Rheology

Rheology is the science that study flow of fluids. Viscosity is the main parameter of flow. Newtonian & non newtonian are the two types of flow behavior according to newtons law of flow. non-newtonian flow can be plastic, pseudoplastic, dilatant, thixotropic, antithixotropic or rheopexy. viscosity can be determined by using various viscometers such as capillary viscometer, cup & bob viscometer, cone & plate viscometer, falling sphere viscometer, brookfield viscometer, etc. factors affeting viscosity are intrinsic, extrinsic or temperature dependence.

1. RHEOLOGY Presented By : Mahewash Sana A. Pathan.

2. RHEOLOGY Rheo - To flow Logos - science/ study “ Rheology is the science that concerns with the flow of liquids & deformation of solids” Invented by Bingham & Crawford . 2

3. VISCOSITY “ viscosity is an index of resistance of a liquid to flow.” OR “ Viscosity is resistance provided to a layer of liquid when it moves over another layer of liquid.” η = F/G Where, η = Viscosity/ dynamic viscosity/ coefficient of viscosity F = Shear stress G = Rate of shear. Units of viscosity: SI unit: Pascal second (Pa. s) CGS unit: Poise ( dy /sq. cm) 3

4. Terms related to rheology Fluidity: Φ = 1 / η Kinematic viscosity: ή = η / ρ Relative viscosity: η r = η / η Specific viscosity: η sp = η - η / η Reduced viscosity: η red = η sp / c Where, ρ = density, η = Viscosity of solvent, c = concentration 4

5. Factors affecting viscosity Intrinsic factors: Molecular weight- heavier the molecular wt, greater will be the viscosity Molecular shape- Liquids with large & irregularly shaped molecules are more viscous. Intermolecular forces- As the intermolecular bonding increases, viscosity of fluids also increases. Extrinsic factors: Pressure: increase in pressure increased cohesive forces increased viscosity. Addition of electrolytes: can increase or decrease viscosity. Temperature: Increase in temperature break the cohesive forces Viscosity decreases 5

6. NEWTON’S LAW OF VISCOSITY “ The shear stress in flowing fluid is directly proportional to the rate of shear.” F α G F = η G Where, F = shear stress G = Rate of shear η = Coefficient of viscosity When data are plotted by taking F on x-axis & G on y-axis, a flow curve is obtained called as ‘ Rheogram ’ or consistency curve. 6

7. NEWTONIAN FLUID “A system or fluid that follows Newton’s law of viscosity” OR “A fluid whose viscosity does not change with the rate of deformation or shear strain” Viscosity of such fluids is constant at a given temperature & pressure. E.g. water, glycerin, chloroform, solutions of syrups, very dilute colloidal solutions. 7

8. NON-NEWTONIAN FLUID “ fluid in which shear stress is not proportional to rate of shear” OR ““A system or fluid that does not follows Newton’s law of viscosity” E.g. Suspensions, emulsions, semisolids Non- newtonian phenomenon may be- Time independent: Plastic flow Pseudoplastic flow Dilatant flow Time dependent: Thixotropy Rheopexy 8

9. PLASTIC FLOW The substance initially behaves like an elastic body & fails to flow when less amount of stress is applied. Further increase in shear stress leads to increase in shear rate which progressively get linearized . The amount of shear stress required to break the inter-particle contacts so that particles behave individually is called as Yield value . Plastic flow follows Newtonian flow above yield value. 9

10. Plastic flow is associated with presence of flocculated particles. Materials that show plastic flow are called as ‘ BINGHAM BODIES’. Quantitative behavior is expressed in terms of Bingham equation. U= F – f / G Where, U = plastic viscosity F = Shear stress f = Yield value G = Rate of shear Yield value Flow 10

11. PSEUDOPLASTIC FLOW The consistency curve for pseudoplastic flow begins at the origin. As the shear stress increases, shear rate also increases, but not linear. E.g. Emulsions, suspensions, polymer dispersions such as tragacanth in water, Sod. Alginate in water, methyl cellulose, sod. CMC, etc. The materials which show pseudoplastic flow behaviour are called as ‘ SHEAR THINNING MATERIALS ’. Stress 11

12. Pseudoplastic flow rheogram can be described by following formula- F^N= ή G Where, N = number given to the exponent ή = Viscosity coefficient For pseudoplastic fluids, N is higher than 1. 12

13. DILATANT FLOW The system show increase in viscosity with increasing rate of shear. Dilatant materials are also called as ‘ SHEAR THICKENING SYSTEM’. E.g. Suspension containing high concentration of solids (> 50%), suspension of starch in water. 13

14. F^N = ή G Where, N = number given to the exponent ή = Viscosity coefficient For dilatent fluids, N is less than 1 & decreases as the viscosity increases. 14

15. THIXOTROPY ( GEL-SOL-GEL) “ It is an isothermal & comparatively slow recovery, on standing of a material, of a consistency lost through shearing.” At rest Multipoint contacts Gel state (High consistency/ high viscosity) On Shear Contacts break down Sol state (Equilibrium) (Low Viscosity) Set aside Particle contacts re-established Gel state (removal of stress) (high viscosity) 15

16. Rheogram of thixotropic material depends on- Rate at which shear is increased or decreased. Length of time during which a sample is subjected to rate of shear. Fig : Thixotropy 16

17. NEGATIVE THIXOTROPY Antithixotropy or negative thixotropy represents an increase in viscosity on the down curve. E.g . Mgnesia magma At rest (on storage) Individual particles are in large no. of small flocs (Low viscosity) Gel state On shear (Equilibrium) Particle collisions, interparticle contacts more (High viscosity) Gel state Set aside (Removal of stress ) Flocs contacts break, individual particles (Low viscosity) Sol state 17

18. Fig: Negative thixotropy 18

19. RHEOPEXY It is a phenomenon in which a sol transform to a gel state more readily rather than keeping a sol at rest. A gentle shaking or low rate of shear is sufficient to transform a sol into gel. 19

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21. DETERMINATION OF VISCOSITY 21

22. OSTWALD/ CAPILLARY VISCOMETER Used to determine viscosity of Newtonian fluids. Both dynamic & kinematic viscosities can be obtained. PRINCIPLE: when a liquid flows by gravity, the time required to pass between two marks ( A & B) through a vertical capillary tube is determined. η1 = ρ 1 t1/ ρ 2 t2 . η2 22

23. FALIING SPHERE VISCOMETER Principle: Hoeppler viscometer η = t ( Sb – Sf ) B Where, t = time taken for the ball to fall between two points Sb = specific gravity of ball Sf = specific gravity of test fluid B = constant for ball 23

24. CUP & BOB VISCOMETER η = Kv w/v Where, η = apparent viscosity of liquid Kv= constant for instrument v = rpm, rate of shear w = weight placed on hanger, shear stress 24

25. CONE & PLATE VISCOMETER For newtonian system, η = C T/v Where, C= instrument constant v = speed of cone (rpm) For plastic flow, U = Cf T- Tf / v Cf = instrument constant Tf = torque at shearing stress axis 25

26. BROOKFIELD VISCOMETER Brookfield viscometers employ the well-known principle of rotational viscometry ; they measure viscosity by sensing the torque required to rotate a spindle at constant speed while immersed in the sample fluid. The torque is proportional to the viscous drag on the immersed spindle, and thus to the viscosity of the fluid. 26

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