rheology of liquids Flashcards
difference between rheology and viscosity
rheology - flow prop of materials
viscosity - resistance of a liquid to flow when subjected to stress
rheology affects what ( 6 )
- packing
- mixing
- lfow thru pipes
physical stability
drug absorption
spreading and adherence
draw the diagram with f , a , dv and dr
draw the graphs of determining n
and the eqn relating shear stress and shear rate
-shear stress = n x rate
examples of newtonian liquids
water, organic solvents, oils and true solutions
true solutions like saline, dextrose, salt solution and sugar solution
suspensions and emusiosn that are q dilute
what happens to the graph if axis are swapped
the graph becomes reflected like a mirror image
shear thinning vs shear thickening
thinning - viscosity dec when stress applied
thickening - viscosity inc when stress applied
plastic flow draw the graph and what are the properties
- 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
pseudoplastic flow draw the graph and what are the properites
- 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 )
pseudoplastic flow for which kind of materials
aq dispersions of hydrocolloids such as tragacanth, alginates, methylcellulose, and synthetic materials like polyvinylpyrrolidone
dilatant flow graph and properties
- move immediately
- shear thickening , convex shape
- apparent viscosity dependant on the shear rate
eqn : N < 1
F^N = n’G
common w deflocculated particles
why is shear thickening problematic for production
- liquid can freeze when high speed stirring bc of resistance to flow, exert overload on the motor and itll burn
dilatant flow for which kind of materials
suspensions with high concentration > 50% of small deflocculated particles
non - newtonian liquid consists of what
solid component with flocculated, deflocculated particles or long molecules
when shearing of non newtonian liquids occur, structural changes occur which are dependant on
- rate of shear
- duration of shear
- freq of shear
structural changes as a result of shearing of non newtonian liquids is reversible or non reversible
often reversible
how to tell if instantaneous or not
in a graph with shear rate as y axis
superimposable
what is the hysterisis looop and what does it say
abc of the diff of curves when inc and dec shear rate
- shows extent of structural change
what is thixotropy and what kind of diagram shows that
- 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
which kind of liquids show thixotropy normally and why
describe the process
- 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
how to reduce recovery time of thixotropic liquids
apply gentle rolling or rocking motion
helps reformation of the structure
viscosity / dynamic viscosity calc and unit (n)
( can be apparent or absolute )
kinematic viscosity calc and units (v)
relative viscosity calc and units (nr)
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
what must be controlled when measuring rheological props
temp ! bc can affect viscosity
capillary viscometer - usage and eqn
for newtonian
measure flwo from one point to another reference to another lqiuid of known viscosity
u tube viscometer appropriate size
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
precautiosn for capillary viscometer
- avoid vibration to streamline flow
- no air bubble
- clean before use
- clamped vertically
- control temp
rotational viscometer use and eqn
concentric cylinder viscometer
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
problems with concentric cylinder
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
how does cone and plate viscometer work
- 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
advantages of cone and plate viscometer
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 )
efflux viscometer use
for comparing
- time for given vol of liquid to flow thru small orifice is determined
efflux time is arbitrary measurement of viscosity
redwood viscometer used for
in bp to determine viscosity of standard solution of tragacanth relative to that of water
concentric cylinder eqn
angular deflection/ angular velocity
cone and plate eqn
torque/ radial velocity
spindle size relation
smaller number= large spindle = lower viscosity liquid
