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Lecture 10 – Viscosity and Flow (Ch. 6)

Lecture 10 – Viscosity and Flow (Ch. 6). This chapter is a study of the shear stress as a function of the shear rate for Newtonian and non-Newtonian biological materials. Lecture 10 – Viscosity and Flow (Ch. 6).

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Lecture 10 – Viscosity and Flow (Ch. 6)

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  1. Lecture 10 – Viscosity and Flow (Ch. 6) This chapter is a study of the shear stress as a function of the shear rate for Newtonian and non-Newtonian biological materials. BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

  2. Lecture 10 – Viscosity and Flow (Ch. 6) “When a fluid or semisolid is subjected to a constant shearing force it flows, ie., it deforms continuously at a velocity that increases as the applied shearing force increases.” Viscosity: quantifies the resistance of the fluid to flow BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

  3. Lecture 10 – Viscosity and Flow (Ch. 6) Liquids and semisolids are usually pumped during processing Viscosity plays a huge part in pump and conveyance system design Viscosity may be dependent on moisture content, concentration, composition and prior treatments. BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

  4. Lecture 10 – Viscosity and Flow (Ch. 6) Newtonian Fluids (Newton 1687) Simplest model Covers most, but not all, ag products Velocity behaves linearly w/ distance Shear stress is linear function of the shear rate Dynamic viscosity: proportionality constant for this relationship BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

  5. Lecture 10 – Viscosity and Flow (Ch. 6) The viscosity can be measured where the fluid of interest is sheared between two flat plates which are parallel to one another Known as planar Couette flow. The shear stress is the ratio of the tangential force F needed to maintain the moving plate at a constant velocity V to the plate area A. Couette flow:Low-speed, steady motion of a viscous fluid between two infinite plates moving parallel to each other. • http://www.answers.com/topic/viscosity?cat=biz-fin BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

  6. Lecture 10 – Viscosity and Flow (Ch. 6) Dynamic viscosity (Figure 6.1) BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

  7. Lecture 10 – Viscosity and Flow (Ch. 6) Kinematic viscosity: dynamic viscosity/density (no force involved) BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

  8. Lecture 10 – Viscosity and Flow (Ch. 6) Non-Newtonian Fluids Relationship between shear stress and shear rate is NOT linear Some also have a yield stress which must be obtained before flow begins. BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

  9. Lecture 10 – Viscosity and Flow (Ch. 6) Most common: pseudoplastic…convex curve towards the shear stress axis (Fig. 6.1b) Apparent viscosity will decrease as shear rate increases Dilatant fluids: concave toward shear stress axis (corn flour, wet beach sand: stiffens when walked on..select pumps carefully!) Apparent viscosity increases as shear rate increases Plastic: linear but intercept is at the yield stress (toothpaste: must stay on brush but must be exudable) Casson-type plastic: has a yield stress but is not linear (chocolate) BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

  10. Lecture 10 – Viscosity and Flow (Ch. 6) Apparent viscosity = shear rate ratio at any given shear rate Pseudoplastic and Dilatant materials, eqtn. 6.2, Table 6.2) Newtonian: n=1, k=dynamic viscosity BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

  11. Lecture 10 – Viscosity and Flow (Ch. 6) Plastic and Casson-type plastic behavior (more general case…Herschel-Bulkley model, eqtn. 6.3 Table 6.3) Chocolate and other Casson materials follows this where N = ½ and the yield stress is taken to the ½ power BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

  12. Lecture 10 – Viscosity and Flow (Ch. 6) Temperature Dependency: Viscosity decreases with an increase in Temp. Typically 2% per degree C For some materials (fruit juices) the T effect follows an Arrhenius relationship (Eqtn. 6.5 page 193) BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

  13. Lecture 10 – Viscosity and Flow (Ch. 6) Time dependent Viscosity (figure 6.2 page 196) BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

  14. Lecture 10 – Viscosity and Flow (Ch. 6) Time dependent Viscosity Thixotropic examples (viscosity decreases with time) Gelatin, shortening, cream, paints Rheopectic examples (viscosity increases with time) Highly concentrated starch solutions…gravy BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

  15. Lecture 10 – Viscosity and Flow (Ch. 6) Flow in a pipe: Darcy-Weisbach Newtonian -Non-newtonian BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

  16. Lecture 10 – Viscosity and Flow (Ch. 6) Examples of viscometers BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

  17. Viscosity and Flow of Liquids and Semisolids…Chapter 6 BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

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