Rheology in Pharmacy

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Questions and Answers

What is the unit of viscosity defined as the shear force required to produce a velocity of 1 cm/sec between two parallel planes of liquid?

Dyne

Poise (correct)

Newton

Pascal

In a rheogram for Newtonian fluids, how is the relationship between shear rate and shear stress represented?

Horizontal line

Straight line passing through origin (correct)

Vertical line

Curved line that does not pass through origin

Which of the following types of fluids exhibits a decrease in viscosity with an increase in shear rate?

Dilatant fluids

Pseudoplastic fluids (correct)

Ideal fluids

Bingham bodies

What characterizes plastic flow in Bingham bodies?

Yield value must be exceeded to initiate flow Signup and view all the answers

What does fluidity represent in terms of viscosity?

It is the reciprocal of viscosity Signup and view all the answers

In non-Newtonian fluids, which of the following accurately describes pseudoplastic flow?

The viscosity decreases with increased shear rate Signup and view all the answers

What happens to the molecules in a pseudoplastic fluid during shear stress application?

They align in the direction of flow, reducing internal resistance Signup and view all the answers

Which statement best describes the mobility of a fluid as depicted on a rheogram?

Mobility is the reciprocal of fluidity Signup and view all the answers

What characterizes dilatant flow?

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

How does a plastic system behave in relation to a Newtonian system?

It resembles a Newtonian system only after yielding. Signup and view all the answers

What happens to interparticle void volume in a dilatant system as shear stress increases?

It increases as the bulk system expands. Signup and view all the answers

What is thixotropy in liquid pharmaceutical systems desired for?

A high consistency in the container and ability to spread easily. Signup and view all the answers

What does the hysteresis loop indicate in thixotropic materials?

A breakdown of consistency during shearing. Signup and view all the answers

What defines pseudoplastic flow?

Viscosity increases at higher shear rates. Signup and view all the answers

In what circumstance would a system's viscosity be directly influenced by the concentration of particles?

Dilatant flow where particles are deflocculated. Signup and view all the answers

Which of the following statements is correct regarding Newtonian and non-Newtonian fluids?

Newtonian fluids show identical flow curves under varying shear rates. Signup and view all the answers

What does viscosity measure in a liquid?

The resistance of liquid to flow Signup and view all the answers

In a Newtonian fluid, how is shear stress related to the rate of shear?

Shear stress is directly proportional to the rate of shear. Signup and view all the answers

Which characteristic distinguishes non-Newtonian fluids from Newtonian fluids?

Non-Newtonian fluids do not exhibit a direct proportionality between shear stress and rate of shear. Signup and view all the answers

What is the primary application of rheology in pharmacy?

Consistency and smoothness of medicinal formulations. Signup and view all the answers

What is thixotropy?

The property of a material to become less viscous over time when subjected to a constant shear stress. Signup and view all the answers

In rheograms, how do Newtonian fluids typically appear compared to non-Newtonian fluids?

They show linear behavior with constant slope. Signup and view all the answers

How is shear stress defined mathematically?

It is the ratio of force to the area over which it is applied. Signup and view all the answers

What type of flow behavior is exhibited by pseudoplastic fluids?

They show a decrease in viscosity with an increase in shear rate. Signup and view all the answers

Study Notes

Rheology

The science that explains how liquids flow and how solids deform
Viscosity is the resistance of liquids to flow
Applications in Pharmacy: Formulation and analysis of emulsions, pastes, suppositories, and tablet coating
Applications in Pharmacy: Consistency and smoothness of medicinal and cosmetic creams, pastes and lotions
Applications in Pharmacy: Involved in mixing and flow of materials, their packaging into containers and their removal prior to use
Applications in Pharmacy: Rheological properties influence the selection of process equipment

Newtonian Systems

Liquids that follow Newton’s Law of Flow
Involves shearing stress (F), rate of shearing (G), and viscosity (η)
Newton's Law of Flow: Shearing stress is directly proportional to the rate of shear

Non-Newtonian Systems

Liquids that don’t follow Newton’s Law of Flow
Three classes of non-newtonian liquids:
- Plastic
- Pseudoplastic
- Dilatant

Plastic Flow

Flow curves don’t pass through the origin
The material does not flow until a certain "yield value" of shear stress is exceeded
Viscosity decreases with increasing shear stress

Pseudoplastic Flow

Viscosity decreases with increasing shear stress
The rheogram starts at the origin
The rheogram results from a shearing action on linear polymers
Viscosity decreases with increasing shear stress because alignment of long axes in the direction of flow reduces internal resistance

Dilatant Flow

Viscosity increases with increasing shear stress
The dilatant flow exhibits by heavy suspensions
At rest, particles are closely packed
The amount of vehicle in the suspension permits particles to move relative to one another at low shear rates
As shear stress increases, the bulk system expands and leads to an increase in the interparticle void volume
The amount of vehicle becomes insufficient to fill the voids, thus increasing the resistance to flow

Thixotropy

Isothermal and slow recovery, on standing of a material, of a consistency lost through shearing.
The shear rate of a thixotropic material is increased in a constant manner and then decreased at the same rate resulting in a hysteresis loop.

Thixotropy in Formulation

A desired property in liquid pharmaceutical systems that should have high consistency in the container and pour and spread easily.

Rheogram

A graphical presentation of shear rate (G) and shear stress (F)
Also known as consistency curve and flow curve
The slope of the line represents fluidity

Viscosity

The force required to produce a velocity of 1cm/sec between two parallel planes of liquid 1 cm2 in area and separated by a distance of 1cm.
The unit of viscosity is "poise"

Fluidity

The reciprocal of viscosity

Modern Viscometers

Equipment used to determine the viscosity of liquids and semisolids

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Description

Explore the fundamental concepts of rheology, focusing on how liquids flow and solids deform. This quiz covers Newtonian and Non-Newtonian systems, viscosity, and their applications in the pharmaceutical industry, such as in the formulation of emulsions and creams. Test your knowledge on these critical principles and their real-world implications.