Understanding Rotational Rheometers-1 — Lesson 1

This lesson covers the transport phenomena of Non-Newtonian fluids, focusing on rotational rheometers. It begins with a recap of previous lessons, discussing capillary viscometers, shear stress, and shear rate. The lesson then delves into the errors that can occur when using capillary viscometers and how to correct them. It further explores the concept of rotational rheometers, explaining how they cause deformation in fluids and how this can be used to measure shear stress and shear rate. The lesson also discusses the measurement of normal stress differences, which is not possible with capillary viscometers. The lesson concludes with an explanation of how to measure shear stress and shear rate in cases where the gap between the cylinders is either very narrow or very large.

Video Highlights

01:05 - Discussion on possible sources of errors when using capillary viscometers and how to correct for these errors.
13:01 - Explanation of how to use the continuity and momentum equations in cylindrical coordinates to solve for shear stress and shear rate.
24:35 - Discussion on how to measure shear stress in concentric cylinders using torque balance.
30:25 - Explanation of how to calculate shear rate in concentric cylinders and the importance of the ratio between the radius of the inner and outer cylinder.
46:54 - Conclusion of the lecture with a summary of the equations derived for shear stress and shear rate in concentric cylinders.

Key Takeaways

• Capillary viscometers are useful for measuring shear stress and shear rate, but they can be prone to errors.
• Rotational rheometers allow for the measurement of shear stress and shear rate by causing deformation in fluids.
• The gap between the cylinders in a rotational rheometer can affect the measurement of shear stress and shear rate.
• Normal stress differences can also be measured using rotational rheometers, unlike capillary viscometers.