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RHEOLOGY
SUBMITTED BYMrs. Uzma Ali
GROUP MEMBER
Swaira Shafique
Neelum Haleema
Zahra Abbas
Maham Ahmad
Hina Shafique
Memoona Nasir
OUTLINE
• Definition• Viscosity• Newtonin fluid• Non-Newtonin fluid• Determination of flow properties• Application
Rheology
• “ Rheology is concerned with the flow and deformation of materials experiencing an applied force.”
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.
Coefficient of viscosity
• “ Ratio of applied stress and rate of shear”• If viscosity increase then shear stress increase:
Shear stress
Rate of shear
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
Newtonian fluid
• Fluids which obey Newtonian equation is called Newtonian fluid.
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
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
Types of Non-Newtonian fuid
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.”
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
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
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
Thixotropy
• To change by touch• It is defined as a reversible transition from a gel to a
sol• Bentonite gel is good example
Thixotrophy
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
Negative thixotropy
• Transformation of dilute concentration into viscous concentration
• Occur due to gradual growth in molecular structures over the time
Determination of flow properties
1) Capillary viscometer: Ostwald U-tube viscometer Suspended level viscometer
2) Rational viscometer Concentric cylinder viscometer Cone plate viscometer
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
CE
F
W
A
V
G
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 CF
B
A
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.”
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
Concentric cylinder viscometer
• C =4ᴫ hὠᶯ • 1/ r1 – 1/r2
• C= Torsion constant• h= height of inner cylinder• ὠ = angular velocity• r1 & r2 = radius of outer and inner cylinder
2 2
θ
Disadvantages
• Shear rate not uniform throughout the process• Frictional drags increase in temperature• Cleaning difficult
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
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
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
2S 1
S 1
2
1
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.
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.
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