Whither compressional rheology?

Macroscopic dynamics of flocculated colloidal suspensions

“…propose an irreversible plastic constitutive model for the isotropic component of T N under uniaxial compression, which effectively encodes negligible recoverable (elastic) volumetric strain with strain hardening. In reality flocculated colloidal suspensions consolidate poroelastically with negligible elastic strain and irreversibly strain harden (Buscall, submitted for publication-a; Lié tor- Santos et al, 2009;Manley et al, 2005;Kim et al, 2007), which in 1D is indistinguishable from plastic failure (Stickland and Buscall, 2009) and hence consistent with the original BW theory. Conversely, the yield of the particulate network under shear involves failure at a critical (and very small) elastic strain, beyond which the shear stresses are borne as a hydrodynamic viscosity as quantified by (21).…”

“…Moreover, the shear-rate controlled nature of these experiments probe suspension behavior after yield has occurred, and hence probe the multidimensional flow characteristics of the suspension. As such, in contrast to Stickland and Buscall (2009), we argue that a yield surface model may still quantify the limit of elastic strain borne by the suspension; however, once yield occurs, a classic plastic model does not capture the full dynamics of combined shear and compression.…”

“…However, in general, suspensions are subject to combined shear and normal stresses and so generalized tensorial constitutive models are required. Presently much is unknown as to the rheology of flocculated colloidal suspensions under combined stresses (Stickland and Buscall, 2009), and so at this juncture it is necessary to speculate as to the most probable behavior, whilst bearing in mind the implications of such assumptions or otherwise. Superposition of both shear and compressive stresses admits three distinct scenarios:…”

“…The pioneering combined shear and compression experiments carried out by Channell (2000) at fixed shear rates suggest that the apparent compressive yield strength is maintained at a level determined by the shear rate, which appears to contradict the notion of yield surface altogether (Stickland and Buscall, 2009). In contrast, a plastic yield model would predict total collapse of the suspension to the close packing limit so long as the shear flow is maintained (Stickland and Buscall, 2009).…”

“…In contrast, a plastic yield model would predict total collapse of the suspension to the close packing limit so long as the shear flow is maintained (Stickland and Buscall, 2009). In these experiments, the shear velocity gradient is oriented normal to the axial direction of compression in a Taylor-Couette device, and so it is plausible that annular shear banding may be occurring, which would explain the shear-rate dependence of the apparent compressive yield strength.…”