Rheology of liquids

Question 1

difference between rheology and viscosity

Answer 1

study of the flow properties of materials vs resistance of a liquid to flow when it is subjected to stress

Question 2

formula for viscosity

Answer 2

viscosity (n) = Shear stress/Shear rate

n = F/A divided by dv/dr

Question 3

what’s the difference between Newtonian and non-newtonian fluid?

Answer 3

newtonian = constant gradient = constant viscosity; always passes through the origin as a straight line

non-newtonian fluid = varying viscosity; curves passing or not passing through the origin

Question 4

examples of Newtonian fluid

Answer 4

water, organic solvent (chloroform, ethanol), oil, true solutions (saline soln)

some suspensions and emulsions (but have to be very dilute)

Question 5

3 types of deviation for non-newtonian liquids

Answer 5

plastic (bingham), pseudoplastic, dilatant flow

Question 6

at low shear stress, plastic flow liquids behave as

Answer 6

elastic solid

Question 7

when does appreciable flow begins for bingham flow?

Answer 7

shear stress must be equivalent to the yield value (fB)

Question 8

at stress above yield value, the plastic fluid resembles a ___________

Answer 8

Newtonian system

Question 9

which two non-newtonian fluid exhibit shear thinning?

Answer 9

plastic and pseudoplastic flow

Question 10

plastic flow is associated with the presence of ______ in a ____________

Answer 10

presence of flocculated particles in a concentrated suspension

Question 11

bingham’s equation

Answer 11

U = (F-f)/G
U = plastic viscosity, F = shear stress, f = yield value, G = shear rate

Question 12

for pseudoplastic flow, viscosity ______ with _____________ shear rate

Answer 12

decreases; increasing

Question 13

at higher shear stress, pseudoplastic flow curve tends towards __________

Answer 13

linearity

Question 14

pseudoplastic flow equation

Answer 14

log G = N log F - log n’
G = shear rate, F = shear stress, n’ = viscosity coefficient, N = index for pseudoplasticity (N > 1),

A Straight line with limited applications

original eqn: F^N = n’ x G

Question 15

pseudoplastic flow is associated with

Answer 15

polymers in solution (aqueous dispersions of hydrocolloids such as tragacanth, alginates, methylcellulose and synthetic materials such as polyvinylpyrrolidone.

polymer = long and high MW molecules

Question 16

for dilatant flow, the viscosity ________ with _________ shear stress

Answer 16

increases, increasing

Question 17

dilatant flow liquids exhibit

Answer 17

shear-thickening

Question 18

dilatant flow equation

Answer 18

F^N = n' G
G = shear rate, n' = viscosity coefficient, N = index of dilatancy (N<1), F = shear stress

Question 19

example of dilatant flow

Answer 19

high concentrations of deflocculated particles

Question 20

non-newtonian liquid consists of

Answer 20

solid component (flocculated/long/deflocculated)

Question 21

structural changes can happen to a non-newtonian liquid via 3 ways

Answer 21

rate of shear, frequency of shear, duration of shear

Question 22

most reversible changes for non-newtonian liquids are

Answer 22

not instantaneous

Question 23

the upcurve and downcurve are usually ________ and form the ______

Answer 23

not superimposable; hysteresis loop

Question 24

the area of the hysteresis loop indicates

Answer 24

the extent of structural breakdown

Question 25

which systems can be described as exhibiting thixotropy?

Answer 25

plastic & pseudoplastic

Question 26

thixotropy is characterised by:

Answer 26

- decrease in apparent viscosity upon application of shearing forces (shear thinning) - slow recovery of apparent viscosity on standing of the system (upcurve and downcurve are not superimposable)

Question 27

what can we do to reduce the recovery time of thixotropy?

Answer 27

gentle rolling or rocking motion helps with reformation of the structure

Question 28

thixotropy is a desirable property in which type of liquid preparations?

Answer 28

suspensions and emulsions

Question 29

formula for dynamic viscosity

Answer 29

n = shear stress / shear rate

Question 30

formula for kinematic viscosity

Answer 30

v = (n) dynamic viscosity / density | where n is the dynamic viscosity

Question 31

Absolute viscosity applied to what type of fluid

Answer 31

Newtonian fluid

Question 32

Apparent viscosity applies to what type of fluid

Answer 32

non-Newtonian fluids

Question 33

what viscometers are used for netwonian fluids? and what are these viscometers alternatively called

Answer 33

capillary viscometers: U-tube, suspended-level - 'one point' instruments note: U-tube more commonly used and found in BP

Question 34

viscometers used for non-netwonian fluids? and what are these viscometers alternatively called

Answer 34

Rotational viscometer: concentric cyclinder viscometer, cone and plate viscometer OR efflux viscometers (esp for tragacanth = pseudoplastic flow) these viscometers are alternative called: 'multiple point' instruments

