Huan+Jia's Weekly+Lecture+Summary

= __Rheology__ =

__What is rheology?__
Rheology is a phenomenon where we encounter it in our daily life.We pour water everyday to drink, squeeze the lemon to have the juice to drink and perhaps,we spread the jam on the toast.Rheology is the study of the flow of fluids and the deformation of the solids when some forces are applied to it such as pouring,squeezing and others.Rheology is applicable to all materials, from gases to solids.When a solid is applied to a force,it will deform, whereas when liquid is subjected to a force,it will flow.Rheology is also concerned with relationship between stress,strain and time.

 For instance,honey in the packaged bottle.Honey is made to be able to flow easily when it is poured onto some food.It will also flow back to the original condition which is become flat rapidly. For the example of j am which has higher viscosity compared to honey,it is more difficult to flow back to its original condition.

 

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.
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__Non-Newtonian fluids__
- Any liquid that show deviation from newtonian behavior. <span style="font-family: Arial,Helvetica,sans-serif; font-size: 15px;">- Shear viscosity is dependent on shear rate but may be dependent or independent on the time of shearing. <span style="font-family: Arial,Helvetica,sans-serif; font-size: 15px;">- Classified into Non-newtonian time dependent and Non-nwtonian time independent fluids. <span style="font-family: Arial,Helvetica,sans-serif; font-size: 15px;"> - Non-newtonian time dependent fluids : Pseudoplastic, Dilatant, Bingham and Casson plastic. <span style="font-family: Arial,Helvetica,sans-serif; font-size: 15px;">- Non-newtonian time Independent fluids : Thixotropic and Rheopectic. <span style="font-family: Arial,Helvetica,sans-serif; font-size: 15px;">- Examples : ketchup, salad dressing, lithographic ink, mayonnaise, skin cream, hair gel, toothpaste, custard and shampoo, etc.

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 * [[image:imk209/270px-Thixotropic-rheopectic.svg.png caption="270px-Thixotropic-rheopectic.svg.png"]] ||
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<span style="color: #ff0000; font-family: Arial,Helvetica,sans-serif; font-size: 16px;">Here are some links that show the examples of the non-newtonian fluids with the demonstration: <span style="color: #0000ff; font-family: Arial,Helvetica,sans-serif; font-size: 16px;">[] <span style="color: #0000ff; font-family: Arial,Helvetica,sans-serif; font-size: 16px;">[]

<span style="font-family: Arial,Helvetica,sans-serif; font-size: 15px;">** __Non-Newtonian, Time Independent Fluids Non-Newtonian, Time Dependent Fluids__ **

===<span style="font-family: Arial,Helvetica,sans-serif; font-size: 15px;">‍ - Non-Newtonian, Time Independent Fluids ===
 * <span style="font-family: Arial,Helvetica,sans-serif; font-size: 15px;">Shear-Thinning---A decrease in viscosity with increasing shear rate. (referred to as Pseudoplasticity)
 * <span style="font-family: Arial,Helvetica,sans-serif; font-size: 15px;">Shear-Thickening---An increase in viscosity with increasing shear rate. (reffered to as Dilatancy)
 * <span style="font-family: Arial,Helvetica,sans-serif; font-size: 15px;">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.


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<span style="color: #808000; font-family: Arial,Helvetica,sans-serif; font-size: 15px;">- Non-Newtonian, Time Dependent Fluids

 * <span style="font-family: Arial,Helvetica,sans-serif; font-size: 15px;">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.
 * <span style="font-family: Arial,Helvetica,sans-serif; font-size: 15px;">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.

=
<span style="font-family: Arial,Helvetica,sans-serif; font-size: 15px;">The yield stress is the applied stress we must exceed in order to make a structured fluid flow.Yield stress involves the occurrence of the stress onto certain materials in which the materials absorb the stress and start to break down. Thus, there is an increasing in the viscosity. For example, when there is no force applied to a ketchup, it cannot flow out of the bottle. In contrast, it can flow out of the bottle if there is a force applied to it(which means that we pour it out). =====




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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)

<span style="color: #008080; font-family: Arial,Helvetica,sans-serif; font-size: 15px;">- Empirical test

 * <span style="font-family: Arial,Helvetica,sans-serif; font-size: 15px;">Test and see
 * <span style="font-family: Arial,Helvetica,sans-serif; font-size: 15px;">Measurement made without changes in product shape, or control of deforming process
 * <span style="font-family: Arial,Helvetica,sans-serif; font-size: 15px;">Examples : Bostwick consistometer, falling ball viscometer,glass capillary viscometer

<span style="color: #008080; font-family: Arial,Helvetica,sans-serif; font-size: 15px;">- Fundamental test

 * <span style="color: #000000; font-family: Arial,Helvetica,sans-serif; font-size: 15px;">'Physics based approach". Use well defined text fixture(geometry) and test condition.
 * <span style="color: #000000; font-family: Arial,Helvetica,sans-serif; font-size: 15px;">units/values are independent of sample geometry and data can be validated with another geometry.
 * <span style="font-family: Arial,Helvetica,sans-serif; font-size: 15px;">Examples : Rotational viscometer