Abstract
The objective of this research is to advance the understanding of the effect of electrical clamping forces on the mechanics of particulate solids. These forces are produced when an electric field is applied to a densely-packed bed of particles by two electrodes which are in direct contact with the particles. Particle-electrode contacts and inter-particle contacts cause electric current to flow through the bed of particles. At the contact points, the current is constricted to flow through small regions of contact. Due to the high electrical resistance near the point of contact, the local electric field across the surfaces surrounding the contact area is greatly enhanced. Hence, the electrical clamping force is generated across the surfaces. These forces play a significant role in a number of industrial applications of electric fields. In particular, the Electromechanical Valve for Solids, which was developed at the University of Surrey for controlling the flow of granular materials, is considered in this work. Using existing models for the electrical clamping force at a single contact, the interactions between the electrical and mechanical forces in bulk particle assemblies of cohesionless glass spheres are analysed. Wall and internal yield stresses are analysed by transforming the microscopic single contact electrical force into macroscopic representations of electrical stress. Using a continuum mechanics approach, the electrical stresses are superimposed on the mechanical stresses as additional external stresses. The important assumption here is that the yield locus remains unchanged with application of the electric field. This approach does not provide a satisfactory description of the bulk failure of the particulate solids as the electrical stresses are substantially overpredicted by theory as compared to the experimental results. The single contact electrical clamping force models are also used to analyse the formation of arches under influence of an electric field. The investigation of arching is undertaken because halting of flow in the Electromechanical Valve for Solids occurs by the formation of stable arches over the electrode openings. The Mohr-Coulomb yield criterion requires the existence of cohesion to support the formation of stable arches. Therefore, it is speculated that the electrical clamping force may act as a cohesive force rather than an external compressive body force on the particle. Analysis of arching in a wedge-shaped hopper using both approaches is inconclusive. A close look at the models of the electrical clamping force reveals that the discrepancy between the theoretical predictions and experimental results may be due to the assumption in the models of Hertzian contact deformation in response to the electric field. In view of the difficulties with the existing models, it is decided to make a direct evaluation of the electrical clamping force experimentally. The electrical clamping forces at single contacts have been measured here for the first time. These measurements required the development of a technique which uses a modified nanoindentation device capable of measuring very small forces. The detachment forces between glass spheres and a polished stainless steel plate with an electric field have been measured. The experimental results are one to two orders of magnitude smaller than predicted by the models, corroborating the macroscopic yield stress results. It is concluded that the single contact models do not satisfactorily describe the contact deformation under an electric field and hence are unreliable. Attempts are made to quantify the contact area by inference from measurements of electrical resistance using the Sphere Resistance theory. However, this method is highly sensitive to the value of surface resistivity, which cannot be measured accurately, and is therefore unsuitable for evaluation of the contact area.