Evaluation of the single contact electrical clamping force

When bulk particulate solids are subjected to an electric field, a remarkable phenomenon occurs as manifested, e.g. in the electro rheological fluids and in a device developed for controlling the flow of granular materials, the electromechanical valve for solids (EVS). It has generally been established that the resistance to shear deformation is enhanced by the application of an electric field. However, no satisfactory description has yet been given for the microscopic mechanisms that are involved in the process. For the EVS it has been found that it is essential to maintain continuous inter-particle contacts for the satisfactory operation of the device. The flow of current through the bed of particles, albeit very small, is considered to be the key to the mechanism involved. In this paper, the measurements of the electrical clamping force across a contact between a resistive sphere and a conductive plate are reviewed. The results are shown to be consistent with the measurements of the macroscopic bulk shear strength under the influence of an electric field. However, they are in disagreement with the predictions of the models of the electrical clamping force available in the literature. For the latter it is considered that the cause of the discrepancy is due to the shortcoming in assuming that the contact deformation follows a Hertzian pattern. An analysis is presented based on the sphere resistance model, which clearly demonstrates the shortcomings in the models of the electrical clamping force.