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Diffusion is not limited to molecules, from a general (mathematical formulation) point-of-view a lot of processes are diffusive. What diffusion is or not is, is best to discuss by means of maths because that is the most accurate and objective way to write down things with. Things like "diffusion is the movement of molecules from high to low chemical potential/concentration, sometimes against the concentration gradient!" I would like to see in it's own section, i.e. not stated as general fact in the first few lines of this article.145.94.65.212 (talk) 07:08, 1 October 2008 (UTC)
It is to specialistic for the general reader looking up diffusion.145.116.1.3 (talk) 07:45, 5 June 2008 (UTC)
Exactly, there seems to be a problem with the management of this page, I tried to put in a general definition in the introduction (see revision of early september) and it disappears; in its place childish statements like "Einstein was clever".
Also there is a problem of definition. I come from a physical chemistry research background where diffusion is only used in the sense of the movement of molecules due to their thermal energy within chemical systems. The transfer of heat may follow the same maths but is not diffusion (to me). I am willing to have a physicist clarify this, but there still needs to be a separate page. The example of the spoon getting hot is confusing and I believe wrong. Manikato (talk) 03:22, 30 September 2008 (UTC)
I would like to keep the page on diffusion general, to fit all the different (natural) processes that show the same behaviour. To me (physicist), diffusion is not restricted to particles/molecules alone, since there can be diffusion of heat, or diffusion of wave function in quantum mechanics (for example). 145.94.65.212 (talk) 19:23, 1 October 2008 (UTC)
Diffusion is the movement from a high to low concentration along the concentration gradient. In ternary solutions particles can be tranported from low to high concentrations. Joao
This can happen in binary alloys, too Olof
Diffusion as noted in the last paragraph generally implies a dilution or mixing of the medium. Hence I'm not totally happy by a contrast with radiation as this is itself a diffusion mechanism. Indeed the reference to heat through a window is indeed diffusion by radiation. Rjstott
Heat through a window was a bad example. Maybe heat through a window at nighttime was what I was thinking. Perhaps heat through an oven door is a better example. However, I think its correct to say radiation and diffusion are completely different transport phenomena: in heat transport by radiation, photons get emitted by thermal activation and carry the energy until the photons are absorbed somewhere else. heat transport by diffusion is governed by the thermal conductivity of the material and the temperature gradient. Olof
Way back in school, there were three heat transport mechanisms, conduction, convection and radiation. I believe that heat can be diffused by any of these three. Light is an electromagnetic radiation and diffusion is often associated with light too. Perhaps an example based on light diffusion might work better? Rjstott
You are right about the three mechanisms: Radiation is when photons carry the energy from one place to another. Heat disappears from a material when the photon gets created, and reappears at the place where the photon gets absorbed. The key part there is that the material needs to be hot enough so that it can thermally activate a photon creation, as when a metal is 'red hot'. Of course, at lower temperatures, the photons are lower energy and there is some radiative heat transport. Convection is when, for example, the hot air next to the oven rises to the ceiling, and the air that used to be near the ceiling moves to.... and so on. It is only applicable to fluids. Conduction is when heat diffuses through a material, and it is the only one of the three mechanisms which is strictly a diffusion. By this I mean that the mathematical equation which is used to model the heat flow is the diffusion equation. Olof
I was also thinking of adding a section on computer graphics diffusion: the process whereby an image is blurred by an image editing program also follows the same math. Olof
I think the 'in biology' section is completely wrong. Amino acids cannot diffuse across a cell membrane, they must be transported across by specific sodium-dependent transport proteins. Moreover, the citation is not very good - many introductory textbooks are notoriously wrong. —Preceding unsigned comment added by 128.61.94.19 (talk) 02:24, 24 October 2008 (UTC)
I hate it when one page is used for two topics. Something needs to be done! Either create a page for Diffusion (physics) and Diffusion (anthropology) or kill the anthropology on the diffusion page.
Osmosis= diffusion of water not general diffusion surely. Equally my GCSE biology teacher could have been lying to me...
