rheology

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Questions and Answers

What happens to a material that exhibits viscoelasticity during storage?

It behaves like a solid to prevent sedimentation. (correct)

It transforms into a gas to reduce pressure.

It behaves like a fluid to facilitate flow.

It becomes completely rigid and immovable.

What is the phase angle for a purely elastic material?

45°

0° (correct)

90°

60°

What does a higher phase angle indicate about a material?

It has no elasticity.

It is more viscous. (correct)

It is less viscous.

It is more elastic.

What condition must be met for flow to occur in a material exhibiting yield stress?

A certain shear stress must be applied. Signup and view all the answers

How are stress and strain related in purely viscous materials?

They are 1/4 cycle out of phase. Signup and view all the answers

What characterizes the phase angle behavior of liquid soaps in a frequency sweep?

The phase angle is relatively independent of frequency. Signup and view all the answers

In the context of pressure-sensitive adhesives, how does the relationship between G' and G'' change during a frequency sweep?

G'' is greater than G' at lower frequencies. Signup and view all the answers

What is a primary purpose of applying a constant stress in a creep test?

To ensure the applied stress does not alter the material’s internal structure. Signup and view all the answers

During the creep test, what happens to the energy stored in the viscoelastic material after the stress is removed?

It is partially recovered to restore the rest state. Signup and view all the answers

What is primarily observed in the phase angle of gels and hydrogel films during a frequency sweep?

The phase angle remains nearly constant across all frequencies. Signup and view all the answers

What is the formula for calculating dynamic viscosity?

η = K ρ t Signup and view all the answers

What is the formula for shear stress?

σ = F / A Signup and view all the answers

In a falling-sphere viscometer, what is used to calculate the dynamic viscosity?

A spherical ball Signup and view all the answers

Which of the following defines shear strain?

Displacement / Height Signup and view all the answers

What characterizes Bingham flow?

It begins when yield stress σy is reached Signup and view all the answers

What does viscosity represent in relation to shear stress and shear rate?

η = σ / γ’ Signup and view all the answers

How does thixotropic behavior manifest in a fluid?

It exhibits time-dependent shear-thinning characteristics Signup and view all the answers

What does the area within the hysteresis loop indicate?

The degree of thixotropic behavior of the material Signup and view all the answers

What is the unit of shear stress?

N/m2 Signup and view all the answers

What best describes shear strain rate?

The rate of change of strain over time Signup and view all the answers

What effect does thixotropy have on a fluid when it is mixed?

It transforms into a liquid Signup and view all the answers

What does the equation σ = σy + η γ’ represent in Bingham flow?

The balance between shear stress and yield stress Signup and view all the answers

Which statement correctly reflects the relationship between shear stress and velocity gradient?

Shear stress is proportional to velocity gradient Signup and view all the answers

How is shear rate defined?

γ’ = Shear strain / Shear time Signup and view all the answers

What happens to a thixotropic material after the application of stress is removed?

It returns to its original state Signup and view all the answers

If the bottom plate is stationary and the top plate moves, what happens to the liquid layers?

Layers move at different velocities creating a velocity gradient Signup and view all the answers

What is the unit of dynamic viscosity in the SI system?

Pascal.second Signup and view all the answers

How is kinematic viscosity calculated?

Kinematic viscosity = dynamic viscosity / density Signup and view all the answers

If a Newtonian liquid has a dynamic viscosity of 0.65 Pa.s, what is its kinematic viscosity if the density is 650 kg/m³?

0.001 m²/s Signup and view all the answers

What characterizes Newtonian fluids?

Viscosity remains constant regardless of shear stress Signup and view all the answers

Which instrument is commonly used to measure the viscosity of Newtonian fluids?

Capillary viscometer Signup and view all the answers

What relationship does the graph of shear stress vs shear rate depict for Newtonian fluids?

Linear relationship with a constant gradient Signup and view all the answers

In the context of an Ostwald U-tube viscometer, what does the equation η = K ρ t represent?

Dynamic viscosity related to time and gravity Signup and view all the answers

What is the correct value of the instrument constant in the capillary viscometer question provided?

1 x 10-7 m² s-2 Signup and view all the answers

What does rheology primarily study?

The science of deformation and flow Signup and view all the answers

Why is rheology important in quality control?

It evaluates physical stability and consistency in quality Signup and view all the answers

How does rheology contribute to process optimization?

By understanding material behavior under stress and strain Signup and view all the answers

What is a potential application of rheological testing?

Evaluating how well a formulation spreads on the skin Signup and view all the answers

Which of the following is NOT a focus area mentioned for rheology?

Assessing the aesthetic qualities of packaging Signup and view all the answers

What does viscometry testing primarily measure?

The flow behavior of liquids and semisolids Signup and view all the answers

What are some outcomes of understanding rheological properties?

Improved drug formulation and delivery methods Signup and view all the answers

Newton's experiment illustrates which aspect of rheology?

