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Introduction to Rheology

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Title: Introduction to Rheology


1
Introduction to Rheology
By Dr. Pawan Singh Takhar pawan.takhar_at_ttu.edu D
epartment of Animal and Food Sciences Texas Tech
University
2
References
  • Books
  • Rheological methods in food process engineering.
    By James F. Steffee
  • Viscoelastic properties of polymers. By John D.
    Ferry
  • Food Chemistry. By Fennema

3
What is Rheology?
  • Study of flow and deformation behavior of
    materials
  • Examples
  • Liquid Foods
  • Flow of water, ketchup, mayonnaise, salad
    dressings etc.
  • Solid Foods
  • Creep of apples, grains, cheese etc.

4
Examples of Rheological Properties
  • Viscosity
  • Power law parameters
  • Elasticity
  • Stress relaxation function
  • Creep compliance function

5
Applications of Rheology
  • Geology and mining
  • Concrete technology
  • Soil mechanics
  • Plastics industry
  • Tribology (study of lubrication)
  • Food industry

6
Two Extremes of Material Flow Behavior
Newtonian Liquids
Most Foods
Elastic Solids
7
Stress and Strain
  • Elastic Spring

L
Area, A
?L
Force, F
8
SI Units of Stress and Strain
  • Stress Pa (N/m2)
  • Strain ?

9
Liquid Foods
10
Dynamic Viscosity
Shear stress
  • Lack of slipperiness
  • Resistance to flow
  • Interlayer friction

SI Unit Pascal-second
Shear rate
1 centi-Poise milli Pascal-second
11
  • Typical Viscosities (Pa.s)
  • Asphalt Binder ---------------
  • Polymer Melt -----------------
  • Molasses ----------------------
  • Liquid Honey -----------------
  • Glycerol -----------------------
  • Olive Oil -----------------------
  • Water --------------------------
  • Acetic Acid --------------------

100,000 1,000 100 10 1 0.01 0.001 0.00001
Courtesy TA Instruments
12
Different types of Fluids
Shear stress
Non Newtonian Fluids
Shear rate
13
Newtonian Foods
Shear stress
Shear rate
  • Examples
  • Water
  • Milk
  • Vegetable oils
  • Fruit juices
  • Sugar and salt solutions

14
Pseudoplastic (Shear thinning) Foods
Shear stress
Shear rate
  • Examples
  • Applesauce
  • Banana puree
  • Orange juice concentrate
  • Oyster sauce
  • CMC solution

15
Dilatant (Shear thickening) Foods
Shear stress
Shear rate
  • Examples
  • Liquid Chocolate
  • 40 Corn starch solution

16
Bingham Plastic Foods
Shear stress
Shear rate
  • Examples
  • Tooth paste
  • Tomato paste

17
Mathematical Model
  • Herschel Bulkley Model

18
Model parameters for different types of fluids
Yield stress?
0ltnlt1
n1
Shear stress
1ltn
Shear rate
19
Importance of yield stress
  • When stress is less than yield stress, material
    does not flow. It behaves like a solid

Important for development of materials like
butter, yogurt, cheese spread
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