60 Questions
What is the term defined as the reciprocal of viscosity?
Fluidity
What is the unit of kinematic viscosity?
Stoke
What is the viscosity of water at 25°C?
0.8904 cp
What happens to the viscosity of a liquid as temperature is raised?
It decreases
What is the equation that expresses the dependence of the viscosity of a liquid on temperature?
ƞ = Ae^(EvRT)
What is the characteristic of the majority of fluid pharmaceutical products?
They are non-Newtonian systems
What is the literal meaning of the term 'rheology'?
Science of flow
What is the primary characteristic of Newtonian systems?
Their rheologic properties can be expressed by a single value
Why is rheology significant in pharmaceutical products?
It influences patient acceptability, physical stability, and biologic availability
What type of flow does a dilatant system exhibit?
Shear-thickening flow
What is the purpose of determining rheologic properties in pharmaceutical products?
To produce products with acceptable consistency and smoothness
What is the significance of analytic methods of rheology in pharmaceutical product development?
They provide valuable information for formulating better pharmaceutical products
What is the hysteresis loop in a rheogram?
The area between the up curve and down curve of the rheogram
Why is it important to consider the time of shear, rate of shear, and degree of structure in the sample when measuring thixotropy?
Because the rheogram for the same product is different depending on these factors
What is the thixotropic coefficient B, as mentioned in the text?
The rate of breakdown with time at constant shear rate
What is the purpose of calculating the plastic viscosities U1 and U2 in the equation for the thixotropic coefficient B?
To determine the rate of breakdown with time at constant shear rate
What is the main difference between the two approaches to estimate the degree of thixotropy in plastic systems?
The first approach uses a constant shear rate, while the second approach uses increasing shear rates
What is the significance of the area of the hysteresis loop in measuring thixotropy?
It represents the degree of thixotropic breakdown
What is the primary difference between rheopexy and antithixotropy?
The equilibrium state of the system
Why are single-point instruments insufficient for characterizing non-Newtonian systems?
They cannot provide a complete rheogram
What is the main advantage of multipoint instruments over single-point instruments?
They can operate at a variety of shear rates
Which type of viscometer is suitable for use with both Newtonian and non-Newtonian systems?
Cup-and-bob viscometer
What is the relationship between shear rate and shearing stress in a Newtonian system?
Shear rate is directly proportional to shearing stress
What is the purpose of extrapolating a line through a single point on the rheogram to the origin?
To complete the rheogram for a Newtonian system
What information is obtained from the difference in plastic viscosities for two separate downcurves?
Thixotropic coefficient
At what shear rates does the rheologic analysis of magnesia magma exhibit negative thixotropy or antithixotropy?
Greater than 30 sec-1
How does dilatancy differ from antithixotropy in terms of solid dispersed phase volume?
Antithixotropic systems contain more than 50% solid phase
What is the formula to calculate the thixotropic coefficient, M, based on the plastic viscosities U1 and U2?
M = (U1 - U2) / ln(U2)
When observing normal thixotropy, where does the downcurve appear in relation to the upcurve?
To the left of the upcurve
How does antithixotropy differ from rheopexy?
Antithixotropy is a decrease in consistency, while rheopexy is an increase
What is the most suitable way to compare pseudoplastic systems?
By comparing their entire consistency curve
What is the significance of the exponent N in the equation FN = ƞ ‘G?
It indicates the flow becomes increasingly non-Newtonian
What is the characteristic of dilatant flow?
The material increases in volume when sheared
What is the difference between pseudoplastic and dilatant systems?
Pseudoplastic systems are shear-thinning, while dilatant systems are shear-thickening
What can be used to describe dilatancy in quantitative terms?
The equation FN = ƞ ‘G
What is the term used to describe materials that exhibit an increase in resistance to flow with increasing rates of shear?
Dilatant
What is rheology used to describe?
The flow of liquids and the deformation of solids
What type of systems can be described in terms of absolute viscosity?
Simple liquids
What is the significance of rheology in pharmaceutical product development?
All of the above
What are the two main classes of materials based on types of flow?
Newtonian and non-Newtonian systems
Why is it important for manufacturers to understand rheology in producing medicinal and cosmetic products?
To ensure the products have a consistent texture and smoothness
What is the relationship between viscosity and flow in a fluid?
The higher the viscosity, the lesser the flow
What is the primary method of obtaining the area of hysteresis in a rheogram?
Using a planimeter or other suitable technique
What is the significance of the time of holding before reducing shearing stress in a plastic system?
