22 Questions
What is the reciprocal of the slope of the line in a stress-strain graph?
Viscosity (ŋ)
What type of fluid does not follow a linear relationship between stress and strain?
Non-Newtonian fluids
What is the characteristic of plastic fluids?
No flow occurs until a certain transition point (yield value) is reached
What is the mechanism behind the behavior of plastic fluids?
A certain force is needed to break the bonds between particles before flow begins
What is the characteristic of pseudo-plastic fluids?
High viscosity at rest, but flows immediately after shaking (stress)
What is the mechanism behind the behavior of pseudo-plastic fluids?
Rearrangement and orientation of polymer chains or building blocks under stress
What is the characteristic of dilatant fluids?
Increase in viscosity with stress
What type of suspensions typically exhibit dilatant behavior?
Suspensions with high solid content (usually more than 50%)
What happens to the viscosity of a thixotropic material when it is subjected to shear stress?
The viscosity of a thixotropic material decreases with the application of shear stress.
What is the term for the time-dependent reversible viscosity of a material?
Thixotropy
Why is thixotropy a desirable property in pharmaceutical applications?
It allows for sustained drug delivery in suspensions and emulsions.
What is the effect of temperature on the viscosity of liquids?
Viscosity of liquids decreases with temperature.
What is the effect of temperature on the viscosity of gases?
Viscosity of gases increases with temperature.
What is the role of solvent in the mechanism of non-Newtonian fluids, specifically dilatant fluids?
The solvent fails to remain in contact with the particles due to high solute content.
What is the relationship between thixotropy and the stability of suspensions?
Thixotropy is related to the stability of suspensions, as it affects the stability of the suspension over time.
What is an example of a pharmaceutical application that utilizes thixotropy?
Intramuscular depot injections.
What is rheology and what does it describe?
Rheology is the study of the flow of liquids and the deformation of solids.
What is stress in the context of rheology?
Stress is the force per unit area.
What is viscosity and what does it measure?
Viscosity is the resistance of gases or fluids to flow due to shear stress.
What is the difference between Newtonian and non-Newtonian fluids?
Newtonian fluids have a linear relationship between stress and strain, while non-Newtonian fluids have a non-linear relationship.
What is fluidity and how is it related to viscosity?
Fluidity is the reciprocal of viscosity, and it represents the ease of flow of a fluid.
What is the assumption made by Newton about the viscosity of materials?
Newton assumed that viscosity is independent of the shear rate at a given temperature.
Study Notes
Rheology
- Rheology is the study of the flow of liquids and the deformation of solids
- It is involved in the mixing and flow of materials, their packaging, and the pouring from containers
Flow
- Flow occurs when stress is applied to a material
- Stress is the force per unit area, and strain is the amount of deformation caused by the shear stress
- Viscosity is the resistance of gases or fluids to flow due to shear stress
- Fluidity is the inverse of viscosity, and the slope of the line on a graph is fluidity
Materials (Fluids)
- Newtonian fluids have a linear relationship between stress and strain
- Non-Newtonian fluids have a non-linear relationship between stress and strain
Newtonian Fluids
- Newtonian fluids have a viscosity that is independent of the shear rate
- The slope of the line on a graph is viscosity (ŋ)
Non-Newtonian Fluids
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Plastic fluids have a yield value, which is the minimum shear stress required for the material to deform and begin to flow
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Yield value affects the quality of creams, such as a skin cream
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Mechanism: a certain force is needed to break the bonds between particles before it starts to flow
-
Pseudo-plastic fluids (Shear Thinning fluids) have a high viscosity at rest but flow immediately after shaking (stress)
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Mobility and viscosity change along the line
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Mechanism: polymer chains or building blocks of the substance rearrange and orient according to the direction of the applied force, decreasing viscosity and increasing flow
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Dilatant fluids (Shear Thickening fluids) have a viscosity that increases with stress
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Suspensions with high solid content (usually more than 50%) usually have this property
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Mechanism: sudden formation of vacuum gaps in between cluster of material particles, leading to an increase in viscosity and decrease of flow
Thixotropy
- Thixotropic materials have a time-dependent reversible viscosities
- They are shear thinning materials that return to their pre-shear conditions when stress is removed
- The decrease in viscosity with application of stress is also time-dependent
- This property is useful in pharmaceutical applications, such as in suspensions and emulsions for sustained drug delivery systems
Factors Affecting Viscosity
- Intrinsic factors: molecular size, particle shape, and intermolecular forces
- Extrinsic factors: temperature and additives
- Viscosity of liquids decreases with temperature, while viscosity of gases increases with temperature
This quiz covers the introduction to rheology, the study of the flow of liquids and deformation of solids, and its importance in physical pharmacy, including its impact on patient acceptability.
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