Rheology 3 Flashcards
What point can be used to calculate viscosity for Newtonian fluids?
Any point of the flow curve
What are Capillary Viscometers used for?
They are generally used for Newtonian fluids.
Why is it difficult to use
Capillary Viscometers
for non-newtonian fluids?
Because the shear rate cannot be controlled(or adjusted)
What are the advantages and Disadvantages of Capillary Viscometers?
Simple and easy to use
Good for Newtonian systems
But
Difficult to control the shear rate
Generally not suitable for non-Newtonian systems
What kind of Viscometers are more commonly used?
Rotational
Explain what a Rotational Viscometer is and how it is used to calculate Viscosity?
A motor turns a disc, bar or cylinder, immersed in a liquid.
The more viscous the liquid, the stronger the resistance, and hence higher the torque.
Viscosity can be calculated from measured rotation speed vs applied torque.
Advantages of Rotational viscosity?
This equipment can be applied over a wide range of shear stress and rate.
And allows a continuous monitoring of a sample over a range of: stress, rate, time, temperature.
Advantages of Concentric Cylinders?
Little possibility of sample evaporation
Can measure low shear stress – useful for emulsions and suspensions
Easy to load liquid samples
Disadvantages of Concentric cyclinders?
Possible air incorporation (air bubbles can be trapped).
Not easy to load semi-solids
Large volumes of materials needed
Shear rate across the gap is not constant. Can lead to slip of ‘plug flow’
Advantages of Cone and plate?
Gives shear rate across radius
Can give high shear rates possible
Small sample of volume is required
Easy to fill and clean, even with highly viscous samples
Little disturbance and sample structure
Rapid temperature equilibrium due to thin film.
