Rheology Part 3 - Flow Profiles - A Video Tutorial by samMorell.com

Summary

This video explains the different types of fluid flow behaviors and their relevance in rheology. It covers Newtonian fluids, whose viscosity remains constant regardless of shear rate, and delves into non-Newtonian fluids like pseudoplastics (e.g., mayonnaise), dilatants (e.g., quicksand), and thixotropic fluids (e.g., ketchup). The video highlights how these materials behave under varying shear forces and emphasizes their applications, such as in coatings and paints. Viewers gain insights into how fluids' viscosities change with shear rates and time, making the content engaging for those interested in material science and fluid dynamics.

Takeaways

• 😀 Newtonian fluids have a constant viscosity regardless of the shear rate, with water being a prime example.
• 😀 Non-Newtonian fluids have a viscosity that changes with shear rate, which includes three types: pseudoplastic, dilatant, and thixotropic.
• 😀 Pseudoplastic fluids, like mayonnaise, become thinner with increased shear rate, due to the untangling of polymer chains.
• 😀 Dilatant fluids, such as quicksand, thicken with increased shear rate, displaying a shear thickening effect.
• 😀 Quicksand demonstrates dilatancy: when struggling, it thickens, making it harder to escape.
• 😀 Thixotropic fluids, like ketchup, have a time-dependent viscosity change that decreases with shear rate but recovers slowly over time.
• 😀 The difference between pseudoplastic and thixotropic fluids is that thixotropic fluids recover more slowly after shear is removed.
• 😀 The degree of hysteresis in thixotropic systems refers to the difference in the viscosity curves when shear is increased versus when it’s removed.
• 😀 Thixotropy is beneficial in paints and coatings, as it helps achieve uniform mixing, spreading, and reduced sagging after application.
• 😀 Paints use thixotropic behavior to reduce brush marks and improve leveling by recovering viscosity slowly after application.
• 😀 Materials like mayonnaise, corn starch, ketchup, and paint have distinct rheological behaviors, including shear thinning, shear thickening, and time-dependent recovery.

Q & A

What is rheology and why is it important?

Rheology is the study of how materials flow and deform under various forces. It helps us understand the behavior of liquids and pastes when subjected to shear forces, which is crucial for designing and optimizing products in industries like food, cosmetics, and coatings.

What are Newtonian fluids and how do they behave under shear stress?

Newtonian fluids are liquids whose viscosity remains constant regardless of the shear rate applied to them. Water is an example of a Newtonian fluid, as its viscosity does not change with increasing or decreasing shear stress.

What differentiates non-Newtonian fluids from Newtonian fluids?

Non-Newtonian fluids differ from Newtonian fluids because their viscosity changes with the shear rate. These fluids can exhibit shear thinning (viscosity decreases with increased shear rate), shear thickening (viscosity increases with increased shear rate), or other time-dependent behaviors.

What is pseudoplastic flow, and what is an example?

Pseudoplastic flow, also known as shear-thinning, is a type of non-Newtonian flow where the viscosity decreases with increasing shear rate. An example of a pseudoplastic fluid is mayonnaise, which becomes thinner when mixed or shaken.

Why does mayonnaise become thinner when mixed or shaken?

Mayonnaise contains polymers that, when at rest, are coiled up due to stabilizing molecular forces. When shear is applied, such as through mixing or shaking, these polymer chains untangle and align with the flow direction, reducing internal resistance and decreasing viscosity.

What is dilatancy, and what is an example of this behavior?

Dilatancy, or shear-thickening, is a type of non-Newtonian flow where the viscosity increases with increasing shear rate. A good example of dilatancy is quicksand, which becomes thicker and harder to move through as you struggle, due to the increased shear stress.

How does the behavior of corn starch demonstrate dilatancy?

Corn starch mixed with water exhibits dilatancy. When mixed slowly, the mixture is smooth and buttery, but when mixed rapidly or subjected to high shear, it becomes thick and paste-like. This behavior is due to the formation of void spaces between particles under high shear, causing an increase in viscosity.

What is thixotropy, and how does it differ from pseudoplastic and dilatant behaviors?

Thixotropy refers to a time-dependent shear-thinning behavior. Unlike pseudoplastic fluids, whose viscosity decreases immediately with increased shear rate, thixotropic fluids take longer to recover their original viscosity after shear stress is removed. Ketchup is a common example, as it flows easily when shaken but becomes thicker when left to rest.

How do thixotropic fluids like ketchup behave during use?

When you shake a ketchup bottle, the shear stress reduces its viscosity, allowing it to pour. However, after pouring, the viscosity increases again as the shear rate decreases, making the ketchup thicker and slower to flow, which is why it may 'stick' to the bottle.

What are the practical benefits of thixotropic behavior in paints and coatings?

Thixotropic behavior in paints and coatings allows for easier application (e.g., brushing or spraying) because the viscosity decreases under shear. Once applied, the viscosity recovers more slowly, giving the paint time to level out, reduce brush marks, and prevent sagging on vertical surfaces.