I just made a major change to the layout of the page. Since there were alot different types (and sub-types) of diffusion listed inthe article, I grouped them all in one section and moved the relevant information to separate pages on the particular types of diffusion. Hope that made the page a bit clearer than before. Cheers Karol 18:01, 15 September 2005 (UTC)
I think diffusion is not connected to turbulence, the example of smoke dissipating into the air is a bad example since it involves convection. I have removed the parts accordingly. Lirnup 20:28, 28 September 2005 (UTC)
Good point on the diffusion not being connected to turbulence, some sort of convection would really be necessary as I would consider the velocity of diffusion occurring within some characteristic length to not be able to obtain Reynolds numbers normally associated with turbulence. I consider the dissipation of smoke a laminar flowBiotechscientist (talk) 02:37, 19 November 2010 (UTC)
At 11h04 today (31 october 2005), I added a link to collective diffusion in this article. At 11h06, or 2 minutes later, this link was deleted by user:Karol Langner, on the basis that I should first create the collective diffusion article (I created this article "only" at 11h17). So Karol, a question: are you going to delete all the red links that have existed for more than 2 minutes in this encyclopedia, or should I conclude that you are specifically after my edits? ThorinMuglindir 12:32, 31 October 2005 (UTC)
OK sorry Karol I was a bit upset when I dscovered you had canceled my edit. Now, admit the real reason why you canceled it is because it conflicted with yours, and having to cope with those 2 text boxes is a mess. So that you thought that since your edit as big and mine was just a link, it wouldn't be much of a deal for me to add it back... I prefer that to you being specifically after my edits. ThorinMuglindir 14:27, 31 October 2005 (UTC)
BTW in hopes that you'd visit it quickly, I added a red link to the following page:
I find this page only gives the physical view. There is also the mathematical term of a diffusion (process). Of course these concepts are related, but it's not the same. LARS 18:58, 19 November 2005 (UTC)
Somebody might want to remove the giant animated GIF from this page. Just sayin'.
Isn't it because warmer systems have more energy and higher energy molecules move fast? too bad i don't have a reference for that Tsinoyboi 16:16, 21 September 2006 (UTC)
The example of the sugar cube in WATER is wrong, unless we could guarantee that water is completely still. Otherwise there will be some transport due to convection, and it is also likely that the "sugar water" neat the sugar cube will sink to the bottom of the glass due to its higher density and the gravitational force.—The preceding unsigned comment was added by 201.231.152.118 (talk • contribs) .
Could it be convection and diffusion? i'm finding in my chemistry textbook that convection in an isolated system such as a closed flask are examples. Maybe the difference is that it be closed and a room has too much outside energy such as people or light. Tsinoyboi 17:14, 21 September 2006 (UTC)
The following are NOT examples of diffusion:
According to my textbook, Chemistry & Chemical Reactivity, 6th edition, Thomson Brooks/Cole ISBN 0-495-01013-8, cover seen here http://www.newtexts.com/newtexts/book.cfm?book_id=3038, those count as diffusion that ultimately they become scattered. It says so on page It's supposed to be closed containers so they can be closed systems, it's just that those systems are open to other sources of energy to affect the rate system. Rate doesn't mean it's not diffusion. Warmer molecules will just mix faster. Being open systems, heat may be introduced like windows or people in a room, or escape like water evaporating from a beaker. The listed items are still molecules diffusion.
