Disperse systems 5

Question 1

Viscosity

Answer 1

The resistance of a fluid to flow/ movement
The more viscous the liquid, the greater the force required to make it move at a given rate
Viscosity is the internal friction of a fluid, produced by the movement of the molecules against each other

Question 2

Factors influencing viscosity

Answer 2

Size of molecule
Shape of molecule
Chemistry of molecule
Concentration of molecule
Nature of solvent
Temperature

Question 3

Assumptions of Einstein equation

Answer 3

It is a dilute suspension
The particles are rigid and spherical
There are no interparticulate interactions
The liquid behaves in a simple predictable way
Brownian motion effects are unimportant

Question 4

Temperature

Answer 4

As the temperature of a liquid increases, the viscosity decreases
As the temperature of a gas increases, its viscosity also increases

Question 5

Newtonian flow

Answer 5

Substance continues to flow in the same way, regardless of the forces acting on them
Their behaviour is simple and predictable, so they behave as expected when disturbed or agitated; when pushed or pulled they move or change shape in proportion to the force applied
Water is Newtonian because it continues to behave in the same way regardless of how fast it is stirred or mixed

Question 6

Non-Newtonian systems

Answer 6

They do not have a constant viscosity i.e. their viscosity changes with the applied shear force
Their properties change in response to movement and deformation

Question 7

Plastic flow

Answer 7

Plastic behaviour is characterised by a minimum force beyond which the material flows
Seen in concentrated suspensions with flocculated particles and high viscosity continuous phases

Question 8

Pseudoplastic flow

Answer 8

Viscosity decreases with the rate of shear force
Shear thinning fluid
Flocculated dispersions containing long, high molecular weight molecules

Question 9

Dilatant flow

Answer 9

Viscosity increases with the rate of shear
Shear thickening fluid
Dispersions containing a high concentration of small, deflocculated particles

Question 10

Rheology in pharmacy

Answer 10

Manufacturing issues: mixing, passing through machinery, pouring/ packing bottles
Patient issues: physical stability, ease of use, safety
During storage a high viscosity is required to prevent/ decrease sedimentation, in addition to this a low viscosity on shaking is preferred