40 Questions
What is the definition of fluidity, φ?
The reciprocal of viscosity
What is the unit of kinematic viscosity?
Stoke
What is the viscosity of castor oil at 20°C?
1000 cp
What happens to the viscosity of a liquid when the temperature is raised?
It decreases
What is the equation that describes the temperature dependence of viscosity?
ƞ = Ae^(EvRT)
What is the characteristic of the majority of fluid pharmaceutical products?
They are non-Newtonian systems
What is the origin of the term 'rheology'?
From the Greek 'rheo' meaning 'to flow' and 'logos' meaning 'science'
What is viscosity an expression of?
The resistance of a fluid to flow
Which type of dispersions have rheologic properties that can be expressed by a single value?
Simple liquids
What is a significant application of rheology in pharmaceutical products?
Formulating products with acceptable consistency and smoothness
What is affected by the rheologic properties of a pharmaceutical system?
Patient acceptability, physical stability, and biologic availability
How are materials classified according to their types of flow?
Into 2 classes: Newtonian and Non-Newtonian systems
What is a characteristic of a plastic flow system?
A yield value exists due to the contacts between adjacent particles.
What is the main difference between plastic and pseudoplastic flow systems?
Pseudoplastic systems are composed of polymers in solution, while plastic systems are composed of flocculated particles.
What is the relationship between the rate of shear and the viscosity of a pseudoplastic substance?
The viscosity of a pseudoplastic substance decreases with increasing rate of shear.
What can be obtained from the slope of the tangent to the consistency curve of a pseudoplastic material at a specified point?
The apparent viscosity of the material.
What is the reason for the existence of a yield value in a plastic flow system?
The force of flocculation between adjacent particles.
What is a characteristic of the consistency curve of a pseudoplastic material?
The curve begins at the origin.
What is a characteristic of substances that exhibit dilatant flow properties?
High concentration of small, deflocculated particles
What is the term for the phenomenon where a material recovers its consistency over time after shearing?
Thixotropy
What is the difference between the upcurve and downcurve in a rheogram of a pseudoplastic material?
The upcurve shows a higher consistency than the downcurve
What is the behavior of a system when N approaches 1?
The system becomes increasingly Newtonian
What is the key factor that influences the rheograms of thixotropic materials?
Rate of shear and time of shear
What is the term for the phenomenon where the downcurve is identical to the upcurve in a rheogram?
Newtonian behavior
What is the characteristic of a rheopectic system?
It forms a gel more readily when gently shaken
What is the limitation of single-point instruments in determining rheological properties?
They are unable to provide a complete rheogram
What is the difference between capillary and falling-sphere viscometers?
One is a single-point instrument, the other is a multipoint instrument
What is the purpose of a multipoint instrument in rheology?
To determine the flow properties of a non-Newtonian system
What is the characteristic of a gel in a rheopectic system?
It is the equilibrium state
Which type of viscometer can be used with both Newtonian and non-Newtonian systems?
Cup-and-bob viscometer
What is a characteristic of a system that exhibits rheopexy?
It forms a sol more readily when gently shaken.
Why are multipoint instruments necessary for non-Newtonian systems?
Because they can produce a complete rheogram.
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 main difference between rheopectic and antithixotropic systems?
The equilibrium state of the system.
Which type of viscometer can only be used with Newtonian systems?
Falling-sphere viscometer.
What is the purpose of operating a viscometer at a variety of shear rates?
To produce a complete rheogram.
Why are single-point instruments virtually useless for non-Newtonian systems?
Because they cannot produce a complete rheogram.
What is the result of extrapolating a line through a single point on the rheogram to the origin?
A complete rheogram.
Which type of instrument is capable of producing a complete rheogram for non-Newtonian systems?
Multipoint viscometer.
What is the relationship between the shear rate and the rheogram in a non-Newtonian system?
The rheogram is a complex function of the shear rate.
Study Notes
Rheology
- Rheology is the study of the flow and deformation of materials, including liquids and solids.
- The term "rheology" comes from the Greek "rheo" meaning "to flow" and "logos" meaning "science".
Newtonian Systems
- Newtonian systems are materials that exhibit a linear relationship between shear stress and shear rate.
- The viscosity of a Newtonian fluid is constant and independent of shear rate.
- Examples of Newtonian fluids include water, olive oil, and castor oil.
Non-Newtonian Systems
- Non-Newtonian systems are materials that do not exhibit a linear relationship between shear stress and shear rate.
- There are three types of non-Newtonian systems: plastic, pseudoplastic, and dilatant.
Plastic Flow
- Plastic flow occurs when a material requires a certain amount of shear stress to initiate flow, known as the yield value.
- Once the yield value is exceeded, the material begins to flow and its viscosity decreases with increasing shear rate.
- Plastic flow is often observed in suspensions with flocculated particles.
Pseudoplastic Flow
- Pseudoplastic flow occurs when a material's viscosity decreases with increasing shear rate, but does not exhibit a yield value.
- Pseudoplastic flow is often observed in polymers and gums in solution.
Dilatant Flow
- Dilatant flow occurs when a material's viscosity increases with increasing shear rate.
- Dilatant flow is often observed in suspensions with deflocculated particles at high concentrations.
Thixotropy
- Thixotropy is a phenomenon where a material's viscosity decreases with increasing shear rate and then returns to its original viscosity when the shear rate is reduced.
- Thixotropy is often observed in pseudoplastic systems.
Rheopexy
- Rheopexy is a phenomenon where a material forms a gel more readily when gently shaken than when allowed to form the gel while the material is kept at rest.
Determination of Rheological Properties
- Rheological properties can be determined using various viscometers, including capillary, falling-sphere, cup-and-bob, and cone-and-plate viscometers.
- Single-point instruments are suitable for Newtonian materials, while multipoint instruments are necessary for non-Newtonian materials.
Important Equations
- η = F / G (Newton's law of flow)
- U = F - f / G (plastic flow equation)
Units of Viscosity
- The unit of viscosity is the poise (P), where 1 P = 1 dyne sec/cm².
- A more convenient unit for most work is the centipoise (cp), where 1 cp = 0.01 P.
Test your knowledge of Rheology, a branch of physics that deals with the flow and deformation of materials. This chapter covers Newtonian and Non-Newtonian systems, different types of flow, and the determination of rheologic properties. Rheology is an important concept in pharmaceutical applications.
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