Talk:Diffusion

Less technical explanation would be useful to a larger range of readers

I dont want to discourage the improvement of the technical information in the article. I think this is of great value to those who can understand it, but I would guess this to be less than 5% of the readers. I would like to see a greatly simplified qualitative description of the process, so the reader who has heard the term, but has no background in physics and is mathematically challenged, can develop an idea of what diffusion means. For example, In diving theory diffusion is an important process in decompression and decompression sickness, but most divers only need to have a qualitative gut feel for why the gas particle move one way and not the other, and why the rate changes. This level of understanding would probably be useful to a larger number of users than the mathematical analysis. Peter (Southwood) ^(talk): 19:20, 1 June 2012 (UTC)

Diffusion is important in thousands of processes, indeed. I just do not know how to introduce diffusion without the ideas of advection, bulk motion and diffusion flux. It may be a proper place between the historical part and the more formal parts for the explanation (for example Section "Diffusion in real life and technology"). Unfortunately, this is not my profession. I will try to continue the physical and mathematical part (and promise to try to be not too formal) and will be happy if somebody will write this "real life and technology" section. A good source of information gives the journal "Diffusion Fundamentals" http://www.uni-leipzig.de/diffusion/ -Agor153 (talk) 21:38, 1 June 2012 (UTC)

like yeah man; this article is NOT, repeat not at the level of a general encylopedia; for instance, where is the simple equation using high school algebra that relates the distance a molecule diffuses given time and the diffusion coefficient ?

I'm sorry, this is just not right; you have done a lot of work, but it is not the right work; sorry to be blunt — Preceding unsigned comment added by 50.245.17.105 (talk) 17:50, 17 February 2015 (UTC)

for instance, these two urls are the right level http://www.physiologyweb.com/calculators/diffusion_time_calculator.html and http://biophysics.med.jhmi.edu/courses/pdf/diffusion2.PDF a table like at key bionumbers ?? a general encylopedia article does not, by definition, use calculus until AFTER a non math, and then a simple algebra example.

sorry to be blunt again, but I havve written a lot of technical articles for wiki, and you really need to bet off your PhD i'm proud of my math high horse and do it right. and I shouldn't have to tell you this by now; that is why I'm mad - you are not doing the right job and I shouldn't have to tell yo uthat — Preceding unsigned comment added by 50.245.17.105 (talk) 17:56, 17 February 2015 (UTC)

Just keep on doing what you know best. I will try to write up something that is accessible without being too wrong. Or someone else can... Formal is fine, just that in isolation it closes the door on most people. As an example, the comment higher up on this page that diffusion is the movement from high free energy to low free energy will be totally meaningless to most people, while high concentration to low concentration probably makes some sense to them. Could you check your Diffusion Fundamentals link, I got a 404 not found error. I sorted it out, a space was needed. Cheers, Peter (Southwood) ^(talk): 06:50, 2 June 2012 (UTC)

I feel that what you say is true... a lot of technical details. In general, however, if you say so, then I think that most of the science-related contents in Wikipedia, (and not only science but in other fields as well), a lot of the data given contain a lot of technical details. If you want to change Wikipedia to be more accessible for those with handicap in maths/science/... etc., I would say that that would require a lot of effort.Luke KMS (talk) 08:45, 9 August 2014 (UTC)

I feel that me, as a lower secondary school student, would not be able to understand all of the terms used by the people writing this thus I feel that it should be made simpler to understand. Skye251 (talk) 03:41, 23 July 2020 (UTC)

The removal of the definition of Diffusion is an indication of the challenges STEM education faces

The definition of diffusion as the movement of a substance from a region of high concentration to a region of low concentration was the result of hundreds of years of research by scientists in many disciplines.

The removal of this definition is an utter disgrace. Moreover, the author of this page, Agor153, (I refer to them as the author as they appear to have made this page their personal mission) actually provides the very essence of the classic definition:

Consider: In Latin, "diffundere" means "to spread out". Logically, if something is "spreading out", it is moving from a region of high concentration to one of a lower concentration, i.e., it is DIFFUSING! Consider also that this very definition is illustrated in each of the three images/animations provided on the page.

How on earth is a young student supposed to grasp the most fundamental aspects of diffusion when the actual definition of the process is omitted?

Wikipedia is, unfortunately, the first port of call for many students who are trying to understand science - what chance do they have when they are met with a physics-driven personal project?

