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Questions and Answers
What does shear viscosity measure in a fluid?
What does shear viscosity measure in a fluid?
What characterizes a Newtonian fluid?
What characterizes a Newtonian fluid?
How is kinematic viscosity defined?
How is kinematic viscosity defined?
Which unit is not typically associated with viscosity?
Which unit is not typically associated with viscosity?
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What is the relationship between shear stress and shear rate for a Newtonian fluid?
What is the relationship between shear stress and shear rate for a Newtonian fluid?
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What characterizes viscoelastic behavior in materials?
What characterizes viscoelastic behavior in materials?
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Which statement best describes the difference between Newtonian and non-Newtonian fluids?
Which statement best describes the difference between Newtonian and non-Newtonian fluids?
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What indicates the liquid-like nature of a material in rheological terms?
What indicates the liquid-like nature of a material in rheological terms?
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In oscillation measurement, which equation represents the storage (elastic) modulus?
In oscillation measurement, which equation represents the storage (elastic) modulus?
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Which type of material is characterized as viscoelastic?
Which type of material is characterized as viscoelastic?
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How does dynamic viscosity (η’) relate to shear viscosity under the Cox-Merz rule?
How does dynamic viscosity (η’) relate to shear viscosity under the Cox-Merz rule?
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What effect does adding water to an elastic solid have in terms of rheological behavior?
What effect does adding water to an elastic solid have in terms of rheological behavior?
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Which of the following statements is true regarding the characteristics of Newtonian fluids?
Which of the following statements is true regarding the characteristics of Newtonian fluids?
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What is the relation between shear stress and shear rate in Bingham flow?
What is the relation between shear stress and shear rate in Bingham flow?
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Which behavior best describes thixotropic materials?
Which behavior best describes thixotropic materials?
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What happens when the yield stress, σy, in a Bingham fluid is exceeded?
What happens when the yield stress, σy, in a Bingham fluid is exceeded?
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In a falling-sphere viscometer, what is primarily measured to calculate viscosity?
In a falling-sphere viscometer, what is primarily measured to calculate viscosity?
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Which statement describes Newtonian fluids?
Which statement describes Newtonian fluids?
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What does the area within the hysteresis loop in thixotropic materials indicate?
What does the area within the hysteresis loop in thixotropic materials indicate?
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How can the viscosity of a fluid be defined in terms of dynamic viscosity and shear stress?
How can the viscosity of a fluid be defined in terms of dynamic viscosity and shear stress?
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Which of the following is a characteristic of non-Newtonian fluids?
Which of the following is a characteristic of non-Newtonian fluids?
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What is a key difference between Newtonian and non-Newtonian fluids in terms of stress response?
What is a key difference between Newtonian and non-Newtonian fluids in terms of stress response?
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What primarily affects the dynamic viscosity of a fluid?
What primarily affects the dynamic viscosity of a fluid?
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What does the term viscosity (η) signify in rheology?
What does the term viscosity (η) signify in rheology?
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In the context of Newton's experiment, what is shear stress?
In the context of Newton's experiment, what is shear stress?
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Which of the following best describes shear strain (γ)?
Which of the following best describes shear strain (γ)?
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Which factor primarily influences the rate of strain in viscoelastic materials?
Which factor primarily influences the rate of strain in viscoelastic materials?
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How is shear strain rate (γ’) defined?
How is shear strain rate (γ’) defined?
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What does oscillatory rheological testing primarily evaluate?
What does oscillatory rheological testing primarily evaluate?
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Which one of the following conditions characterizes a Newtonian fluid?
Which one of the following conditions characterizes a Newtonian fluid?
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What can rheology help determine in a product during quality control?
What can rheology help determine in a product during quality control?
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Which of the following statements is true about non-Newtonian fluids?
Which of the following statements is true about non-Newtonian fluids?
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What happens when a viscoelastic material is subjected to an applied stress?
What happens when a viscoelastic material is subjected to an applied stress?
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Study Notes
Rheology
- Rheology is the science of deformation and flow, derived from the Greek word "rheos" meaning "stream."
- Rheology is important in characterizing and classifying materials, especially liquids and semisolids.
- Rheology helps understand raw materials, finished products, and quality control.
- This includes analyzing physical stability, consistency, patient compatibility, drug bioavailability, potential contamination, mixing quality, and variability.
- Rheology is used in process optimization, including selecting suitable equipment (e.g., toothpaste).
- It aids product development by optimizing flow, spreading, and firmness.
- Rheology is also used in predicting behavior under different conditions (e.g., temperature, pressure).
- Rheology is crucial for research and development related to exploring molecular structures, interactions, and new materials.
Viscometry Testing
- Viscometry is the measurement of a fluid's resistance to flow.
