rheology of liquids

Question 1

difference between rheology and viscosity

Answer 1

rheology - flow prop of materials

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

Question 2

rheology affects what ( 6 )

Answer 2

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

Question 3

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

Answer 3

-shear stress = n x rate

Question 4

examples of newtonian liquids

Answer 4

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

Question 5

what happens to the graph if axis are swapped

Answer 5

the graph becomes reflected like a mirror image

Question 6

shear thinning vs shear thickening

Answer 6

thinning - viscosity dec when stress applied

thickening - viscosity inc when stress applied

Question 7

plastic flow draw the graph and what are the properties

Answer 7

• 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

Question 8

pseudoplastic flow draw the graph and what are the properites

Answer 8

• 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 )

Question 9

pseudoplastic flow for which kind of materials

Answer 9

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

Question 10

dilatant flow graph and properties

Answer 10

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

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

Question 11

why is shear thickening problematic for production

Answer 11

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

Question 12

dilatant flow for which kind of materials

Answer 12

suspensions with high concentration > 50% of small deflocculated particles

Question 13

non - newtonian liquid consists of what

Answer 13

solid component with flocculated, deflocculated particles or long molecules

Question 14

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

Answer 14

• rate of shear
• duration of shear
• freq of shear

Question 15

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

Answer 15

often reversible

Question 16

how to tell if instantaneous or not

Answer 16

in a graph with shear rate as y axis

superimposable

Question 17

what is the hysterisis looop and what does it say

Answer 17

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

- shows extent of structural change

Question 18

what is thixotropy and what kind of diagram shows that

Answer 18

• 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

Question 19

which kind of liquids show thixotropy normally and why

describe the process

Answer 19

• 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

Question 20

how to reduce recovery time of thixotropic liquids

Answer 20

apply gentle rolling or rocking motion

helps reformation of the structure

Question 21

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

kinematic viscosity calc and units (v)

relative viscosity calc and units (nr)

Answer 21

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

Question 22

what must be controlled when measuring rheological props

Answer 22

temp ! bc can affect viscosity

Question 23

capillary viscometer - usage and eqn

Answer 23

for newtonian

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

Question 24

u tube viscometer appropriate size

Answer 24

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

Question 25

precautiosn for capillary viscometer

Answer 25

- avoid vibration to streamline flow - no air bubble - clean before use - clamped vertically - control temp

Question 26

rotational viscometer use and eqn | concentric cylinder viscometer

Answer 26

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

Question 27

problems with concentric cylinder

Answer 27

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

Question 28

how does cone and plate viscometer work

Answer 28

- 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

Question 29

advantages of cone and plate viscometer

Answer 29

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 )

Question 30

efflux viscometer use

Answer 30

for comparing - time for given vol of liquid to flow thru small orifice is determined efflux time is arbitrary measurement of viscosity

Question 31

redwood viscometer used for

Answer 31

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

Question 32

concentric cylinder eqn

Answer 32

angular deflection/ angular velocity

Question 33

cone and plate eqn

Answer 33

torque/ radial velocity

Question 34

spindle size relation

Answer 34

smaller number= large spindle = lower viscosity liquid