However, in a quiescent medium, gas could distribute evenly across across a room, and the dissolving of sugar will distribute sugar within the water, and the ink particles distributing is diffusion again. I think the word NOT should have been simply removed. Wiki editors, I am new to this, but removing content that is worthwhile to the general public, and relevant, I think you should correct the inaccuracies, and not just remove content. I know about my little section of the world, but so do other people, even if it is the other way around.Biotechscientist (talk) 02:47, 19 November 2010 (UTC)
Theses have nothing to do with diffusion, they are examples of active transport, require energy and specialized transporters/engulfing machinery:
Besides Pinocytosis is an intake of small droplets of liquid not lipids, phagocytosis has nothing to do with transporting glucose, it's engulfing of solid particles by cell membranes, read their articles for more info.Enemyunknown (talk) 13:24, 12 December 2008 (UTC)
Diffusion and Thermodynamics are definitely related but can you say that diffusion occurs as a result of the second law of thermodynamics? isn't it more that The second law of thermodynamics works because of diffusion of heat? Tsinoyboi 16:15, 21 September 2006 (UTC)
Also, i found on Thermodynamic equilibrium #Equilibrium overview, "diffusive equilibrium" is used but there's no page for it. Tsinoyboi 16:34, 21 September 2006 (UTC)
hmm, i guess in Chemical potential, it states that entropy remains unchanged at equilibrium, but then again how is chemical potential related to diffusion? I still stand by diffusion as a result of the second law is incorrect. Tsinoyboi 16:34, 21 September 2006 (UTC)
I changed the third paragraph of nature of diffusion. I even cited my source --Tsinoyboi 07:28, 29 September 2006 (UTC)
Chemical potential gradients are what cause diffusion. This is why diffusion of, for example, matter, does not have to occur 'down a concentration gradient' - it will go in any direction which reduces the chemical potential. If this weren't true, we couldn't get spinodal decomposition. Mike 17:07, 17 May 2007 (UTC)
I re-added a sentence about the chemical potential gradient's importance to diffusion, and in particular up-hill diffusion. I think it is of great importance to be aware that diffusion in reality is NOT an effect of concentration gradients but of the interactions between individual atoms/molecules, and how these prefer to stick together or to spread out, to minimize the system energy.(Johnsarelli (talk) 12:12, 4 August 2008 (UTC))
The example animation on the page shows a clear example of convection and bulk flow; the higher-density dye sinks in the glass of liquid (with turbulent fluid flow); barely any of the visible mixing is due to diffusion. After all, diffusion constants for small molecules tend to be a few hundred microns squared per second, meaning that it would take millions of seconds (weeks) for diffusion to equilibrate the dye concentration in the glass.
Does anyone want to keep that graphic as an example of "mixing that isn't diffusion"? Otherwise, even though it looks cool, I'll just delete it. Ichoran 04:29, 15 October 2006 (UTC)
You are right. http://en.wikipedia.org/wiki/Image:Diffusion.gif must be deleted from the diffusion or edited.
-V.I. 17 November 2006
WHY?????? goddammit, quit deleting my edit...Will987654321 06:08, 1 December 2006 (UTC)will987654321
Because it is NOT diffusion -V.I.
I agree that the illustration is NOT an example of diffusion in the Fickian sense that this article is referring to. It is an example of convection. The time it would take to see Fickian diffusion at that scale is BIG. I think that the animation is very misleading and should be removed. When I get some time I will make a simulation of Fickian diffusion. -ZBS
The introduction implied that Fick's law and Fourier's law were alternate names of the diffusion equation. Fick's law is used to derive the diffusion equation, but is not identical to it. Also, I think the important aspect of Fick's law is not that it labels the diffusing substance as mass (as opposed heat), which was implied, but that the net transport is proportional to the concentration gradient. There are other possibilities for purely diffusive systems! I tried to clarify this in the introduction. Danrs 20:04, 15 August 2007 (UTC)
See my comment in the Spontaneous Process talk page.
Is it possible for us to add a section into the article where this process is explained simply? I'm staring at the screen and cringing when I read this - maybe I just have unnaturally low intelligence. The Last Melon 01:53, 26 June 2007 (UTC)
I've tried to make the introductory paragraph more accessible to non-experts. Ichoran 19:03, 17 July 2007 (UTC)
"A concentration gradient is the difference between the high concentration and the low concentration. It also determines how fast diffusion occurs."
Isn't it more like the concentration gradient is a measure of how fast the diffusion is? // habj 21:17, 3 August 2007 (UTC)
Thermal diffusion ≠ Heat flow. Currently Thermal diffusion redirects here, and then this article sends it off to "Heat flow", which redirects to Heat transfer). This is not the best understanding of Thermal diffusion as in the OED or most places. — DIV (128.250.204.118 06:03, 21 September 2007 (UTC))
The diffusion experiment with ammonia and cotton wool seems a little suspect to me. Won't the rapid evaporation of ammonia increase the local pressure and thus cause slight bulk flow within the tube? Wouldn't stabbing gelatin with a needle with ink be a better experiment? The key to doing a good experiment that demonstrates diffusion is to choose a system where non-diffusive flow is greatly retarded by viscosity or somesuch. Ichoran 19:55, 3 October 2007 (UTC)
The term diffusion is derived from the Latin verb husionere which means "to break" but can also mean "leave way"
There is no such word "husionere" in Latin. In fact, I can only assume this is vandalism, since it doesn't bear even a passing resemblance to reality.
diffuse
dis-
http://archives.nd.edu/latgramm.htm
diffusilis -e [capable of spreading , elastic]
fusus (1) -a -um partic. from fundo; q.v.
fundo (1) fundere fudi fusum: of liquids [to pour , pour out]; of metals, [to melt, cast]. Transf., [to pour out, shower, give abundantly]; [to squander]; 'se fundere', [to rush, stream]; of sounds, [to utter]; with emphasis on distribution, [to spread, extend, scatter]; milit., [to rout, defeat, scatter, put to flight]. Hence partic. fusus -a -um, [spread out, extended]; 'crines', [flowing free]; of speech, [diffuse]; adv. fuse, [widely, copiously].
Proving once again that an encyclopedia predicated on popular consensus... sucks.
--75.63.48.18 (talk) 08:02, 8 January 2008 (UTC)
I proposed to move this article to Molecular diffusion.
Reason: to better define the scope of the article. "Diffusion" is a large and complex topic.
The obviously central yet missing article is that on the molecular diffusion of mass.
Under the current title, the basic definitions are disputed. The article does not appear to develop into a useful one, despite an overall large number of edits.
Cheers. Stan J. Klimas (talk) 21:32, 22 October 2008 (UTC)
The article gives a few examples for typical diffusion speeds such as "In gas: 100 mm per minute". Diffusion is inherently something that is described by the square of length per time unit and these examples (which suggest that 100 mm per 60 seconds equals about 1.5 mm per second) are misleading. Han-Kwang (t) 09:14, 2 February 2009 (UTC)
The See also section is now rather long with 13 entries. Would it be better to shorten this and rely on the Diffusion Category to aid further navigation? --Art Carlson (talk) 11:35, 9 March 2009 (UTC)
I just suggested that the page Diffusion equilibrium be merged into this one. There is only a small amount of material on that page, and this article already has a section on non-equlibrium systems, so it seems like it would fit here. Shanata (talk) 21:29, 20 October 2009 (UTC)
The animation on carrier diffusion in an intrinsic semiconductor bar is inaccurate. The greater hole concentration relative to electron concentration is attributed to the higher diffusion coefficient of electrons in most semiconductors. While this is true for most values of electron and hole diffusion coefficients, under this configuration the photogenerated electrons and holes actually diffuse at the same rate, i.e. they should be in equal concentration for each position in the bar. The reason for this is the formation of an electric field between the electrons and holes as the electrons attempt to diffuse more rapidly. This electric field drives the electrons back towards the holes and vice versa as any concentration mismatch develops. Because of the induced drift, the carriers actually diffuse at the same rate, otherwise known as ambipolar diffusion. This phenomenon is not a good example to show since it fundamentally involves both diffusion and drift. I would suggest changing to an example not involving the diffusion of charged particles due to this complication, so that it is more accurate.
[moved here from private talk page:]
In October of 2010 I edited the diffusion page with the example of spraying cologne. Agreed, maybe I should have been more specific about the transport phenomena occuring, but the edit you have made of this example is clearly incorrect. When the cologne leaves the nozzle of the sprayer convective flow will be dominating; however as the cologne "cloud" increases its volume it will eventually reduce its velocity to zero at which point it will be at atmospheric pressure. Please note, this phase of transport is primarily driven by pressure and is therefore the convective regime of this process; however there will still be a concentration gradient between the cologne cloud and the room air(so long as the room is sufficiently large enough to accomodate the cologne cloud at atmospheric pressure and temperature). Transport will then continue via brownian motion, ie diffusion, until the cologne is eventually distributed throughout the room. This process will occur in the presence of no other gradients (pressure, temperture, electromagentic) because of the random motion of the molecules. Or it could be looked at as, it will always occur because the system with a cologne cloud and air is more ordered than a system where the cologne molecules evenly distributed throughout the room. This is to say the process is entropically driven. Since entropy always increases the process will occur spontaneously at any temperature above 0 kelvin. Another way of looking at it, is that the pressure driven flow is an enthalpic process where diffusion is an entropic process.
