Dynamic viscosity : ? (eta)
It is defined by the NEWTON equation: and quantify measurement of internal friction of fluid.
His determination needs to apply to the fluid a Shear rate (D), and to measure the resistant Shear stress (?) to this rotation.
Shear rate : D (shear rate)
is the shearing which subjected by the product in the application. It is known for measurement geometries with small gap. It is not the speed of rotation of the bob (in rpm !).
Either a sheared fluid, by a laminar move (dV), between two parallel plates with a surface (S) and separate by a distance dx.
Shear stress : ? (Tau)
There is the shearing force (F), with which the sample answers to the shear rate (D), divided by the contact surface (S).
There is the « science » of « flow ».
Associated physical measurements, realised with the hand of Rheometers, enables the visualisation of the behaviour of the product in various flow , temperature and time conditions .
Displayed curves of the flow behaviour of a fluid.
The curves ? = f(D) enables, by adapted fitting, the access to direct related parameters with the application.
a – With coaxial cylinders
The fluid is sheared between two coaxial cylinders, with radius Ri and Ra and a length L, by a laminar move which are breaking down in multi-layer with different angular speed from 0 (for the layer in contact with the fixed cylinder) to ?0 (for the layer in contact with the rotating bob). The relative move of layers towards others give, a shear rate D and one Shear stress ?.
By imposing ?0 and measuring M, the resisting torque to this rotation, we calculate D and ? according :
b- With Cone-Plate :
The fluid is placed between a Plate and a Cone with angle ? (< 3° ).
The cone, maintained to a constant speed induce a laminar shearing move. In those conditions, ? and D are constant in the gap, according :
Study of different rheological behaviours
One thixotropic product is a sample for which the variation of viscosity in function of shear rate is associated to a variation trough the time.
Owe talk about Thixotropy or Rheopexy, with the condition of REVERSIBLE Transformations: frozen or solidification.
Causes of thixotropy :
– Molecular structure
– Ball loose Package
– « Château de cartes » with layers
– Particules mixing