Non-Newtonian fluids, unlike their more predictable Newtonian counterparts, exhibit behaviours that vary with shear rate and/or time.
In mining, commonly encountered behaviours can have a significant influence on extraction, transportation, and safety considerations:
- Shear-Thinning. The apparent viscosity decreases with increasing shear rate; i.e., the material thins out when it’s sheared faster.
- Thixotropy. Apparent viscosity decreases with time when sheared; i.e., the material thins out over time. This decrease in viscosity is reversible and the material will regain its “strength” when at rest.
- Yield Stress/Viscoplasticity. Viscoplastics behave like a solid until a critical stress is exceeded, above which the material flows like a liquid. If a material is also thixotropic, then this critical yield stress can also change with time and shearing.
- Some less common phenomena – in mining at least – are shear-thickening, rheomalaxis, and viscoelasticity; but still occur in certain situations.
Understanding a material’s rheology is key for a variety of facets in mine planning; basic plant design and equipment selection and sizing depend heavily on the rheology of the slurries being handled. In simulations of dam breach events, material rheology is extremely important; a thick paste-like fluid can have a large lateral spread, while a thixotropic slurry may be more prone to channelization and can flow much farther than expected based on its “unsheared” rheology.
To gain this understanding and characterize a material’s rheology, we perform a range of measurements to determine which non-Newtonian phenomena are present and the magnitude of their impacts. There is no “one standard” measurement that will characterize the rheology of any material, different types of tests are needed for different materials. Moreover, the specific application for which the rheology data will be used dictates what type of measurements are most relevant, e.g., for dam breach modeling the yield stress and viscosity at low shear rates is often more important than the viscosity at high shear rates.