THE RHEOLOGY AND PHASE-STRUCTURE OF STEADY UNIAXIAL COMPACTION

We present first-principles calculations using large-scale computer si mulations, and a theoretical analysis of the rheology and microstructu re of a particulate model undergoing the process of uniaxial compactio n. Such a fundamental process underlies many areas of science and engi neering, covering many length scales. If the particles are of the scal e of atoms or molecules then the system models, as well as the shock w ave studies pioneered by Hoover and Holian, polymer injection molding, for particles of larger scales it describes extrusion, filtration and sedimentation of powders, pastes, or suspensions. The role which none quilibrium Molecular Dynamics modelling plays in the understanding and improvement of these processes is elucidated. We present numerical re sults from a 'reference' system of monodisperse hard spheres, with the minimum number of well-defined system and system-state parameters whi ch uniquely specify the compaction process. We also present theoretica l methods for relating this reference system to systems with additiona l or more complex interactions. We find that the processing and materi al properties are closely related. Uniaxial compaction processes displ ay a linear behaviour at low extension rates, however with strong non- Newtonian theology. The system obeys Trouton's rule at very low extens ion rates. At high extension rates, the compaction process becomes non linear, and processing timescales become comparable to natural relaxat ion times in the material. As a result, complex materials, such as a c olloidal glass, can be formed. Other microstructural reorderings of th e particles, such as a nonequilibrium perturbation of the equilibrium freezing transition, are also seen, and summarised in an effective pha se diagram.