Effect of shape and orientation on rigid particle rotation and matrix deformation in simple shear flow

The rotation behaviour of rigid particles from a wide range of symmetry classes was analysed in a series of three-dimensional analogue experiments, in which the particles were embedded in a transparent linear–viscous matrix deformed to large shear strains. Comparison of the measured trajectories with theoretical periodic paths for rigid ellipsoids shows that quadratic, slightly orthorhombic and monoclinic particles have trajectory paths and rotation rates closest to theory. Orthorhombic and monoclinic particles, with a minimum aspect ratio >1.5, develop non-periodic trajectories. The results establish that the simplified theoretical model for ellipsoidal forms is a good approximation for a wide range of natural particle shapes. The relationship between the rotation of a rigid particle and deformation of the adjacent matrix was also determined in a series of experiments by placing the particle at different depths relative to an imprinted grid. The results are well illustrated by the surface passing through the centre of the particle and initially parallel to the shear plane. Two deformed regions in the matrix are recognised. In the first region, the matrix rotates with the particle, together forming an approximately ellipsoidal object. The second region is dominated by folds, of limited extent along the rotation axis and greatest extent in a direction that rotates asymptotically toward the shear direction. The folds rotate more slowly than the principal direction of maximum finite stretch.