Shape fabrics of particles in low concentration suspensions: 2D analogue experiments and application to tiling in magma

Jefferyʹs equations ascribe a theoretical cyclic nature to the shape fabric of non-interacting rigid particles immersed in a viscous fluid undergoing simple shear flow. This theoretical behaviour is confirmed at ‘low’ shear strains (γ < 6) by two-dimensional experiments in a torsion apparatus, inducing shape fabric development of particles evenly distributed on the surface of a silicon fluid and at low particle concentrations (13–14% in area). For larger shear strains however (6 < γ < 20), the shape fabric orientation tends to remain close to the shear plane, its magnitude remains at low values and the cyclicity of the fabric disappears. This is due to interactions between particles, forming tiling features with variable shape ratios. Interactions rapidly increase in number for γ > 5 (first experiment: 134 identical particles) or γ > 1 (second experiment: 178 particles with two size classes), then become stable at 17% (first experiment) and at more than 50% (second experiment) of the population of particles. Due to the contribution of the tiled particles, the shape fabric becomes asymmetrical in its orientation distribution, with a maximum lying above the shear plane. The latter result provides a new shear sense indicator, in addition to the statistical determination of the tiled features. The study also suggests that crystalline fabrics in magmas could be acquired at high melt fractions, i.e. early in the crystallization history of the magma.