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Questions and Answers
What is the definition of fluidity, φ?
What is the definition of fluidity, φ?
- The sum of viscosity and density
- The ratio of viscosity to temperature
- The product of viscosity and density
- The reciprocal of viscosity (correct)
What is the unit of kinematic viscosity?
What is the unit of kinematic viscosity?
- Centistoke
- Poise
- Stoke (correct)
- Pascal
What is the viscosity of castor oil at 20°C?
What is the viscosity of castor oil at 20°C?
- 0.1 cp
- 100 cp
- 10 cp
- 1000 cp (correct)
What happens to the viscosity of a liquid when the temperature is raised?
What happens to the viscosity of a liquid when the temperature is raised?
What is the equation that describes the temperature dependence of viscosity?
What is the equation that describes the temperature dependence of viscosity?
What is the characteristic of the majority of fluid pharmaceutical products?
What is the characteristic of the majority of fluid pharmaceutical products?
What is the origin of the term 'rheology'?
What is the origin of the term 'rheology'?
What is viscosity an expression of?
What is viscosity an expression of?
Which type of dispersions have rheologic properties that can be expressed by a single value?
Which type of dispersions have rheologic properties that can be expressed by a single value?
What is a significant application of rheology in pharmaceutical products?
What is a significant application of rheology in pharmaceutical products?
What is affected by the rheologic properties of a pharmaceutical system?
What is affected by the rheologic properties of a pharmaceutical system?
How are materials classified according to their types of flow?
How are materials classified according to their types of flow?
What is a characteristic of a plastic flow system?
What is a characteristic of a plastic flow system?
What is the main difference between plastic and pseudoplastic flow systems?
What is the main difference between plastic and pseudoplastic flow systems?
What is the relationship between the rate of shear and the viscosity of a pseudoplastic substance?
What is the relationship between the rate of shear and the viscosity of a pseudoplastic substance?
What can be obtained from the slope of the tangent to the consistency curve of a pseudoplastic material at a specified point?
What can be obtained from the slope of the tangent to the consistency curve of a pseudoplastic material at a specified point?
What is the reason for the existence of a yield value in a plastic flow system?
What is the reason for the existence of a yield value in a plastic flow system?
What is a characteristic of the consistency curve of a pseudoplastic material?
What is a characteristic of the consistency curve of a pseudoplastic material?
What is a characteristic of substances that exhibit dilatant flow properties?
What is a characteristic of substances that exhibit dilatant flow properties?
What is the term for the phenomenon where a material recovers its consistency over time after shearing?
What is the term for the phenomenon where a material recovers its consistency over time after shearing?
What is the difference between the upcurve and downcurve in a rheogram of a pseudoplastic material?
What is the difference between the upcurve and downcurve in a rheogram of a pseudoplastic material?
What is the behavior of a system when N approaches 1?
What is the behavior of a system when N approaches 1?
What is the key factor that influences the rheograms of thixotropic materials?
What is the key factor that influences the rheograms of thixotropic materials?
What is the term for the phenomenon where the downcurve is identical to the upcurve in a rheogram?
What is the term for the phenomenon where the downcurve is identical to the upcurve in a rheogram?
What is the characteristic of a rheopectic system?
What is the characteristic of a rheopectic system?
What is the limitation of single-point instruments in determining rheological properties?
What is the limitation of single-point instruments in determining rheological properties?
What is the difference between capillary and falling-sphere viscometers?
What is the difference between capillary and falling-sphere viscometers?
What is the purpose of a multipoint instrument in rheology?
What is the purpose of a multipoint instrument in rheology?
What is the characteristic of a gel in a rheopectic system?
What is the characteristic of a gel in a rheopectic system?
Which type of viscometer can be used with both Newtonian and non-Newtonian systems?
Which type of viscometer can be used with both Newtonian and non-Newtonian systems?
What is a characteristic of a system that exhibits rheopexy?
What is a characteristic of a system that exhibits rheopexy?
Why are multipoint instruments necessary for non-Newtonian systems?
Why are multipoint instruments necessary for non-Newtonian systems?
What is the relationship between shear rate and shearing stress in a Newtonian system?
What is the relationship between shear rate and shearing stress in a Newtonian system?
What is the main difference between rheopectic and antithixotropic systems?
What is the main difference between rheopectic and antithixotropic systems?
Which type of viscometer can only be used with Newtonian systems?
Which type of viscometer can only be used with Newtonian systems?
What is the purpose of operating a viscometer at a variety of shear rates?
What is the purpose of operating a viscometer at a variety of shear rates?
Why are single-point instruments virtually useless for non-Newtonian systems?
Why are single-point instruments virtually useless for non-Newtonian systems?
What is the result of extrapolating a line through a single point on the rheogram to the origin?
What is the result of extrapolating a line through a single point on the rheogram to the origin?
Which type of instrument is capable of producing a complete rheogram for non-Newtonian systems?
Which type of instrument is capable of producing a complete rheogram for non-Newtonian systems?
What is the relationship between the shear rate and the rheogram in a non-Newtonian system?
What is the relationship between the shear rate and the rheogram in a non-Newtonian system?
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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.
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