One of the Smallest Machines Ever Assembles Itself

At some point in the future I suppose we'll all have a good laugh about the days when we used to make things out of stuff, where stuff is material harvested from the Earth as petroleum or wood or iron ore. These olden days materials didn't even do things like build themselves into complex machines or solar panels; we had to do it for them, expending yet more energy and surely wasting some of that material in the process. The materials (the stuff) of the past were stupid and as crude as forming bullets from molten lead must have once seemed.

In this future, it will no doubt be noted that one of the landmark observations in materials science came nearly by accident, or was at least quite unexpected. US Air Force-funded researchers working with silver superlattices—alternating layers of crystalline silver nanoparticle clusters and an organic buffer material—noted that not only did their material successfully self-assemble, the result featured a peculiar mechanical action: when the material is compressed, the nanoparticle structures rotate like very, very, very tiny gears. The result: a machine, of sorts. The corresponding study is published in the just-out editon of the journal Nature Materials.

"As we squeeze on this material, it gets softer and softer and suddenly experiences a dramatic change," said Uzi Landman, of the Georgia Institute of Technology. "When we look at the orientation of the microscopic structure of the crystal in the region of this transition, we see that something very unusual happens. The structures start to rotate with respect to one another, creating a molecular machine with some of the smallest moving elements ever observed." In each layer of the superlattice, those gears move in opposite directions to each other, and when pressure is taken again removed from the material, researchers found that these gears return to their original positions.

Potential uses for the material include molecular-scale switching, sensing, and maybe energy absorption. It's possible to imagine a future use in which force is converted to mechanical motion by the material. One might dream up a scenario in which steam turbines in power plants are replaced by a material such as this, that can convert pressure into nanoscale rotation for nanoscale generators (my own speculation).

The gear motion is the result of the particular self-assembly process of the material. "Self-assembly" here is maybe not all that you expect; the material is not some from-scratch autonomous creation. Rather, the material is begun as a solution of molecules with a predisposition for bonding with each other at particular angles via hydrogen atoms/hydrogen bonds.

"These bonds are directional and cannot vary significantly, which restricts the orientation that the molecules can have," said Landman. What results are unusually rigid structures (that must maintain particular angles internally) within a superlattice sheet. To maintain their correct orientations, these structures can only rotate as the superlattice has forced applied to it. The material is compressed, and in order for that to happen its layers flex with respect to each other. A further effect of these rotating gears/structures is that once the superlattice has been compressed by about 6 percent, it quite suddenly becomes much softer and further compression takes much less effort.

Weird behavior is what should be expected in a material like this. Essentially, what occurs is that the already weird properties of particles are being realized at the level of the macro-world; if you construct a full-scale system out of nanometer-scale objects, you wind up with the properties of the nanometer-scale objects at full-scale. "We make the small particles, and they are different because small is different," said Landman. "When you put them together, having more of them is different because that allows them to behave collectively, and that collective activity makes the difference."