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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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