Physical Pharmacy: Rheology

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Questions and Answers

What is the relationship between stress and strain in a linear relationship?

Directly proportional

What is the physical property represented by the reciprocal of the slope of the line in a linear relationship between stress and strain?

Viscosity (ŋ)

What type of fluid does not exhibit a linear relationship between stress and strain?

Non-Newtonian fluids

What is the characteristic feature of plastic fluids?

Yield value Signup and view all the answers

What is the mechanism behind the flow of plastic fluids?

Breaking of bonds between particles Signup and view all the answers

What is the characteristic feature of pseudo-plastic fluids?

Shear thinning Signup and view all the answers

What is the mechanism behind the flow of pseudo-plastic fluids?

Rearrangement and orientation of polymer chains Signup and view all the answers

What type of fluid exhibits an increase in viscosity when subjected to shear stress?

Dilatant fluids Signup and view all the answers

What does the term 'rheology' describe?

The flow of liquids and the deformation of solids Signup and view all the answers

What happens to the viscosity of a thixotropic material when shear stress is applied?

The viscosity of a thixotropic material decreases when shear stress is applied. Signup and view all the answers

What type of fluids are dilatant fluids?

Non-Newtonian fluids Signup and view all the answers

What is the force per unit area that causes fluid movement?

Stress (Shear stress) Signup and view all the answers

What measures the resistance of a fluid to flow?

Viscosity Signup and view all the answers

Why is thixotropy a desirable property in pharmaceutical applications?

It allows for sustained drug delivery systems. Signup and view all the answers

What is the relationship between stress and strain in a Newtonian fluid?

Linear Signup and view all the answers

What is the relation between thixotropy and the stability of suspensions?

Thixotropy is related to the stability of suspensions and emulsions. Signup and view all the answers

What is the characteristic of a fluid with a non-linear relationship between stress and strain?

Non-Newtonian Signup and view all the answers

How does temperature affect the viscosity of liquids?

The viscosity of liquids decreases with temperature. Signup and view all the answers

According to Newton's assumption, what is the relationship between force and flow rate in a fluid?

Twice the force would move the fluid twice as fast Signup and view all the answers

How does temperature affect the viscosity of gases?

The viscosity of gases increases with temperature. Signup and view all the answers

What is the effect of molecular size on viscosity?

Larger molecular size increases viscosity. Signup and view all the answers

What happens to the viscosity of a thixotropic material when the stress is removed?

The viscosity returns to its original state. Signup and view all the answers

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

Plastic fluids have a yield value, which is the minimum shear stress required for the material to deform and begin to flow
Yield value affects the quality of creams, such as a skin cream
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)
Mobility and viscosity change along the line
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
Dilatant fluids (Shear Thickening fluids) have a viscosity that increases with stress
Suspensions with high solid content (usually more than 50%) usually have this property
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

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Description

Learn about rheology, the study of the flow of liquids and deformation of solids, its importance in pharmacy, and its impact on patient acceptability.