Viscosity Flashcards
Define: Viscosity
A measure of a fluidβs resistance to flow. The perception of thickness
Define: Shear viscosity
The ratio of the applied shear stress (π) to the resulting rate of shear (πΈ Μ)
Why is measuring viscosity important
- Ingredient specifications may require a certain viscosity - can affect food quality e.g sedimentation - Food safety e.g. different gels can stop bacterial growth - Health issues. something more viscous in the small bowel can change the rate at which something is absorbed - processing. moving it through pipes in a factory - Pharmaceutical application - NPD
Define: Shear stress
The stress acting in a plane as the result of a force acting parrallel to the plane π =πΉ/π΄ π = shear stress F = force applied A = surface area It is measured in pascals (Pa)
Define: Shear rate
The rate at which we are moving the material
How can we measure viscosity
Parallel plate Cone and plate Concentric cylinder
How do the different methods of measuring viscosity differ
A larger surface area allows for more precise measurements. In terms of measurements parallel plate < cone and plate < Concentric cylinder
How is rotational viscosity measured
Rotating the measuring device in one direction and analysing the amount of force applied
What is complex (dynamic) viscosity?
Measuring the viscosity through oscillating (backwards and forwards)
Complex (or Dynamic) Viscosity (Ξ·*) is denoted as: Ξ·* = G*/ Ο
Ο = frequency of oscillation
G* = (Gβ 2 + Gββ 2) 1/2
Define: Storage modulus
Also known as Gβ, it is the measure of the deformation energy stored in the system . Reflects the solid (elastic) part of the system
Define: Loss modulus
Also known as Gββ, it is a measure of the liquid (viscous) part of the sample
Explain this graph
Graph shows two different newtonian fluids. In Newtonian fluids if we change the viscosity it remains proportional to the shear rate e.g. honey, oil, milk
Explain the purple line in this graph
Graph shows a non-newtonian fluid which is sheer thickening. Viscosity increases as we increase the shear rate
Explain the green line in this graph
Graph shows a non-newtonian fluid which is sheer thinning. Viscosity decreases as we increase the shear rate
Define: Non-Newtonian fluid
A fluid thats viscosity is dependent on its shear rate
Define: Newtonian fluid
A fluid thats viscosity remains constant independent of shear rate
Explain the blue line on this graph
Graph shows a newtonian fluid. The viscosity remains constant irrelevant of the shear rate
Explain what this graph is showing
Straight line on log:log graph suggests that the system is particulate
Explain what this graph is showing
Graph shows that chocolate is sheer thinning.
Explain what this graph is showing
The size of the molecule affects the viscosity and sheer thinning ability.
When looking at a Cox-Merz graph what are we looking at information for
The viscoelastic properties of a fluid during oscillation
Explain what the clear and coloured triangles on this graph show
Both coloured (dynamic viscosity) and clear triangles (complex viscosity) are on the same line. Therefore in a newtonian solution you get the same viscosity whether measured by oscillation or rotation
What can affect viscosity behaviour
- Microstructure (rods, macromolecular, polymer solution)
- Particle size
- Concentration
- Particle properties (hardness or softness)
- Temperature
How does a rod shaped microstructure affect viscosity behaviour
At rest the fibres will be randomly oriented. Apply shear rate and the fibres will slowly orient in flow direction. With increasing orientation due to increasing shear rate the individual fibre will present less resistance to the flow and viscosity will decrease. Shear thinning
How do macromolecular structures affect viscosity behaviour
At rest the solutions are tangled and coiled. Under high sheer the molecules may deform, disentangle or completely unravel into fibres resulting in decreased viscosity. Shear thinning
How do dispersions affect viscosity behaviour
particles can diffuse between laters in flowing suspensions. At high shear rates they can become trapped between ayers causing shear thickening or thinning.
How does particle size affect viscosity
Larger particles interact with one another more frequently therefore the liquid is thicker. Starch granules swell under temperature hence why is is a good thickener.
Define:
Yield Point
The yield point or yield stress is the minimum force that must be exceeded in order to break down a sampleβs structure at rest, and thus make it flow.
Why do different substances have different viscosities?
All liquids are composed of molecules. When put into motion, molecules and particles are forced to slide along each other. They develop a flow resistance caused by internal friction.
Larger components present in a fluid are the reason for higher viscosity values.
Describe ideally elastic behaviour and give an example of a substance with these properties.
A substance that has no viscous portion and will return to its original size and shape when force is removed.
(e.g. Eiffel Tower)
Describe ideally viscous behaviour and give an example of a substance with these properties.
A substance that has no elastic portion and whose size and shape are entirely dependant on the container they are in.
(e.g. Water)
Define:
Brownian Motion
The random movement of microscopic particles in a fluid as a result of continuous bombardment from molecules of the surrounding medium.
