Rheology

Question 1

What is rheology?

Answer 1

The study of the flow and deformation properties of matter

Question 2

What is viscosity?

Answer 2

the resistance of a fluid to flow or movement

Question 3

what is the coefficient of dynamic viscosity?

Answer 3

-the constant of proportionality where the rate of flow (gamma)-shear rate is proportional to the applied force (sigma)-shear stress

Question 4

what is shear stress?

Answer 4

-the force applied (N) per unit of surface (m2) parallel to the fluid layers (i.e force per lateral area )

Question 5

what is shear rate?

Answer 5

the rate at which the velocity of flow (m/s) decreases depending on the distance (m) from the point in which the force is applied

Question 6

what is a newtonian fluid?

Answer 6

a fluid with a constant viscosity and has zero shear rate at zero sheer stress
-the shear rate is directly proportional to the shear stress

Question 7

state all the expressions of viscosity…

Answer 7

-kinematic viscosity
-relative viscosity
-specific viscosity
reduced viscosity

Question 8

what is kinematic viscosity?

Answer 8

-the resistance to floe of a liquid under gravity

Question 9

what is the expression for kinematic viscosity?

Answer 9

K=n/p dynamic viscosity/density

Question 10

what is the expression for relative viscosity?

Answer 10

nrel=dynamic viscosity/dynamic viscosity of the solvent

Question 11

what is the expression for specific viscosity?

Answer 11

nrel-1

Question 12

what is the expression for reduced viscosity?

Answer 12

nsp/C(% w/v)

Question 13

what does Poiseulle’s law state for a single fluid flowing though a capillary tube?

Answer 13

-that the viscosity= radius of capillary x time of flow x pressure difference/8 x length of capillary x volume of liquid

Question 14

with poiseuille’s law how can you compare two fluids with the came viscometer?

Answer 14

-you can divide the equation applied to one liquid by the equation applied to the second one

Question 15

poiseuille’s law

Answer 15

𝑡1𝜌1𝜂2/
𝜂1 =
𝑡2𝜌2

Question 16

what are examples of newtonian fluids?

Answer 16

-pure gases
-pure liquids
-solutions of low MW molecules

Question 17

what are examples of non-newtonian fluids?

Answer 17

majority of pharmaceutical systems

Question 18

what happens in materials with a plastic or Bingham flow?

Answer 18

-the rheogram does not pass through the origin
-the rheogram intersects the y axis in a point defined as yield value
-the fluid does not flow until a minimum stress of σy is applied
- If the shear stress is < σy the material
behaves as a solid

Question 19

what is plastic/bingham flow?

Answer 19

-where materials behave as solids until a certain stress threshold is exceeded after which they flow like a fluid

Question 20

give examples of materials with a plastic/bingham flow?

Answer 20

-concentrates suspensions
-dispersions with flocculated particles

Question 21

what happens in the pseudo plastic flow?

Answer 21

-the material flows as soon as shear stress is applied
-the relationship between shear stress and shear rate is not linear
-these are shear thinning fluids

Question 22

what property changes with the shear stress in materials with a pseudoplastic flow?

Answer 22

-viscosity changes so we look at the apparent viscosity at a defined shear stress value and temperature

Question 23

give examples of materials with a pseudo plastic flow…

Answer 23

-solutions of natural or chemically modified hydrocolloids (tragacanth,methylcellulose)
-synthetic polymers (polyacrylic acid,PVP)

Question 24

what happens to long chains in a pseudo plastic flow?

Answer 24

-at rest, the long chains in solution are entangled,with immobilised solvent

Question 25

what happens as a solution with a pseduoplastic flow undergoes sheer stress?

Answer 25

-the long chain =s disentangle, align and flow one past the other, freeing the solvent and reducing the viscosity of the system

Question 26

what happens in a dilatant flow?

Answer 26

-the viscosity increases with an increase in shear stress -happens with shear thickening liquids

Question 27

what are examples of materials with a dilatant flow?

Answer 27

-dispersions of highly concentrated small deflocculated particles, mainly during dispersion preparations -wet sand

Question 28

what does newtons law state?

Answer 28

the flow rate is proportional to the stress applied

Question 29

when plotting the shear rate vs shear stress for a newtonian fluid, how can the dynamic viscosity be obtained?

Answer 29

-by calculating the slope of the line

Question 30

how can the flow properties of a simple or newtonian fluid be determined?

Answer 30

-by the use of a capillary viscometer and a particular one is the Ostwald viscometer

Question 31

what is a Ostwald viscometer used to do?

Answer 31

- to determine viscosity by measuring the time a given volume of liquid takes to flow through a capillary of a given size at a stated temperature and under the influence of gravity -the capillary size varies,the most appropriate one to use would give a flow time above 200 secs

Question 32

what are the factors affecting the rheological behaviour of pharmaceutical liquids and semisolids?

Answer 32

-presence of particles -electrostatic interctions -steric hindrance -polymer entanglement

Question 33

how can the presence of particles affect the rheological behaviour of pharmaceutical liquids and semisolids?

Answer 33

-it can lead to obstacles, longer paths, more resistance, higher viscosity

Question 34

what electrostatic interactions can affect the rheological behaviour of pharmaceutical liquids and semisolids?

Answer 34

-ioniic interactions -van der Waals interactions -hydrogen bonding

Question 35

why can steric hindrance affect the rheological behaviour of pharmaceutical liquids and semisolids?

Answer 35

-changes in the chemical composition can affect the hindrance of the particles

Question 36

what do hydrocolloid do?

Answer 36

-they can affect the rheological behaviour of pharmaceutical liquids and semisolids as they contribute to polymer entanglement by being used as suspending agents or thickeners