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form and tech > rheology of liquids > Flashcards

rheology of liquids Flashcards

1
Q

difference between rheology and viscosity

A

rheology - flow prop of materials

viscosity - resistance of a liquid to flow when subjected to stress

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2
Q

rheology affects what ( 6 )

A
  • packing
  • mixing
  • lfow thru pipes
    physical stability
    drug absorption
    spreading and adherence
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3
Q

draw the diagram with f , a , dv and dr
draw the graphs of determining n
and the eqn relating shear stress and shear rate

A

-shear stress = n x rate

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4
Q

examples of newtonian liquids

A

water, organic solvents, oils and true solutions
true solutions like saline, dextrose, salt solution and sugar solution
suspensions and emusiosn that are q dilute

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5
Q

what happens to the graph if axis are swapped

A

the graph becomes reflected like a mirror image

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6
Q

shear thinning vs shear thickening

A

thinning - viscosity dec when stress applied

thickening - viscosity inc when stress applied

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7
Q

plastic flow draw the graph and what are the properties

A
  • dosent move immediately
  • shear thinning
  • fb yield value must be applied before appreicable flow begins
  • apparent viscosity dependant on the shear rate
  • for flocculated particles
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8
Q

pseudoplastic flow draw the graph and what are the properites

A
  • move straight away
  • shear thinning
  • for long polymer molecules
  • linearity @ high stress values

eqn for this is F^N = n’G
N > 1 ( index for pseudoplasticity )

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9
Q

pseudoplastic flow for which kind of materials

A

aq dispersions of hydrocolloids such as tragacanth, alginates, methylcellulose, and synthetic materials like polyvinylpyrrolidone

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10
Q

dilatant flow graph and properties

A
  • move immediately
  • shear thickening , convex shape
  • apparent viscosity dependant on the shear rate

eqn : N < 1
F^N = n’G
common w deflocculated particles

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11
Q

why is shear thickening problematic for production

A
  • liquid can freeze when high speed stirring bc of resistance to flow, exert overload on the motor and itll burn
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12
Q

dilatant flow for which kind of materials

A

suspensions with high concentration > 50% of small deflocculated particles

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13
Q

non - newtonian liquid consists of what

A

solid component with flocculated, deflocculated particles or long molecules

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14
Q

when shearing of non newtonian liquids occur, structural changes occur which are dependant on

A
  • rate of shear
  • duration of shear
  • freq of shear
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15
Q

structural changes as a result of shearing of non newtonian liquids is reversible or non reversible

A

often reversible

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16
Q

how to tell if instantaneous or not

A

in a graph with shear rate as y axis

superimposable

17
Q

what is the hysterisis looop and what does it say

A

abc of the diff of curves when inc and dec shear rate

- shows extent of structural change

18
Q

what is thixotropy and what kind of diagram shows that

A
  • shear thinning AND slow recovery ( not instaneous, not superimposable )

thixotropy characterised by :
- dec in apparent viscosity upon application of shearing forces ( shear thinning )
and - slow recovery of apparent viscosity on standing of the system ( non instantaneous )

  • graph shld be rate on y axis and
  • up curve - to indicate shear thinning

if downcurve thats shear thickening

19
Q

which kind of liquids show thixotropy normally and why

describe the process

A
  • flocculated or long particles

at rest -loose 3d structure
on shearing - structure breaks down
particles gradually become aligned and flow easily
when shearing stops - structure starts to reform but takes time

20
Q

how to reduce recovery time of thixotropic liquids

A

apply gentle rolling or rocking motion

helps reformation of the structure

21
Q

viscosity / dynamic viscosity calc and unit (n)
( can be apparent or absolute )

kinematic viscosity calc and units (v)

relative viscosity calc and units (nr)

A

n = shear stress/shear rate , dyne cm ^-2
Pa.s, or Poise (P) or Centipoise ( cps)

kinematic (v) = dynamic / density
m^2 s^-1 or Stoke (St) or Centistoke (cSt)

relative (nr ) = solution viscosity/ solvent viscosity
no units - comparative

22
Q

what must be controlled when measuring rheological props

A

temp ! bc can affect viscosity

23
Q

capillary viscometer - usage and eqn

A

for newtonian

measure flwo from one point to another reference to another lqiuid of known viscosity

24
Q

u tube viscometer appropriate size

A

flow time of at least 300s for size A and at least 200s for all the other sizes obtained
- to stream line the flow

sizes A to H avail

25
Q

precautiosn for capillary viscometer

A
  • avoid vibration to streamline flow
  • no air bubble
  • clean before use
  • clamped vertically
  • control temp
26
Q

rotational viscometer use and eqn

concentric cylinder viscometer

A

inner cylinder hung freely by torsion wire
immersed in test liquid contained in the outer cylinder

either inner or outer is rotated
this produces torque to the inner cylinder ( if outer cylinder is spun )
stress indicated by angular deflection of the pointer which mvoes past the scale
more viscous => greater deflection

smaller number = larger spindle
= lower viscosity liquid

27
Q

problems with concentric cylinder

A

shear rate of test liquid across the gap is not constant
frictional heating is significant at high shear rate
filling is difficult if gap is small
larege volume of test liquid needed if gap is big

28
Q

how does cone and plate viscometer work

A
  • also for non newtonian

flat circular plate with wide angle cone places above
tip of cone touches the plate and test liquid loaded into the included gap

plate is rotated at a fixed speed and torque transmitted throught he test liqudi to the con e
= then measured

29
Q

advantages of cone and plate viscometer

A

small vol of test liquid needed
filling and cleaning easy
shear rate constant thruout the test liquid bc gap is small
( preferred oer the other one )

30
Q

efflux viscometer use

A

for comparing

  • time for given vol of liquid to flow thru small orifice is determined
    efflux time is arbitrary measurement of viscosity
31
Q

redwood viscometer used for

A

in bp to determine viscosity of standard solution of tragacanth relative to that of water

32
Q

concentric cylinder eqn

A

angular deflection/ angular velocity

33
Q

cone and plate eqn

A

torque/ radial velocity

34
Q

spindle size relation

A

smaller number= large spindle = lower viscosity liquid

form and tech (9 decks)

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