UNC-CH Physics and Astronomy PhD Defense

Patrick Doyle

“Impedance Study of Wet Granular Systems”

Granular systems are collections of macroscopic particulate material. A pile of sand at the beach, an hourglass, or Saturn’s rings are common examples that demonstrate the scale and ubiquity of these systems. It has been argued that they should be considered an additional state of matter because they behave differently from the more familiar solids, liquids, and gases. These systems have an additional freedom to involve other material within the intergranular space. In particular, the introduction of water has dramatic consequences for the granular system’s properties. Adding water to the pile of sand on the beach, for instance, allows it to be shaped into a sandcastle.

Granular systems that possess water also have very different electrical properties from the dry system. This concept is the basis for our work. A large body of literature exists which describes significantly enhanced dielectric and conductive properties found in these systems. The strengthened response is a result of interfacial polarization which creates areas of concentrated free charge around grain surfaces, giving the system significant polarizability on sub-GHz time scales. Our work takes advantage of these water-enabled electrical enhancements and amplifies them further by using highly polarizable metal grains. Metal grains can be mixed with insulating material to create systems that produce a controlled response against a well-defined background. The contrast can be employed in the study of other systems such as remote geological fractures, or it can be used to study properties of the granular system itself. I will discuss both of these ideas, along with some recent work detecting structural transitions in percolative systems.