Influence of Measuring Geometry on Rheomalaxis of Macrocrystalline Wax–Oil Gels: Alteration of Breakage Mechanism from Adhesive to Cohesive

Abstract

Rheological measurement of wax–oil gel breakage is highly susceptible to the phenomenon of adhesive breakage, hindering instrument-scale replication of cohesive breakage processes. Adhesive breakage measurements are notoriously irreproducible due to strongly nonaffine gel deformation. Efforts to ensure mechanical fixation give rise to spatially inhomogeneous deformation fields in the measuring geometry, particularly with respect to azimuthal and radial location. To elucidate the functional role of mechanically fixating geometries during gel breakage processes, three model solutions were prepared containing 5, 7.5, and 10 wt % macrocrystalline wax in dodecane. Rheograms were acquired in controlled deformation mode at imposed shear rates in the range of 0.1–1.0 s–1 using a vane or a cone and plate geometry. Yield stress values, nominally ascribed to primary peak height, were established based on 95% confidence intervals. Yielding trends confirm that adhesive breakage is particularly pronounced in high solid-fraction gels. A solid-fraction threshold delineates cohesive breakage in low solid-fraction gels from inherent adhesive breakage in high solid-fraction gels. Mechanical fixation in a vane geometry precludes wall slippage, ensuring cohesive breakage; resultant yield stress values follow a modified power-law dependency on the total wax content, characterized by a power-law exponent of ∼1.25. Nonuniform deformation within the vane geometry confers a modest (artificial) reduction in apparent yield stress value as a consequence of azimuthal integration of the torque signal. Nonuniform deformation also confers a distinct (artificial) broadening of the breakage peak and is accompanied by the appearance of a new shoulder peak located at a deformation value of ∼5. Conversely, in the cone and plate geometry, adhesive breakage occurs inherently for high solid-fraction gels and is manifested by a substantial reduction in measured yield stress, albeit without a concomitant peak broadening. Hence, the practical utility of the cone and plate geometry is limited to low solid-fraction gels that inherently exhibit cohesive breakage behavior. Mechanical fixation afforded by the vane geometry effectively precludes wall slippage, enhancing measurement reproducibility while ensuring a cohesive breakage of high solid-fraction wax-gels that otherwise rupture in adhesive mode.