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Related Concept Videos

Viscosity of Fluid01:19

Viscosity of Fluid

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Viscosity measures the resistance a fluid offers to flow and deformation. It results from internal friction between layers of fluid moving relative to one another. Dynamic viscosity, denoted by the Greek letter mu (μ), quantifies the force needed to move one fluid layer over another. For Newtonian fluids like water and air, the relationship between the shearing stress and the rate of shearing strain is linear, meaning their viscosity remains constant regardless of the applied stress.
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Viscosity01:17

Viscosity

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When water is poured into a glass, it falls freely and quickly, whereas if honey or maple syrup is poured over a pancake, it flows slowly and sticks to the surface of the container. This difference in the flow of different kinds of liquids arises due to the fluid friction between the liquid layers and the liquid and the surrounding material. This property of fluids is called fluid viscosity. In this example, water has a lower viscosity than honey and maple syrup.
The SI unit of viscosity is...
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Pressure Variation in a Fluid at Rest01:11

Pressure Variation in a Fluid at Rest

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In a fluid at rest, the pressure at any point beneath the fluid surface depends solely on the depth, not on the container's shape or size. This principle, known as hydrostatic pressure, arises because, in stationary fluids, there is no acceleration, meaning the forces within the fluid balance out. Only vertical forces, caused by the weight of the fluid above, contribute to pressure changes with depth.
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Related Experiment Video

Updated: Jan 4, 2026

Rapid Viscoelastic Characterization of Airway Mucus Using a Benchtop Rheometer
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Corneal Viscous Properties Cannot Be Determined From Air-Puff Applanation.

Mathew Francis, Himanshu Matalia, Rudy M M A Nuijts

    Journal of Refractive Surgery (Thorofare, N.J. : 1995)
    |November 12, 2019
    PubMed
    Summary

    Air-puff applanation did not detect significant corneal viscosity in patients. Advanced modeling showed differences in corneal stiffness and viscosity between normal and keratoconic eyes, but viscous response remained undetectable.

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    Area of Science:

    • Ophthalmology
    • Biomechanics
    • Corneal Physiology

    Background:

    • Corneal viscoelastic properties are crucial for understanding corneal biomechanics.
    • Air-puff applanation is a common method for assessing corneal properties.
    • Distinguishing between elastic and viscous components of corneal deformation is important for diagnosing conditions like keratoconus.

    Purpose of the Study:

    • To determine if corneal viscous properties can be measured using air-puff applanation.
    • To compare corneal stiffness and viscosity in normal, fellow, and keratoconic eyes.
    • To evaluate the effectiveness of different biomechanical models in analyzing corneal deformation data.

    Main Methods:

    • Analyzed Corvis ST deformation data from 312 normal eyes, 107 fellow eyes, and 289 keratoconic eyes.
    • Utilized a standard linear solid model (elastic) and a two-compartment Kelvin-Voigt model (visco-elastic).
    • Calculated corneal stiffness, viscosity, and hysteresis area, including virtual phase shifting of deflection amplitude.

    Main Results:

    • Both models yielded similar corneal stiffness values, which significantly differed between normal, fellow, and keratoconic eyes (P = .001).
    • Corneal viscosity and deflection hysteresis area were small in magnitude and detected only when deflection amplitude was virtually phase shifted.
    • No significant corneal viscous response was directly detected through standard air-puff applanation analysis.

    Conclusions:

    • Standard air-puff applanation, as analyzed, does not appear to reliably measure corneal viscous properties.
    • While corneal stiffness and viscosity differ between patient groups, the viscous component's detection requires advanced analytical techniques.
    • Further research may be needed to refine methods for assessing corneal viscoelasticity.