Rheology Basics and Applications

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Questions and Answers

Which of the following describes dilatant materials?

Their viscosity decreases with increased shear rate.

They behave like Newtonian fluids under stress.

They exhibit spontaneous deformation upon stress removal.

They become thicker or more viscous with increased shear rate. (correct)

What characterizes thixotropic systems?

They show an increase in viscosity over time under constant shear.

Their viscosity changes instantaneously with shear rate.

They become less viscous with sustained shear stress. (correct)

They maintain a constant viscosity regardless of shear conditions.

In shear-thinning phenomena, what occurs in a fluid?

The fluid remains unchanged regardless of applied shear stress.

The viscosity decreases as the shear stress increases. (correct)

The viscosity increases in response to higher shear rates.

The fluid experiences permanent deformation without recovery.

Which of the following best describes Newtonian fluids?

They have a direct and proportional relationship between shear stress and shear rate. Signup and view all the answers

What type of flow is characterized by the need for a minimum yield stress to begin flowing?

Bingham flow Signup and view all the answers

In non-Newtonian fluids, what happens to the viscosity when shear rates vary?

Viscosity decreases or increases depending on the type of non-Newtonian fluid. Signup and view all the answers

Which condition describes pseudoplastic flow?

Viscosity lowers with increasing shear rate. Signup and view all the answers

Which unit is used to express the coefficient of viscosity?

Centipoise Signup and view all the answers

Which of the following best describes pseudoplastic flow?

The viscosity decreases with increasing shear rates. Signup and view all the answers

What happens to a plastic material when the shear stress is below the yield value?

It behaves like an elastic material. Signup and view all the answers

What characterizes a Newtonian fluid as opposed to a non-Newtonian fluid?

Exhibits constant viscosity irrespective of stress or shear rate. Signup and view all the answers

How is the yield value of a plastic material related to the force of flocculation?

Stronger flocculation leads to higher yield values. Signup and view all the answers

Which equation relates to the plastic viscosity in a Bingham plastic?

$ au = f_B + U D$ Signup and view all the answers

What is the primary effect of shear on pseudoplastic materials at the particulate level?

Molecules align and reduce friction with increased shear. Signup and view all the answers

In dilatant materials, what is the behavior of viscosity as shear rate increases?

Viscosity increases. Signup and view all the answers

What is the primary characteristic of dilatant materials?

They resist stirring or shaking due to caking. Signup and view all the answers

What describes the slope of the rheogram in a plastic material?

Plastic viscosity. Signup and view all the answers

In thixotropic systems, what happens to the structure when shear is applied?

The structure breaks down and viscosity decreases. Signup and view all the answers

Which statement best describes shear-thinning systems?

They have a hysteresis loop in their rheogram. Signup and view all the answers

What does thixotropy specifically refer to?

Slow recovery of consistency after shearing. Signup and view all the answers

What distinguishes non-Newtonian fluids from Newtonian fluids?

Non-Newtonian fluids can show changes in viscosity with varying shear rates. Signup and view all the answers

What happens to an asymmetric particle structure in a thixotropic system at rest?

It conforms to a gel-like state. Signup and view all the answers

What is a common application of thixotropic materials in formulation?

To maintain high consistency while being easy to pour. Signup and view all the answers

What occurs during the flow of a thixotropic material when stress is applied?

The structure becomes aligned and viscosity decreases. Signup and view all the answers

Study Notes

Rheology

Rheology is the study of how matter deforms under stress
Viscosity measures a fluid's resistance to flow; higher viscosity means greater resistance
Stress applied to a system causes strain, which can be elastic (reversible) or viscous (irreversible) or viscoelastic
Units of viscosity: centipoise (cP), 1 cP = 1 mPa·s

Newtonian Systems

Newtonian fluids have a direct relationship between shear stress and shear rate
Newton's Law: Shear rate is directly proportional to shear stress
Imagine a fluid as layers; layers slide past each other at progressively decreasing velocity
Uppermost layer moves, lower layers move less

Non-Newtonian Systems

Most pharmaceutical fluids don't follow Newton's Law, viscosity changes with shear rate
Need to measure viscosity over a wide range of shear rates
Examples of flow behaviors include plastic, pseudoplastic, and dilatant flows

Plastic Flow

Flow starts above a certain yield stress (yield value)
Below yield value, material is like an elastic solid
Above yield value, material behaves like a Newtonian fluid
Viscosity stays constant above the yield value
Plastic viscosity: (Shear Stress - Yield Value) / Shear Rate

Pseudoplastic Flow

Viscosity decreases as shear rate increases
Common in polymer solutions and some natural gums (like tragacanth)

Dilatant Flow

Viscosity increases as shear rate increases
Suspensions of highly concentrated, small, deflocculated particles
Resistance to flow increases with higher shear rate
Volume of the material increases with shear

Thixotropy

Viscosity decreases with increasing shear rate but recovers over time when shear is reduced
Characteristic "downward curve" versus "upward curve" on rheograms
Rheology is useful in evaluating how pharmaceutical systems respond to shear

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Description

This quiz covers fundamental concepts in rheology, including the behavior of fluids under stress, viscosity measurement, and the distinctions between Newtonian and Non-Newtonian systems. Explore terms such as shear stress, shear rate, and various flow behaviors relevant in pharmaceutical applications.