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
How are non Newtonian fluids classified?
According to their DEVIATION from ideal Newtonian behaviour. The 3 classes are; Plastics (Bingham flow), Pseudoplastic flow, Dilatant flow
When does a plastic (Bingham body) flow?
When a critical elastic shear stress is exceeded (at high stresses)
At lower stresses the plastic behaves as a solid elastic type material and does not flow!
What is the yield stress value?
The POINT AT WHICH THE MATERIAL FLOWS!
Yield stress value = Fb symbol
What types of substance usually has plastic flow?
Concentrated suspensions, particularly if the continuous phase is highly viscous and the particles FLOCCULATE (loosely aggregate) eg in toothpaste.
Toothpaste is an example of a non Newtonian fluid; it doesn't come out the tube easily as its highly viscous and doesn't flow well, have the squeeze the tube and exert a force to make it move!!
What is Plug flow and it's advantage/ disadvantage?
Parts of a sample exceed the Bingham yield stress (Fb) while other portions of the sample don't.
This is good because it allows pastes and other viscous materials to be squeezed out of tubes (toothpaste)
Bad because needs a lot of detailed rheological properties to make it this consistency
Remember Plug= toothpaste properties (plug/sink/toothpaste!!)
With plastic systems, when does FLOW occur?
When Bingham yield stress (Fb) is reached!
With Bingham flow (plastic system), what happens once yield stress is exceeded.
The flow behaves in a Newtonian fashion (flows easily above this stress!)
What may Bingham flow be completely described by? What's the formula?
By the PLASTIC VISCOSITY (U) of the system
U= ( σ - f) / y
What do you know about Pseudoplastic flow?
The material with this property will flow as soon as shear stress is applied ( unlike plastics where a certain stress level must be reached)
However fluidity will still increase linearly with increasing the shear rate (shear stress and shear strain as seen on the rheogram)
Also known as SHEAR RATE THINNING (becomes thinner/less viscous)
PHARMACY FLUIDS COMMONLY HAVE THIS PROPERTY
Opposite to Dilatant flow! (shear rate thickening)
With Pseudoplastic flow, what happens to dynamic viscosity with increasing shear rate?
Dynamic viscosity becomes lower and lower (flows more and more easily with increasing shear rate)
What is Pseudoplastic flow exhibited by?
Emulsions & creams,
Aq hydro colloidal dispersions (alginates),
Do materials with Pseudoplastic flow have a single value of viscosity?
NO, Dynamic viscosity becomes increasingly lower with increased shear rate! Changes at different strain rates a shear stresses (shear rate) as seen on the rheogram for Pseudoplastics.
What do you know about Dilatant flow?
This is the opposite type of flow to Pseudoplastic flow.
Viscosity INCREASES with an increase in shear rate!!
These molecules become more viscous upon shearing, therefore volume increases and that's why they're called DILATANT! (they thicken).
Exhibit shear rate thickening.
Less common than plastics/ Pseudoplastics.
Disastrous if this occurs in the body.
Exhibited by dispersions with a high concentration of small deflocculated particles.
With Dilatant mechanism, what happens at zero shear?
Particles are closely packed, low viscosity and the liquid can flow easily at zero shear rate
With Dilatant mechanism, what happens at low shear rates?
This is when you pour the fluid. Particles clump together increasing the viscosity of the system.
With dilatant mechanism, What happens at higher shear rates?
Viscosity increases until a paste-like material forms.
The effect of increasing shear rate and increasing the viscosity is Reversible!!
Tell me about quicksand?
A NON Newtonian fluid
Usually a solid gel, but minor change in stress results in a decrease in its viscosity (Pseudoplastic property) ie when you stand on it you put stress on it, it becomes less viscous and more water- like and you sink!
Can exhibit Dilatant properties after initial perturbation (disturbance ie changes to stress) remember non Newtonian behaviour can be reversible!
What is shear thinning?
Shear thinning leads to a decrease in viscosity.
Molecules tend to align themselves with the direction of flow, therefore when the fluid is moving (being poured) molecules tend to slide over each other more easily reducing viscosity of the substance.
Makes it easier to mix things
What is shear thickening?
This is when viscosity increases.
This is because hydrodynamic forces overcome the repulsive forces between particles allowing them to clump together and aggregate (agglomerate). This forms a rigid material which decreases the ease of flow.
Is behaviour in non Newtonian systems time dependent?
Difference between viscoelastic and thixotrophic?
If recovery after stress exerted is fast= viscoelastic
If recovery after stress exerted is slow = thixotrophic
What does thixotrophicity mean?
To change by TOUCH
a material which exhibits a reversible time dependent decrease in viscosity is thixotrophic
It is the relatively slow recovery of the viscosity lost on shearing (increase or decrease shear rate) on Pseudoplastics, dilatants, and plastics. I.e the time it takes for viscosity to return back to normal as these processes are reversible
How is the extent of thixotrophy quantified?
The area of the hysteresis loop between the ideal Newtonian fluid line and the curved line for the non newtonian fluid on the rheogram ( see slide for diagram)
Measure of thixotrophy = area of loop- kind of potato wedge shape
When do we see thixotrophic behaviour?
In micellar formulations.
In applications of creams and lotions- if viscosity decreases upon shearing ( ie rubbing into skin applying pressure and stress) this makes the application easier. However once it's rubbed in the viscosity slowly returns back to a thicker cream/lotion which stops it feeling wet on your skin. Understanding rheology properties allows you Make application of creams/ lotions better for a patient!
How can the flow properties a fluid be changed?
Flow may be altered by obstacles the fluid passes or the thing the fluid has to flow through (tube) the narrower the area the fluid flows through the slower the flow
What did Reynolds experiment show?
Flow conditions effected by 4 factors:
Diameter of the pipe
Density of fluid
Viscosity of fluid
Velocity of fluid
How do you work out Reynolds number (Re)?
Re= ρ x u x d / η.
ρ= fluid density
Remember this is inertial force / viscous force(viscosity)
What does Reynolds number explain?
Expresses the ratio of the inertial forces to the viscous forces (inertial force/ viscous force)
Predicts the type of flow that will occur in a particular situation (laminar, turbulent, transitional flow)
it is DIMENSIONLESS
For a cylindrical pipe, What does a value below 2000, above 4000 and between 2000-4000 for Reynolds number mean??
Below 2000= streamline / laminar flow occurs
Above 4000= turbulent flow
2000-4000= transitional flow
Differences between laminar, transitional and turbulent flow?
Laminar; happens with small pipes, water of low velocity, flow quite slow, with small volume, straight stream of water
Transitional; mixture of laminar and turbulent flow, turbulent in middle of pipe, laminar at edges
Turbulent flow; vortices, eddies (currents formed at certain viscosities) and wakes make the flow unpredictable. Turbulent flow happens in general at high flow rates and with larger pipes.
Turbulent and laminar flow can be predicted with Reynolds number
What are the ways you can measure Newtonian flow?
•Falling Sphere Techniques
•Simple Rotational Viscometer (Concentric Cylinders)
If the viscosity of a solution stays the same when shaken up, what type of solution must it be?
A Newtonian solution (viscosity not effected by stress!)
If heat is produced when a material returns back to its original shape after deformation, what type of material have we got?
What is waters dynamic viscosity ?
1.002 this is ideal
Don't need to learn equations just have a rough idea of them!!
Look back at diagrams from lecture and include in answers for better marks!!
What happens when you walk on Pseudoplastic?
It gets thinner as viscosity decreases!
What is best for drugs, viscous or non viscous?
Better to have less viscosity as the blood doesn't like viscous things going into it as it will disturb its flow