The interaction between parallel plates and liquid flow Signup and view all the answers

Study Notes

MPharm Programme PHA114 - Rheology

Rheology is the science of deformation and flow.
It originates from the Greek word "rheos," meaning a stream or anything that flows.
Aulton's Pharmaceutics Ch6 is a relevant text.

Viscometry Testing

Viscometry measures a fluid's resistance to flow.

The Importance of Rheology

Characterisation & classification of materials: Rheological measurements describe the flow behavior of liquids and semi-solids, providing insights into viscosity, elasticity, and viscoelasticity to understand the material's structure.
- This applies to raw materials and finished products.
Quality control: Rheology aids in assessing physical stability, consistency, patient compatibility, drug bioavailability, and identifying potential issues like contamination, poor mixing, or variability in quality. This can be relevant, for example, in the performance of transdermal patches.
Process optimisation: Understanding how materials behave under stress and strain assists in selecting suitable equipment for optimal processing.
Product development: Rheology assists in optimizing flow, spreading, and other product characteristics (e.g., firmness).
Predict behaviour: Rheology enables predictions of a product's behavior under different conditions (e.g., temperature and pressure).
Research & Development: Exploration of molecular structure, interactions, and new materials.

Rheology as a Quality Control Tool

Rheology can quantify the characteristics of processing and application, addressing issues such as product spreadability on the skin, pouring from containers, particle sedimentation during storage, and adhesive properties.
For example, rheology can characterize the patch detachment from the skin.

Learning Outcomes

Rheology and rheological testing
Introduction to viscometry
Thixotropy and yield stress
Oscillatory rheological testing
Rheological viscoelasticity parameters

Newton's Experiment

Imagine a cube of liquid sandwiched between two parallel plates.
The bottom plate is fixed, while the top plate slides horizontally.
Intermolecular forces cause the liquid to adhere to both plates.
Applying a force to the top plate moves it at a constant velocity, with the bottom plate remaining stationary.
Liquid layers move at varying velocities, generating a velocity gradient. Internal resistance to flow (shear stress) is proportional to the velocity gradient.

Viscometry - Definitions of Terms

Shear stress (σ): The torsional force per area.
Shear strain (γ): The resulting displacement divided by the sample height.
Shear rate (γ'): The change in strain over a specific time.
Viscosity (η): Shear stress divided by shear rate.

Shear Strain (γ)

Shear strain is often abbreviated to strain, a dimensionless quantity.
It is expressed as a percentage or millistrain.

Shear Stress (σ)

Shear stress is calculated as Force divided by Area.
The unit for shear stress is N/m² or Pascals (Pa).
1 N/m² = 1 Pa

Shear Rate (γ')

Shear rate is the rate of change of strain.
The unit for shear rate is s⁻¹ (reciprocal seconds).

Shear Viscosity (η)

Viscosity is calculated as Shear Stress divided by Shear Rate.
Units include Pascal-second (Pas) and Poise (P).
Conversions: 1 Pa.s = 10 P, and 1 mPa.s = 1 cps

Viscosity

Dynamic viscosity (η): Resistance of a fluid to flow (Nm²s or Pas).
Kinematic viscosity (ν): Normalised value of viscosity (m²s⁻¹), calculated as dynamic viscosity divided by density. It is sort of like velocity of flow.

Newtonian Flow

Liquids with constant viscosity regardless of applied shear stress (force per unit area).
Resistance to flow remains constant with varying speed or flow force.
The graph of shear stress vs shear rate is linear.
Gradient of this graph is equal to the viscosity.

Ostwald U-Tube Viscometer

Laminar flow is under the influence of gravity.
Time taken for the liquid to flow from one point to another is measured.
Dynamic viscosity (η) can be calculated using the formula: η = Kρt, where K is an instrument constant, ρ is the density, and t is the time taken for the liquid to flow.

Capillary Viscometer Question

Example calculation: Dynamic viscosity of a Newtonian liquid using the instrument constant, the liquid density, and the mean flow time.

Falling Sphere Viscometer

Uses Stokes' Law (V = 2r²g(ρ−σ)/9η) to determine the dynamic Viscosity of a fluid.
A small test sphere falls through a tube of liquid.
Timing is used to determine the viscosity of the substance.

Rotational Rheometer

Measures non-Newtonian flow using parallel plates (e.g., stainless steel, titanium), cone plates.
Upper mobile plate versus a lower stationary plate.
Variables for experimental settings include size of plate (20mm, 40mm), gap size between plates, and temperature.

Typical Rheology Instrumentation

Demonstrates examples of cup and bob viscometers and rotational rheometers.

Typical Measuring Geometries

Visual representation of cone and plate and cup and bob geometries.

Factors that Affect Viscosity

Variables affecting viscosity include temperature (T), pressure (P), time (t), thixotropy systems, and shear stress (σ).
Newtonian systems versus non-Newtonian systems.

