Introduction to Rheology

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Introduction to Rheology
By Dr. Pawan Singh Takhar pawan.takhar_at_ttu.edu D
epartment of Animal and Food Sciences Texas Tech
University
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References

• Books
• Rheological methods in food process engineering.
  By James F. Steffee
• Viscoelastic properties of polymers. By John D.
  Ferry
• Food Chemistry. By Fennema

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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.

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Examples of Rheological Properties

• Viscosity
• Power law parameters
• Elasticity
• Stress relaxation function
• Creep compliance function

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Applications of Rheology

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

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Two Extremes of Material Flow Behavior
Newtonian Liquids
Most Foods
Elastic Solids
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Stress and Strain

• Elastic Spring

L
Area, A
?L
Force, F
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SI Units of Stress and Strain

• Stress Pa (N/m2)
• Strain ?

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Liquid Foods
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Dynamic Viscosity
Shear stress

• Lack of slipperiness
• Resistance to flow
• Interlayer friction

SI Unit Pascal-second
Shear rate
1 centi-Poise milli Pascal-second
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• 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
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Different types of Fluids
Shear stress
Non Newtonian Fluids
Shear rate
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Newtonian Foods
Shear stress
Shear rate

• Examples
• Water
• Milk
• Vegetable oils
• Fruit juices
• Sugar and salt solutions

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Pseudoplastic (Shear thinning) Foods
Shear stress
Shear rate

• Examples
• Applesauce
• Banana puree
• Orange juice concentrate
• Oyster sauce
• CMC solution

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Dilatant (Shear thickening) Foods
Shear stress
Shear rate

• Examples
• Liquid Chocolate
• 40 Corn starch solution

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Bingham Plastic Foods
Shear stress
Shear rate

• Examples
• Tooth paste
• Tomato paste

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Mathematical Model

• Herschel Bulkley Model

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Model parameters for different types of fluids
Yield stress?
0ltnlt1
n1
Shear stress
1ltn
Shear rate
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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