Hydrodynamic transport properties of concentrated suspensions

Viscosity Model for Nanoparticulate Suspensions Based on Surface Interactions

In this paper, a widely mechanistic model was developed to depict the rheological behaviour of nanoparticulate suspensions with solids contents up to 20 wt.%, based on the increase in shear stress caused by surface interaction forces among particles. The rheological behaviour is connected to drag forces arising from an altered particle movement with respect to the surrounding fluid. In order to represent this relationship and to model the viscosity, a hybrid modelling approach was followed, in which mechanistic relationships were paired with heuristic expressions. A genetic algorithm was utilized during model development, by enabling the algorithm to choose among several hard-to-assess model options. By the combination of the newly developed model with existing models for the various physical phenomena affecting viscosity, it can be applied to model the viscosity over a broad range of solids contents, shear rates, temperatures and particle sizes. Due to its mechanistic nature, the model even allows an extrapolation beyond the limits of the data points used for calibration, allowing a prediction of the viscosity in this area. Only two parameters are required for this purpose. Experimental data of an epoxy resin filled with boehmite nanoparticles were used for calibration and comparison with modelled values.

Complex Fluids and Hydraulic Fracturing

Nearly 70 years old, hydraulic fracturing is a core technique for stimulating hydrocarbon production in a majority of oil and gas reservoirs. Complex fluids are implemented in nearly every step of the fracturing process, most significantly to generate and sustain fractures and transport and distribute proppant particles during and following fluid injection. An extremely wide range of complex fluids are used: naturally occurring polysaccharide and synthetic polymer solutions, aqueous physical and chemical gels, organic gels, micellar surfactant solutions, emulsions, and foams. These fluids are loaded over a wide range of concentrations with particles of varying sizes and aspect ratios and are subjected to extreme mechanical and environmental conditions. We describe the settings of hydraulic fracturing (framed by geology), fracturing mechanics and physics, and the critical role that non-Newtonian fluid dynamics and complex fluids play in the hydraulic fracturing process.

Rheology of corn stover slurries at high solids concentrations--effects of saccharification and particle size

The rheological characteristics of untreated and dilute acid pretreated corn stover (CS) slurries at high solids concentrations were studied under continuous shear using plate-plate type measurements. Slurry rheological behavior was examined as a function of insoluble solids concentration (10-40%), extent of pretreatment (0-75% removal of xylan) and particle size (-20 and -80 mesh). Results show that CS slurries exhibit shear-thinning behavior describable using a Casson model. Further, results demonstrate that the apparent viscosity and yield stress increase with increasing solids concentration (which corresponds to a decrease in free water). Dilute acid pretreatment leads to lower viscosity and yield stresses at equivalent solids concentrations, as does smaller particle size. Taken together, these findings are consistent with the hypothesis that the availability of free water in the slurry plays a significant role in determining its rheological behavior. In particular, as the free water content of the slurry decreases, e.g., with increasing solids concentration, the greater interaction among particles likely increases the apparent viscosity and yield stress properties of the slurry. The results also suggest that the availability of free water, and thereby slurry rheological properties, depend on the chemical composition of the corn stover as well as its physical characteristics such as particle size and porosity. Hydrophilic polymers within the cell wall, such as xylan or pectin, or larger pores within bigger particles, facilitate sequestration of water in the solid phase resulting in decreased availability of free water. Thus, dilute acid pretreated slurries, which contain smaller size particles having significantly lower xylan content than slurries of untreated milled stover, exhibit much lower viscosities and yield stresses than untreated slurries containing large particles at similar solid concentrations.