Newtonian Fluids

Newton's Law of Flow: σ/γ' = constant
The flow curve remains a linear plot from zero shear rate.
Dynamic viscosity (η) is the proportionality constant.

Examples of Newtonian Fluids

Viscosity values for water, petrol, glycerol, and hydrogen gas at 20°C.

Non-Newtonian Fluids

Shear stress dependent viscosity:
- Shear-thinning flow (e.g., creams)
- Shear-thickening flow (e.g., concentrated suspensions)
- Bingham flow (e.g., toothpaste).
Time dependent viscosity:
- Thixotropic flow (e.g., bentonite, ketchup).

Shear-thinning Flow

Flow begins when shear stress is applied and viscosity is high at low shear rates.
Power law equation: σ = ηγⁿ where n < 1.

Shear-thickening Flow

The opposite behavior of shear-thinning.
Viscosity is high at high shear rates.
Power law equation: σ = ηγⁿ where n > 1.

Bingham Flow

Newtonian flow begins when a specific yield stress (σy) is reached.
Equation: σ = σy + ηγ'.

Thixotropic Behaviour

Thixotropy is a time-dependent shear-thinning behavior.
The material flows as a liquid after mixing, but reverts to a more viscous state over time or after application. Forms thicker.

Hysteresis Loop (Thixotropic Loop)

The area of the loop indicates the thixotropic nature of the material.

Yield Stress

No flow occurs until a specific yield stress is applied.

Viscoelasticity in Products

Many materials exhibit viscoelastic behavior; they act like viscous liquids in some processes and like elastic solids in others.
Examples: concentrated suspensions, which need to be solid for storage, but fluid for application.

Oscillation Principles

Shows input stress and measured strain graphs on a time axis.
Phase angle (δ) is a measure of the material's elasticity and viscosity.

Phase Angle (δ)

Purely elastic materials: Stress and strain are in phase (phase angle = 0).
Purely viscous materials: Stress and strain are 90° out of phase.
Higher phase angle = more viscous; lower phase angle = more elastic.

Complex Modulus (G*)

Calculated from the ratio of stress and strain amplitudes (in oscillation tests).

Calculated Parameters in Oscillation

Loss (viscous) modulus (G"): Calculated by Stress x Sin(δ) / Strain.
Storage (elastic) modulus (G'): Calculated by Stress x Cos(δ) / Strain.
Dynamic viscosity (η'): Calculated from G'/frequency.

Test Modes

Viscometry (shear): Measures the viscosity of a substance.
Oscillation: Used for observing elasticity and viscosity of substances.
Creep and Recovery: A test of how material responds over time under a constant stress (like the change in shape over time of a plastic toy under stress).
'Wet sponge model' describes the behavior of sponge-like materials. Elastic-solid behavior (storage modulus) in dry state, and highly viscous behavior when wet. Can be affected by materials like honey.

Rheological behaviour of cosmetic ingredients/finished products

Shows different material states (liquids, semisolids, solids) and their corresponding rheological classifications (viscous, viscoelastic, elastic).
Categorises the substances based on different states and their associated viscous, viscoelastic or elastic properties.

Viscoelastic behaviour

Describes materials that exhibit both viscous and elastic properties.
Examples include polymer melts, polymer gels, and creams. Viscoelastic behavior is the sum of viscous and elastic behavior.

Oscillation Measurement Types

Amplitude sweep: Identify linear viscoelastic region, measures inherent structure, and tests dispersion/suspension stability.
Frequency sweep: Characterizes the material's behavior over different frequencies (different timescales of deformation).

Amplitude Sweep

Identifies the linear viscoelastic region.
Measures inherent structure.
Determines dispersion/suspension stability.

Frequency Sweep

Shows a material's relative process time behavior (long or short timescales). The graph shows G' and G" against frequency. Elastic behavior (G'>G") dominates at lower frequencies, viscous dominated (G'<G") at higher frequencies.

Frequency Sweep (Liquid Soaps, Gels, Hydrogel Films)

Analysis of phase angle for various materials (liquid soap, gel, hydrogel) to assess the effect of frequency (related to how long the materials are subjected to stress). Viscosity generally remains constant.

Frequency Sweep (Pressure Sensitive Adhesives)

Analysis of phase angle for pressure sensitive adhesives.

Viscoelasticity measurements - Creep test

Application: constant stress over a period determines resulting strain.
Applied stress: Low enough to avoid damaging internal structure, yet high to encourage movement.
Viscoelastic behavior: The material stores and dissipates energy. Stored energy is recovered.
Creep compliance (J): Strain divided by stress (J = γ/σ). The measurement of this is important in determining the material's stress, strain, and flow.
Curve description: Viscoelasticity can be described by the Burgers combined model

Studying That Suits You

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Description

Test your knowledge on the properties of viscoelastic materials through various scenarios such as storage, phase angles, and stress-strain relationships. This quiz covers fundamental concepts of viscoelasticity, yield stress, and behavior under different testing conditions.