|MadSci Network: Physics|
Helloooo, Jeff! You sort of asked two questions. The title asks whether or not a solid equivalent of Brownian Motion exists. The message body asks about motion of atoms inside a solid. They're not quite the same question, and your Mad Scientist will try to explain both. Brownian motion is the random visible (in a microscope) wiggling of small particles because of collisions with the moving atoms in the fluid containing the particles. I suppose a solid equivalent does exist (motion of inclusions of one solid in another) but as far as I know it has no name. The amplitude of motion is vastly smaller, as atoms in solid are constrained to moving less than the crystal lattice spacings (on the order of a few Angstroms or so), and so not visible in a microscope (well, possibly in a very sophisticated transmission electron microscope, but I doubt it). Don't go away disappointed, though. At least two forms of atomic motion exist inside solids. The first is little packets of vibrational energy called phonons. Think of a lattice as a bunch of solid balls (atoms)held together with springs (chemical bonds). Wiggle one and the whole array tends to jiggle. After struggling though some horrid mathematics, it turns out that the jiggling is constrained to occur in only certain directions and energies. In essence these constrained vibrations of mechanical energy, called phonons, are standing waves in the crystal. 1-D Analogy: a taut violin string will only oscillate at certain frequencies. 2-D Analogy: a drum head only vibrates at certain frequencies and in certain patterns. In addition, we have point defects. A crystal with all its atoms exactly in place in the lattice is a very high-entropy state, and not likely to occur at any temperature much above absolute zero. It turns out that some of the atoms acquire enough thermal energy to manage to get loose, becoming what are called interstitials, and sit between the atoms still in the lattice. They leave behind "holes" in the lattice, called vacancies. At any given temperature, an equilibrium exists between the thermal generation of interstitial-vacancy pairs and their annihilation (interstitial "falling" back into a vacancy), causing a specific concentration of the point defects to exist in the lattice. Interstitials and vacancies both move through a crystal lattice, fairly rapidly as these things go. By the way, other atomic species can enter a lattice as interstitials, too. This is called solid-state diffusion and can change the characteristics of the material dramatically. For example, some metamorphic rocks are created when atomic species from a hot magma diffuse into the surrounding wall rocks and change their makeup into an entirely new set of minerals. So, in summary, no visible equivalent to Brownian motion exists in solids, but yes, certainly the atoms move around, both as part of the lattice vibrations (phonons) and as interstitials/vacancies. Hmm, yet another yes and no answer. Never can get a straight answer out of these Mad Scientists :-)
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