Rheology

food

1. Unit Two Introduction to Rheology

2. What is Rheology Why we study it How is it related to fluids or fluid mechanics

3. Rheology Science describing the flow and deformation of matter under stress. Rheo = the flow

4. Properties of Fluid Fluid flow takes place when force is applied on a fluid. Force per unit area is defined as stress. When force acting on a surface is perpendicular to it, the stress is called normal stress. More commonly, normal stress is referred to as pressure.

5. When the force acts parallel to the surface, the stress is called shear stress, σ. When shear stress is applied to a fluid, the fluid cannot support the shear stress; instead the fluid deforms, or simply stated, it flows. The influence of shear stress on solids and liquids leads to a broad category of such materials as plastic, elastic, and fluid.

6. Elastic Solids In the case of an elastic solid, when shear stress is applied, there is a proportional finite deformation, and there is no flow of the material. On removal of the applied stress, the solid returns to its original shape.

7. What are plastic materials

8. Soft Cheese and Jell 0

9. What is Fluid

10. Compressiblity When normal stress or pressure is applied on a liquid, there is no observed appreciable effect. Thus, liquids are called incompressible fluids, whereas gases are compressible fl uids , since increased pressure results in considerable reduction in volume occupied by a gas.

11. Viscosity

12. Mathematical Definition of Viscosity

13. Refer to above diagram for infinitesmall movements

14. Equations for handling Newtonian Liquids Mathematically Continuity equations Reynolds Number Entrance region and fully developed region

15. Non Newtonian Liquids Liquids offer interesting properties. They flow under gravity and do not retain their shape. They may exist as solids at one temperature, and liquid at another (e.g., ice cream and shortenings). Products such as apple sauce, tomato purée, baby foods, soups, and salad dressings are suspensions of solid matter in liquid. When droplets of one liquid are submerged in another, we obtain emulsions—milk, for example.

16. Classification of Non Newtonian Liquids

17. Relationship b/w shear stress and shear rate

18. The properties of non-Newtonian liquids can be classifi ed as time independent and time-dependent ( Fig. 2.42 ). The time-independent non-Newtonian liquids respond immediately with a flow as soon as a small amount of shear stress is applied. Unlike Newtonian liquids, the relationship between shear stress and shear rate is nonlinear, as shown in Figure 2.43 . There are two important types of time-independent non-Newtonian liquids, namely, shear-thinning liquids and shear thickening liquids. The differences between these two types of liquids can be understood easily by considering another commonly used term, apparent viscosity .

19. An apparent viscosity is calculated by using a gross assumption that the non-Newtonian liquid is obeying Newton’s law of viscosity

20. Determination of apparent viscosity from plot of shear stress vs shear rate.

21. Thus, at any selected shear rate, a straight line is drawn from the selected point on the curve to the origin ( Fig. 2.44 ). The slope of this straight line gives a value for the apparent viscosity. Using this method, it should be evident that the value obtained for apparent viscosity is dependent on the selected shear rate. Therefore the apparent viscosity must always be expressed along with the value of shear rate used to calculate it; otherwise, it is meaningless.

22. What are Shear Thinning Liquids For a shear-thinning liquid, as the shear rate increases, the apparent viscosity decreases; thus, the name shear thinning is used to describe the behavior of these liquids.

23. Shear Thinning Liquids Shear-thinning liquids also are called pseudoplastic or power law liquids . Some common examples of shear-thinning liquids are condensed milk , fruit purées, mayonnaise, mustard , and vegetable soups . When shear-thinning products are shaken in a jar, they become more “ fluid . ” Similarly , if these products are mixed at high intensity in a mixer , their viscosity decreases, which may aid in their mixing. There are several reasons to explain shear-thinning behavior .

24. A liquid that appears homogenous to the naked eye may actually contain microscopic particulates submerged in it. When these liquids are subjected to a shear, the randomly distributed particles may orient themselves in the direction of flow ; similarly, coiled particulates may deform and elongate in the direction of flow . Any agglomerated particles may break up into smaller particles. These types of modifications due to shearing action improve the flow of such fluids , and an increased “ fluidity ” is observed. T hey are also usually reversible .

25. Thus, when the shearing action is stopped, after a time lag, the particulates return to their original shape—the elongated particulates coil back, and separated particles may again agglomerate . Note that changes in viscosity at a very low shear rate ( 0.5 per s) or a very high shear rate ( 100 per s1 ) are usually quite small, as seen in Figure on next slide Therefore , in measuring rheological properties of power law fluids , a shear rate between 0.5 per s and 100 per s is used.

26. ■ Figure Apparent viscosity vs shear rate .

27. With some liquid foods, the processing steps may alter their flow properties . For example, raw egg at 21C is a Newtonian fluid , but when frozen whole egg is thawed, its response changes to that of a shear-thinning liquid. Similarly , single-strength apple juice is a Newtonian liquid, but concentrated apple juice ( depectinized and filtered) is a shear-thinning liquid

28. Dilatant Liquids If the increase in shear rate results in an increase in apparent viscosity , then the liquid is called a shear-thickening liquid (or sometimes referred to as dilatant liquid ). Examples of shear-thickening liquids include 60% suspension of corn starch in water . With shear-thickening liquids , the apparent viscosity increases with increasing shear rate. These liquids become “ stiffer ” at higher shear rates . Mostly, these liquids are suspensions—solid particles in a liquid that acts as a plasticizer .

29. Why behind dilatant liquids At low shear rates, the liquid is sufficient to keep the solid particles well lubricated, and the suspension flows almost as a Newtonian liquid. But as shear rate increases, the solid particles begin to separate out, forming wedges while increasing the overall volume. Hence , they are called dilatant liquids. The liquid is then unable to act as a plasticizer. As a result, the overall suspension becomes more resistant to flow .

