Rheology• “ Rheology is concerned with the flow anddeformation of materials experiencing anapplied force.”
VISCOSITY• “The viscosity of the fluid is the internal resistance orfriction involved in the relative motion of onelayer 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 stressRate of shear
Classification of Materials According toFlow and Deformation• When classifying materials according to the types offlow and deformation, it is customary to place themin two categories.• (i) Newtonian system• (ii) Non-Newtonian system
Newtonian fluid• Fluids which obey Newtonian equation is calledNewtonian 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 atparticular 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 untila 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 theaxis of shearing stress is called yield value.”
Plastic flow• A Bingham body does not begin to flow until ashearing stress, corresponding to the yield valueexceeded.• 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 materialsdeceases with the increase in the rate of shearS =K du/dx n=1n
Dilatant flow• A type of flow characterized by an increase inviscosity as shear stress is increased• Example is given as• · Starch in cold water,· Deflocculated particles• Apparent viscosity must increase with increase inshear 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 asol• Bentonite gel is good exampleThixotrophy
Rheopexy• Rheopexy is the rare property of some non-Newtonian fluids to show a time-dependent increasein viscosity; the longer the fluid undergoes shearingforce, the higher its viscosity.• Time lag thatcan be reducedby a slow shakingor rolling motion.Rheopexy
Negative thixotropy• Transformation of dilute concentration into viscousconcentration• Occur due to gradual growth in molecular structuresover the time
Ostwald viscometer• Work under influence of gravity• Liquid is introduced through arm Vuntil G• Pipette is used to avoid wetting of tubeabove G• Liquid is suck into arm W above E• Time taken for falling E to F is notedCEFWAVG
Suspended level viscometer• Liquid is fill to bulb C through tube V• Ventilating tube Z is then closed; liquid isdrawn into C by applying suction at W untilliquid is above mark E• Liquid is held by closing W & Z is opened• W is finally opened & time taken for fallingof liquid from E to F• Associated with readjustment of volume whenmeasurement taken as series of temperatureV W ZECFBA
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= ct4 Kinematicviscosity“The absolute viscositydivided by thedensityof a liquid at definitetemperature.”
Concentric cylinder viscometer• In which liquid whose viscosity is measured fills thespace between 2 coaxial cylinder, the inner onesuspended by torsion wire• Outer cylinder is rotated at constantrate & resulting torque on innercylinder is measured• Inner cylinder is rotated at constantrate & resulting torque on outercylinder is measured
Concentric cylinder viscometer• C =4ᴫ hὠᶯ• 1/ r1 – 1/r2• C= Torsion constant• h= height of inner cylinder• = angular velocityὠ• r1 & r2 = radius of outer and inner cylinder2 2θ
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 sampleto cone is measured• Viscosity calculated from equation• ᶯ = 3G/2ᴫRΩ/Ψ• G= torque on cone• R= radius of cone• Ω= radial velocity of plate• Ψ = angle between cone and plate2
Falling sphere viscometer• 3ᴫᶯdu=ᴫ/6d g( ρ – ρ )• Left side of equation showsviscous drag• Right side of equation showsforce of gravity• Use for Newtonin fluid• Temperature of falling sphereand liquid is same3s 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 ) F18uρ2S 12S 1S 121
Red wood viscometer• Involve determining the time taken for given volume of liquidto flow through narrow orifice• Redwood viscometer is an empirical instrument• Efflux times are arbitrary measurementof viscosities usually expressedas Redwood seconds.
Application in Pharmacy• It is involved in formulation and analysis of pharmaceuticalproducts 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 therepacking in containers.• The poloxamers are block polymers and are used in dermatologicbases or topical ophthalmic preparations because of their lowtoxicity & ability to form clear water based gels.• Also used in study of paints, inks, doughs, road building materials,cosmetics, dairy products & other materials.