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Van Der Waals Wildcats Away!

By Alaina G. Levine

I make it no secret that my alma mater and former employer is the University of Arizona (UA). I spent many years there amongst the strange attractors in the Physics and other science and engineering departments, soaking up as much fascinating why-the-world-works-the-way-it-does knowledge as possible.

I still have quite an affinity for the Wildcats who roam the halls of the Physics and Atmospheric Science Building, mostly with their heads down as they contemplate problems associated with GUTs, AGN, ATLAS, and many other acronyms. For some, I always wondered why they didn’t bump into walls, given that their eyes were always transfixed to the floor instead of objects ahead.

This week I learned that a team of UA physicists had published interesting revelations relating to another acronym, VDW, aka the van-der-Waals force. The scholars discovered a novel method to calculate how individual atoms interact with various surfaces, which is dictated by this fabulous force. The paper is published in Phys Rev Letters, and the accompanying data could provide a new baseline for theorists to test their ideas about how atoms cooperate, and allow scientists and engineers to more successfully develop nanostructures.

The group, consisting of associate professor Alex Cronin, Will Holmgren, and Cathy Klauss, and lead by Vincent Lonij, a graduate student, wanted to better understand how this extremely weak force works. The van-der-Waals force can make “anything stick to anything, provided the two are extremely close to each other,” according to the UA press release written by Daniel Stolte. It is a fundamental force in many areas of science, such as chemistry, physics and biology, but because it is so weak, it is very difficult to analyze.

The cohort orchestrated an experiment to calculate the interactions between individual atoms and a surface, using quantum mechanical principles. As Stolte explains it:

We shoot a beam of atoms through a grating, sort of like a micro-scale picket fence," Lonij explained. "As the atoms pass through the grating, they interact with the surface of the grating bars, and we can measure that interaction."As the atoms pass through the slits in the grating, the van-der-Waals force attracts them to the bars separating the slits. Depending on how strong the interaction, it changes the atom's trajectory, just like a beam of light is bent when it passes through water or a prism.A wave passing through the middle of the slit does so relatively unencumbered. On the other hand, if an atom wave passes close by the slit's edges, it interacts with the surface and skips a bit ahead, "out of phase," as physicists say."After the atoms pass through the grating, we detect how much the waves are out of phase, which tells us how strong the van-der-Waals potential was when the atoms interacted with the surface."

This is all well and good, but “the most significant discovery was that an atom's inner electrons, orbiting the nucleus at a closer range than the atom's outer electrons, influence the way the atom interacts with the surface,” wrote Stolte."We show that these core electrons contribute to the atom-surface potential," said Lonij, "which was only known in theory until now. This is the first experimental demonstration that core electrons affect atom-surface potentials.""But what is perhaps more important," he added, "is that you can also turn it around. We now know that the core electrons affect atom-surface potentials. We also know that these core electrons are hard to calculate in atomic theory. So we can use measurements of atom-surface potentials to make the theory better: The theory of the atom."

So to sum up: we can now construct better nanobots, understand what the heck is going on at surfaces with individual atoms, created a baseline for physicists to investigate their theories, and developed an awareness that the electrons influence the behavior of movement at these surfaces. Go Wildcats!


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