rheology

Question 1

Define rheology

Answer 1

the study of flow and deformation of matter

Question 2

Why is rheology important to pharmaceutics?

Answer 2

Flow behaviour of systems change when exposed to different stress conditions. This affects formulation stability, biological activity and patient acceptability

Question 3

define flow

Answer 3

Flow is the tendency of a material to deform following the application of force

Question 4

How can deformation be measured?

Answer 4

By the application of shear stress (F) at a shear rate (G).

Question 5

What is viscocity?

Answer 5

The resistance to flow that a material offers.

Question 6

What is the relationship between flow and viscosity?

Answer 6

The higher the viscosity of a material, the more shear stress is needed to deform it at a shear rate.

Question 7

Define shear stress

Answer 7

The force per unit area required to bring about deformation

Question 8

What is the shear rate?

Answer 8

The rate at which a material is being deformed

Question 9

Describe newtonian flow

Answer 9

Shear stress increases in a linear fashion upon increasing the shear rate
- viscosity does not change with increased shear rate.

Question 10

Describe dilatant flow

Answer 10

Also known as shear thickening, this involves an increase in viscosity when increasing shear rate.
This occurs as it involves high concentrations of undissolved particles suspended in a liquid, so application of force displaces solid particles and causes themm to clump.

Question 11

Describe pseudoplastic flow

Answer 11

Also known as shear thinning, application of force to a material will cause viscosity to decrease. This is as networks of these materials will be displaced with force - where these is an initial high viscosity.

Question 12

Describe behaviour of bingham plastics and pseudoplastics

Answer 12

These materials require a initial application fo force before demonstrating flow behaviour.
Bingham plastics show newtownian flow properties after this inital force and bingham pseudoplastics behave as they normally would after an initial application of force.

Question 13

What is thixotropy?

Answer 13

This is the ability of materials to regain their initial structural integrity after the application of force.
This is time dependent.

Question 14

What is the loop of hysteresis

Answer 14

This shows the difference between flow and thixotropy, where a larger loop represents a slower rate of thixotropy, meaning that the product has experienced greater deformity.

Question 15

Describe the application of shear force, flow and thixotropy in context.

Answer 15

Example:
A ketchup bottle can be shaken (application of force) to cause it to become less viscous (pseudoplastic flow) and be squeezed out of the bottle. After leaving the bottle it will regain its initial viscosity (thixotropy)

Question 16

How does viscosity affect formulation stability, biological activity and patient acceptability?

Answer 16

Formulation stability: more viscous = sedimentation of undissolved particles.
Biological activity: more viscous = slower movement of drug out of formulation
Patient acceptability: less viscous = easier to pour and swallow.

Question 17

How can flow behaviour be tested?

Answer 17

Viscometry is used where a spindle is immersed into a substance and is rotated at different shear rates. This then gives out a reading of viscosity.