3
$\begingroup$

When an air bubble is formed inside water, and while it's still inside, does a force due to surface tension act on it? Why and where does this force act?

I read somewhere that a bubble would detach from the bottom of a container filled with a liquid if the buoyant force exceeds the force due to surface tension. If surface tension is a property of a liquid surface, why does it work at the bottom, well inside the liquid? I think it follows from something very basic, but I just can't grasp it.

EDIT : I think I figured it out. Is it beacuse the liquid pulls the layer of liquid surrounding the bubble?

$\endgroup$
3
$\begingroup$

Surface tension is rightly named: it is a tension in the surface that separates a liquid and another fluid, air in your case. This tension is at the origin of forces: e.g, if the surface is curved, the force tends to straighten it (as when you exert a tension on a piece of string: it will straighten). In a small spherical bubble which is far from the walls of the container and from the surface, surface tension acts all over the surface, exerting a force toward the center of the bubble: this force is resisted by an opposing pressure in the bubble. This is known as Laplace law.

If the bubble is at the bottom, then surface tension is also at the origin of a force at the line of contact of the water-air interface and the bottom of the container. This force is proportional to the value of the surface tension (a constant that only depends on the fluids you have and temperature) times the length of that line.

If the bubble is small enough, this force can resist the buoyancy force. Imagine now that you inflate the bubble, the volume and thereofre buoyancy will grow faster than the perimeter of the contact, and at some point the buoyancy force will be able to detach the bubble.

(Note that in order to have a complete picture, one should also consider the shape evolution of the bubble, which is governed by other surface energies: the solid-water energy and solid-gas energy, as the angle of contact governs the proportionality mentioned above)

| cite | improve this answer | |
$\endgroup$
  • $\begingroup$ Can you explain why surface tension acts towards the centre of the AIR bubble? In the case of a liquid drop, liquid is inside the drop. If we consider one hemisphere of the LIQUID drop, the other surface is also liquid and hence pulls the hemisphere considered along the periphery by a total net force S(surface tension of liquid) times circumference of the hemisphere. But in case of a AIR bubble, where will surface tension act? Its confusing me. How is difference in pressure inside and outside an AIR bubble still 2S/R where S is surface tension of LIQUID $\endgroup$ – Hola Apr 1 '18 at 10:48
  • $\begingroup$ Hi @Ola, thanks for your questions. It's not the right place to answer them as a comment, please post a new question with this... once you've read en.wikipedia.org/wiki/Surface_tension . $\endgroup$ – Joce Apr 2 '18 at 18:16
  • $\begingroup$ I dont understand. I commented since my doubt and problem in concept is ** precisely** the actual question. I just want to know how the direction of surface tension on the air bubble is towards the centre of the bubble, the words which you mentioned in your answer without giving complete reasoning and proof as to why it happens.The water in contact with the air bubble must be pulled away from the centre due to surface tension right? Since there are water molecules surrounding the water in contact with air bubble, it must experience a force ourward right? Why is thisforce towards center $\endgroup$ – Hola Apr 4 '18 at 18:44
  • $\begingroup$ If you still feel that this question is not related to the answer you gave (I dont know how you feel that) and you will not answer it here in the comments then I shall post it as another question. But once again all I require is the proper reasoning behind why "surface tension acts all over the surface, exerting a force toward the center of the bubble "which are your own words without explanation as how this force arises. So if you could please help me out, Id like the reasoning . If not please let me know and I will post a new question $\endgroup$ – Hola Apr 4 '18 at 18:47
  • $\begingroup$ Please ask a new question stating what you know and what you are wondering about. $\endgroup$ – Joce Apr 5 '18 at 21:28
3
$\begingroup$

Surface tension happens at the boundary of a liquid and something else. In your case this something else is either the air of the bubble or the bottom of the container.

Look at bubbles form in a glass filled with a carbonated drink. Some form on the walls and on the bottom. They stick there until they get big enough so that the buyant force pulls them off.

| cite | improve this answer | |
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.