# Dictionary Definition

diffused adj

1 (of light rays) subjected to scattering by
reflection from a rough surface or transmission through a
translucent material; "diffused light"

2 (of light) not bright or glaring; "a softer
diffused radiance" [syn: softened]

# User Contributed Dictionary

## English

### Pronunciation

### Verb

diffused# Extensive Definition

In physics, chemistry and biology, diffusion denotes the
mixing of two or more substances or the net motion of a substance
from an area of high concentration to an area of low concentration.
The theory is that both of these result from the random motion of
micro-scale individual agents (such as molecules) giving rise to
net changes on the macro-scale. While originally formulated within
the framework of the physical sciences, the concept of diffusion
has been applied to phenomena such as the manner in which
information is spread amongst a population. The chemistry
definition of diffusion is the movement of a fluid from an area of
higher concentration to an area of lower concentration.

Diffusion is an abstract topic and is often only
explained as theoretical model. It is part of transport
phenomena in general, and often accompanied by the much quicker
convection (making it
hard to observe 'pure' diffusion). A few examples are shown
below.

## The diffusion equation

To verify any microscopic model we may think up, we need to calculate its consequences and compare these to observation. Another way of arriving at a microscopic model is to write down a general equation and solve it mathematically (i.e. start from what you already know). This general equation, not refering to any microscopic model, is the diffusion equation\partial_t c (\mathbf,t) = D\nabla^2
c(\mathbf,t).

This equation is composed out of two true
statements. One of these is the continuity
equation

\partial_t c(\mathbf , t) = - \mathbf \cdot
\mathbf(\mathbf , t).

And the other Fick's
law

\mathbf (\mathbf , t) = - D \mathbf c (\mathbf,
t),

where \mathbf (\mathbf , t) is the flux, D is the diffusion
constant, and c (\mathbf, t) is the concentration of diffusing
material.

The continuity equation is the mathematical
equivalent to a piggybank. Your savings
increase by the amount that you put in, they decrease by the amount
you take out, no more and no less. Fick's law, on the other hand,
was born as an empirical law which means that it describes
observations and is not derived from any argument.

One general solution to the diffusion equation is
a Gaussian one. This suggest an uncorrelated random walk as a
microscopic model, completely in line with Robert Brown's
observations.

## Einstein relation

Einstein showed that Fick's law (empirical) can be derived by writing the flux in terms of the chemical potential, and taking this potential to be that of an ideal gas. This last step is valid for not-too-dense concentrations of particles in general (in a gas, but in a liquid as well). The result is\mathbf (\mathbf , t) = - \frac\mathbf c
(\mathbf, t),

where \gamma is the drag
coefficient (the inverse of the mobility). The
Einstein relation follows directly to be

D = \frac,

which is the most general expression for the
diffusion coefficient, not refering to any microscopic model.

## Entropy and diffusion

- Brownian motion, for example of a single particle in a solvent
- Collective diffusion, the diffusion of a large number of (possibly interacting) particles
- Ellusion of a gas through small holes.
- Electronic diffusion, resulting in electric current
- Facilitated diffusion, present in some organisms.
- Gaseous diffusion, used for isotope separation
- Heat flow
- Itō diffusion
- Knudsen diffusion
- Momentum diffusion, ex. the diffusion of the hydrodynamic velocity field
- Osmosis is the diffusion of water through a cell membrane.
- Photon diffusion
- Reverse diffusion
- Rotational diffusion
- Self-diffusion
- Surface diffusion
- Active transport, pumping material across a cell membrane
- Pinocytosis, "cell drinking" intake of small droplets of lipids
- Phagocytosis, carrier proteins are used to transport glucose

Metabolism and respiration rely in part upon
diffusion in addition to bulk or active processes. For example, in
the alveoli of mammalian lungs, due to differences in
partial pressures across the alveolar-capillary membrane, oxygen diffuses into the blood
and carbon
dioxide diffuses out. Lungs contain a large surface area to
facilitate this gas exchange process.

## An experiment to demonstrate diffusion

Diffusion is easy to observe, but care must be
taken to avoid a mixture of diffusion and other transport
phenomena.

It can be demonstrated with a wide glass tubed
paper, two corks, some
cotton wool soaked in ammonia solution and some red
litmus
paper. By corking the two ends of the wide glass tube and
plugging the wet cotton wool with one of the corks, and litmus
paper can be hung with a thread within the tube. It will be
observed that the red litmus papers turn blue.

This is because the ammonia molecules travel by
diffusion from the higher concentration in the cotton wool to the
lower concentration in the rest of the glass tube. As the ammonia
solution is alkaline, the red litmus papers turn blue. By changing
the concentration of ammonia, the rate of color change of the
litmus papers can be changed.

## References

- Investigations on the Theory of the Brownian Movement

## See also

## External links

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diffused in Chinese: 扩散