, 2003, 9:49am » |
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So I see the consensus is that these "water bubbles" exist. Now for the hypothesizing:
1) Water bubbles only last for a very short period of time. Why?
2) Water bubbles are much less common than air bubbles. Why?
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prince
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Re: Water Bubbles -- Fiction or Reality?
« Reply #7 on: Jan 31st, 2003, 3:11pm » |
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on Jan 31st, 2003, 9:49am, James Fingas wrote:
1) Water bubbles only last for a very short period of time. Why?
2) Water bubbles are much less common than air bubbles. Why? |
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I would't necessarily agree that water bubbles are much less common than air bubbles (rain, a shower, etc.). If the question is why do water bubbles on water last a short time, SWF addresses this.
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SWF
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Re: Water Bubbles -- Fiction or Reality?
« Reply #8 on: Jan 31st, 2003, 8:02pm » |
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Thank you for acknowledging my comments. Prince. I was beginning to think that I am the only one who can see them.
However I would like to make a partial retraction what I had said. I think for the same volume of bubble the forces squeezing fluid out of the thin membrane will be the same whether is a supported drop of water or a floating bubble of air. I stand by the remarks about it being easier to squeeze air out of thin gap.
There are probably many other arguments one could use to explain the lack of water bubbles. Another one is that when water is splashed, water droplets strike the surface with full force while air bubbles, having to float through more viscious water, arrive at the surface more gently. This allows air bubbles to better survive impact with the water/air interface.
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aero_guy
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Re: Water Bubbles -- Fiction or Reality?
« Reply #9 on: Jan 31st, 2003, 8:26pm » |
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SWF, so you are saying that the forces on the membrane are identical whether the membrane is made up of the liquid or the gas and that is the viscosity, or resistance of the material that determines the duration of the bubble?
The effect is not just between a gas and liquid, but between any two non-mixing fluid. The border between oil and vinegar in a container of salad dressing seems to contain both oil and vinegar bubbles. If I am not mistaken, though, you see more bubbles of oil, the more viscous liquid, last longer which seems counter intuitive based on the last arguement. This calls for an experiment.
I have also noticed that water bubbles are more likely to persist on a hot liquid, the cup of coffee being the prime example. Is this perhaps because the vapor barrier between the liquids is being continuiously fed new gas by the vaporization of the surface water molecules? Any ideas? This would seem to be leading up to the Leidenfrost effect or something similar.
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SWF
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Re: Water Bubbles -- Fiction or Reality?
« Reply #10 on: Feb 1st, 2003, 5:08pm » |
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on Jan 31st, 2003, 8:26pm, aero_guy wrote:| SWF, so you are saying that the forces on the membrane are identical whether the membrane is made up of the liquid or the gas and that is the viscosity, or resistance of the material that determines the duration of the bubble? |
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That is my guess. The effect of gravity on a drop water surrounded by air, gives a net downward force equal to volume times the density difference between water and air. The air density is included because of bouyant force on the water drop due to being surrounded by air. For a bubble of air surrounded by water the effect of gravity has the same magnitude but it acts upward, since the bouyant force from the surrounding water is greater than the weight of the air.
I think high frequency vibrations of the liquid surface will help prolong life of water bubbles.
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aero_guy
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Re: Water Bubbles -- Fiction or Reality?
« Reply #11 on: Feb 3rd, 2003, 3:47am » |
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Really? I would have figured any disturbance of the surface would break the tension holding the bubble there, so you would be popping your bubble.
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James Fingas
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Re: Water Bubbles -- Fiction or Reality?
« Reply #12 on: Feb 4th, 2003, 11:39am » |
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I think the surface tension would be a more important metric than the viscosity. I agree very much with the idea of it being easier to squeeze the thin layer of air out, but air bubbles can last indefinitely--the surface tension seems to counteract the 'squeezing' effect.
As for temperature, maybe that is just a "worms on the sidewalk" effect. I have seen these bubbles canoeing, and the water was certainly not very warm...
One further observation: water bubbles seem to last when they are in motion. I'm thinking this could be due to an "aeroplaning" effect. As soon as the water bubbles slow down, they seem to pop. Since I don't drink coffee myself, I can't say whether or not water bubbles in a cup of coffee can survive when they're stationary.
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SWF
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Re: Water Bubbles -- Fiction or Reality?
« Reply #13 on: Feb 4th, 2003, 7:46pm » |
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With the same composition of air and water, the surface tension of an air/water interface is the same whether the interface is outside an air bubble or a water bubble. However certain impurities like soap molecules and other scum are attracted to the surface (and act to reduce surface tension). A larger body of water is more likely to have an impure surface. This can prolong bubble life by several mechanisms: increased 'surface viscosity', reduced evaporation rate from the thin film, and the lower surface tension means less of a driving force for the bubble to pop (which reduces energy). An air bubble floats to the same surface where scum collects, while a droplet formed by spashing probably has cleaner surfaces.
It is interesting that a steel needle can be supported by surface tension for a long time- much longer than a water bubble will last.
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