Abstract
Abstract Wall slip or, more usually, liquid-solid phase separation at the boundary wall when measuring the rheological properties of particulate suspensions is normally considered an undesirable source of error. However, exclusion of a structure consisting of multiple particulates at a planar boundary can, in turn, reveal the nature of that structure and the way it interacts with other elements in the dispersion. Using a system of surface-treated ground calcite particles, designed to control lyophilicity, dispersed, respectively, in two comparative liquids, hexadecane (dispersive surface tension component only) and linseed oil (both dispersive and polar surface tension components), the relative wettability of the particulate surface can be studied. The static state is viscoelastic, with the elastic component reflecting the network of interacting forces acting to structure the particles together and/or to trap liquid within the long-range particle-particle matrix. As strain is applied under plate-plate geometry, selected aggregate structures become size-excluded at the wall, leading to a loss of shear coupling with the bulk polydisperse suspension. At high strain, given optimal solids content, this results in a stochastic transition between two discrete stress data sets, i.e. that with full shear coupling and that with only partial coupling. Stress recovery is subsequently monitored as strain is step-wise reduced, and the progress toward loss of the stochastic transient phenomenon, together with its parallel change in magnitude, is used to describe the re-formation of primary agglomerates. Cessation of the phase separation indicates re-build of the close-to-static structure. Under certain conditions it is observed that the cessation may be accompanied by a secondary relaxation of state, indicating the build of a secondary but weaker structure, likened to the well-known dual-level flocculation in aqueous colloidal suspension. Rheo-optical observations using small angle light scattering illumination (SALS) are used to confirm a structure model switching from static (uncoupled with shear) to rotating (fully coupled to the boundary-defined shear) and finally uniformly sheared.
Highlights
The study of suspension rheology is full of examples where experimental measures are taken to ensure accurate, reproducible equilibrium rheometrical conditions
As solids content of the suspension is increased from 20 w/w% to 80 w/w%, it is noticeable how both systems increase in their viscosity remaining separated by a decade until the very highest solids content when they cross over at the highest shear rate
This suggests that the untreated (UT ground calcium carbonate particles (GCC)) sample does not initially undergo any further aggregation/flocculation as a function of solids content before or during the first stages of shear
Summary
The study of suspension rheology is full of examples where experimental measures are taken to ensure accurate, reproducible equilibrium rheometrical conditions. Display properties of particle-particle and particle-liquid interaction, to include flocculation and shear-induced aggregation, and apparent wall-slip [3,4,5]. The phenomenon seen in flocculated and structured colloidal suspension systems is more generally that of solids depletion by size or structure exclusion at the wall, resulting in an enrichment of the liquid phase and depletion of the solid phase, with the result that the observer measures the rheology of the thin liquid-rich layer, which either matches that of the liquid alone, usually Newtonian, or that of the liquid containing fine colloidal particles that have not been size excluded, usually thixotropic [8, 9]. The coupling of shear with the bulk suspension is either lost by potential shear banding or compromised by coupling via a progressive increase in polydispersity (inclusion of coarse component) as a function of distance from the wall; topics, which have been discussed widely in the scientific literature and reviewed by Divoux et al 2016 [8]
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