a torsional rheometer apparatus for determining rheological properties of a specimen is provided . a stationary plate and a rotatable plate are in generally coaxial position and structured to receive a specimen therebetween . in one embodiment , at least one of the plates and preferably both have roughened specimen engaging surfaces to serve to reduce undesired slippage between the plate and the specimen . a motor is provided to rotate the rotatable plate and a transducer for monitoring forces applied to the stationary plate and generating output signals to a computer which determines the desired rheological properties are provided . in one embodiment , the roughened surfaces consist of projections extending toward the specimen . where granular material is being evaluated , it is preferred that the roughness of the plate is generally equal to the average size of the granular material being processed . in another embodiment , an air - solid mixture is processed and the roughened portions are pore openings in the plates . air flows through the region between the two pore containing plates to maintain the solid materials in suspension . in yet another embodiment , the base of the stationary plate is provided with a deformable capacitance sensor and associated electronic means .