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

4. Lecture 10 – Viscosity and Flow (Ch. 6)
Newtonian Fluids
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 10 – Viscosity and Flow (Ch. 6)
1/13/2019
BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

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

7. Lecture 10 – Viscosity and Flow (Ch. 6)
Kinematic viscosity: dynamic viscosity/density
Gives a quantity that is dependent only on the type of fluid, not density or concentration
1/13/2019
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.
Reference article:
1/13/2019
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)
1/13/2019
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
1/13/2019
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
1/13/2019
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)
1/13/2019
BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

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

15. Flow in a pipe: Darcy-Weisbach Newtonian -Non-newtonian
Lecture 10 – 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 10 – Viscosity and Flow (Ch. 6)
Examples of viscometers
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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

18. HW#6 Assignment Due 2/18
Problem 1: Name three applications where knowing the viscosity of a food product would be important. Are the food products Newtonian or non-Newtonian? How do you know which it is? (because the book says so is not the right answer!)
Problem 2: Pick one of the different kinds of viscometers, explain briefly how it works and where or how it would be used.
Problem 3: 6.1 in the Stroshine Book
1/13/2019
BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity