YIELD STRESS

1. YIELD STRESS

2. 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

3. It tends to set up

4. The official way to make it flow

7. 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

8. Does it really exist? • Everything flows given enough time • From a practical standpoint, it is an engineering reality

9. . . . 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

11. . . . On the down side • Causes problems in gravity feed systems • Excess residue on sides of bottles

12. 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

13. Static yield stress Equilibrium flow curve Shear Stress Dynamic yield stress Shear Rate

14. 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”

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

16. Model fit such as power law or Casson Often, data is unavailable at low shear stresses . . . The very part we are most interested in.

17. 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

18. Such as for testing concrete

19. Devices similar to the USDA Consistometer have been used to study canned pumpkin and similar products h h’

21. 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.

23. Vane Method 1: Measure Minimum Torque Required to Initiate Flow • Note that by using vanes we increase the sensitivity of the measurement

24. h d

25. Mo Torque Time

26. time evolution of the stress for imposed shear rate experiments at different imposed rates

27. Different shear rates may be used, and the shear stress extrapolated to zero shear rate

28. Shear Stress Shear Rate

29. One could also linearly change the stress, and note where the material gives

30. % Strain Yield stress Stress (Pa)

31. Cone Penetrometer Method A cone is brought into the sample. Several experiments are run at different speeds

33. 0.10 0.08 Force 0.04 Time

34. Yield stress dh/dt