Why do bubbles coalesce in liquids?

[chef]

TL;DR Summary

Why do the bubbles in a glass of soda get drawn to each other and join.

I noticed whilst watching the tiny bubbles rising up in a glass of tonic that if they are close enough to each other they come together and coalesce. I tried to note the distance and my best guess was that at 8 times the bubble diameter the bubble attracts others. Sometimes they get attracted to the glasses edge dependent on position.

I presume the gas is CO2 and the fluid is gin and tonic, no ice or lemon.

At first thought I wondered if it was surface tension that somehow relaxed with distance and that was the attractor. Then I wondered if the bubble distorts the fluid surface and gives it a gradient around the bubble that other bubbles fall into similar in a way that a low pressure weather system lifts the sea surface level.

If anyone knows of work done on this or an answer I would like to know.
Thanks Chef

 

[A.T.]

At first thought I wondered if it was surface tension that somehow relaxed with distance and that was the attractor.

Yes, surface tension acts to minimize the surface area, and one bigger bubble has less surface area than two small ones.

 

[chef]

Thanks A T
I kinda thought as much but when they sit on the surface they only get attracted when the 2 bubbles are within the 8 times diameter of each other so is there some limit to how far the surface tension acts. I would have thought that the surface tension was like a field that diminishes with the square of the radius so why do bubbles say 15 diameters away not get attracted?

If the reason the 2 bubbles coalesce is because 1 bigger bubble has a lower surface tension than 2 small ones what is the method that they 'communicate' to realize that fact?

Thanks Chef

 

[mfb]

The force gets weaker with distance, at some point it becomes too weak to move the bubbles. I'm surprised you see an effect over 8 times the bubble diameter.

 

[Khashishi]

There are some non-isotropic stresses in the fluid near the surfaces due to surface tension. Consider two bubbles on the left and the right, with some fluid between them. The fluid in the center between the bubbles feels some stress up and down. Normally, in a stationary fluid the stresses are balanced by the opposite stress of the bits of fluid above and below it. But the bit of fluid above and below the center are farther from the surfaces and at an angle to the surfaces, so the surface tension effect on the stress is lower. I think overall, the stress in the vertical direction is slightly lower above and below the center. So fluid in the center will flow up and down. This creates a void which causes the bubbles to move together.

 

[A.T.]

...so is there some limit to how far the surface tension acts.

There is no hard limit, but at some point other factors, like the chaotic fluid motion will have more influence.

If the reason the 2 bubbles coalesce is because 1 bigger bubble has a lower surface tension than 2 small ones what is the method that they 'communicate' to realize that fact?

The bubbles aren't 'communicating'. The fluid particles interact though cohesive forces.

 

[chef]

Kashishi I think we are talking about bubbles on the surface and not rising up in the fluid, that would be a very different scenario I think. Also way more complex with buoyancy, motion, viscosity etc so it is just the simpler case of the surface.

I recommend a glass of gin and tonic and just watch the bubbles and see them do their work. Still not convinced though that the surface does not change shape and have valleys and ridges due to the bubbles.

mfb - the bubbles seem to join when the distance is 8 diameters from one bubble and also 8 from the other so 16 in all for equi sized bubbles.

Best Chef

 

[anorlunda]

If the surface is not static but dynamic, things change. Bubbles can "surf" on ripples. Subsurface rising bubbles and bursting bubbles can create ripples.

The discussion reminds me of "dual walker" videos that I love to watch. Those are drops rather than bubbles, but there may be some overlap in the physics of drops and bubbles. Surface tensions play a role in both cases.

 

[A.T.]

Kashishi I think we are talking about bubbles on the surface and not rising up in the fluid

Additionally to surface tension, there can be flows on the surface, resulting from the circulation within the fluid. These flows push bubbles on the surface together. This is more relevant for hot drinks, but many rising bubbles could generate such flows in cold drinks too.

 

[chef]

The bubbles I see are around 2 or 3 maybe 4 mm in dia and i presume this will cause some circulation as they rise. Difficult to estimate I presume.

I still think the surface will have some deformation due to the bubbles presence and also the surface tension. Again hard to estimate or model but there must be some knowledge on this somewhere.

I remember reading Lord Rayleigh's description of how a jet of water broke into droplets due to surface tension as far back as late 1800's (when he wrote it and not when I read it)

Chef