PhD student Abe Clark in the Behringer Lab
During this past summer, five students, including a Duke undergraduate and four students from local high schools, worked in the Behringer Lab in the Department of Physics at Duke. Each student had an independent project, and was teamed with either Duke post-doc, Joshua Dijksman, or Duke Ph.D. student, Abe Clark. Colton Brown, a Duke Physics major, worked with Abe Clark on several projects. He helped assemble a novel gas-fluidization stage that will allow experiments on special granular particles that are effectively gravity-free. In fact, this apparatus was designed and built by Siyuan Sun, at the time, a Duke Physics undergraduate, and now Harvard graduate student. Colton then helped Abe with a novel granular impact experiment that allows us to probe what happens if a heavy object, like a meteorite, strikes the surface of the earth. And, before leaving, he put together a different apparatus that was used by one of the high school students to study how friction might affect a phenomena in granular materials known as jamming.
The student who then ran with this apparatus, acquiring novel data on shear-jamming, was David van Vleet. Abe Clark was also his mentor. David's results are really very valuable to the Duke group and to scientists elsewhere, who want to understand jamming. Ryan Pasca, teamed with mentor Joshua Dijksman, worked on adapting novel, nearly transparent, granular particles that are made from hydrogels for experiments in our lab. These particles are special for a number of reasons, but most particularly because they can be used in a special laser-scanning apparatus, developed by Joshua. This apparatus can track detailed motion of grains as we modify the granular system. The tracking process requires that the index of refraction of particles and surrounding fluid are very well matched. Ryan carried out systematic studies to find out the best way to match the particle and fluid indices of refraction, while varying properties of the fluid. Also working with Joshua was Kirby Hoekstra, but on something very different. Kirby worked studies to investigate how drops spread on a surface, and in particular, how this process is affected by the viscosity of the fluid, and also by rotation of the surface. In fact, the process by which drops spread on a rotation surface is very important in the manufacture of electronic chips. Daniel Kong worked with Abe Clark to understand a very different and novel granular problem that arises when something like a lunar lander approaches the moon's surface. In such a situation, the braking, or retro-rockets on the lander cause significant cratering of the lunar soil or regolith. The nature of this process is of great concern and interest to NASA. Experiments at Duke have been able to characterize this cratering process by using controlled gas jets, and to show that for strong enough jets, there is an instability that may have important consequences for a real lunar lander. The question addressed by Daniel was, what is the best mathematical model to characterize the cratering process? Daniel acquired data on crater formation, and then showed that there are two useful models for characterizing the crater formation, one that works for short times, and another, adapted from the first, that covers the full evolution of the crater.