Please make the appropriate changes to the page. The ink droplet example is also incorrect for similiar reasons. I am not fixing the page because I did not break it.
If you did not make the described changes I apologize. — Preceding unsigned comment added by Anwilson6 (talk • contribs) 17:52, 14 April 2011 (UTC)
== Major Rewrite Proposal ==Diffusion vs convection
[moved here from private talk page:]
In October of 2010 I edited the diffusion page with the example of spraying cologne. Agreed, maybe I should have been more specific about the transport phenomena occuring, but the edit you have made of this example is clearly incorrect. When the cologne leaves the nozzle of the sprayer convective flow will be dominating; however as the cologne "cloud" increases its volume it will eventually reduce its velocity to zero at which point it will be at atmospheric pressure. Please note, this phase of transport is primarily driven by pressure and is therefore the convective regime of this process; however there will still be a concentration gradient between the cologne cloud and the room air(so long as the room is sufficiently large enough to accomodate the cologne cloud at atmospheric pressure and temperature). Transport will then continue via brownian motion, ie diffusion, until the cologne is eventually distributed throughout the room. This process will occur in the presence of no other gradients (pressure, temperture, electromagentic) because of the random motion of the molecules. Or it could be looked at as, it will always occur because the system with a cologne cloud and air is more ordered than a system where the cologne molecules evenly distributed throughout the room. This is to say the process is entropically driven. Since entropy always increases the process will occur spontaneously at any temperature above 0 kelvin. Another way of looking at it, is that the pressure driven flow is an enthalpic process where diffusion is an entropic process.
Please make the appropriate changes to the page. The ink droplet example is also incorrect for similiar reasons. I am not fixing the page because I did not break it.
If you did not make the described changes I apologize. — Preceding unsigned comment added by Anwilson6 (talk • contribs) 17:52, 14 April 2011 (UTC)
Eau-de-Cologne in air and ink-in-water are popular examples for mass transport because they are vivid real-life cases known to everybody. In real life, however, these tracers spread in space because of convection, period. All you say is correct in some hypothetical limit of air or water being perfectly at rest, with walls being at perfectly homogeneous and constant temperature - but in this case, diffusion takes extremely long, and you have lost any connection to the real-life situation that originally motivated the choice of these examples. -- Marie Poise (talk) 09:17, 18 April 2011 (UTC) First, please show me the SIMPLE equation that you use to show that the flux of cologne can ONLY be by convection. I want your general equation and assumptions. Convection is a bulk property. It is either forced or natural (as with density gradients). If I just opened the bottle of cologne up, there would be ZERO velocity due to convection. I have not applied a force and natural convective flow is absent. This is the same for the drop dye aside from the original movement into the water. Additionally, you have described diffusion as EXTREMELY SLOW/LONG TIME. Are you kidding me? Diffusion of a gas in a gas is THE FASTEST of any molecule in a medium. I would use the Nernst-Planck equation. Is there an electric field being applied? No. Is there a velocity of the bulk cologne? Sure. Is there a concentration gradient? Yes. Explain to me then how both aren't occurring simultaneously? Regardless of the first persons entropic/enthalpic point, which is valid, you have given ZERO points to validate your claims in this post and the diffusion wikipage. I BEG YOU, assuming you are educated or in the process of a obtaining a degree, to bring your Diffusion Wikipedia page post to a reputable professor of chemical engineering or mechanical engineering and have them explain to you the fundamental mass flux process. — Preceding unsigned comment added by JC44567 (talk • contribs) 02:48, 22 April 2011 (UTC
This page needs major work. I wouldn't mind doing it, but I'd like to know if I have the community's approval first, since I'm new to this.
