Questions about bubble behaviour

[Chiefly]

Watching the bubble that formed on top of my Gin and Tonic was interesting.
First I noticed a fine stream of bubbles coming up under a larger surface bubble. The fine stream, about 5-10 per second were just visible.
The surface bubble moved around some 1/4 or 1/2 diameter of the bubble in what seemed random directions.

I assumed this was surface tension from the micro bubbles affecting the big bubble. Fascinating.

Then one instance the bubble moved away by say 10mm or 2 x dia of the bubble. The fine stream of micro bubbles now turned toward the surface bubble in a gentle arc from what appeared to be 20mm below the surface. The stream of micros continued to merge with the bubble until it burst.

By the way it was my first G&T so the mind was clear.

What forces would pull the micros to the bubble even though it was up to 15 or 20mm away from the overhead. I cannot think surface tension can extend underwater so what is the force or attraction involved here?

Thanks

 

[berkeman]

By the way it was my first G&T

I was going to ask about that; thanks for clarifying.

 

[Chiefly]

Don't worry it is a new day, so nearly time to set up today's experiment.
As they say, 'good experiments are repeatable by peers'.

 

[Baluncore]

What forces would pull the micros to the bubble even though it was up to 15 or 20mm away from the overhead. I cannot think surface tension can extend underwater so what is the force or attraction involved here?

I believe you have it backwards. The surface bubble does not control the rising stream of micro-bubbles, the rising stream drives the surface bubble on, or across, the surface.

The bubbles form at depth on a nucleation point, maybe an imperfection, or a grain of sugar, on the surface inside the glass. As the bubble grows, it reaches a point where its buoyancy overcomes surface tension. Bubbles are then released, to rise as a train of similar sized small bubbles, from the one nucleating point. The rising bubbles entrain the surrounding liquid, to form a density convection stream that rises towards the surface, and then spreads out radially at the surface.

Surface bubbles can be held at the centre of the stream on the surface, as small bubbles rise, join, to build a bigger surface bubble. The surface bubble can rotate about a horizontal axis in the surface, as it grows. At some point, like a ball, stable on at the top of a vertical jet, asymmetry in the rising stream can push the surface bubble sideways, where it appears to be followed by the train of bubbles, still rising in the same, now asymmetric, fluid convection cell.

You can study the process by dropping the smallest grain of sugar into a carbonated liquid, that grain will then nucleate and release a train of CO₂ bubbles. Use a large clear glass. Investigate the way that the concentration of juniper oil, used as flavour in the Gin, affects the surface tension, to moderate the size of bubble released, and the way that bubbles merge at the surface.

Does the oil seem to calm your stormy seas, in the late afternoon?
Do you find watching the rising bubbles therapeutic?

 

[Chiefly]

Baluncore - A very detailed and convincing reply. The asymmetric flow caused by the bubble stream is partially visible in the fluid due to the different fluid types being visible. Maybe caused by the oil you mentioned.
I studied a bubble over a rising stream, and as before, this one moved off sideways. But when the bubble burst, the stream continued, but immediately reverted to a vertical stream rather than the curved stream when the bubble was present. Somehow, that seems to indicate the surface bubble is interacting with the stream?

I imagine that as the stream approaches the surface, it senses the surface tension, which certainly could be felt some mm's below the surface.

The calming of the stormy seas was achieved long ago when I retired. And I mean that literally.

I find the rising bubbles interesting, as I worked with CFD for many, many years, but alas, had to relinquish my seat. I know that is an opening for wisecracks, but it is the correct term. I am not sure if CFD could even model such a phenomenon.

I will continue my research tomorrow.

 

[Baluncore]

But when the bubble burst, the stream continued, but immediately reverted to a vertical stream rather than the curved stream when the bubble was present. Somehow, that seems to indicate the surface bubble is interacting with the stream?

Yes, but it is a lesser effect. The bubble rests in the surface as it contains heavy, higher pressure gas, compressed by the bubble's surface tension. The bottom of the bubble is more or less flat, while the top is spherical. The hydrostatic pressure is therefor lower below a bubble's floor, which will gently attract the rising train of bubbles, to deviate towards the nearest edge of a surface bubble's floor.

There will also be some effect due to the container contact meniscus. Wax the glass, or wipe it with windscreen treatment, to change the meniscus profile. What detergents are used by your housekeeper when cleaning the glass?

 

[Chiefly]

Agreed, the surface of the fluid under the bubble is curved down. The weight of the bubble displaces a small amount of fluid due to bouyancy much like a 50 ton hovercraft displaces as much as a 50 ton vessel.
But why should the hydrostatic pressure be lower? If it were lower, then fluid would be pulled in to fill the 'gap'.

Having my own experimental rig is useful in allowing various tests to be performed. The first experiment was to gently blow the bubble away by some 10-20 mm and the stream curves to follow it. So that shows the bubble is the influencer rather than an asymmetric flow induced by the rising stream.
Second, was to pop the bubble deliberately and watch the curved stream revert to a vertical one. As predicted, it is easily verified. It does seem as though the effect is only just below the surface even though the stream starts to curve way deeper, which may be caused by a micro bubble influencing the one below it. Possibly setting up streamlines in the fluid.

Hence, I am still left with no exact reason for the curved stream.

What detergents are used by your housekeeper when cleaning the glass?

I have asked the butler, but he says he will have to ask the night shift when they arrive.

 

[Baluncore]

Hence, I am still left with no exact reason for the curved stream.

I agree with what you write, but I think we need to look more closely at the circulating cell below the bubble, and the density variation within that cell. I am reasonably certain that the bottom of a large bubble is flat, but with a peripheral meniscus.

The fact that the miscible oil-alcohol mix, can be seen moving in the fluid, (due to differences in refractive index), tells me that the composition of the fluid must therefore vary. One complexity is the sugar content of the tonic water, and of the gin, both of which can alter both RI and density. I am wondering if the curved-bubble-train effect disappears after being shaken or stirred, if the Gin is somehow immiscible and layered in the tonic water or dilute carbonic acid. That layering may be disrupted by the rising bubbles of CO₂.

Maybe I should start compiling a list of RIs and Dords for the juniper oil and Geneva alcohol involved. https://en.wikipedia.org/wiki/Dord

Unfortunately, I only have access to the cheap 'Gedanken Brand' of Gin here, so will need an invitation to visit an hospitable friend, one with an experimental scientific bent, who is willing to establish a research team to advance the science.