Rheolog Flashcards
What is rheology
study of the flow properties of materials
what is viscosity
resistance of a liquid to flow when it is subjected to stress
viscosity = shear stress/ shear rate
What does rheological properties of material affect?
efficiency of mixing
flow through pipes
ease of packaging into and removal from containers
physical stability of preparation
rate of drug absorption
spreading and adherence of preparation to skin
What are the factors that make up shear stress and shear rate?
Shear stress = Force / Area
Shear rate = dv/dr = velocity gradient
What are the 2 types of liquids based on rheological properties?
newtonian and non-newtonian liquids
Newtonian liquids follow the Newton’s Law of Flow, what is the equation telling us about this?
Shear rate is directly proportional to the shear stress
shear stress = n * shear rate
the flow curve of a newtonian liquid shows a straight line passing through the origin.
newtonian liquids have constant viscosity.
When the shear rate is on the y axis and the shear stress is on the x axis,
what does the gradient tell us for a newtonian liquid?
1 / gradient = coefficient of viscosity
what are the 3 types of non-newtonian liquids in the lecture?
Plastic flow (Bingham flow) Pseudoplastic flow Dilatant flow
What are the characteristics of plastic flow (bingham flow)
liquid behaves as an elastic solid at low shear stress, upon applying a certain shear stress Fb, then there will be an appreciable flow.
at shear stress above Fb, the liquid resembles a newtonian system.
shear thinning (i.e the more force u add, the viscosity will fall)
What is Bingham’s equation
U = (F-f) / G where U is plastic viscosity F is shear stress f is yield value G is shear rate
plastic flow is associated with the presence of flocculated particles in a concentrated suspension.
Similarities and differences of plastic and pseudoplastic flow?
Both have shear thinning
Both flow curves tend to linearity at higher shear stress
Pseudoplastic will flow as soon as shear stress is applied whereas plastic need at least Fb.
What is pseudoplastic flow associated/ likened with?
polymers in solutions, the more stress applied, the more disentangled the polymers will be and the liquid will behave like newtonian liquid.
Dilatant flow properties?
Similar to Pseudoplastic, the liquid will flow as soon as shear stress is applied.
Exhibits shear thickening
F^N = n’G
same empirical equation as depicted in pseudoplastic flow. in this case N value is <1
N is the index of dilatancy.
what is dilatant flow associated/ likened with?
high concentrations of deflocculated particles i.e suspensions with high concentration (>50%) of small, deflocculated properties.
why does dilatant flow properties pose a problem in production?
When producing liquids, we use mixers like turbine, propeller and paddle mixers.
At high rotation rates in propeller mixers, liquid displaying dilatant flow will exhibit strong shear thickening and the liquid may ‘freeze’, which hinders the process of manufacturing.
What can affect the degree of structural changes in a non-newtonian liquid system when sheared?
Rate of shear
duration of shear
frequency of shear
The area of hysteresis loop is formed by taking the upcurve and downcurve area loop, what does the area indicate
extent of structural breakdown
bigger the area, the bigger the structural breakdown of the liquid
What are the 2 important factors of thixotropy?
Exhibits shear thinning
Slow recovery of apparent viscosity on standing
Why is thixotropy good for liquid preparations?
Upon standing, viscosity of liquid is high. Based on stokes law, the rate of creaming will be slower.
Upon shaking, drug can be easily homogenised due to shear thinning properties, also makes it easy to pour the drug out.
Liquids that are composed of flocculated particles or long molecules usually exhibit what property?
thixotropy
Difference between kinematic viscosity vs dynamic viscosity?
dynamic viscosity = shear stress/ shear rate
whereas kinematic viscosity = dynamic viscosity / density
Absolute viscosity vs apparent viscosity?
absolute used to measure newtonian fluids
apparent used to measure non-newtonian fluids
Does temperature affect viscosity?
yes
What type of instruments is the best for newtonian liquids.
Since a single point determination is adequate, we can use a one point instrument that operate at a single shear rate i.e capillary viscometer.
What types of instruments can be used for non-newtonian liquids
Since shear rate not directly proportional with shear stress, we need multiple point instruments that operate over a range of shear rates to get a complete rheogram. Example: rotational viscometer
Example of capillary viscometers?
U-tube viscometer
suspended level viscometer
example of rotational viscometer?
concentric cylinder viscometer
cone and plate viscometer
What can we obtain from poiseuilles law to calculate relative viscosity of 2 liquids?
n1/n2 = t1p1/t2p2
since kinematic viscosity: v = n/p
v1/v2 = t1/t2
What are the sizings for U-tube viscometer?
Size A to H
How long should flow time be from size A to size H of U-tube viscometers?
At least 300s for A
at least 200s for all other sizes
What are the sizings for Suspended-level viscometer?
9 sizes:
1, 1A, 2, 2A, 3, 3A, 4, 4A, 5
Size 1 should flow at least 300s
other sizes at least 200s
What are the advantages of Suspended-level viscometer over U-tube viscometer?
no need to fill the viscometer with a precise volume of liquid
can be used for very viscous liquid (20000 to 100,000 mm^2s^-1, 10x higher than that of u-tubes)
what are certain precautions to look out for while using capillary viscometers
Ensure streamline flow by avoiding vibration and maintaining a minimum flow time
liquid should be free from air bubbles
ensure viscometer is clean before use
ensure viscometer is clamped vertically
ensure accurate temperature control (in a water bath)
The searle type of concentric cylinder viscometer is defined by the rotation of the _______ cylinder. Whereas the Couette type is defined by rotation of the _______ cylinder.
Inner, outer.
What are the terms that can be varied to show the change in viscosities in the concentric cylinder viscometer?
angular deflection and angular velocity of the outer cylinder.
What is the most commonly used viscometer for non-newtonian liquids
brookfield viscometer
What are problems of concentric cylinder viscometers?
shear rate of test liquid across the gap is not constant
end effects can be significant
frictional heating is significant at high shear rate
filling is difficult if gap is small
large volume of test liquid is required if the gap is big
Describe the cone and plate viscometer
consists of flat circular plate with a wide angled cone placed centrally above
the angle between plate and cone should be small to avoid edge effect
What terms can be varied to measure the viscosity of the liquid in a cone and plate viscometer?
G = torque on the cone omega = radial velocity of the plate
Main advantages of the cone and plate viscometer
volume of test liquid needed is small
filling and cleaning is easy
shear rate is constant throughout the test liquid
examples of efflux viscometers?
Redwood viscometer and flow cups
function of redwood viscometer?
used in BP to determine the viscosity of a standard solution of tragacanth relative to that of water (as a qualitative test)
How does Efflux viscometers work?
They measure the time taken for a given volume of liquid to flow through a small orifice, gives a comparative result instead of a quantitative result.
