1. Lecture 9 – 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.
1/13/2019
BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

2. Lecture 9 – 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
1/13/2019
BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

3. Lecture 9 – 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.
1/13/2019
BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

4. Lecture 9 – 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
1/13/2019
BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

5. Lecture 9 – 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.
1/13/2019
BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

6. Dynamic viscosity (Figure 6.1)
Lecture 9 – Viscosity and Flow (Ch. 6)
Dynamic viscosity (Figure 6.1)
1/13/2019
BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

7. Kinematic viscosity: dynamic viscosity/density (no force involved)
Lecture 9 – Viscosity and Flow (Ch. 6)
Kinematic viscosity: dynamic viscosity/density
(no force involved)
1/13/2019
BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

8. Lecture 9 – 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.
1/13/2019
BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

9. Lecture 9 – 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)
1/13/2019
BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

10. Lecture 9 – 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
1/13/2019
BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

11. Lecture 9 – 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
1/13/2019
BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

12. Lecture 9 – 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)
1/13/2019
BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

13. Time dependent Viscosity (figure 6.2 page 196)
Lecture 9 – Viscosity and Flow (Ch. 6)
Time dependent Viscosity (figure 6.2 page 196)
1/13/2019
BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

14. Lecture 9 – 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
1/13/2019
BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

15. Flow in a pipe: Darcy-Weisbach Newtonian -Non-newtonian
Lecture 9 – Viscosity and Flow (Ch. 6)
Flow in a pipe:
Darcy-Weisbach
Newtonian
-Non-newtonian
1/13/2019
BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

16. Examples of viscometers
Lecture 9 – Viscosity and Flow (Ch. 6)
Examples of viscometers
1/13/2019
BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

17. Viscosity and Flow of Liquids and Semisolids…Chapter 6
1/13/2019
BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity