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Related Concept Videos

Diffusion on Chromatography Columns01:07

Diffusion on Chromatography Columns

In column chromatography, when an analyte is introduced as a narrow band at the top of the column, the solutes begin to separate and broaden, developing a Gaussian profile. This broadening occurs due to various factors, such as longitudinal diffusion.
Longitudinal diffusion occurs when the solute molecules in the mobile phase diffuse from the more concentrated center of the chromatographic band to the more dilute regions on either side, both towards and against the flow direction. This...
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Velocity Potential

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Relation between Poisson's ratio, Modulus of Elasticity and Modulus of Rigidity

Deformation occurs in axial and transverse directions when an axial load is applied to a slender bar. This deformation impacts the cubic element within the bar, transforming it into either a rectangular parallelepiped or a rhombus, contingent on its orientation. This transformation process induces shearing strain. Axial loading elicits both shearing and normal strains. Applying an axial load instigates equal normal and shearing stresses on elements oriented at a 45° angle to the load axis.
Normal Strain under Axial Loading01:20

Normal Strain under Axial Loading

Normal strain under axial loading is an important concept in the field of mechanics of materials. Axial loading implies the application of a force along the axis of a material, like a column or bar. This force can either compress or stretch the material. In the context of axial loading, normal strain is the deformation experienced by the material in the direction of the loading force. It's calculated as the change in length divided by the original length of the material. This unitless ratio...
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In analyzing a thin-walled hollow shaft subjected to torsional loading, a segment with width dx is isolated for examination. Despite its equilibrium state, this segment faces torsional shearing forces at its ends. These forces are quantitatively described by the product of the longitudinal shearing stress on the segment's minor surface and the area of this surface, leading to the concept of shear flow. This shear flow is consistent throughout the structure, indicating a uniform distribution of...

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Updated: Jun 25, 2026

The Diffusion of Passive Tracers in Laminar Shear Flow
08:01

The Diffusion of Passive Tracers in Laminar Shear Flow

Published on: May 1, 2018

About "axial" and "radial" diffusivities.

Claudia A M Wheeler-Kingshott1, Mara Cercignani

  • 1University College London, Institute of Neurology, Department of Neuroinflammation, London, UK. c.wheeler-kingshott@ion.ucl.ac.uk

Magnetic Resonance in Medicine
|March 3, 2009
PubMed
Summary
This summary is machine-generated.

Researchers should carefully interpret axial and radial diffusivities, especially in crossing fibers. Changes in these metrics may not accurately reflect underlying tissue properties like myelin or axonal density.

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Last Updated: Jun 25, 2026

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Area of Science:

  • Neuroimaging
  • Diffusion Tensor Imaging (DTI)

Background:

  • Axial and radial diffusivities are key metrics in DTI, often interpreted as indicators of axonal and myelin density.
  • However, their interpretation can be confounded by complex tissue microstructures, such as crossing fibers.

Purpose of the Study:

  • To investigate potential problems in interpreting axial and radial diffusivities derived from diffusion tensor eigenvalues.
  • To assess the reliability of these metrics in reflecting biophysical properties in simulated and in vivo data.

Main Methods:

  • Simulations were performed to evaluate the impact of crossing fibers on axial and radial diffusivities.
  • In vivo DTI data from healthy and multiple sclerosis subjects were analyzed to compare principal eigenvector directions.

Main Results:

  • Simulations showed that changes in radial diffusivity can falsely alter axial diffusivity and vice versa in voxels with crossing fibers.
  • In vivo data revealed significant angular deviations (>45 degrees) between principal eigenvectors in areas of low anisotropy, pathology, and partial volume.
  • White matter pathology exhibited altered principal eigenvector directions and increased radial diffusivity compared to healthy tissue.

Conclusions:

  • Interpreting axial and radial diffusivities solely based on underlying tissue structure is discouraged.
  • Thorough investigation of mathematical and geometrical properties is crucial before inferring biophysical properties from these DTI metrics.