- Viscosity is a fluid's resistance to flow (e.g., viscous liquid versus a solid like a sponge).
- Important definitions:
- Shear stress: force per area.
- Strain: resultant displacement divided by sample height.
- Shear rate: the rate of change of strain.
- Viscosity: shear stress divided by shear rate.
- Units:
- Pascal-second (Pa·s) is the SI unit for viscosity.
- Poise (P) is the CGS unit for viscosity.
- Dynamic viscosity (η): Resistance of a substance to flow, and is calculated by dividing shear stress by shear rate.
- Kinematic viscosity (v): Normalised viscosity which is a measure of velocity of flow, and is found by dividing dynamic viscosity by density (ρ).
Learning Outcomes
- Rheological and rheological testing
- Introduction to viscometry
- Thixotropy and yield stress (shear-thinning, shaking in container, thickens back up when stopping).
- Oscillatory rheological testing
- Rheological viscoelasticity parameters
Rheology as Quality Control
- Rheology can describe and quantify product characteristics.
- Examples of process problems it can assess:
- Spreadability on the skin
- Pouring from containers
- Particle sedimentation during storage.
- Adhesive strength of patches to the skin
Newtonian Experiment
- Imagine a cube of liquid between two parallel plates.
- The bottom plate is fixed, and the top plate slides horizontally.
- Liquid molecules stick to both plates (intermolecular forces).
- Applying a force to the top plate causes it to move at a constant velocity.
- Bottom plate is stationary.
- Liquid layers move at different velocities, creating a velocity gradient (from zero at the bottom to V at the top).
- Internal resistance to flow is called shear stress, which is proportional to the velocity gradient.
Measuring Geometries
- Rotational rheometers are used for non-Newtonian flow measurement.
- They include parallel plates, cone and plate, and cup and bob designs.
- Essential parameters during testing also include plate size, gap size, and temperature.
Factors Affecting Viscosity
- Temperature and pressure affect viscosity in opposing ways. Increasing temperature lowers viscosity, and higher pressure increases viscosity.
- Time also affects viscosity: thixotropic systems are affected by time, changing viscosity over time.
- Shear Stress also affects viscosity: Newtonian materials exhibit stable viscosity no matter what shear stress is applied. Non-Newtonian materials viscosity changes based off level of applied shear stress
Newtonian Fluids
- Newton's Law defines flow as a constant ratio of stress to strain rate.
- Plots of Newtonian fluids show a linear relationship between shear stress and shear rate.
- Dynamic viscosity (η) is the constant of proportionality in Newton's Law
- Examples of Newtonian fluids are water, petrol, glycerol, and hydrogen gas (with varying viscosities).
Non-Newtonian Fluids
- Non-Newtonian fluids behave differently than Newtonian fluids, changing viscosity based on shear stress or time.
- Shear-dependent viscosity: There are three main types:
- Shear thinning flow: (e.g. cream, suspensions) Viscosity decreases with increasing shear rate.
- Shear thickening flow: (e.g. concentrated suspensions) Viscosity increases with increasing shear rate.
- Bingham flow: (e.g. toothpaste) Viscosity remains constant below a certain shear stress value (yield stress). No flow until the yield stress is exceeded.
- Time-dependent viscosity: Thixotropy- Viscosity changes over time (e.g., bentonite, ketchup).
Viscoelasticity
- Many materials exhibit viscoelasticity, acting like viscous liquids in some circumstances and elastic solids in others.
- An example is a concentrated suspension. During storage, the suspension needs to act like a solid to prevent sedimentation; During application it needs to act like a fluid to be evenly applied (e.g. paint).
- Different testing modes include amplitude, frequency, temperature sweeps, and time experiments (e.g., thixotropy).
Oscillation
- Amplitude sweep: helps identify the linear viscoelastic region, important for understanding the material's inherent structural make up. Also can be used to test dispersion/suspension qualities.
- Frequency sweep: gives a unique fingerprint that shows relative process time behaviour- determines if material is elastic or viscous dominated during flow processes.
- Time experiment: determines rheological property of a thixotropic material.
Creep Test
- A constant stress over time is applied, and the resulting strain is measured.
- The stress needs to be low enough to not damage the material's internal structure yet high enough to cause changes in material flow.
- Stored energy changes in the material as the applied stress is removed; this is measured in creep compliance J (strain divided by stress).
- Examples of materials that demonstrate creep include adhesives and paints.
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Description
Test your understanding of viscosity concepts in fluid mechanics with this quiz. Explore the measures and characteristics of both shear and kinematic viscosity, as well as the properties of Newtonian fluids. Perfect for students looking to deepen their knowledge in fluid dynamics.