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Related Concept Videos

Diffusion01:21

Diffusion

Diffusion is a type of passive transport. In passive transport, a substance tends to move from an area of high concentration to an area of low concentration until the concentration is equal across the space. For example, take the diffusion of substances through the air. When someone opens a perfume bottle in a room filled with people, the perfume is at its highest concentration in the bottle and is at its lowest at the edges of the room. The perfume vapor will diffuse, or spread away, from the...
Diffusion01:12

Diffusion

Diffusion is the passive movement of substances down their concentration gradients—requiring no expenditure of cellular energy. Substances, such as molecules or ions, diffuse from an area of high concentration to an area of low concentration in the cytosol or across membranes. Eventually, the concentration will even out, with the substance moving randomly but causing no net change in concentration. Such a state is called dynamic equilibrium, which is essential for maintaining overall...
Physiological Pharmacokinetic Models: Blood Flow-Limited Versus Diffusion-Limited Models00:57

Physiological Pharmacokinetic Models: Blood Flow-Limited Versus Diffusion-Limited Models

Physiological pharmacokinetic models, often called flow-limited or perfusion models, typically assume a swift drug distribution between tissue and venous blood, creating a rapid drug equilibrium. This premise is based on the idea that drug diffusion is extremely fast, and the cell membrane presents no barrier to drug permeation. In this scenario, where no drug binding occurs, the drug concentration in the tissue equals that of the venous blood leaving the tissue. This greatly simplifies the...
Behavior of Gas Molecules: Molecular Diffusion, Mean Free Path, and Effusion03:48

Behavior of Gas Molecules: Molecular Diffusion, Mean Free Path, and Effusion

Although gaseous molecules travel at tremendous speeds (hundreds of meters per second), they collide with other gaseous molecules and travel in many different directions before reaching the desired target. At room temperature, a gaseous molecule will experience billions of collisions per second. The mean free path is the average distance a molecule travels between collisions. The mean free path increases with decreasing pressure; in general, the mean free path for a gaseous molecule will be...
Passive Diffusion: Overview and Kinetics01:17

Passive Diffusion: Overview and Kinetics

Passive diffusion is a critical process that allows small lipophilic drugs to cross the cell membrane along a concentration gradient. This mechanism's efficiency depends on four primary factors: the membrane's surface area, the drug's lipid-water partition coefficient, the concentration gradient, and the membrane's thickness.
When administered orally, drugs establish a substantial concentration gradient between the gastrointestinal (GI) lumen and the bloodstream, expediting their diffusion into...
Theories of Dissolution: Diffusion Layer Model01:15

Theories of Dissolution: Diffusion Layer Model

Dissolution, the process by which drug particles dissolve in a solvent, is explained by the diffusion layer model, a theoretical framework that simulates the absorption of oral drugs and allows us to analyze experimental data.
This process starts with a thin layer, saturated with the drug, forming at the interface between the solid and liquid. The solute then diffuses from this layer into the main solution. The Noyes-Whitney equation suggests that the rate of dissolution relies on the diffusion...

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The Diffusion of Passive Tracers in Laminar Shear Flow
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The Diffusion of Passive Tracers in Laminar Shear Flow

Published on: May 1, 2018

Diffusion approximation revisited.

Manabu Machida1, George Yu Panasyuk, John C Schotland

  • 1Department of Bioengineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA. mmachida@seas.upenn.edu

Journal of the Optical Society of America. A, Optics, Image Science, and Vision
|May 5, 2009
PubMed
Summary
This summary is machine-generated.

The diffusion approximation (DA) for radiative transport equations (RTE) is improved by setting reduced intensity I(r)=0. This simplifies calculations and enhances accuracy for radiative transfer studies.

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Area of Science:

  • Physics
  • Applied Mathematics
  • Radiative Transfer Theory

Background:

  • The radiative transport equation (RTE) describes light propagation in participating media.
  • The diffusion approximation (DA) simplifies the RTE for optically thick media.
  • Various definitions of reduced intensity exist within the DA framework.

Purpose of the Study:

  • To evaluate different definitions of reduced intensity in the diffusion approximation (DA) of the radiative transport equation (RTE).
  • To determine the optimal definition of reduced intensity for maximizing DA accuracy.
  • To identify conditions for the applicability of the DA.

Main Methods:

  • Comparison of DA results with exact solutions of the RTE in infinite homogeneous space.
  • Analysis of different definitions for the reduced intensity I(r).
  • Investigation of simplified RTEs without scattering.

Main Results:

  • The DA achieves highest accuracy when the reduced intensity I(r) is defined to be zero.
  • Separating specific intensity into reduced and diffuse components is shown to be unnecessary for optimal DA.
  • Conditions for the valid application of the diffusion approximation are discussed.

Conclusions:

  • A simplified approach to the diffusion approximation, by setting I(r)=0, yields superior accuracy.
  • The findings offer a more efficient and accurate method for solving radiative transport problems.
  • This study clarifies the theoretical underpinnings and practical application of the DA.