Title: Cohesion amongst Grains in Charged Powders

Author (Poster): Charles Lewis, UMass

Abstract:

We present an experimental study of the ionic conductivity of granular materials and its effects on the mechanical properties of a planar granular pile. We apply a net charge on the sample using a corona discharge device to attain surface potentials up to 2,000 V in magnitude. Using hydrophobic sand grains, we find that the sample becomes rigid and cohesive, whereas (untreated) hydrophilic sand grains become unstable. To quantify the rigidity, we apply shear stress on an assembly and measure the yield stress. The yield stress increases with the surface potential. In separate experiments we measured the rate of electrical discharge of charged granular piles. Under a wide range of conditions, we find empirically a characteristic scaling of the surface potential vs time. These findings may point the way to new methods to control the mechanical properties of charged powders. This project is partially supported by the Xerox University Affairs Committee.