Diffusion Totally Explained

Everything about Diffusion totally explained

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. 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.

Background and theories

The mechanism of diffusion is Brownian motion whereby a particle makes a random walk about a central location since by kinetic theory the mean velocity of a particle is zero if it isn't subject to any external forces. Due to collisions with neighboring particles the motion is characterized by a mean free path. Diffusion can be simulated with kinetic Monte Carlo methods.
The diffusion equation

partial_t c (mathbf, t)

Fick's law is an assumption that may not hold for a given diffusive system (for example, the diffusion may depend on concentration in addition to concentration gradient), in which case the motion wouldn't be described by the normal (simple, Fickian) diffusion equation. An analogous statement of Fick's law, for heat instead of concentration, is Fourier's law.

Thermodynamic approach

Diffusion increases entropy, decreasing Gibbs free energy, and therefore is thermodynamically favorable. Diffusion operates within the boundaries of the Second Law of Thermodynamics because it demonstrates nature's tendency to wind down, as evidenced by increasing entropy.

Equilibrium with applied force

Diffusion is often important in systems experiencing an applied force. In a conducting material, the net motion of electrons in an electrical field quickly reaches a terminal velocity (resulting in a steady current described by Ohm's law) because of the thermal (diffusive) motions of atoms. The Einstein relation relates the diffusion coefficient to the mobility of particles.

In biology

In cell biology, diffusion is a main form of transport for necessary materials such as amino acids through cell membranes.

Types of diffusion

The spreading of any quantity that can be described by the diffusion equation or a random walk model (for example concentration, heat, momentum, ideas, price) can be called diffusion. Some of the most important examples are listed below.

Atomic 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
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 surfed ace 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 processes.
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.

Further Information

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