Rheology Characteristics
- Rheological characteristics are based on the viscosity, viscosity shear dependency and plasticity of a given substance. Viscosity represents the ease with which the layers of a fluid or semi-fluid substance move past each other. Maple syrup, for example, has a higher viscosity than water. Scientists measure viscosity in a unit called the poise. A poise is defined in terms of the shear rate of a fluid. Shear is the amount of force required to move the layers of the measured fluid past each other. Shear rate is the rate of change in this shear stress, or force. The viscosity of a substance that requires a shear stress of one dyne per square centimeter per second, for example, is one poise. Mazola corn oil has an approximate viscosity of 100 centipoise, or one poise. Finally, if a substance has plastic properties, or plasticity, it behaves differently when different amounts of stress are applied to it. A Bingham plastic -- a theoretically ideal plastic material -- behaves like a solid when it is static, or motionless. When the yield value of a Bingham plastic is exceeded, however, the plastic flows. Ketchup is a Bingham plastic. Yield value is the amount of force that must be applied to a plastic substance before flow will occur.
- Newtonian fluids behave in predictable ways. Water and olive oil, for example, are Newtonian fluids. The relationship between viscosity and shear rate is constant in Newtonian fluids. As soon as shearing stops in a Newtonian fluid, all stress in the fluid drops to zero. Further, the relationship between shear stress and shear rate is fixed. As shear rate increases, shear stress also increases in a linear, or straight-line, fashion. Fluids that do not have these characteristics are non-Newtonian. Non-Newtonian fluids are the focus of rheology.
- There are three types of time-dependent, non-Newtonian fluids or substances: pseudoplastic, dilatant and Bingham. Pseudoplastic fluids show a decreasing viscosity with increasing shear rate. The shear rate of a pseudoplastic fluid also increases at a greater rate as shear stress increases. This is also referred to as sheer-thinning. Paints are known to exhibit pseudoplastic characteristics. In dilatant fluids, viscosity increases with shear rate and shear rate increases with shear stress, but the rate of increase for both, though initially high, eventually slows. Sand-and-water mixtures and clay slurries, for example, behave in this way: They are referred to as sheer-thickening. The viscosity of a Bingham plastic, though initially high, rapidly decreases to a low level as shear rate increases. Shear rate and shear stress, on the other hand, show a linear relationship in a Bingham plastic. As soon as a Bingham fluid has reached its yield value and started to flow, it can behave as a dilatant, a pseudoplastic or a Newtonian fluid.
- The characteristics of time-dependent non-Newtonian fluids derive from viscosity as related to time when shear rate is held constant. Thixotropic fluids lose viscosity over time at constant shear rates. This is a characteristic of yogurt, greases and other substances that are gels at rest but become a fluid under stress. Thixotropic fluids are often found in the food and chemical industries. Rheopectic fluids, on the other hand, show an increase in viscosity over time at a constant shear rate. Rheopectic fluids are rare. Gypsum paste is an example.