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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.
Speaking of ice, have folks here ever heard of Pykrete? And would this explain why Pykrete melts so slowly?
Supposedly tissue paper works as well as sawdust. So you can tell all your friends you know how to beat someone to death with a wet paper towel.
--grendel drago
Laws do not persuade just because they threaten. --Seneca
At least the way the article describes the study.. it doesnt seem like it models the problem well.. but something tells me these arent the greatest writers here... For instance:
"So Yodh's team made some big atoms. Specifically, they made see-through crystals that are like small beads and are visible in an optical microscope."
By "see-through crystals" i'm assuming they mean optically transparent crystals constructed from small beads, not crystals that are like beads that then form a larger crystal structure, although from the wording, it's impossible to tell.
"The spheres swell or collapse significantly with small changes in temperature, and they exhibit other useful properties that allow them to behave like enormous versions of atoms for the purpose of our experiment,"
As far as I know.. atoms dont significantly change size when temperature changes.... they change how fast they move. I dont really see how size-changing beads model water molecules here, unless it's on a macroscale where a molecules are considered to expand as a group with increased temperature... but that sort of would defeat the pupose of the whole study...
On the other hand.... I think that the research is probably solid, espcially if it's being published in Science, a extremely selective journal. I think the article just fails to explain it well, and takes quotes out of context. Sadly, this is all too common in scientific journalism.
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.
super cooled liquids and vapors are easy to make.
I have a small fridge here (absocold) it is kind of like the small fridges students use in dorm rooms.
If I put a bottle of water in the freezer compartment
most of the time it will not freeze.
what is fun is to hand it to someone and ask them to shake it or even let me them drink the water.
I will suddenly turn to slush. It is very strange to have water freeze in you mouth.
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.
If you observe a particle, its wavefunction collapses in an irreversable process. Before the measurement is made, there is no way to know for sure where the particle will collapse to when you observe it (You just have a wavefunction of probability amplitudes), and so it the position of the particle is not deterministic. Thus Einstein's comment "God does not play dice". The weird thing is that if you could never actually observe anything, the universe _would_ be deterministic. The wavefunctions that describe the probability amplitudes would just spread out and interact in a perfectly predictable way via wave mechanics.
The underlying point here is the techniques materials scientists normally use to examine material properties. Techniques like FTIR, SEM, STEM and x-ray diffraction work well on materials in one state but any time phase change occurs they are too simple to examine the change as it occurs. Even an environmental SEM that can examine certain materials at higher temperatures tends to still be too simplistic to examine a phenomena like melting closely at the atomic scale. For melting energy really one of the few useful techniques is DSC (differential scanning calorimetry) and that still won't let you observe the melting mechanism itself, only detect the energy needed to reach the melting point. In this area, the physicists actually have us beaten because they at least have particle detectors that can observe the effect of high energy collisions at the sub-atomic scale. That's why this experiment is important, they are developing techniques to circumvent the limitations of the instrumentation.
What would Richard Feynman do, if he were here right now? He'd do some math and he'd follow through!