Ifat Glassman Posted July 26, 2006 Report Share Posted July 26, 2006 I'm referring to bubbles of gas inside liquid. What is the reason that the gas arranges itself in that form? Quote Link to comment Share on other sites More sharing options...
Blinky Posted July 26, 2006 Report Share Posted July 26, 2006 Answer can be found here under the heading surface tension and shape. It´s due to surface tension. Quote Link to comment Share on other sites More sharing options...
Maarten Posted July 26, 2006 Report Share Posted July 26, 2006 In short it is like this: (let's hope I remember this correctly ) The molecules in the liquid are attracted to each other. It is energetically favorable for a molecule to be surrounded by others. This is described by the free energy of a certain system, and any system will try to make this free energy as low as possible (usually a negative value for energetically favorable situations). However, at the surface of the liquid there are only molecules pulling at every side except the surface side, and the molecule experiences is basically pulled back into the liquid. As there have to be some molecules on the outside, the liquid will try and minimize the total surface area, and a sphere is the most efficient way to do this. Due to other factors interacting here the shape may change; i.e. a drop of water is not spherical because of gravitational forces attracting the molecules, which causes more of them to be present at the bottom of the drop, so to speak. Ahh, and to answer your question I think it works similarly for gases. Quote Link to comment Share on other sites More sharing options...
Vladimir Berkov Posted July 26, 2006 Report Share Posted July 26, 2006 Due to other factors interacting here the shape may change; i.e. a drop of water is not spherical because of gravitational forces attracting the molecules, which causes more of them to be present at the bottom of the drop, so to speak. I thought drops of liquid were spherical? I know that lead shot is made by dropping the lead in liquid form from a height sufficient for it to attain terminal velocity, and by the time it reaches the bottom it has cooled into a spherical shape. Quote Link to comment Share on other sites More sharing options...
Ifat Glassman Posted July 26, 2006 Author Report Share Posted July 26, 2006 Ahh, Gibbs' free energy . Wallah! so, Maarten, in essence you're saying that the water (or liquid) favors connections with molecules of it's own kind, and tries to "reject" the gas from interfering with those connections, and by that forces the gas into a shape that would minimise it's area, assuming that the volume has to stay constant (even if the gas had taken another form). What determines the volume of the gas? I think it has to be the fact that the pressure of the liquid and the gas has to be equal. Right? Nice. If I get any other questions I'll post them here. Quote Link to comment Share on other sites More sharing options...
Maarten Posted July 26, 2006 Report Share Posted July 26, 2006 (edited) Well, the volume of a gas is directly related to the number of molecules present in the gas. This only changes with temperature, it's not dependant on the type of gas as far as I know. With solids and liquids you have to take density into account but for gases you don't really have to. There are multiple types of free energy. They are all dependant on slightly different characteristics, so for example the Gibbs free energy takes the number of molecules, pressure and temperature into account. Helmholtz free energy takes number of molecules, volume and temperature into account. For ideal gases it's straightforward to translate pressure into volume and vice versa, so you can convert them rather easily. This is harder for most real substances because there are many more things you have to take into account. Vladimir, if you look at the characteristic shape of a drop of liquid that is falling or dripping it will be bigger at the bottom than the top. That is what I meant. Edited July 26, 2006 by Maarten Quote Link to comment Share on other sites More sharing options...
Vladimir Berkov Posted July 27, 2006 Report Share Posted July 27, 2006 Vladimir, if you look at the characteristic shape of a drop of liquid that is falling or dripping it will be bigger at the bottom than the top. Then what about shot towers? There isn't much use for a piece of shot which is shaped like the illustration you posted. http://en.wikipedia.org/wiki/Shot_tower Also, a cursory search of the internet yeilded the following info stating raindrops are not teardrop-shaped: http://ga.water.usgs.gov/edu/raindropshape.html Quote Link to comment Share on other sites More sharing options...
Adrian Hester Posted July 27, 2006 Report Share Posted July 27, 2006 Well, the volume of a gas is directly related to the number of molecules present in the gas. This only changes with temperature, it's not dependant on the type of gas as far as I know. With solids and liquids you have to take density into account but for gases you don't really have to. Only for ideal gases. For real gases, an accurate equation of state (the equation relating the temperature, pressure, and temperature of the gas) includes substance-specific terms. Quote Link to comment Share on other sites More sharing options...
Maarten Posted July 27, 2006 Report Share Posted July 27, 2006 Ah yes, that is correct. But I think for dilute gases it is still a good approximation to assume they behave as an ideal gas. Heh, didn't know that about the raindrops! Perhaps the viscosity of the fluid is very important here. If we are talking about liquid metal this might be enough to keep it from deforming. I guess that the surface tension contribution to the free energy is a lot bigger than I thought! Quote Link to comment Share on other sites More sharing options...
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