Tauraj Ehtizazi Flashcards
What is Newtonian system?
Within a block of liquid the bottom layer is fixed, there is friction between the layers of the liquid. So when stress is applied the top layer will slide, and layers below will follow as there is friction present
What is shear rate?
Difference in velocity divided by the difference in height - s-1
What is shear stress?
Horizontal force applied per unit area - Nm-2
Pressure - vertical force
What is viscosity?
The ratio of shear stress to shear rate
Shear stress divided by shear rate
What are the units of viscosity?
Poise (P) Or centipoise (cP) 1cP = 0.01P
What is Newtonian fluid?
Viscosity depends on the temperature and not the shear rate or time
Viscosity does not change by prolonged or vigorous shaking (shear rate)
Linear graph which passes through the origin
What is a rheogram?
A plot of shear stress against shear rate for a liquid
What is pseudoplastic flow?
Fluid will flow as soon as shear stress is applied
Viscosity decreases by increasing shear rate (shaking)
Non-linear but passes through the origin
A low polymer concentration is which type of fluid?
Newtonian fluid
A high polymer concentration is which type of fluid?
Non-Newtonian fluid
What is dilatant flow?
Viscosity increases by increasing shear rate (shaking)
Non-linear but passes through the origin
How does dilatant flow occur?
At rest particles are closely packed and inter-particle voids are at a minimum which the vehicle can fill
But during flow the volume of solid material increases so now there is not enough fluid to fill the voids
What is Bingham or plastic flow?
Fluid will only flow once shear stress is above a threshold value
Viscosity usually stays constant
Linear but does not pass through the origin so there is a yield value
How can you determine the rheological properties of liquids?
Use a bob and cup viscometer
What is the usual speed of the bob in a bob and cup viscometer?
0.01-600 rpm
What does an upward curve mean in a rheogram?
Increasing shear rate increases shear stress
What does a downward curve mean on a rheogram?
Decreasing shear rate decreases shear stress
Upward and downward curves for non-Newtonian and pseudoplastic fluids are identical? True or False
False - Newtonian and pseudoplastic fluids have identical upward and downward curves
Are the upward and downward curves on a thixotropic fluid identical?
No they are not identical
What is thixotropic flow?
Viscosity depends on shear rate and its duration
The more shear rate applied over a prolonged time the less viscous the fluid
What is the best fluid system to implement in a suspension?
Thixotropy - when bottle is shook viscosity is lowered
Disperses particles so uniform dose given
Particles maintained in suspended state so harder to sediment out
What is the best system to implement in a parenteral formulation?
Thixotropy - fluid structure breaks down upon injection
consistency is recovered when in the muscle
sustained release
BUT NEVER DILUTE THE FLUID
What is relative viscosity?
Ratio of viscosity of solution to viscosity of solvent
What flow type has a Reynolds number below 2000?
Laminar
What flow type has a Reynolds number above 4000?
Turbulent
What flow type has a Reynolds number between 2000-4000
Transitional