First, and my main gripe, it conflates the relaxation of concentration gradients with the motion of individual particles, which makes it difficult to choose a point of view from which to explain things. I'd like to separate out the continuum concepts of concentration field relaxation from the idea of random molecular motion. That way the thermodynamics and continuum mechanics ideas, like convective mixing, shear-induced dispersion, chemical potential gradients, etc, could be focused on in one article while activated hopping, collisions, and so forth can be focused on in the other. The commonly observed phenomenon, chemical or concentration diffusion, should be detailed separately from its underlying mechanism, particle diffusion, to avoid information overload for people only interested in one or the other. Each one will retain references to the other, but in less confusing mix than right now.
Second, there are numerous misleading statements, starting from the first sentence where it is implied that solids don't undergo diffusion. I'll try to clean these up, using standard engineering and physical chemistry texts like Bird Stewart and Lightfoot and Atkins as terminology references.
Third, there's too much left out. I'll fill in a bit of the history, some additional phenomenology comparing liquid, gas, and solid phase diffusion, some discussion of mathematical methods for dealing with it, e.g. grid-based and meshfree advection-diffusion PDE solvers, stochastic schemes for individual particle trajectories like Brownian and Stokesian dynamics, and so on. I'd also like to flesh out the equilibrium vs. non-equilibrium, linear vs. nonlinear, and frequency-dependent vs frequency independent cases, but I'll wait on that till linear response theory is a respectable page of its own.
That's what I'd like to do. If there are objections, please let me know. Also, I'm not a professor, just a grad student, and I'm not going to want to proofread everything, so it's a sure bet that I'll confuse some terms. Help me out if you see any mistakes. — Preceding unsigned comment added by ChE Fundamentalist (talk • contribs) 05:43, 29 July 2011 (UTC)
Welcome, ChE Fundamentalist! There is definitely lot of work to be done. If you are new to Wikipedia, I just suggest you start by small improvements here and there before you attack the major restructuring the diffusion articles definitely deserve. If you need help, feel free to contact me on my talk page. Again: welcome! -- Marie Poise (talk)
I've gotten a bit of experience wikifying, cleaning up typos, and writing big sections on articles, so now I'm planning to return to this. I've come up with a layout, and I'd like to know what anyone watching this thinks.
First, I'm going to go ahead with the separation into Molecular diffusion and Chemical diffusion. Chemical diffusion should be the first one that viewers come across if they're just interested in the mixing process, but both should have disambiguation notices just below the titles. Is there a tool I could use to sort links from other pages to these and make sure each link goes to the right one?
In the Chemical diffusion article, my planned layout is the following:
And then the Molecular diffusion page will have the following layout:
I'm not trying to make this go straight from start-class to good article status; I know there are things I've left out that should be covered in these articles. If there are any that you think I've left out that simply must be in any remotely acceptable version of these articles, please wait for me to finish this and then add them yourself or type them up on this talk page so I can copy and paste them in. On the other hand, if you have any objections to the general structure that I'm proposing, please let me know immediately so I can consider changing it. ChE Fundamentalist (talk) 18:49, 21 August 2011 (UTC)
There is also molecular diffusion in solids. This is the origin of aging processes in many solid materials. I would therefore suggest to remove the "(liquid or gas)" in the first sentence "Molecular diffusion, often simply called diffusion, is the thermal motion of all (liquid or gas) particles at temperatures above absolute zero." — Preceding unsigned comment added by 128.176.210.208 (talk) 17:14, 10 September 2019 (UTC)
Hey authors, I only had a quick view on this article and I find that already the introductory sentence "Molecular diffusion, often simply called diffusion, is the thermal motion of all (liquid or gas) particles at temperatures above absolute zero." misses some precision. To my knowledge and according to the definition in many basic textbooks in physics, diffusion is the mixing process resulting from random motion, not the random motion itself. The random motion of the fluid particles is rather termed thermal motion. It seems I cannot edit the introductory section. Please help! Thanks (Pelicanelson (talk) 10:53, 10 September 2020 (UTC)).
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