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Introduction to Rheology

Introduction to Rheology. By: Dr. Pawan Singh Takhar pawan.takhar@ttu.edu Department of Animal and Food Sciences Texas Tech University. References. Books: Rheological methods in food process engineering. By: James F. Steffee Viscoelastic properties of polymers. By: John D. Ferry

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Introduction to Rheology

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  1. Introduction to Rheology By: Dr. Pawan Singh Takhar pawan.takhar@ttu.edu Department 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. Area Force L h 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. Casson Plastic Bingham Plastic Pseudoplastic (or Shear thinning) Newtonian Dilatant (or Shear thickening) 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. Casson Plastic Bingham Plastic Pseudoplastic (or Shear thinning) Newtonian Model parameters for different types of fluids Yield stress? 0<n<1 n=1 Shear stress 1<n 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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