Question 35

precautions to take note for capillary viscometers

Answer 35

- ensure streamline flow by avoiding vibration and maintaining a minimum flow time - free from air bubbles - clean viscometer before use - clamp viscometer vertically - accurate temperature control

Question 36

equations used in capillary viscometers (what is the law used?) equation relating viscosity and time

Answer 36

Poiseuille's Law: n = (pi x r4 x t x P)/ 8 x l x V ``` r = radius of capillary tube t = time of flow P = pressure diff across capillary tube ends l = length of capillary tube V= vol of liquid ``` --> n = K t p where k is a constant, p here refers to density n1/n2 = (t1 x p1) / (t2 / p2) OR V1 / V2 = t1/ t2 v = kinematic viscosity n = (dynamic) viscosity 1 and 2 refers to test and reference liquids respectively

Question 37

flow time requirement for U-tube (Ostwald) viscometer

Answer 37

at least 300s for size A, at least 200s for all other sizes (B to H) - ensures less possibility of turbulence - also ensure a streamline flow is formed to follow the Poiseuille's law eqn

Question 38

flow time requirement for suspended-level (ubbelohde) viscometer

Answer 38

at least 300s for size 1, at least 200s for all other sizes (1A to 5) - ensures less possibility of turbulence - also ensure a streamline flow is formed to follow the Poiseuille's law eqn

Question 39

2 types of concentric cylinder viscometers

Answer 39

Searle type = inner cylinder is rotated Couette type = outer cylinder is rotated

Question 40

the inner cylinder is also called as what for the concentric cylinder viscometers

Answer 40

spindle

Question 41

what do rotational viscometers measure

Answer 41

measurement based on viscous drag exerted on a body when it its rotated in a liquid

Question 42

what do capillary viscometers measure

Answer 42

time of flow by gravity

Question 43

what do we measure using the couette type?

Answer 43

torque transmitted to the inner cylinder stress exerted on the inner cylinder translate to the angular deflection of the point = equal to the viscosity

Question 44

equation relating viscosity with angular deflection

Answer 44

n = (K x tetha) / w where k is a constant, tetha = angular deflection w = angular velocity of outer cylinder n = (dynamic) viscosity original eqn: C x tetha = ( 4 x pi x h x w x n)/ (1/r1^2 - 1/r2^2) ``` r1 = radius of inner cylinder r2 = radius of outer cylinder h = height of inner cylinder covered by test liquid ```

Question 45

diff sizes of spindle can cause diff defraction, give examples

Answer 45

larger spindle for lower viscosity = larger area, greater defraction (i think it's deflection) smaller spindle for greater viscosity = lower deflection

Question 46

example of a type of concentric cylinder viscometer

Answer 46

brookfield viscometer (i think its a searle type: inner cylinder is rotated)

Question 47

problems of concentric cylinder viscometers

Answer 47

- shear rate of test liquid across the gap is not constant - end effects can be significant - frictional heating significant at high shear rate - large vol of test liquid required if gap is big

Question 48

equation connecting viscosity and torque on cone + radial velocity of plate for cone and plate viscometer

Answer 48

n = (K x G) / ohm G = torque on cone K is a constant ohm = radial velocity of plate

Question 49

important point for cone and plate viscometer

Answer 49

angle should be small to avoid edge effect (referring to angle c in diagram)

Question 50

advantages of cone and plate viscometer compared to concentric cylinder

Answer 50

- vol of test liquid needed is small (unlike the concentric cylinder) - filling and cleaning is easy - shear rate is constant throughout the test liquid (unlike the concentric cylinder)

Question 51

what does efflux viscometers measure

Answer 51

time taken for liquid to flow though a small orifice longer the efflux time = more viscous the liquid

Question 52

examples of efflux viscometers

Answer 52

redwood viscometer, flow cups

Question 53

what is the main use of redwood viscometer

Answer 53

determines viscosity of standard solution of tragacanth relative to that of water

Question 54

diff bet u-tube (ostwald) viscometer and suspended-level (ubbelohde) viscometer

Answer 54

suspended-level (ubbelohde) viscometer have a U-tube + side arm

Question 55

advantages of suspended-level (ubbelohde) viscometer (two adv)

Answer 55

(1) no need to fill viscometer with precise vol of liquid | 2) can be used for very viscous liquids (10x higher than U-tube viscometer

Question 56

why non-newtonian liquids uses multiple point instrument

Answer 56

shear rate is not directly proportional with shear stress viscosity is not constant multiple point instruments allow to find apparent viscosities

Question 57

why newtonian liquids uses single point instrument

Answer 57

shear rate is directly proportional with shear stress viscosity is constant single point determination at a specific shear rate is adequate