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Rheology of liquids Flashcards

1
Q

difference between rheology and viscosity

A

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

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2
Q

formula for viscosity

A

viscosity (n) = Shear stress/Shear rate

n = F/A divided by dv/dr

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3
Q

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

A

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

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4
Q

examples of Newtonian fluid

A

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

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

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5
Q

3 types of deviation for non-newtonian liquids

A

plastic (bingham), pseudoplastic, dilatant flow

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6
Q

at low shear stress, plastic flow liquids behave as

A

elastic solid

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7
Q

when does appreciable flow begins for bingham flow?

A

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

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8
Q

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

A

Newtonian system

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9
Q

which two non-newtonian fluid exhibit shear thinning?

A

plastic and pseudoplastic flow

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10
Q

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

A

presence of flocculated particles in a concentrated suspension

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11
Q

bingham’s equation

A 
U = (F-f)/G
U = plastic viscosity, F = shear stress, f = yield value, G = shear rate
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12
Q

for pseudoplastic flow, viscosity ______ with _____________ shear rate

A

decreases; increasing

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13
Q

at higher shear stress, pseudoplastic flow curve tends towards __________

A

linearity

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14
Q

pseudoplastic flow equation

A

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

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15
Q

pseudoplastic flow is associated with

A

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

polymer = long and high MW molecules

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16
Q

for dilatant flow, the viscosity ________ with _________ shear stress

A

increases, increasing

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17
Q

dilatant flow liquids exhibit

A

shear-thickening

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18
Q

dilatant flow equation

A 
F^N = n' G
G = shear rate, n' = viscosity coefficient, N = index of dilatancy (N<1), F = shear stress
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19
Q

example of dilatant flow

A

high concentrations of deflocculated particles

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20
Q

non-newtonian liquid consists of

A

solid component (flocculated/long/deflocculated)

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21
Q

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

A

rate of shear, frequency of shear, duration of shear

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22
Q

most reversible changes for non-newtonian liquids are

A

not instantaneous

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23
Q

the upcurve and downcurve are usually ________ and form the ______

A

not superimposable; hysteresis loop

24
Q

the area of the hysteresis loop indicates

A

the extent of structural breakdown

25
Q

which systems can be described as exhibiting thixotropy?

A

plastic & pseudoplastic

26
Q

thixotropy is characterised by:

A
  • 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)
27
Q

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

A

gentle rolling or rocking motion helps with reformation of the structure

28
Q

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

A

suspensions and emulsions

29
Q

formula for dynamic viscosity

A

n = shear stress / shear rate

30
Q

formula for kinematic viscosity

A

v = (n) dynamic viscosity / density

where n is the dynamic viscosity

31
Q

Absolute viscosity applied to what type of fluid

A

Newtonian fluid

32
Q

Apparent viscosity applies to what type of fluid

A

non-Newtonian fluids

33
Q

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

A

capillary viscometers: U-tube, suspended-level
- ‘one point’ instruments

note: U-tube more commonly used and found in BP

34
Q

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

A

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

35
Q

precautions to take note for capillary viscometers

A
  • 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
36
Q

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

A

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

37
Q

flow time requirement for U-tube (Ostwald) viscometer

A

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
38
Q

flow time requirement for suspended-level (ubbelohde) viscometer

A

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
39
Q

2 types of concentric cylinder viscometers

A

Searle type = inner cylinder is rotated

Couette type = outer cylinder is rotated

40
Q

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

A

spindle

41
Q

what do rotational viscometers measure

A

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

42
Q

what do capillary viscometers measure

A

time of flow by gravity

43
Q

what do we measure using the couette type?

A

torque transmitted to the inner cylinder

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

44
Q

equation relating viscosity with angular deflection

A

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
45
Q

diff sizes of spindle can cause diff defraction, give examples

A

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

smaller spindle for greater viscosity = lower deflection

46
Q

example of a type of concentric cylinder viscometer

A

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

47
Q

problems of concentric cylinder viscometers

A
  • 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
48
Q

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

A

n = (K x G) / ohm

G = torque on cone
K is a constant
ohm = radial velocity of plate

49
Q

important point for cone and plate viscometer

A

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

50
Q

advantages of cone and plate viscometer compared to concentric cylinder

A
  • 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)
51
Q

what does efflux viscometers measure

A

time taken for liquid to flow though a small orifice

longer the efflux time = more viscous the liquid

52
Q

examples of efflux viscometers

A

redwood viscometer, flow cups

53
Q

what is the main use of redwood viscometer

A

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

54
Q

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

A

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

55
Q

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

A

(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

56
Q

why non-newtonian liquids uses multiple point instrument

A

shear rate is not directly proportional with shear stress

viscosity is not constant

multiple point instruments allow to find apparent viscosities

57
Q

why newtonian liquids uses single point instrument

A

shear rate is directly proportional with shear stress

viscosity is constant

single point determination at a specific shear rate is adequate

Solid (16 decks)

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