Diffusion describes the spread of particles through random motion from regions of higher concentration to regions of lower concentration. The time dependence of the statistical distribution in space is given by the diffusion equation. The concept of diffusion is tied to that of mass transfer driven by a concentration gradient. Diffusion is invoked in the social sciences to describe the spread of ideas.

Diffusion in physics

Diffusion is particles move/spread. In molecular diffusion, the moving entities are small molecules which are self propelled by thermal energy and do not require a concentration gradient to spread out through random motion. They move at random because they frequently collide. Diffusion is this thermal motion of all (liquid and gas) molecules at temperatures above absolute zero. Diffusion rate is a function of only temperature, and is not affected by concentration. Brownian motion is observed in molecules that are so large that they are not driven by their own thermal energy but by collisions with solvent particles.

The following image shows change in excess carriers being generated (green:electrons and purple:holes) with increasing light intensity (Generation rate /cm3) at the center of an intrinsic semiconductor bar. Electrons have a higher diffusion constant than holes, leading to fewer excess electrons at the center as compared to holes.

While Brownian motion of large molecules is observable under an electron-microscope, small-molecule diffusion can only be probed in carefully controlled experimental conditions. Under normal conditions, molecular diffusion is relevant only on length scales between nanometer and millimeter. On larger length scales, transport in liquids and gases is normally due to another transport phenomenon, convection.

Therefore, some often cited examples of diffusion are wrong: If cologne is sprayed in one place, it will soon be smelled in the entire room, but a simple calculation shows that this is due to diffusion. If ink is dropped in water, one usually observes an inhomogeneous evolution of the spatial distribution, which clearly indicates convection; diffusion dominates only in perfect thermal equilibrium.

In contrast, heat conduction through solid media is an everyday occurrence (e.g. a metal spoon partly immersed in a hot liquid). This explains why the diffusion of heat was explained mathematically before the diffusion of mass.

Other types of diffusion


  1. ^ D. Ben-Avraham and S. Havlin (2000). Diffusion and Reactions in Fractals and Disordered Systems. Cambridge University Press. http://havlin.biu.ac.il/Shlomo%20Havlin%20books_d_r.php. 
  2. ^ Weiss, G. (1994). Aspects and Applications of the Random Walk. North-Holland. 

Diffusion in a Bipolar Junction Transistor Demo

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  • diffusion — [ difyzjɔ̃ ] n. f. • 1587; lat. diffusio, de diffundere 1 ♦ Action de se répandre, de se diffuser. La diffusion de la lumière, de la chaleur, du son dans un milieu. Phys. Phénomène par lequel les diverses parties d un fluide deviennent homogènes… …   Encyclopédie Universelle

  • Diffusion — Dif*fu sion, n. [L. diffusio: cf. F. diffusion.] 1. The act of diffusing, or the state of being diffused; a spreading; extension; dissemination; circulation; dispersion. [1913 Webster] A diffusion of knowledge which has undermined superstition.… …   The Collaborative International Dictionary of English

  • Diffusion [1] — Diffusion. In Gasen und Lösungen gleichen sich lokale Unterschiede der Konzentration allmählich durch Wanderung der Materie aus. Diesen Vorgang bezeichnet man im ersten Falle als Gas (Atmolyse nach Graham), im zweiten als Hydrodiffusion. Unter… …   Lexikon der gesamten Technik

  • diffusion — DIFFUSION. s. f. Terme de Physique. Il se dit Des fluides en parlant De l action de se répandre, ou de l état de ce qui est répandu. Diffusion de lumière, de la lumière. Il se dit figurément Da discours, du style, lorsqu il est trop abondant en… …   Dictionnaire de l'Académie Française 1798

  • diffusion — diffusion, diffusionism Diffusion refers to the spread of cultural attributes from one culture to another through contact between different cultural groups. Diffusion theory developed in the eighteenth and nineteenth centuries, in opposition to… …   Dictionary of sociology

  • Diffusion — Diffusion, 1) Weitläufigkeit; 2) Diffusion der Gase, die Eigenschaft der elastischen Flüssigkeiten, sich, den Gesetzen der Schwere entgegen, sowohl in anderen Gasen, als auch in Flüssigkeiten u. festen Körpern zu vertheilen, s.u. Gas …   Pierer's Universal-Lexikon

  • Diffusion — (lat., »Ergießung, Ausbreitung«), der Vorgang der allmählichen Mischung zweier miteinander in Berührung befindlicher Körper, insbes. von Flüssigkeiten oder Gasen. Gießt man Weingeist vorsichtig auf in einem Gefäß befindliches Wasser, so haben… …   Meyers Großes Konversations-Lexikon

  • diffusion — UK US /dɪˈfjuːʒən/ noun [U] ► MARKETING, COMMUNICATIONS the process by which information, ideas, changes in behaviour, etc. spread through a group of people: »The research looks at factors influencing the diffusion of innovative products in… …   Financial and business terms

  • Diffusion — Diffusion. См. Диффузия. (Источник: «Металлы и сплавы. Справочник.» Под редакцией Ю.П. Солнцева; НПО Профессионал , НПО Мир и семья ; Санкт Петербург, 2003 г.) …   Словарь металлургических терминов

  • Diffusion [2] — Diffusion des Lichts, s. Reflexion …   Lexikon der gesamten Technik

  • Diffusion [3] — Diffusion im Bergbau, s. Wetterwirtschaft …   Lexikon der gesamten Technik

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