Rheology Flashcards
What is Rheology?
This is the study of flow, and how much force to add to something in order to make it move
What is the force per unit area needed to cause the liquid to flow?
‘Shearing stress’ (σ) = F/A (force/area)
The higher the force needed, the higher the shearing stress.
The difference in velocity between two layers at a given distance apart is known as:
Rate of shear (γ) , Υ=dv/dx
dv = difference in velocity
dx = distance between two layers
What is the rate of flow directly related to?
It is directly related to the force applied (σ) with viscosity (η) as a constant
What happens to the rate of flow if more force is applied?
The rate of flow will increase.
What will happen to the force required to get a flow if the dispersion is very viscous?
The force required (σ) will be greater to get a flow if the dispersion is very viscous.
Newtonian flow: How does a liquid behave to be classed as having Newtonian flow? (2)
1) The liquids continue to flow in the same way, and are simple and predictable. i.e. if a a small amount of pushing got a response of 1 and a large mount of pushing gave a response of 3, could predict a medium amount of pushing would give a response of 2.
2) Their behaviour is simple and predictable, and they move and change shape in proportion to the force applied.
Give an example of a Newtonian liquid:
Water is a Newtonian liquid - it continues to behave in the same way regardless of how fast it is stirred or mixed i.e. we always know what the water will do when we pour it into a glass based on the force that we pour.
Viscosity η = σ/γ. Explain why viscosity is always a constant for each individual Newtonian systems?
Rate of flow (shear γ) is directly proportional to the applied force (stress σ). So σ+γ are always proportional e.g. 1/1, 2/2, so viscosity is a constant
Non-Newtonian Systems: What are these?
Fluids that behave in an unepected, non-linear way. They do not have a constant viscosity (so it changes as the force changes) and their properties change in response to movement and deformation.
What are the 3 types of non-Newtonian flow?
1) Plastic flow
2) Pseudoplastic Flow
3) Dilatant Flow
Explain Plastic Flow:
It is not often seen in pharmacy, but does happen in very concentrated suspensions with flocculated particles in a continuous phase with high viscosity.
It is characterised as a minimum force beond which the material will flow i.e. when first applying stress, there is no response until a level of stress is reached and there is a response and the material starts to flow.
CHARACTERIZED BY GAP AT BEGINNING OF GRAPH WHERE NOTHING HAPPENS
It is non-newtonian
What happens in the period of plastic flow when the force is not enough to make the material flow?
The force is not being used for flow it is used to make particles all line up and untangle, once all lined up, particles can all pass by each other = YIELD STRESS, and the material will flow
How do you use the graph to caluclate the applied force (stress σ)?
If a force of 2 gives a flow of 2, what will a force of 1 give?
Extrapolate the linear line back to x axis, and mark this point as σy, then use the same equation as the Newtonian flow, only with σy added: σ = σy + ηΥ
A force of 1 will give a flow of less than 1 because the initial force goes to align the particles ready for flow.
Explain Pseudoplastic flow:
The viscosity decreases with the rate of sheer i.e. when force is put to the fluid (like shaking), the fluid becomes less viscous as the particles align and untangle e.g. Calpol. Fluids with this type of flow are flocculated dispersions with long, high molecular weight molecules.
Also called “shear thinning” fluids.
It is non-newtonian
