rheology

Question 1

what is rheology

Answer 1

study of flow properties of materials

Question 2

what is viscosity

Answer 2

resistance of a liquid to flow when it is subjected to stress; usually mentioned as dynamic viscosity

Question 3

how to count shear stress

Answer 3

force/ area

Question 4

how to count shear rate

Answer 4

= velocity gradient = dv/dr

Question 5

how to count viscosity

Answer 5

shear stress/shear rate

Question 6

what are newtonian liquids

Answer 6

liquids that have constant viscosity

- flow curve (rheogram) shows straight line passing through origin

Question 7

plastic flow

Answer 7

bingham flow:

• behaves as an elastic solid at low shear stress
• a certain shear stress equivalent to yield value must be exerted before appreciable flow begins
• at shear value above yield value- resembles newtonian system
• apparent viscosity dependent on shear rate
• liquid experiencing plastic flow are shear thinning

Question 8

how to count plastic flow

Answer 8

plastic viscosity= (shear stress - yield value ) / shear rate

Question 9

psuedoplastic flow

Answer 9

• liquid flow as soon as shear stress is applied
• viscosity decreases when increasing shear rate
• flow curve tends otwards linearity at higher shear stress
• liquid exhibiting psuedoplastic flow are shear thinning
• apparent viscosity dependent on shear rate

Question 10

what is thixotropy

Answer 10

decrease in apparent viscosity upon application of shearing forces (shear thinning), slow recovery of apparent viscosity on standing
- desirable property to have in suspensions and emulsions

Question 11

type of viscometer for netwonian liquids

Answer 11

capillary viscometers- utube, suspend level

Question 12

type of viscometer for non-newtonian liquids

Answer 12

rotational viscometer - concentric cylinder, cone and plate

Question 13

advantage of suspend level (ubbelohde) viscometer

Answer 13

1. no need fill viscometer with precise volume of liquid

2. can be used for very viscous liquids (x10 more than utube)

Question 14

precaution of capillary viscometer

Answer 14

1. ensure streamline flow by avoiding vibration and maintaining a minimum flow time
2. liquid should be free from air bubble
3. ensure viscometer is clean before use
4. ensure viscometer is clamped vertically
5. ensure accurate temperature control

Question 15

problems of concentric cylinder viscometeres

Answer 15

1. shear rate of test liquid across the gap is not constant
2. end effects can be significant
3. frictional heating is significant at high shear rate
4. filing is difficult if gap is small
5. large volume of test liquid is required if the gap is big

Question 16

main advantage of cone and plate viscometer

Answer 16

1. volume of test liquid needed is small
2. filling and cleaning is easy
3. shear rate is constant throughout the test liquid