Wednesday, April 25, 2007

Water Flows Like Molasses on the Nanoscale

Scientists at Georgia Tech has discovered that when confined to a width of less than 2 nanometers, water flows with a viscosity equivalent to molasses.

"Since water usually has a low viscosity, the force you would expect to feel as you compress it should be very small," said Riedo, assistant professor in Georgia Tech's School of Physics. "But when we did the experiment, we found that when the distance between the tip and the surface is about one nanometer, we feel a repulsive force by the water that is much stronger than what we would expect."

There are many things that do not really behave in the same fashion as you change its dimensionality. Take for instance, charge carriers that we are so familiar with. In 3D, ordinary metals behave in the "usual" fashion, and typically can be accurately described by Landau's Fermi Liquid Theory. However, try confining it to 1D. Then these charge carriers (be it electrons or holes) will start behaving in very unusual manner. The weakest possible interaction between themselves could possibly produce what is known as the spin-charge separation. This is where the spin and charge properties appear to "flow" differently, as if the charge carrier has split itself into 2 components, one that carries the spin, while the other carries the charge.

This is purely a many-body effect in 1D, and can be described by the Tomonaga-Luttinger Liquid model. However, it again illustrates that simply by changing the dimensionality, the physics can be VERY different. Now this also ties in with my previous "rants" about the bastardization of quantum mechanics. There seems to be a free-wheeling idea that some understanding in one area can be EASILY extrapolated into another. People are applying QM into human interactions, or using Relativity to justify social policies, etc. All of these people are exhibiting the classical symptom that I've called "imagination without knowledge is ignorance waiting to happen". What it means is that they took the superficial knowledge of certain physics principles, but are completely ignorant of other parts of physics that simply cannot be isolated from such principles. For example, there is a very important area of physics called "phase transition". This is the study where a system undergoes an ABRUPT CHANGE in one or more properties as goes through that transition. Water turning into ice is an example. A metal going into a superconductor is another example. Its resistivity drops abruptly to zero as it goes just below the transition temperature Tc. It isn't a gradual process. But what is more important is that the behavior and temperature dependence of the resistivity above Tc cannot be extrapolated all the way down to, say, T=0, because the characteristics of the resistivity is very different below and above Tc. When things undergo a phase transition, this is what happens. You cannot assume that the behavior in one phase is the same as the behavior in another phase.

Now, if that is the case, then what is there to say that one can actually extrapolate what we know for systems obeying quantum mechanical behavior to, let's say, a human interaction scale? There's nothing that say we can. There's no indication that social and collective human behavior are "quantum systems". When people use physics principles and extrapolate them into such scales, they are only aware of those "sexy" ideas of physics while ignoring the fact that physics are often aware of when such extrapolations can and cannot be applied. They should also take what we have learned from the study of phase transition and be aware that not everything can be extrapolated that easily.


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