Flashcards in PA20285 MEDICINES DESIGN; rheology Deck (68)
What is the definition of rheology??
The study of the DEFORMATION and the FLOW of matter
What is the DEBORAH NUMBER equation?
The relaxation time of a material / time of the process being observed. De= Tow/ T
What does the Deborah number show?
It characterises the fluidity of materials under specific flow conditions
What was the Pitch Drop experiment used to illustrate?
Illustrates that some substances that appear to be solid are actually very high viscosity fluids. Pitch (a viscoelastic solid polymer, a plant) flows at a very slow rate and it can take several years for a single drop to form. Only 8 drops have fallen over 8 years!
Why is rheology important in pharmacy?
It's all about flow properties and viscosity, understanding the rheology of a substance is essential for the preparation and dosage form of a drug.
The viscosity of a drug is important as it may alter the dissolution process and absorption of the drug.
Viscoelastic properties has a link to a drugs bioavailability and it's function.
Remember viscosity links to BIOAVAILABILITY!
What are examples of Newtonian fluids?
Water , air, liquid paraffin, glycerol
Examples of Non Newtonian fluids?
Mostly High viscous fluids eg. Ketchup, cornflour, oobleck
What are Hookean solids used for?
Used for predicting the stress - strain behaviour of materials undergoing large deformations ie with forces exerted on them (Mass placed on them)
What is stress & it's formula & units?
Stress = force per unit area
σ = F/A (force divided by area)
units are Nm^2 or Pa (pascals)
What is strain, it's formula and units?
Strain is RELATIVE EXTENSION
y= dL / L
Or y= Lo - L / Lo
Strain is DIMENSIONLESS ( no units) as its a ratio of 2 like properties
What does Youngs Modulus (E) measure and what's the equation?
Measures stiffness of an elastic material. It's the ratio of stress over strain in the range that Hookes law holds.
If you plot stress against strain the gradient of the line = E
E = stress / strain = σ / y
What do ideal / elastic solids do? (when deformed)
When deformed under stress they spring back to their original shape instantaneously (eg stretching an elastic band)
What do viscoelastic solids do?(when deformed)
When deformed these take time to return to there original shape. They return as a function of time. dt= time it takes.
What happens when you deform a plastic?
It will not return back to its original shape ( eg plastic containers don't change shape once moulded)
What is the main property that distinguishes a fluid from a sold?
FLUID CANNOT MAINTAIN A SHEAR STRESS FOR ANY LENGTH OF TIME
If a shear stress is applied to a fluid, it will move under shear
What is viscosity?
The measure of ease with which a fluid moves/ yields
Dynamic Viscosity describes a fluids internal resistance to flow
Less viscous = greater ease of movement
What does dynamic viscosity describe?
The shear forces which exist in a moving liquid and is a measure of the internal resistance (friction of the fluid)
What gives rise to shear stress?
A FORCE acting over an area, A, gives rise to shear stress across the liquid.
The liquid responds to this applied stress by flowing.
What gives rise to shear rate?
If a force is applied on a liquid it will start to flow. A velocity gradient forms between the surface of the liquid and the surface it is on. This velocity gradient gives us shear rate.
VISCOSITY= the ratio of shear stress to shear rate
Formulas for shear stress and shear strain?
Shear stress ( σ ) = F/ A
Shear strain = dv/ dt
dv is the flow velocity
dt is the thickness of the liquid layer
F is the tangential force applied to the upper most layer of the liquid
What is Newtons law of a perfect Newtonian liquid?
F/ A = η x (dv/dt)
η is the coefficient of viscosity
1/ η= y/ σ = Φ. Φ= fluidity!
What is a Flow curve/ rheogram?
The plot of strain rate vs shear stress
The gradient of the line is fluidity
1/ gradient is the coefficient of viscosity
Newtonian liquid/ fluid? (plot)
Plot of shear stress to strain rate is LINEAR with Newtonian liquids/ fluids
Non - Newtonian liquid/ fluid? (plot)
Plot of shear stress to strain rate is NON LINEAR for non Newtonian liquids/ fluids
What does shear stress quantify?
The MAGNITUDE of force acting on a certain area driving a flow process ( making something flow)
What does shear rate quantify?
Used to quantify the speed of flow deformation.
Pharmacists measure shear rate as a function of shear stress using a flow curve/ rheogram
What are Newtonian fluids completely described by?
The coefficient of viscosity (η)
What are the coefficient of viscosity ( also known as dynamic viscosity) units??
Nm^2.s or Pa.s = 10 Poise ( P )
Why do most pharmaceutical fluids not obey Newtonian behaviour?
This is because viscosity of solution (η) VARIES with shear (y), they are not equal as seen with newtons law!! η is not equal to y, but varies with it.
This variation is due to the make-up of pharmaceutical fluids as they may contain; suspensions, emulsions, gels, polymer based systems