Strain-rate- and capillary-number-dependent deformation of weak viscous particles

Abstract

Traditional strain analysis assumes that a particle deforms affinely with its matrix and that the magnitude and geometry of particle deformation are representative of the bulk strain. However, there are forces, lumped together under the terms of competency or viscosity contrast, which oppose particle deformation. For small weak particles, surface tension may also be important in resisting particle deformation. We performed analogue model experiments to investigate the effect of surface tension on the deformation of a weak viscous particle (bubble or droplet) enclosed in a viscous matrix. The experiments were performed using a ring-shear apparatus allowing large magnitudes of plane strain, simple shear. The results demonstrate the controlling influence of capillary number on particle deformation. The particles did not continuously elongate with increasing shear strain but achieved a steady-state value dependent on capillary number. Because capillary number represents the ratio of shear stresses, which deform particles, to the surface (interfacial) stresses restraining elongation, there should be a strong dependence of particle strain on the deformed particle size and bulk strain rate. The most important geological implications of this study are in the field of melt segregation in deforming anatectic systems.