Thermal and Statistical Physics

Prof. Carlson
Thermal and Statistical Physics Podcast

Thermal and Statistical Physics Purdue University Phys 416

  1. 11/16/2005

    Lecture 22: Nucleation in First Order (Abrupt) Phase Transitions

    Supercooling Demonstration (thanks to special guest Prof. Ken Ritchie): Put filtered water in a plastic bottle in your freezer for, say, 4 hours. Now, carefully remove it from the freezer, and shake the bottle vigorously. We did this, and saw ice crystals begin to slowly form in the water, because the liquid water was supercooled, and the ice phase was technically more stable. (Some crystals even resembled snowflakes, and grew larger as they floated to the top.) You may have to experiment with how long you leave the bottle in the freezer. Too short a time, and nothing happens. If you freeze the bottle longer, a vigorous shake will turn the whole bottle white as crystals form everywhere. Too long, and it will all freeze in the freezer. Do try this at home! Today we discuss nucleation in first order (abrupt) phase transitions. The ice crystals in our supercooled bottle of water formed through nucleation -- tiny ice crystals grew larger over time. The arctic cod is a supercooled fish, living in water too salty to freeze even though it's at -1.9 degrees Celsius! The reason the fish doesn't freeze solid is due to antifreeze glycoproteins, which inhibit the growth of nucleated ice crystals. We calculate the energy barriers to nucleation at the liquid-gas transiton, and find that a nucleated liquid bubble in the gas phase must be large enough before it will turn the whole substance liquid. If it's too small, the bubble is unstable and converts back into gas. We also discuss: Slushy ice -- where is that on our phase diagram? Surface tension and faceting in crystals. Plant-eating bacteria which secrete enzymes that encourage ice nucleation on plants. And quite a bit about how snowflakes form. Much of today is from Jim Sethna's statistical mechanics book, and the part about snowflakes and ice formation is from research at my alma mater, Caltech, as presented at www.snowcrystals.com. Lecture Audio

  2. 11/08/2005

    Lecture 19: Symmetries, Order Parameters, and the Failure of Reductionism

    We finish the van der Waals equation of state, and use it to illustrate the liquid-gas phase transition. It turns out that at low pressure, the van der Waals equation of state has a wiggle where (dp/pV)0. Since this would cause an explosion, the system instead undergoes phase separation so that part of the container has liquid, and part has gas in it. More is different: We discuss the failure of reductionism. Reductionism is the idea that you will learn everything about an object by breaking it into its smallest bits -- like atoms, then electrons and protons, then quarks, then strings. But large collections of particles (like liquids, gases, and solids) have many properties which aren't really due to their constituents per se, but rather are due to larger organizing principles, and the symmetry of the associated phase. Example: All solids are hard, even though they're made out of different substances. So the property "hardness" is not actually caused by the particular form of the potentials for the particular atoms in that solid. Rather, it's due to the symmetry of the regular crystalline structure the atoms take, and is independent of the type of atom. To illustrate, we discuss several phases of matter, and identify the corresponding "order parameter", which is a measurable quantity that captures the symmetry of the phase. Visual Aids: Rotini pasta to demonstrate twisted nematic phases. Specimens from my rock collection: quartz, amethyst, hematite, and others to see how all crystals are similar, despite being made from different atoms. The "sameness" manifests itself in the basic property of a solid: being hard. The "differenc" manifests itself in color, and in the shape of the crystals, which reveal the underlying quantum mechanics of how the chemical bonds form from atom to atom. Plus, the return of the squishy crystal to illustrate phonons. 0 Lecture Audio

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Thermal and Statistical Physics Purdue University Phys 416

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