1. 10/11/2015BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity 1 This chapter is a study of the shear stress as a function of the shear rate for Newtonian and non-Newtonian biological materials. Lecture 9 – Viscosity and Flow (Ch. 2 and notes)

2. 10/11/2015BAE2023 Physical Properties of Biological Materials Lecture 9 Viscosity 2 “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 Lecture 9 – Viscosity and Flow

3. 10/11/2015BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity 3 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. Lecture 9 – Viscosity and Flow

4. 10/11/2015BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity 4 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 Lecture 9 – Viscosity and Flow

5. 10/11/2015BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity 5 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 tangential Couette flow:Low-speed, steady motion of a viscous fluid between two infinite plates moving parallel to each other.Couette flow Lecture 9 – Viscosity and Flow http://www.answers.com/topic/viscosity?cat=biz-fin

6. 10/11/2015BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity 6 Dynamic viscosity (Figure 6.1) Lecture 9 – Viscosity and Flow

7. 10/11/2015BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity 7 Kinematic viscosity: dynamic viscosity/density (no force involved) Lecture 9 – Viscosity and Flow

8. 10/11/2015BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity 8 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. Lecture 9 – Viscosity and Flow

9. 10/11/2015BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity 9 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) Lecture 9 – Viscosity and Flow

10. 10/11/2015BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity 10 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 Lecture 9 – Viscosity and Flow

11. 10/11/2015BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity 11 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 Lecture 9 – Viscosity and Flow

12. 10/11/2015BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity 12 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) Lecture 9 – Viscosity and Flow

13. 10/11/2015BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity 13 Time dependent Viscosity (figure 6.2 page 196) Lecture 9 – Viscosity and Flow

14. 10/11/2015BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity 14 Time dependent Viscosity –Thixotropic examples (viscosity decreases with time) »Gelatin, shortening, cream, paints –Rheopectic examples (viscosity increases with time) »Highly concentrated starch solutions…gravy Lecture 9 – Viscosity and Flow

15. 10/11/2015BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity 15 Flow in a pipe: Darcy-Weisbach –Newtonian –-Non-newtonian Lecture 9 – Viscosity and Flow

16. 10/11/2015BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity 16 Examples of viscometers Lecture 9 – Viscosity and Flow

17. 10/11/2015BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity 17 Viscosity and Flow of Liquids and Semisolids…Chapter 6