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. 1/17/2019 BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

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 1/17/2019 BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

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. 1/17/2019 BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

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 1/17/2019 BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

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 1/17/2019 BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

Dynamic viscosity (Figure 6.1) Lecture 10 – Viscosity and Flow (Ch. 6) Dynamic viscosity (Figure 6.1) 1/17/2019 BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

Kinematic viscosity: dynamic viscosity/density (no force involved) Lecture 10 – Viscosity and Flow (Ch. 6) Kinematic viscosity: dynamic viscosity/density (no force involved) 1/17/2019 BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

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. 1/17/2019 BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

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) 1/17/2019 BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

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 1/17/2019 BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

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 1/17/2019 BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

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) 1/17/2019 BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

Time dependent Viscosity (figure 6.2 page 196) Lecture 10 – Viscosity and Flow (Ch. 6) Time dependent Viscosity (figure 6.2 page 196) 1/17/2019 BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

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 1/17/2019 BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

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 1/17/2019 BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

Examples of viscometers Lecture 10 – Viscosity and Flow (Ch. 6) Examples of viscometers 1/17/2019 BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity

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

HW Due 2/19 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 1/17/2019 BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity