RHEOLOGY

What is Rheology?

We encounter rheology in our daily life. We eat breakfast using a range of spread of toast. We and knead bread dough and squeezed toothpaste from the tubes. Rheology is the study of deformation and flow of material that behave in an unusual way under stress and strains. “Unusual way” here means the unusual flow behavior of materials. Oil and water flow in a normal ways, whereas mayonnaise, peanut butter, chocolate, toothpaste and bread dough flow in complex and unusual ways.
Rheology is mainly concerned with relationship between strain, stress and time.

For example, when we open a partly used jar of mayonnaise, the top surface retains the last shape that was left by the last person who used it. When we compare this observation with the behavior of honey, we will notice that the top surface of honey in a jar is always smooth. The surface of honey will be flat again within a few seconds after we carved with a spoon. Honey is able to flow and become flat quite rapidly, while the mayonnaise, even after months, fails to flow, and it retains the last shape carved into it by a knife.

Normal fluids can be different because some fluids are thicker than others; some fluids have higher viscosities than others. But other than having different viscosities, all normal or Newtonian fluids (oil, air, water, honey) will follow the same Newtonian flow laws. However, some fluids do not follow Newtonian flow laws and these are the non- Newtonian fluids, for example, mayonnaise, peanut better, and jam do not follow Newton’s laws of viscosity and therefore is classified by rheology standards for unusual flow.

On the other hand, the “deformation and flow” of material when subjected to external forces includes classical fluid mechanics and elasticity which treat the flow of Newtonian liquids, such as water, oil, honey, ketchup and small deformations of hard solids, such as wood and steel. When subjected to external forces, solids will deform, whereas liquids will flow. We apply external forces such as pouring, pushing, pressing, sucking, scooping, etc. by stress and strain. For example, when we press the plastic bottle of ketchup, external force will initiate the flow of ketchup out of the bottle, when we suck water using a straw, the external suction force will initiate the water to flow up the straw.

Stress and Strain

a) Stress-----Simply defined as force per unit area. (we can think stress as a normalised force or the intensity of force)
b) Strain------Simply a quantitative measure of the extent to which ann element of material has been deformed.
The deformation implies the change of shape. (dimensional change)

Newtonian and Non-Newtonian fluids

Viscosity of fluids are affected by :
- Shear rate
- Time of shearing
- Temperature
- Pressure

Newtonian fluids

- Shear viscosity depends only on temperature and pressure.
- Shear viscosity does not vary with shear rate.
- Examples : water, oil, milk, honey, soft drinks sugar and salt solution, etc.

Non-Newtonian fluids

- Any liquid that show deviation from newtonian behavior.
- Shear viscosity is dependent on shear rate but may be dependent or independent on the time of shearing.
- Classified into Non-newtonian time dependent and Non-nwtonian time independent fluids .
- Non-newtonian time dependent fluids : Pseudoplastic, Dilatant, Bingham and Casson plastic.
- Non-newtonian time Independent fluids : Thixotropic and Rheopectic.
- Examples : ketchup, salad dressing, lithographic ink, mayonnaise, skin cream, hair gel, toothpaste, custard and shampoo, etc.

Non-Newtonian, Time Independent Fluids Non-Newtonian, Time Dependent Fluids

- Non-Newtonian, Time Independent Fluids

• Shear-Thinning---A decrease in viscosity with increasing shear rate. (referred to as Pseudoplasticity)
• Shear-Thickening---An increase in viscosity with increasing shear rate. (reffered to as Dilatancy)
• Rheologists call things like cornflour slurry a dilatant fluid. A dilatant fluid behaves like a liquid when moved slowly, but like a solid when hit hard. Why? Dilatant fluids often contain particles that, when moved slowly, will change shape and slide past each other. When they are moved quickly they lock up - a little bit like people trying to push quickly through an exit door and blocking it up.

Let's experiment
1. Take the cornflour or cornstarch and slowly mix in the water (using fingers or a spoon) until it has the consistency of tomato sauce when you pour it.
2. Now that you have made your slurry, investigate it's ‘magical' properties! Try tapping it with your finger at different speeds - tap it slowly and you should be able to poke your finger right in. Tap it fast and it should be solid. Try picking it up and letting it run through your fingers.

- Non-Newtonian, Time Ddependent Fluids

• Thixotropy---A decrease in apparent viscosity with time under constant shear rate or shear stress, followed by a gradual recovery, when the stress or shear rate is removed.
• Some fluids behave in the opposite manner of dilantant fluid - a thixotropic fluid tends to look like a solid until you stir it, and then it behaves like a runny liquid. Test this witth a bowl of yogurt or tomato sauce.

‍What is yield stress?

The yield stress is the applied stress we must exceed in order to make a structured fluid flow. It is the minimum shear stress that has to be applied to initiate a sample flow. The minimum shear stress is required to overcome the cohesive force that holds. This yield stress is in fact the strength necessary to break the continuous network of interactions between particles throughout the sample.Some common products that exhibit yield stress are ketchup, salad dressing, lithographic ink, mayonnaise, skin cream, hair gel, toothpaste, custard, shampoo, shaving cream and blood.

The yield stress is the applied stress we must exceed in order to make a structured fluid flow.

Yield stress can be further classified into Static and Dynamic Yield Stress. The intersection on the stress axis is then taken as the yield stress, the assumption being that any stress below this is insufficient to cause the sample to flow. Rheologists call this a dynamic yield stress. (refer to figure 1 and figure 2)

Measuring Viscosity

- Empirical test

• Test and see
• Measurement made without changes in product shape, or control of deforming process
• Examples : Bostwick consistometer, falling ball viscometer,glass capillary viscometer

- Fundamental test

• 'Physics based approach". Use well defined text fixture(geometry) and test condition.
• units/values are independent of sample geometry and data can be validated with another geometry.
• Examples : Rotational viscometer

Viscoelasticity

Viscoelasticity material have both viscous (liquid) and elastic (solid) properties.

Weissenberg Effect (Rod-climbing phenomena)

• Rod-climbing phenomena is a manifestation of elastic component in the viscoelastic material.
• The unique manifestation of these effects depend on the ratio of elastic to viscous components in the viscoelastc material.

Spring and Dashpot Model

• Spring---purely elastic response (hookean solid)
• Dashpot---purely viscous response (newtonian liquid)

Tme-dependent Viscoelastic Behavior : The Deborah Number

• What we measure depend on how rapidly we measure.
• Deborah number, De---the ratio of a characteristic relaxation time of a material to a characteristic time of the relevant deformation pocess. De=τ/Τ
• Hookean elastic solid---T is infinite
• Newtonian viscous liquid---T is zero
• High De (>>1) Solid-like behavior
• Low De (<<1) Liquid-like behavior
• IMPLICATION: Material can appear solid-like either because:(1) it has a very long characteristic relaxation time or (2) the relevant deformation process is very

Dynamic Oscillatory Testing

• Small amplitude oscillatory shear (SAOS)
• Dynamic rheological experiment

Figure above show the solid food textures measured as small amplitude oscillatory shear (SAOC). G′ is indicated by triangles and phase angle by squares.

Viscoelastic Parameters

• Complex modulus : measure of materials overall resistance to deformation. G*=Stress*/StrainG*=(G’2 +G”2)1/2
• Elastic (storage) modulus : measure of elasticity of material. The ability of the material to store energy. G' = (stress*/strain)cos δ
• Viscous (loss) modulus : ability of the material to dissipate energy. Energy lost as heat. G" = (stress*/strain) sin δ
• Tan delta : measures of material damping - such as vibration or sound damping. Tan δ = G"/G'

Principles of Texture Analysis

Benefits of instrumental analysis of food texture

1. Force is applied at a precise rate
2. Results are accurate and repeatable
3. Povide an objective measure of costomer acceptance
4. Cost-effective way to provide repeatable results quickly
5. Shorten new product development cycle
6. Provide process control feedback

What is Texture?

Texture encompasses all the rheological and structural ((geometrical&surface) attributes of a food product perceptible by means of mechanical, tactile, visual and auditory receptors.

Types of Instrumental Texture Test

• Fundamental test
• Empirical test
• Imitative test

- Imitative Test

Texture Profile Analysis---Provides textural parameters which correlate well with sensory evaluation parameters.

- Empirical Tests

1. Compression
2. Puncture & penetration
3. Cutting & shearing
4. Fracture & bending
5. Extrusion (forward & backward)
6. Tension

1. Compression
- Assumes : The sample being tested has a surface area equal to or smaller than the diameter of the probe in use.
- Popular for the testing of : breakfast cereals, breadcrumbs, freshness/firmness of cake/bread.

2. Puncture & Penetration
- Assumes : the samples being tested is of larger surface area than the contact area of the probe used.
- Usually involves : small cylinder probes, needle and conocal probes.
- Causes irreversible changes in sample.
- Involves compression and shear force.
- Popular for testing of : biscuit dough consistency, cheese maturity/hardness, gel rupture strength, fruit ripeness.

3. Cutting & Shearing
- Usually involves : Knife/guillotine blade, Warner-Bratzler blade, Volodkevich bite jaws, Wire cutter, Kramer shear cell, Light knife blade, Craft knife.

4. Fracture & Bending
- Three Point Bend rig---measure fracture and break strength of biscuits, chocolate, bread sticks.
---measure freshness of vegetables.

5. Extrusion (forward)
- Sample is forced through orifice in bottom of pot.
- Popular for testing of : viscous liquid, gels, paste, processed fruits and vegetables, creams, toothpaste.

6. Extrusion (backward)
- Sample is placed in a pot and piston is forced through sample.
- Product extrudes around the disc.
- Popular for testing of : viscous liquid, gels, paste, processed fruits and vegetables.