Cool anything down enough, and it becomes a crystal solid, according to traditional physics theories. But that might not always be so, and two scientists think they have found cases where a liquidlike state is more stable than the solid crystal, in a reversal of the norm.
For the experiment, detailed in yesterday's (Aug. 4) issue of the journal Nature Physics, the research duo used a computer simulation to create a liquid-that-is-not-a-liquid. Even though the experiment was done with virtual rather than real molecules, it offers an important insight into how crystals are made, the researchers said.
This knowledge, in turn, can tell scientists more about how to prevent substances from crystallizing when needed, or keep them amorphous. [Twisted Physics: 7 Mind-Blowing Findings]
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Cooling colloids
To get this bizarre liquid, the researchers started with a colloid, or a liquid with tiny particles suspended in it. A classic example is milk, which is mostly water but looks white because of all the bits of fat and protein floating in it. Freeze milk, though, and you get crystallized water — ice — while the white stuff separates and solidifies if it is cold enough.
"A colloid has particles small enough that thermal energy is important," said lead author of the new study, Frank Smallenburg, a physicist at La Sapienza University in Rome.
But if the molecules of the colloid bond to each other in just the right way, the familiar crystallization doesn't happen. Instead, the colloid turns into a stable form that seems solid, but has the molecular structure of a liquid.
Smallenburgsimulated a colloid on a computer, and plugged in the equations describing how it acts as the temperature drops. Using a computer model of molecules with four bonds, he saw that if those bonds were stiff, crystallization happened quickly. If they were flexible, though, the bonds stayed disordered and made lumpy agglomerations. Cooled further, they became like glass — disordered molecules that don't flow but form a kind of amorphous solid.
"When we make the bonds more flexible, the liquid phase remains stable even at extremely low temperatures," Smallenburg said. "The particles will simply never order into a crystal, unless they are compressed to high densities."
Energy and entropy
Molecules with flexible bonds behave this way because of two competing forces in a cooling liquid: energy and entropy, which is a measure of how disordered a system is. In liquids, the molecules all bounce around randomly, whereas in a crystal they are ordered in regular patterns, so liquids have more entropy than crystalline solids. [Video – Mysterious Materials Act Like Liquids and Solids]
As a liquid cools, the molecules move around less and less. They have less energy, so they try to arrange themselves in ways that are easier (take less energy). Molecules like water will bind to each other at a specific angle because it takes less energy to do that; the bond that makes the familiar six-sided crystal pattern is a lower energy state. At the same time, the amount of entropy — disorder — actually decreases when water freezes.
Colloidal molecules with flexible bonds have more ways to connect with their fellows in a liquid. "When the bonds are flexible enough, the number of ways you can connect all particles to four neighbors and form a disordered structure is much larger than the number of bonding patterns that result in a crystal," Smallenburg said.
The result: a liquid that acts sort of like a solid.
The computer simulation does describe some real systems, he said. There are polymers and large organic molecules, like DNA, that have similar characteristics. Even water and silica can be simulated.
The next steps will be experimenting with real materials to study polymers. Smallenburg noted that his group is collaborating with a French team researching polymers that behave like silica when they are heated. With some work, the new simulation could be applied to this case as well, Smallenburg said.
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