Viscosity Concepts in Fluid Mechanics
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Questions and Answers

What does shear viscosity measure in a fluid?

  • The thermal conductivity of the fluid
  • The pressure exerted by the fluid
  • The resistance of a fluid to deform under shear stress (correct)
  • The speed of sound in the fluid
  • What characterizes a Newtonian fluid?

  • It exhibits elastic properties under stress
  • It has a variable viscosity depending on shear stress
  • Its viscosity changes with temperature
  • Its viscosity remains constant regardless of the shear rate (correct)
  • How is kinematic viscosity defined?

  • A measure of the fluid's flow velocity
  • Dynamic viscosity divided by fluid density (correct)
  • The product of shear stress and shear strain
  • Viscosity multiplied by fluid density
  • Which unit is not typically associated with viscosity?

    <p>Cubic meter per second (m³/s)</p> Signup and view all the answers

    What is the relationship between shear stress and shear rate for a Newtonian fluid?

    <p>They are directly proportional</p> Signup and view all the answers

    What characterizes viscoelastic behavior in materials?

    <p>Materials exhibiting both viscous and elastic properties</p> Signup and view all the answers

    Which statement best describes the difference between Newtonian and non-Newtonian fluids?

    <p>Newtonian fluids have a constant viscosity regardless of stress, while non-Newtonian fluids do not.</p> Signup and view all the answers

    What indicates the liquid-like nature of a material in rheological terms?

    <p>High loss modulus, G’’</p> Signup and view all the answers

    In oscillation measurement, which equation represents the storage (elastic) modulus?

    <p>G’ = Stress × Cos(θ)</p> Signup and view all the answers

    Which type of material is characterized as viscoelastic?

    <p>Structured liquids such as polymer gels that show both viscous and elastic behavior</p> Signup and view all the answers

    How does dynamic viscosity (η’) relate to shear viscosity under the Cox-Merz rule?

    <p>It is proportional to the loss modulus divided by frequency.</p> Signup and view all the answers

    What effect does adding water to an elastic solid have in terms of rheological behavior?

    <p>It creates a complex viscoelastic behavior indicating increased flow properties.</p> Signup and view all the answers

    Which of the following statements is true regarding the characteristics of Newtonian fluids?

    <p>Their viscosity remains consistent regardless of the rate of deformation.</p> Signup and view all the answers

    What is the relation between shear stress and shear rate in Bingham flow?

    <p>Shear stress increases linearly after the yield stress is reached.</p> Signup and view all the answers

    Which behavior best describes thixotropic materials?

    <p>They show time-dependent shear-thinning properties.</p> Signup and view all the answers

    What happens when the yield stress, σy, in a Bingham fluid is exceeded?

    <p>The fluid behaves like a Newtonian fluid.</p> Signup and view all the answers

    In a falling-sphere viscometer, what is primarily measured to calculate viscosity?

    <p>The time taken for the sphere to fall.</p> Signup and view all the answers

    Which statement describes Newtonian fluids?

    <p>They have a constant viscosity regardless of shear rate.</p> Signup and view all the answers

    What does the area within the hysteresis loop in thixotropic materials indicate?

    <p>The degree of thixotropic behavior.</p> Signup and view all the answers

    How can the viscosity of a fluid be defined in terms of dynamic viscosity and shear stress?

    <p>Dynamic viscosity equals shear stress divided by shear rate.</p> Signup and view all the answers

    Which of the following is a characteristic of non-Newtonian fluids?

    <p>They display variable viscosity depending on shear rate.</p> Signup and view all the answers

    What is a key difference between Newtonian and non-Newtonian fluids in terms of stress response?

    <p>Newtonian fluids do not have a yield stress, while non-Newtonian fluids do.</p> Signup and view all the answers

    What primarily affects the dynamic viscosity of a fluid?

    <p>The temperature of the fluid.</p> Signup and view all the answers

    What does the term viscosity (η) signify in rheology?

    <p>The measure of a liquid's resistance to shear stress</p> Signup and view all the answers

    In the context of Newton's experiment, what is shear stress?

    <p>The internal resistance related to the velocity gradient</p> Signup and view all the answers

    Which of the following best describes shear strain (γ)?

    <p>It is the displacement divided by the height of the sample</p> Signup and view all the answers

    Which factor primarily influences the rate of strain in viscoelastic materials?

    <p>The shear stress applied</p> Signup and view all the answers

    How is shear strain rate (γ’) defined?

    <p>As the velocity of the top layer over the height of the sample</p> Signup and view all the answers

    What does oscillatory rheological testing primarily evaluate?

    <p>The viscoelastic properties of a sample under different stress conditions</p> Signup and view all the answers

    Which one of the following conditions characterizes a Newtonian fluid?

    <p>Shear stress is proportional to shear rate</p> Signup and view all the answers

    What can rheology help determine in a product during quality control?

    <p>The spreadability and adhesion characteristics of the formulation</p> Signup and view all the answers

    Which of the following statements is true about non-Newtonian fluids?

    <p>Their viscosity can change based on the rate of shear applied</p> Signup and view all the answers

    What happens when a viscoelastic material is subjected to an applied stress?

    <p>It will exhibit both elastic and viscous behavior during deformation</p> Signup and view all the answers

    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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    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.

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