Designer materials: Entropy can lead to order, paving the route to nanostructures

Jul 26, 2012
Shapes can arrange themselves into crystal structures through entropy alone, new research from the University of Michigan shows. Image credit: P. Damasceno, M. Engel, S. Glotzer

(Phys.org) -- Researchers trying to herd tiny particles into useful ordered formations have found an unlikely ally: entropy, a tendency generally described as "disorder."

by University of Michigan scientists and engineers show that the property can nudge particles to form organized structures. By analyzing the shapes of the particles beforehand, they can even predict what kinds of structures will form.

The findings, published in this week's edition of Science, help lay the ground rules for making designer materials with wild capabilities such as shape-shifting skins to camouflage a vehicle or optimize its aerodynamics.

Physicist and chemical engineering professor Sharon Glotzer proposes that such materials could be designed by working backward from the desired properties to generate a blueprint. That design can then be realized with nanoparticles—particles a thousand times smaller than the width of a human hair that can combine in ways that would be impossible through ordinary chemistry alone.

One of the major challenges is persuading the nanoparticles to create the intended structures, but recent studies by Glotzer's group and others showed that some simple particle shapes do so spontaneously as the particles are crowded together. The team wondered if other particle shapes could do the same.

"We studied 145 different shapes, and that gave us more data than anyone has ever had on these types of potential crystal-formers," Glotzer SAID. "With so much information, we could begin to see just how many structures are possible from particle shape alone, and look for trends."

Using computer code written by chemical engineering research investigator Michael Engel, applied physics graduate student Pablo Damasceno ran thousands of virtual experiments, exploring how each shape behaved under different levels of crowding. The program could handle any polyhedral shape, such as dice with any number of sides.

Left to their own devices, drifting particles find the arrangements with the highest . That arrangement matches the idea that entropy is a disorder if the particles have enough space: they disperse, pointed in random directions. But crowded tightly, the particles began forming crystal structures like atoms do—even though they couldn't make bonds. These ordered crystals had to be the high-entropy arrangements, too.

Glotzer explains that this isn't really disorder creating order—entropy needs its image updated. Instead, she describes it as a measure of possibilities. If you could turn off gravity and empty a bag full of dice into a jar, the floating dice would point every which way. However, if you keep adding dice, eventually space becomes so limited that the dice have more options to align face-to-face. The same thing happens to the nanoparticles, which are so small that they feel entropy's influence more strongly than gravity's.

"It's all about options. In this case, ordered arrangements produce the most possibilities, the most options. It's counterintuitive, to be sure," Glotzer said.

The simulation results showed that nearly 70 percent of the shapes tested produced crystal-like structures under entropy alone. But the shocker was how complicated some of these structures were, with up to 52 particles involved in the pattern that repeated throughout the crystal.

"That's an extraordinarily complex crystal even for atoms to form, let alone particles that can't chemically bond," Glotzer said.

The particle shapes produced three crystal types: regular crystals like salt, liquid crystals as found in some flat-screen TVs and plastic crystals in which particles can spin in place. By analyzing the shape of the particle and how groups of them behave before they crystallize, Damasceno said that it is possible to predict which type of crystal the particles would make.

"The geometry of the themselves holds the secret for their assembly behavior," he said.

Why the other 30 percent never formed crystal structures, remaining as disordered glasses, is a mystery.

"These may still want to form crystals but got stuck. What's neat is that for any particle that gets stuck, we had other, awfully similar shapes forming crystals," Glotzer said.

In addition to finding out more about how to coax nanoparticles into structures, her team will also try to discover why some shapes resist order.

Explore further: Protons fuel graphene prospects

More information: "Predictive Self-Assembly of Polyhedra into Complex Structures," by P.F. Damasceno et al., Science, 2012.

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Torbjorn_Larsson_OM
3 / 5 (2) Jul 27, 2012
Good for astrobiology! Besides throwing a monkey wrench into the creationist machinery for disfiguring entropy, it neatly answers my pet peeve when people equates entropy, whether macrostate or microstate, with some loose form of disorder:

Instead, she describes it as a measure of possibilities.

That is what I use: "Entropy (as a microstate fine-grained parameter) is a measure of the availability of states." Over time the tendency for more available energy states, a higher entropy, would ratchet up if it can, purely for probabilistic reasons.

As this physics tells us, whether those states are more ordered or disordered depends on the details of the system.

TL; DR: Entropy =/= disorder.

Entropy may have helped the first cells to form, entropy forces is a (small, IIRC) part of what makes micelles and later membrane protocells assemble. It may even today be part of what constitutes the cell construction.
Lurker2358
not rated yet Jul 27, 2012
They forgot to account for many factors I've mentioned elsewhere, or example, since this is a simulation and therefore a nested reality, they forgot to account for the entropy in the "more fundamental" reality, being the real world. That is to say, the computer uses a certain amount of electricity and produces a certain amount of work on the particles, whilst producing a certain amount of heat waste. The computer creates, updates, and destroys countless temporary variables during the course of simulation, all of which represent "work" being done on the particles, as well as the associated heat waste and entropy not counted by the model itself.

Thus the computer, a machine, is doing "work" in the classical sense, on the data representing the particles, and they are not "spontaneously" assembling themselves, seeing as how classical work in a more fundamental level of reality is being done in order to accomplish the simulation.

This is as bad as the rubber sheet model of space-time...
Lurker2358
not rated yet Jul 27, 2012
Five stars so it may be seen, but NOT for the faulty science behind the article, because they failed to consider the fact that NO computer simulation is a closed system, as described above.

"Work" is being done on the particles by the electricity passing through the transistors and other circuitry representative of said particles.

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