rheology

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AMIST UNIVERSITY MALAYSIA

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RHEOLOGY The term rheology is derived from the Greek words Rheo, ”to flow, "and logos,' science” Rheology-study of the deformation and flow properties of matter. Rheology can be applied to solid, liquid and gaseous Solid-completely resistance Gaseous-non resistance Liquid- less resistance Focuses primarily on liquid and semisolid Emulsion,paste,cream,lotion tablet coating soln,suspention EX. pouring from a bottle Extrusion from tube Passage through a syringe needle

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pouring from a bottle Extrusion from tube Passage through a syringe needle

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Viscosity-resistance to flow of fluid – Units (poise, centi poise, milli poise) Rheology means simply –to study the flow properties of fluids flow viscosity viscosity flow

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Similar to deck of cards Bottom consider as fixed and top plan layer of liquid moved at a greater Velocity. Then each lower layer will move with a velocity. flow of liquid Top layer velocity Second layer velocity Third layer velocity bottom layer velocity constant

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A dv dr dv-velocity dr-distance traveled A-area

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Mathematical equation-viscosity Shear stress = F’/A force per unit area applied to bring liq. flow Flow force Rate of shear Shearing stress Shear stress F (force ) = F’/A G-FLOW F-FORCE Rate of shear G (flow) = dv/ dr , change in velocity with distance Liquid viscosity Shear stress F (force ) to bring rate of shear (flow) apply

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Relationship between shear stress (force ) and rate of shear (flow) Shearing stress α Rate of shear Force α Flow F’/A α dv/ dr (or) F’/A dv/ dr η (or) G-FLOW F-FORCE F = η G η - coefficient of viscosity

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viscosity is calculated by η = F/G Coefficient viscosity is defined as the F/G required to maintain Unit difference in velocity between two parallel layers in the liquid. Units (poise, centi poise, milli poise) Kinematic viscosity defined as viscosity divided by density of the liquid. Units (stokes and centi stokes) Fluidity θ = reciprocal of viscosity θ = 1/ η

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According to flow Newtonian system - liquid that obey newton’s law of flow Non Newtonian systems - liquid that doesn't obey newton’s law of flow

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Rate of shear Shearing stress (F) (G) G-FLOW F-FORCE Shearing stress α Rate of shear Newtonian flow Force α Flow liquid that obey newton’s law of flow eqn is F= η G shear stress-shear rate relationship is normally represented in the form of a cure namely rheogram Ex.water,glycerin,chloroform,Dil.colloidal soln

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G-FLOW F-FORCE Yield value Force Liquid fails to flow ( non linear) Force Liquid start to flow (linear) Linear portion extrapolated to x axis at a point Called yield value Plastic flow Particle-bonded Friction force-high Yield value Particle-individual Friction force-nil Futher stress flow Ex.susp,butter,ointment,paste and gel Bingham bodies

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Rate of shear Shearing stress (F) (G) Pseudoplastic flow Ex. Tragacanth in water Sod. Alginate in water MC in water SCMC in water Curve is not linear Shear thinning system

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Pseudoplastic flow rheogram - F Ν = η ’G where N is a exponent and η viscosity Pseudoplastic flow

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Rate of shear Shearing stress (F) (G) Dilatant flow Shear thickening system Ex. Susp.more than 50% deflo-susp Susp . of starch in water Kaolin 12% in water Zinc oxide 30% in water

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Conti… At rest close packed mini Void volume sufficient vehicle Relatively low consistency Open packed high void volume Insufficient vehicle relatively High consistency Increasing rate of shear

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THIXOTROPY Thixotropy is defined as an isothermal and comparatively slow recovery, on standing Material, of a consistency lost through shearing POSITIVE THIXOTROPY NEGATIVE THIXOTROPY

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Rate of shear Shearing stress (F) (G) POSITIVE THIXOTROPY Plastic system Up curve ab Down curve bc (left) a b c Plastic system Pseudoplastic system Pseudoplastic system Up and down curve superimposed

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Rate of shear Shearing stress (F) (G) Material showing a bulge in the hysteresis Loop(swelling) Ex.aqueous bentonite magma(10-15% by wt) At rest: crystalline plate Are irregularly arranged On shear: crystalline plate Are arranged to form house Of cards

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Rate of shear Shearing stress (F) (G) The spure value represents a sharp point of Structural break down at a low shear rate in the up-curve Ex.procaine penicillin gel in IM inj

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Rate of shear Shearing stress (F) (G) NEGATIVE THIXOTROPY Ex.Magnesia Megma Rheopexy is a phenomenon in which a soln transforms to a gel state more readily rather than keeping a soln at rest (gentle shaking)

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Rate of shear Shearing stress (F) (G) Measurement of thixotropy a b c d t 2 t 1 1/U 1 1/U 2 e

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Rate of shear Shearing stress (F) (G)

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