30. Bingham Plastic and Heschel Bulkley Fluids Another important class of non-Newtonian liquids requires the application of yield stress prior to any response . For example, tomato catsup For these types of liquids, a plot of shear stress against shear rate does not pass through the origin, as shown in last Figure After the application of yield stress, the response of these liquids can be similar to a Newtonian liquid; in that case, they are called Bingham plastic .

31. On the other hand, if the response of a liquid, after the yield stress is applied, is similar to a shear-thinning fl ow , then these liquids are called Herschel– Bulkley fl uids .

32. These liquids that require a yield stress to flow may be viewed as having an interparticle or intermolecular network that resists low-level shear force when at rest. Below the yield stress, the material acts like a solid and does not flatten out on a horizontal surface due to force of gravity. It is only when the applied stress exceeds the forces holding the network together that the material begins to fl ow .

33. Time Dependant Non Newtonian Liquids Time-dependent non-Newtonian liquids obtain a constant value of apparent viscosity only after a certain finite time has elapsed after the application of shear stress. These types of liquids are also called thixotropic materials; examples include certain types of starch pastes.

34. Thixotropy

35. Reason Behind Thixotropy Many dispersions not only show the potential for orientation but additionally for a time-related particle/molecule-interaction. This will lead to bonds creating a three-dimensional network structure which is often called a “gel”. In comparison to the forces within particles or molecules, these bonds -- they are often hydrogen or ionic bonds -- are relatively weak: they rupture easily, when the dispersion is subjected to shear over an extended period of time (Fig. 9).

36. Thixotropy When the network is disrupted the viscosity drops with shear time until it asymptotically reaches the lowest possible level for a given constant shear rate. This minimum viscosity level describes the “sol”-status of the dispersion. A thixotropic liquid is defined by it’s potential to have it’s gel structure reformed, whenever the substance is allowed to rest for an extended period of time. The change of a gel to a sol and of a sol to a gel is reproducible any number of times.

37. Hysteresis Curve of Thixotropy

38. Some thixotropic fluids return to a gel state almost instantly, such as ketchup, and are called  pseudoplastic  fluids. Others such as yogurt take much longer and can become nearly solid. Many  gels  and  colloids  are thixotropic materials, exhibiting a stable form at rest but becoming fluid when agitated.

39. Thixotropy Tests: How to test its behaviour

40. Rotational Test

41. Rheopectic Flow Behavior Rheopective liquids are characterized by a viscosity increase related to the duration of shear. When these liquids are allowed to rest they will recover the original -- i.e. the low -- viscosity level. Rheopective liquids can cycle infinitely between the shear-time related viscosity increase and the rest-time related decrease of viscosity. Rheopexy and thixotropy are opposite flow properties. Rheopexy is very rare.

42. Can you now tell me what is rheology and rheometry

43. Viscoelastic =viscous +elastic

44. Viscoelastic Behaviour

45. Viscoelastic Behaviour of Polyisobutylene (PIB)

46. Shear Deformation or Shear Strain

47. Elasticity Law

48. Creep Test for Visco elastic Materials Ie how to test whether a material is viscoelastic or not

49. Difference between viscoelastic solid and fluid In the curve shown in the last slide the visco elastic solids completely recover ie the viscous strain is zero. But for viscoelastic fluid viscous strain is not zero after removal of the shear stress.

50. How to model the viscoelastic material with the help of spring and dashpot

51. Burger Model

52. Burger Model Here a material is taken for study which consists of macromolecules(parallel connection of spring 2 and damper 2){shown in the centre of burger model. It is linked by springs(spring 1 in a highly viscous oil(damper 3). When we apply shear stress it leads to sudden increase in strain(dilatation of spring elements 1 which are situated in the orientation of the strain)

53. After that strain rate drops. During that time macromolecules get oriented, the twisted springs get tensioned and the macromolecules stretched upto their mechanically maximal possible size(delayed viscoelactic strain of spring 2 and damper 2).

54. If more force is applied the strain increase linearly and if the macromolecules are irreversible disentangles and cause to flow with the viscous matrix mass(viscous strain of damper 3)

55. If the test duration is long enough, all dampers and springs finally show maximum dilatation. During the recovery phase or relieve phase spring 1 returns to its original tension(elasticity) and the parallel connection of spring 2 and damper 2 recovers with a delay( viscoelastic recovery). Damper 3 remains fully displaced so that a partial strain is maintained.

56. If the remaining strain is very small the material is called a viscoelastic solid other wise it is called a viscoelastic liquid.

57. Show from the pdf copy

58. Relaxation Test

59. Relaxation Test Analysis

60. Relaxation Time Spectrum and Retardation Time Spectrum

61. Oscillatory Tests

62. Amplitude Sweep Test

63. Frequency Sweep test

64. Types of Rheometers

65. Controlled Stress

66. When to Use

67. Plate and Cone

68. Plate and Cone

69. Plate and Cone

70. Plate and Cone

71. Parallel Plate

72. Parallel Plate

73. Parallel Plate

74. Capillary Rheometer

75. Shear rate calculation for capillary rheometer

76. Viscosity calculation for capillary rheometer

77. THI X O T ROPY THI X O T ROPY N on- Ne w ton ia n , T i m e De p e nd e nt b e h a v i ou r. De f i n i t i on of Th ix ot r o p y: I t is th e d ec r ease i n v isc o si t y as a fun c t i on of t i m e upon s h ea r i ng , th e n r ec o v e r y of o r i g i n al v isc o si t y as a fun c t i on of t i m e w i thout s h ea r i ng