Rheology

Question 1

What is Rheology?

Answer 1

The science of the characteristics of flow which study the deformation and flow properties of matter

Question 2

What are the main components of Rheology?

Answer 2

Elasticity and Viscosity

Elasticity refers to stickiness or structure; elastic materials (e.g. rubber band)

Viscosity is an expression of resistance of a fluid to flow
Higher viscosity= higher resistance

Question 3

What is fluidity?

Answer 3

The reciprocal of viscosity (1 /η)

Fluidity is additive in an ideal solution

Question 4

What is viscoelasticity?

Answer 4

A phenomenon related to materials that have both elastic behavior and viscous flow (e.g. polymer gel, which at rest retains its shape and upon a mechanical stress, is liquefied and begins to flow)

Question 5

Flow: Newtonian Flow vs Non-Newtonian Flow

Answer 5

Flow is the relationship between viscosity and the applied force

Newtonian flow: viscosity stays constant while the applied force changes (e.g. water)

Non-Newtonian flow: viscosity changes as a function of applied forces

Question 6

Newton’s Law of Flow

Answer 6

The rate of sheer stress is directly related to the shearing stress

Shearing stress =η x sheer rate

Question 7

What is absolute viscosity?

Answer 7

At a certain temperature, regardless of how much sheer stress is applied, it will have the same viscosity value

Question 8

What is kinematic viscosity?

Answer 8

The absolute viscosity divided by the density of the liquid at a specific temperature

Unit= Stokes (s) and centistokes (cs)

Kinematic viscosity= η /ρ

Question 9

What is the effect of temperature on viscosity?

Answer 9

Usually, the fluidity increases as the temperature is raised

Activation energy is required to:

• Move the molecules
• Break intermolecular interactions
• Volume and density

Question 10

What happens in Non-Newtonian Systems?

Answer 10

In many fluids, apparent viscosity changes with changing shear rates (i.e. DO NOT follow Newtons Law of Flow)

Question 11

Examples of Non-Newtonian Systems?

Answer 11

Emulsions
Suspensions
Colloids
Ointments

Question 12

What is plastic flow?

Answer 12

Does not flow until a certain transition point (yield value) is exceeded

Viscosity decreases when shear rate increases

At higher rate of shear, it resembles a Newtonian system
Example: pastes (concentrated suspensions)

Question 13

What is pseudoplastic flow?

Answer 13

No yield value: begin flow instantaneously when shear stress is applied

Nonlinear relationship between shear stress and rate of shear

Viscosity decreases when shear rate increases (shear-thinning system)

Example: ketchup, suspensions/emulsions contain water soluble polymers as viscosity modifiers, suspending agents or emulsifiers

Question 14

What causes pseudoplastic behavior?

Answer 14

Pseudoplastic behavior is associated with the structure formed by the globular confirmation of the polymer (suspending agent,…)

As shearing stress increases, normally disarranged molecules align themselves in the direction of the flow

Increase shearing stress; decrease resistance to flow; decrease in viscosity

Question 15

What is dilatant flow?

Answer 15

A viscosity increase with an increase of shear rate (shear-thickening systems)

Opposite of pseudoplastic systems

Deformation and flow occurs instantaneously

Nonlinear relationship between shear stress and rate of shear

Examples:
Whipping cream
some suspensions with very high solid content

Question 16

What causes dilatant flow?

Answer 16

Increasing rates of shear

Question 17

Thixotrophy phenomenon

Answer 17

A shearing-thinning system has lower viscosity on the downward curve, even though the shear stress is the same
It is due to time for the polymer configuration to be recovered
-Intermolecular structure breaks down when shear rate increases and does not reform immediately when stress is reduced or removed
-Assymetric particles

Question 18

Negative thixotrophy (anti-thixotrophy)

Answer 18

A shearing-thinning system has higher consistency on the downward curve; viscosity increases through shearing

Have low solid content (1-10%), flocculate systems

Question 19

What is an example of a viscosity modifier?

Answer 19

• Cellulose derivatives: carboxymethycellulose
• Sodium alginate: brown seaweed polymer mixer
• Gums: acacia, Xanthan
• Synthetic polymers: poly (acrylic acid) resins, polyethylene glycol, polyvinylpyrrolidone, poloxamers
• Clays: bentonite

Question 20

How to determine Newtonian viscosity?

Answer 20

1. Capillary viscometer (e.g. Ostwald viscometer): measures the time required for the liquid to pass between two marks as it flows by gravity through a vertical capillary tube. One shear stress only
2. Falling-sphere viscometer (e.g. Hoeppler viscometer): viscosity of a liquid is measured from the terminal velocity of a falling sphere

Question 21

How to determine Non-Newtonian viscosity?

Answer 21

• Dynamic viscosity is measured
• General term is rheometer
• Rotational viscometer

1. Cup-and-bob (coaxial cylinder) viscometer
2. Cone-and-plate viscometer

Question 22

Why are all these devices important for Pharmaceutics?

Answer 22

-Rheology of blood
Rheology of mucous
Rheology of mixing
Rheology of dosage forms 
       -dermatology products
       -opthalmic products