YIELD STRESS

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YIELD STRESS
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Some Observations

• A jar of mayonnaise, ketchup, whipped topping or
  chutney may be tilted without the product flowing
• A jar of mayonniase can be gently pushed with a
  spoon, and it may return to its original position
• Such products may behave as thick liquids or as
  semi-solids

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It tends to set up
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The official way to make it flow
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Yield Stress

• If the lateral force on the food is strong
  enough, the mayonnaise will not return to its
  initial position
• If the thick ketchup is shaken or jerked with
  enough force, the ketchup will flow
• The yield stress is the minimum force required to
  make the material flow

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Does it really exist?

• Everything flows given enough time
• From a practical standpoint, it is an engineering
  reality

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. . . It might be good

• May inhibit flow under low stress caused by
  gravity
• May give sag or slump resistance to molten
  chocolate or batters
• May prevent particle settling
• Yield stress plays a big role on how well some
  foods are coated

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. . . On the down side

• Causes problems in gravity feed systems
• Excess residue on sides of bottles

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Static vs dynamic yield stress

• There can be two types of structure in a
  thixotropic fluid
• One structure insensitive to shear rate- defines
  dynamic yield stress
• A weak structure forms over time when the system
  is at rest
• Together, both contribute to resistance to flow
  leading to static yield stress

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Equilibrium flow curve
Static yield stress
Shear Stress
Dynamic yield stress
Shear Rate
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Measuring Yield Stress

• Measuring yield stress can be difficult as yield
  may occur at relatively low stress
• It has been demonstrated that a variation of the
  yield stress of more than one order of magnitude
  can be obtained depending on the way it is
  measured
• Despite the controversial concept of the yield
  stress as a true material property, there is
  generally acceptance of its practical usefulness
  in engineering design and operation of processes
  where handling and transport of industrial
  suspensions are involved

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Traditional viscometry

• Plot shear stress versus shear rate
• Fir data with a model that helps predict yield
  stess

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Model fit such as power law or Casson
Often, data is unavailable at low shear stresses
. . . The very part we are most interested in.
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Slump Test
Sample is placed in a holding cup of specific
dimensions. Material is inverted and
released The distance slumped over a given
time is measured
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Such as for testing concrete
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Devices similar to the USDA Consistometer have
been used to study canned pumpkin and similar
products
h
h
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Vane Rheometry

• Use a vane viscometer
• Vane is lowered slowly into sample
• Increase strain systematically. The sample
  deforms elastically. Stress increases until the
  yield stress occurs and the material flows.

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Vane Method 1 Measure Minimum Torque Required to
Initiate Flow

• Note that by using vanes we increase the
  sensitivity of the measurement

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h
d
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Mo
Torque
Time
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time evolution of the stress for imposed shear
rate experiments at different imposed rates
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• Different shear rates may be used, and the shear
  stress extrapolated to zero shear rate

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Shear Stress
Shear Rate
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• One could also linearly change the stress, and
  note where the material gives

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Strain
Yield stress
Stress (Pa)
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Cone Penetrometer Method
A cone is brought into the sample. Several
experiments are run at different speeds
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0.10
0.08
Force
0.04
Time
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Yield stress
dh/dt