Category: Uncategorized
Foam is everywhere
10/04/09 02:04 , Categories: Uncategorized Link: http://cosmicweb.uchicago.edu/filaments.html
Wherever there is energy, there will be bubbles
wherever there is gravity, bubbles will make foam
Whipped cream, shaving lather, tomato foam, sea scum,… all these materials are complicated composites, so you might think that’s why foam, in practice, is a difficult material to study. But actually, there’s more "simple".
No need even for two different materials, to start studying foam and performing experiments. A mathematical, abstract system of identical particles, in an abstract empty space is allready good anough a model, that can actually be tested on a real existing situation;
No need for fazes, thermodynamics, no need for liquid and gas, or vapor.
Movement energy ( for instance high frequency /high volume sound) can create bubbles of vacuum in any liquid matter. So a very basic phenomenon to study, is a coarse distribution of particles in space. The particles themselves need not be identified. Suffice the forces that act between them and perhaps one external force field. One rule governing the attractive and repulsive force between any two particles, depending on the distance between them. And a second rule, for a force felt by each particle, depending on its place in space. (think of gravity for instance) These forces will determine the evolution in time of the particle distribution. Then, mathematically the problem is reduced to finding what kind of fields and particle interaction is necessary to produce foam.
Actually: the perfect example is the universe, as studied by astronomers. Mass distribution in the universe, on a very large scale actually has a foam-like structure.
It evolved into this structure, coming from a uniform smooth spread of mass.
So in conclusion: foam may be fragile, but it will always be there.
een punt
.
Waar alles middelpunt geworden is, bestaat geen middelpunt meer.
Verschuiming en virtualisering is een proces waar alles zijn eigen (bewegend?) middelpunt wordt.
De elektrische gelijktijdigheid van de informatiebewegingen resulteert in de vibrerende totaalsfeer van de auditieve ruimte waarvan het middelpunt overal is en de omtrek nergens. (Marshall Mc Luhan)
Vrij naar: Peter Sloterdijk, SFEREN.
foam flows versus air flows
17/02/09 21:35 , Categories: Uncategorized The front edge on the wing of an airplane is specially shaped, so that when it cuts through the air fast, the wing is pushed up. The leading edge splits the air so there is an airflow over the top and bottom, but the flow over the top is faster because the curved topside is a greater distance. Because the top flow is faster there is less air pressure and you get lift.
It works under water also. The same shape keeps a surf board from sinking, when you move fast enough.
Upside down wings are put on racing cars to help keep them down on the road.
But the same wing shape works exactly the other way round when it moves through foam.
in the figure below, the arrows indicate the direction of forces: "lift" (in this case rather "push, since it points downwards) and drag.
foam facts
10/02/09 00:09 , Categories: Uncategorized
bubbles & surface tension
Surface-tension is caused by the attraction between the liquid’s molecules by various intermolecular forces. In the bulk of the liquid, each molecule is pulled equally in all directions by neighboring liquid molecules, resulting in a net force of zero. At the surface of the liquid, the molecules are pulled inwards by other molecules deeper inside the liquid and are not attracted as intensely by the molecules in the neighboring medium (be it vacuum, air or another liquid). Therefore, all of the molecules at the surface are subject to an inward force of molecular attraction which is balanced only by the liquid’s resistance to compression, meaning there is no net inward force. However, there is a driving force to diminish the surface area, and in this respect a liquid surface resembles a stretched elastic membrane. Thus the liquid squeezes itself together until it has the locally lowest surface area possible.
In 1887, Lord Kelvin asked how to partition space into cells, all of the same volume, such that the total area of the interfaces between the cells is a minimum. The best partition Kelvin could come up with was made of slightly curved 14-sided polyhedra. For over a century, nobody could improve on Kelvin’s partition. Then in 1993, Denis Weaire and Robert Phelan came up with a partition of space into two kinds of cells (of equal volume, of course) that beat Kelvin’s partition by 0.3% in area.
The physics, as described in “Beating Kelvin’s Partition of Space”, is Denis Weaire and Robert Phelan’s discovery of an improvement on
http://apothdrawer.blogspot.com/2004/05/water-cube-and-foam-theory.html
http://www.susqu.edu/facstaff/b/brakke/kelvin/kelvin.html
A basic kind of foam can be made by mixing a gas and a liquid. The proportions of the quantities of gas and liquid may vary, and different proportions results in foam with different properties. When you shake this mixture vigorously, you obtain for a short moment a proportion of 37% liquid in the total volume of foam. At this point the foam behaves like a liquid. Perfectly round gas bubbles float around in the liquid. But quickly by the force of gravity the bubbles will float to the surface (actually it’s the liquid that drains to the bottom.) Until the gravity balances out with capillary forces and surface tension. At that point the foam on top of the liquid contains only 5% liquid volume and its properties resemble more a solid! The gas is no longer in the shape of perfect round bubbles. The liquid in between partitions the gas into tightly packed polyhedrons with more or less curved faces and vertices.
To make a stable foam, that is, a foam that doesn’t separate into liquid and gas instantly, you need to add other substances. For instance soap.
Looking close at some foam, we check what happens with two adjacent bubbles of different size. The pressure in a small bubble is bigger than in the bigger bubble (just like with a new balloon, you have to blow very hard to inflate it for the first time). So gas will flow from the smaller to the bigger. This will happen even through the surface they share, without causing hole.
The liquid in between the bubbles also may evaporate into the gas of the bubbles themselves.
When a big bubble explodes or implodes in the outer realm of the foam, it leaves a gap behind. When a little bubble explodes, you just get a little bit of movement of the other bubbles. That is, the foam rearanges itself to fill in the gap.
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