From: tjgerman@53iss6.Waterloo.NCR.COM (Trevor German)
Newsgroups: rec.puzzles
Subject: WEIGHTLESS BUBBLE
Date: 16 Oct 92 13:34:10 GMT
Organization: Imaging Systems Division, NCR Corp, Waterloo, Ont., CANADA


	In response to bubble of air in sealed tube of fluid
	under equilibrium conditions in a totally weightless
	environment.




















	OK, let us assume that the container is placed at a point in space
	where there is a complete lack of gravity. (If such a place
	exists).

	If the container were infinite in dimentions the bubble would assume
	a shape that gave it minimum volume driven by the surface tension
	of the fluid. This shape would be a perfect sphere, right .. NOT.

	Consider the following. Take a litre of water and dump it in this
	environment.  Given enough time it
	will settle down with no internal currents and form a sphere. Now
	inject into the cantre of the sphere a bubble of air (Forget the
	fact that the disturbance to the water would ruin the whole effect)
	What forces are acting on the bubble. Initially the bubble will
	remain dead centre and be spherical. However there are forces 
	acting on it. There is a micro-gravity within the sphere of water
	centred at the centre of the sphere. The pressure at the centre is
	oh so much larger than that at the surface. (More water more pressure)
	The air, being less dense than the water wants to float to the
	surface of the sphere. If there were no surface tension the bubble
	would break up into an infinite number of tiny bubbles and float
	out in every direction towards the surface. Surface tension however
	holds the bubble together as long as the bubble is not too big. An
	equilibrium condition does exist in which the bubble could be balanced
	in the middle of the sphere. However, it is practically impossible 
	to establish this equilibrium. Any slight offset unbalances the micro
	-gravity such that the pull on the bubble is more one way than the
	other and hence the bubble will start to move out. Once this move
	begins, there is an imbalance in pressure accross the depth of the
	bubble and that perfect spherical shape will be lost. Actually, even
	if you could balance the bubble in the middle of the sphere, the air 
	within it will eventually disolve and migrate out to the outer surface
	(I guess this is the tiny bubbles I mentioned before, it just takes 
	a while). Anyway, my point is this is not a natuarally occurring
	equilibrium.

		So in a closed container where does the bubble go. This
	depends on the shape. The driving force is the outer surface tension
	of the fluid. The bubble will end up as far away from the centre of
	mass of the fluid as it can get without hanging up on the geometry
	of the container. If the container is a cylindrical tube capped at
	both ends it will head towards one end at the rim. There it will
	deform further such that the volume of the fluid is a minimum.
	



	However, since infinite dimentions would imply the container did not 
	exist, this is assumed to be not the case. Consider the following
	possible solutions. 

 There will therefore
	be forces of attraction between the bubble and the walls of the
	container
--
       >|  "Only average people            | Trevor J German BSc    |<
       >|        never make mistakes."     | NCR, Waterloo, Ontario |<
       >| tjgerman@53iss6.Waterloo.NCR.COM | Canada.                |<