page : continuous mixing comparison, 5. on Because of vacuum that naturally develops above a hopper outlet when the voids in fine powders expand as the material discharges, the resulting counter flow of gas will hinder the solids flow. A sample of powder is placed in a cell and then pre-sheared that is, the sample is consolidated by exerting a normal stress, and then sheared until the measured shear stress is steady. He received his B.S.