Bingham Model or Bingham Equation
A simple rheological model that relates shear stress and shear rate and quantifies yield stress and high-shear viscosity.
Brookfield Viscosity
A simple viscosity value, usually reported in centipoise (cP), obtained with a Brookfield Viscometer, usually fitted with a dip-in spindle but may also be fitted with some specialist accessories such as Cone/Plate, Helipath stand and T-Bar spindles or a concentric cylinder measuring system such as a Small Sample Adaptor or Ultra-Low Accessory. More..
Carreau Model or Carreau Equation
A relative of the Cross Model that is often fitted to viscosity vs shear rate profiles.
Casson Model or Casson Equation
A commonly used rheological model that quantifies yield stress and high shear viscosity, typically used for inks and molten chocolate.
Complex Modulus
The overall resistance to deformation of a material, regardless of whether that deformation is recoverable (elastic) or non-recoverable (viscous).Symbol G*.
Complex Viscosity
Complex Modulus divided by Angular Frequency.
Compliance
The ratio strain/stress. Symbol J
Creep Test
A rheological test method where a constant low stress is applied to a material and it's resulting deformation and flow is monitored over the period of the test.
Cross Model or Cross Equation
A rheological model commonly fitted to viscosity vs shear rate profiles
More about the Cross Model..
Crossover Frequency
In an oscillatory (or dynamic) frequency sweep, the frequency at which the elastic and viscous moduli cross, usually marking the transition from the terminal (viscous) region to the rubbery plateau (elastic) region).
Dilatancy
Shear thickening – non-Newtonian behaviour where viscosity increases with increasing shear rate.
Elastic Modulus
The contribution of elastic (solid-like) behaviour to the complex modulus. Symbol G'.
Herschel Bulkley Model
A rheological model that combines the Power law model with a yield stress variable.
Instantaneous Elastic Compliance
In a creep test, the compliance achieved instantaneously upon the impostion of stress.
Kinematic Viscosity
The dynamic viscosity divided by density. Typically reported in Stokes or centiStokes.
Loss Angle
Another name for Phase Angle
Loss Modulus
Another name for Viscous Modulus. Symbol G".
Loss Tangent
Another name for Tan Delta
Newtonian Fluid
A fluid which exhibits a viscosity that is independent of the current shear conditions.
Non-Newtonian Fluid
A fluid which exhibits a viscosity that is dependent upon the shear conditions.
Ostwald (or Ostwald de Waele) Model
Same as Power law Model
Phase Angle
The phase difference between the stress and strain in an oscillatory test. A measure of the presence and extent of elastic behaviour in a fluid. Symbol δ.
Power Law Model
A useful rheological model that describes the relationship between viscosity or shear stress and shear rate over the range of shear rates where shear thinning occurs in a Non-Newtonian fluid. Quantifies overall viscosity range and degree of deviation from Newtonian behaviour. More about the Power Law Model..
Pseudoplastic
Same as shear-thinning
Relaxation Time
A time constant describing the rate of relaxation of stresses in a material (eg a viscoelastic fluid) that has been deformed to a defined strain.
Rheopexy
Time dependent viscosity increase at constant shear rate – often known as anti-thixotropy.
Shear Rate
The rate of change of shear stress. The velocity gradient perpendicular to the direction of shear flow (dv/dx). Units 1/s or s-1
Shear Stress
The shear force per unit area. Symbol s or t. Units of Pascals (Pa)
Shear Strain
A unit-less quantity, the relative displacement of the faces of a sheared body (for example a layer of fluid) divided by the distance between them.
Shear Thinning
A non-Newtonian fluid which exhibits higher viscosities at lower shear rates and vice-versa.
Storage Modulus
Another name for Elastic Modulus. Symbol G'.
Tan Delta
The tangent of the phase angle – the ratio of viscous modulus to elastic modulus and a useful quantifier of the presence and extent of elasticity in a fluid.
Thixotropy
Time dependent viscosity change at constant shear rate. A thixotropic fluid is a shear-thinning fluid that takes a finite time to reach an equilibrium viscosity following a step change in the applied shear rate.
Vane Spindle
A four or six bladed paddle measuring system that can be employed instead of cones, plates, cylinders or spindles on a rheometer or viscometer. The vane is often used to eliminate slippage and sample damage upon loading.
Viscoelasticity
The phenomenon of exhibiting both elastic (solid-like or energy storing) and viscous (liquid-like or energy dissipating) properties. See here
Viscosity
The resistance of a fluid to flow. In shear deformation viscosity is the ratio of applied shear stress to resulting shear rate. Typcally reported in units of Poise (P) and centipoise (cP) or Pascal seconds (Pa.s) or milliPascal seconds (mPa.s). Symbol, typically h.
Viscous Modulus
The contribution of viscous (liquid-like) behaviour to the Complex Modulus. Symbol G".
Wall slip
A phenomenon seen in dispersions where a low-viscosity slip layer forms at a smooth surface due to localised depletion (wall depletion) of the dispersed phase. Wall slip leads to poor viscosity data at low shear rates but can be eliminated by employing crosshatched (or serrated), roughened or grooved measuring systems. See here for more.
Yield Stress
In a shear-thinning product, the minimum stress that must be applied to initiate significant (i.e. more than creeping) flow and a significant drop in viscosity.
Zero-shear Viscosity
The viscosity at the limit of low shear rate. In essence, the viscosity a product will ultimately attain when at rest and undisturbed.
