Complex Systems and Nonlinear Dynamics
My research interests are in nonlinear dynamics, complex systems, and materials science. Specifically, my experiments have focused on granular materials and solidification interfaces.
While giving a tour to a middle school class, I asked if anyone knew what granular materials were. Although I didn't necessarily expect a response, one student connected the term to "granulated sugar" and correctly guessed that we were talking about a pile of grains. In fact, granular materials are ubiquitous -- important examples including the storage of corn in silos, the mixing of powders for pharmaceuticals, and the transport of gravel, sand, and concrete in construction. You could probably come up with 10 more examples without too much effort. In fact, their abundance and commercial applications are part of the reason that there is an estimated trillion dollar economy in their use. It's been estimated that processing granular materials accounts for around 10% of the world's energy consumption. The collapsing silo on the left is not as uncommon as you might think.
Despite this, they are still surprisingly mysterious. The fact that something as simple as sand can both flow through an hourglass and support our weight in the beach (and even act as a gas in vibrated systems) alludes to the fact that granular systems do not strictly act as one state of matter. There are still deep physical questions to be answered. Some of these are related to the anisotropic stress network in which fluctuations are as large as the mean, the strong history dependence and energy dissipation, and the connection to a broader class of "jammed" systems.
The upshot is that we don't even have good equations to decribe sand flowing out of a bucket, despite the fact that these materials are present in numerous situations in nature and industry.
Photoelastic MeasurementsOne unique feature of granular systems is the complex force network that transmits stress through the system. On the right is an experimental image using photoelastic grains between crossed polarizers. Bright regions indicate large forces. These force networks are inhomogeneous and strongly history dependent. It is an indication that a statistical approach is necessary, but also that understanding the force network is fundamental to understanding the behavior of granular materials at the grain scale.
The ultimate goal of this research is to formulate a general theory of granular flow. Current experiments focus on:
- Measuring the force network during vibrated flows in order to probe the jamming/unjamming transition.
- High speed video measurements of the jamming/unjamming transition in silo and avalanche flows.
Follow this link for a list of publications.
If you are a JMU student interested in getting involved in experimental physics research (or just want to talk about getting research experience in science), please stop by or drop me an email.
Additional details can be found at my previous web page, Duke granular page