The rheology and phase structure of steady uniaxial compaction

We present first-principles calculations using large-scale computer simulations, and a theoretical analysis of the rheology and microstructure of a particulate model undergoing the process of uniaxial compaction. Such a fundamental process underlies many areas of science and engineering, covering many length scales. If the particles are of the scale of atoms or molecules then the system models, as well as the shock wave 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 nonequilibrium Molecular Dynamics modelling plays in the understanding and improvement of these processes is elucidated. We present numerical results from a ‘reference’ system of monodisperse hard spheres, with the minimum number of well-defined system and system-state parameters which uniquely specify the compaction process. We also present theoretical methods for relating this reference system to systems with additional or more complex interactions. We find that the processing and material properties are closely related. Uniaxial compaction processes display a linear behaviour at low extension rates, however with strong non-Newtonian rheology. The system obeys Troutonʹs rule at very low extension rates. At high extension rates, the compaction process becomes nonlinear, and processing timescales become comparable to natural relaxation times in the material. As a result, complex materials, such as a colloidal glass, can be formed. Other microstructural reorderings of the particles, such as a nonequilibrium perturbation of the equilibrium freezing transition, are also seen, and summarised in an effective phase diagram