5. Rheology
• “ Rheology is concerned with the flow and
deformation of materials experiencing an
applied force.”
6. VISCOSITY
• “The viscosity of the fluid is the internal resistance or
friction involved in the relative motion of one
layer of molecules with respect to the next.”
• Unit:
Unit of viscosity is poise.
7. Coefficient of viscosity
• “ Ratio of applied stress and rate of shear”
• If viscosity increase then shear stress increase:
Shear stress
Rate of shear
8. Classification of Materials According to
Flow and Deformation
• When classifying materials according to the types of
flow and deformation, it is customary to place them
in two categories.
• (i) Newtonian system
• (ii) Non-Newtonian system
10. Shear stress
• “ Force per unit area ”
• S= F/A
• S= ᶯ du/dx
• =ᶯ S/du/dx
• =ᶯ F/A/du/dx
• This is Newtonian equation
11. Non-Newtonian fluid
• Fluids which not follow the Newtonian equation
• Because value of ᶯ varies with rate of shear
• Consider apparent viscosity of these system at
particular rates of shear
• Apparent viscosity is ratio of shear stress to shear rate
• Apparent viscosity is time dependent
13. Plastic flow
• “ The material, which fails to flow until
a certain shearing stress has been applied.”
• Bingham Bodies:
“The bodies, which follow the plastic flow,
are called as Bingham Bodies.”
• Yield Value:
“The point at which curve intercept the
axis of shearing stress is called yield value.”
14. Plastic flow
• A Bingham body does not begin to flow until a
shearing stress, corresponding to the yield value
exceeded.
• ᶯpl= S-fb
• du/dx
15. Pseudoplastic flow
• “ Flow show by the materials having polymers in solutions.”
• Example are given as
• · Cellulose ether,· Tragacanth.· Alginates etc.
• There is no yield value so no part of the curve is linear.
• Viscosity:
The viscosity of the pseudoplastic materials
deceases with the increase in the rate of shear
S =K du/dx n=1n
16. Dilatant flow
• A type of flow characterized by an increase in
viscosity as shear stress is increased
• Example is given as
• · Starch in cold water,· Deflocculated particles
• Apparent viscosity must increase with increase in
shear rate up to a maximum level
• S = K du/dx n=‹1n
17. Thixotropy
• To change by touch
• It is defined as a reversible transition from a gel to a
sol
• Bentonite gel is good example
Thixotrophy
18. Rheopexy
• Rheopexy is the rare property of some non-
Newtonian fluids to show a time-dependent increase
in viscosity; the longer the fluid undergoes shearing
force, the higher its viscosity.
• Time lag that
can be reduced
by a slow shaking
or rolling motion.
Rheopexy
19. Negative thixotropy
• Transformation of dilute concentration into viscous
concentration
• Occur due to gradual growth in molecular structures
over the time
21. Ostwald viscometer
• Work under influence of gravity
• Liquid is introduced through arm V
until G
• Pipette is used to avoid wetting of tube
above G
• Liquid is suck into arm W above E
• Time taken for falling E to F is noted
C
E
F
W
A
V
G
22. Suspended level viscometer
• Liquid is fill to bulb C through tube V
• Ventilating tube Z is then closed; liquid is
drawn into C by applying suction at W until
liquid is above mark E
• Liquid is held by closing W & Z is opened
• W is finally opened & time taken for falling
of liquid from E to F
• Associated with readjustment of volume when
measurement taken as series of temperature
V W Z
E
C
F
B
A
23. Poiseuille’s equation
V/t = ᴫPr / 8ᶯ l
V/t ∝ P/ᶯ
P= hρg
V/t ∝ hρg / ᶯ
1/ t ∝ ρ / ᶯ
V=ᶯ/ρ
1/t ∝ 1/V
t V∝
V= ct
4
Kinematic
viscosity
“The absolute viscosity
divided by the
density
of a liquid at definite
temperature.”
24. Concentric cylinder viscometer
• In which liquid whose viscosity is measured fills the
space between 2 coaxial cylinder, the inner one
suspended by torsion wire
• Outer cylinder is rotated at constant
rate & resulting torque on inner
cylinder is measured
• Inner cylinder is rotated at constant
rate & resulting torque on outer
cylinder is measured
26. Disadvantages
• Shear rate not uniform throughout the process
• Frictional drags increase in temperature
• Cleaning difficult
27. Cone plate viscometer
• Plate is rotated at a fixed speed
• Torque transmitted through sample
to cone is measured
• Viscosity calculated from equation
• ᶯ = 3G/2ᴫR
Ω/Ψ
• G= torque on cone
• R= radius of cone
• Ω= radial velocity of plate
• Ψ = angle between cone and plate
2
28. Falling sphere viscometer
• 3ᴫᶯdu=ᴫ/6d g( ρ – ρ )
• Left side of equation shows
viscous drag
• Right side of equation shows
force of gravity
• Use for Newtonin fluid
• Temperature of falling sphere
and liquid is same
3
s 1
29. Falling sphere viscometer
• η= d g(ρ – ρ )
18 u V= η/ρ
• η/ρ= d g ( ρ – ρ )
18uρ1
• V= d g ( ρ – ρ ) F
18uρ
2
S 1
2
S 1
S 1
2
1
30. Red wood viscometer
• Involve determining the time taken for given volume of liquid
to flow through narrow orifice
• Redwood viscometer is an empirical instrument
• Efflux times are arbitrary measurement
of viscosities usually expressed
as Redwood seconds.
31. Application in Pharmacy
• It is involved in formulation and analysis of pharmaceutical
products as emulsions, paste, suppositories and tablet coatings.
• It is involved in manufacture of pastes medicines cream ointments.
• It is also involved in mixing and flow of materials and there
packing in containers.
• The poloxamers are block polymers and are used in dermatologic
bases or topical ophthalmic preparations because of their low
toxicity & ability to form clear water based gels.
• Also used in study of paints, inks, doughs, road building materials,
cosmetics, dairy products & other materials.
