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How Ice Melts
Killer Instinct writes "Ever wonder how ice melts? Until now, scientists could not explain why ice cubes in your drink melt. They've known the basics, but the details remained elusive. A breakthrough new study, announced yesterday, supports a leading theory that melting starts when the fundamental structure of matter begins to crack. Melting is considered a basic phenomenon in physics. An understanding of how it works is crucial to gaining a firm grasp on the physical world."
This is somewhat akin to boiling really, at least from my perspective.. small nucleation points, that spread throughout the liquid or crystal, effecting an overall phase change when the energy distribution reaches a point such that the majority of atoms prefer the gaseous or liquid state (depending on the phase change).
a. Summary is plagiarized from the article, unless I've missed some nested quotes.
b. These guys took this problem because "the earliest phase of melting has never been seen" but they didn't do that either! All they did was make "see-through crystals that are like small beads and are visible in an optical microscope." Doesn't sound like a hell of a lot of progress to me; anyone care to elaborate?
c. Their main result seems to be that the melting process starts at crystal defects and spreads to create liquidy regions within the crystal. Again, can anyone explain why the melting might not start at defects - the weak points?
I'm sure there's something neater here than I'm seeing; it would be nice if the article had more info.
Oh, and don't forget, you can use distilled water to make 2" long ice spikes on your cubes!
using hot water makes it faster than using cold water, right?
Water that's really hot will loose heat more rapidly than cool water in the same surroundings. What people don't get is that once the hot water has cooled off, it now cools at the slower rate.
What actually IS useful about freezing hot water is that there are a lot less air bubbles so the ice doesn't crack and throw shards out when you pour freshly brewed tea over it on a hot summer afternoon.
This reminds me of a similar effect that I often observe while cooking, particularly while stir-frying (or any other high-heat method). That is: a drop of water will evaporate more quickly in a pan on medium heat that it will in a pan on high heat.
The reason? When a drop of water hits a pan on very high heat, the underside is instantly tranformed into a layer of vapor which then acts as a buffer between the pan and the liquid on top. So insulated, the water droplet will then "dance" and roll around the pan like a ball bearing. The drop can remain in the pan for a surprising amount of time, though I have never personally measured.
There they were, sitting in the van with all those dials, and the cat was dead. -V. Marchetti, CIA
If "pre-melting" truly begins at the defect sites, it would be interesting to see whether ultra-low defect containing crystals melt at a higher temperature. Say, purify and grow a chunk of ice through the same procedure used to fabricate semiconductor grade silicon (Czochalski style or epitaxially), and then see if it holds together through warmer temps.
You could imagine two closed cylindrical containers, each initially filled with a substance in a liquid state. The liquid of container A is at a temperature such that the density is the minumum. The liquid in container B is initially at a higher temperature than container A. For simplicity's sake, the only extremum of density with respect to temperature is the minimum I mentioned. You begin cooling both liquids at the top of the cylinder.
As heat is transferred from container A, the density will always be increasing from the top to the bottom of the container in a predictable fashion, i.e., the "heavier" substances will always be on the bottom of the container. This doesn't promote convection. With container B, there are good opportunities for convection, due to the varying density gradient and the effect of gravity. Solid forming on the top of container B could even sink. The convection leads to a higher sustained temperature gradient at the cylinder boundary, leading to faster heat transfer and faster cooling.
Note that this mechanism doesn't require an open system. There are, of course, other possible mechanisms, but this is the simplest one I could come up with.