It affects the degree of structure in the sample
What is the relationship between the plastic viscosities U1 and U2 in the equation for the thixotropic coefficient, M?
U1 and U2 are calculated from different downcurves
What is the purpose of determining the structural breakdown of a plastic system possessing thixotropy when subjected to increasing shear rates?
To estimate the degree of thixotropy
What is the characteristic of a plastic system that exhibits thixotropy when subjected to a constant rate of shear?
The viscosity decreases with time
What is the equation that relates the thixotropic coefficient, M, to the plastic viscosities U1 and U2?
M = ln(U1 / U2)
What is a characteristic of materials that exhibit viscoelasticity?
Both viscous properties of liquids and elastic properties of solids
Which of the following pharmaceutical areas is rheology significant in?
Fluids
What is an example of a pharmaceutical product that exhibits viscoelasticity?
Creams
In which of the following pharmaceutical areas is rheology important for mixing and particle-size reduction?
Fluids
What is a significant aspect of rheology in quasisolids?
Spreading and adherence on the skin
In which of the following pharmaceutical areas is rheology important for the flow of powders from hoppers?
Solids
What type of flow is typically exhibited by polymers in solution?
Pseudoplastic flow
What does the yield value in a suspension indicate?
Force of flocculation
In what kind of system does the consistency curve begin at the origin?
Pseudoplastic system
What is the impact of increasing the rate of shear on the viscosity of a pseudoplastic substance?
The viscosity decreases
Why does a pseudoplastic material not have a yield value like a plastic system?
As there are no flocculated particles in the system
How can an apparent viscosity be calculated in a pseudoplastic material?
By obtaining the slope of the tangent at any specified point
Study Notes
Rheology
- Rheology is the science of the flow and deformation of materials, including liquids and solids.
- Viscosity is an expression of the resistance of a fluid to flow, with higher viscosity indicating greater resistance.
Fluidity (φ)
- Fluidity is the reciprocal of viscosity (φ = 1/η).
- It is a measure of the ease of flow of a fluid.
Kinematic Viscosity
- Kinematic viscosity is the absolute viscosity divided by the density of the liquid at a specific temperature.
- Units of kinematic viscosity are stoke (s) and centistoke (cs).
Absolute Viscosities of Newtonian Liquids
- Castor oil: 1000 cp
- Olive oil: 100 cp
- Water: 1.0019 cp (used as a standard for viscosity of liquids)
Temperature Dependence of Viscosity
- Viscosity of a liquid decreases as temperature increases.
- Fluidity increases with temperature.
- The dependence of viscosity on temperature is expressed by an equation analogous to the Arrhenius equation of chemical kinetics.
Non-Newtonian Systems
- Most pharmaceutical products are not simple liquids and do not follow Newton's law of flow.
- Examples include suspensions, emulsions, and gels.
Thixotropy
- Thixotropy is the property of a fluid that becomes less viscous when agitated and more viscous when left still.
- Thixotropy is measured by the area of the hysteresis loop in a rheogram.
Measurement of Thixotropy
- Two approaches to estimate the degree of thixotropy:
- Determine structural breakdown with time of a plastic system possessing thixotropy when subjected to a constant rate of shear.
- Determine structural breakdown of a plastic system possessing thixotropy when subjected to increasing shear rates.
Pseudoplastic Flow
- Pseudoplastic flow is typically exhibited by polymers in solution.
- The consistency curve for a pseudoplastic material begins at the origin, with no yield value.
- Viscosity of a pseudoplastic material decreases with increasing rate of shear.
Dilatant Flow
- Dilatant flow occurs in certain suspensions with a high percentage of dispersed solids.
- Resistance to flow increases with increasing rates of shear.
- Dilatant systems are often termed "shear-thickening systems."
Rheology in Pharmacy
- Rheology is significant in pharmacy in areas such as:
- Fluids: mixing, particle-size reduction, passage through orifices, and physical stability.
- Quasisolids: spreading and adherence on the skin, removal from jars, and release of the drug from the base.
- Solids: flow of powders from hoppers and into die cavities.
- Processing: production capacity and processing efficiency.
Viscoelasticity
- Viscoelastic measurements are based on the mechanical properties of materials that exhibit both viscous and elastic properties.
- Examples of viscoelastic systems in pharmacy include creams, lotions, ointments, and suspensions.
Test your knowledge on rheology, including topics such as Newtonian systems, Non-Newtonian systems, Plastic Flow, and more. Explore the flow of liquids and the determination of rheologic properties in pharmaceutical areas.
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