This also has knock-on effects. For example, "Facilitated Diffusion" was chosen as a term by scientists decades ago to accurately convey the mechanism for one of the most important cell transport mechanisms. The physiochemical properties of ions or molecules govern whether they can diffuse across cell membranes - from high to low concentration. Lipophobic/hydrophilic substances cannot cross the membrane due to the hydrophobic core of the phospholipid bilayer. Specialized transporter proteins (ion channels, transporter proteins) in the cell membrane "facilitate" (to make easier) the process of diffusion across the cell membrane, hence the name. However, this concept has now been lost within the facilitated diffusion wiki page as that page has also become the personal project for another user.

At this stage, I consider Wikipedia to be one of the biggest threats to STEM education due to the editing of its pages by subjective editors that show little regard for the multi-disciplinary nature of scientific discovery that led to the very definitions underpinning the titles of wiki pages.

[I'm an Associate Professor in the Departments of Physiology & Pharmacology and Large Animal Medicine at The University of Georgia; 20 years experience as a medical researcher and educator] — Preceding unsigned comment added by 96.32.136.122 (talk) 23:15, 27 March 2014 (UTC)

Dear User 96.32.136.122. I can agree that the problem of the short and correct definition of diffusion is not trivial, indeed. The short form "movement of a substance from a region of high concentration to a region of low concentration" is obviously wrong because just the flux of a gas from a region with high pressure to a region with low pressure is not diffusion but 100% satisfies your definition. You should also have in mind non-perfect media where diffusion is driven not by the gradient of concentrations but by the gradient of chemical potential and, as an extreme case, the diffusion phenomena in phase transition when they may move substances from regions of low concentration to regions of high concentration. Could I kindly recall that our expertise outside Wikipedia is not an argument [BTW, I am a full Professor in Biophysics and Applied Mathematics, 40 years experience in research and education and thousands of citations but this does not matter here :)). We have to use published reliable sources and simple general argumentation.] If you can find a general, simple and correct definition of diffusion that is applicable both for gas diffusion and spinodal decomposition and does not confuse diffusion with advection then you are more than welcome. I have spent a lot of time reading available reviews, classical textbooks and papers and have found that it seems to be impossible to define diffusion separately, without discussion of other transport processes (bulk motion). For discussion here (not in the article) I can say even more: in Reality there are the general Transport Phenomena where diffusion does not exist separately. We, for our convenience, separate diffusion from advection to study the simple and useful limit cases. Therefore, we get immediately the problem of separate definition. In any case, thank you for this important question. Perhaps, we should work further to find the proper definition for this Wikipedia article.Agor153 (talk) 12:34, 29 March 2014 (UTC)

Dear User 96.32.136.122. I can agree that the problem of the short and correct definition of diffusion is not trivial, indeed.

I fear that what you have done is to reduce this article from a resource that a student can use to grasp the fundamentals of diffusion, and turned it into something that is now only relevant to a small readership by making it a predominantly theoretical physics discussion.

The short form "movement of a substance from a region of high concentration to a region of low concentration" is obviously wrong because just the flux of a gas from a region with high pressure to a region with low pressure is not diffusion but 100% satisfies your definition.

Fluxes of gas from high to low pressure is bulk flow, thereby driven by pressure gradients, and not by concentration gradients.

You should also have in mind non-perfect media where diffusion is driven not by the gradient of concentrations but by the gradient of chemical potential and, as an extreme case, the diffusion phenomena in phase transition when they may move substances from regions of low concentration to regions of high concentration.

If something is moving from low to high concentration, then it is not diffusing, - it is not “spreading out”. — Preceding unsigned comment added by CogentEducator (talk • contribs) 21:27, 18 April 2014 (UTC) CogentEducator (talk) 22:03, 18 April 2014 (UTC)

Dear CogentEducator, you have the right to think that diffusion is what you like but you have added a wrong definition. Just the flux of gas from high density to low density is "moving from high to low concentration" but it is not diffusion. I kindly ask you to delete the wrong definition. If you like, you can read any advanced textbook. Wikipedia should be simple and clear, indeed, but it should not deliver mistakes. (BTW, you did not support your "definition" by any source. Is it your own invention?)Agor153 (talk) 01:29, 21 April 2014 (UTC)

Dear CogentEducator, your definition of gradient is also fantastic: "A gradient is the change in the value of a quantity (e.g., concentration, pressure, temperature) with the change in another variable (e.g, distance)." Where did you find such a definition?Agor153 (talk) 01:41, 21 April 2014 (UTC)

In addition, you remark "Fluxes of gas from high to low pressure is bulk flow, thereby driven by pressure gradients, and not by concentration gradients" is surprising: under constant temperature gradients of concentration and of pressure in perfect gas are proportional with a constant coefficient because P=nRT (P- pressure, n- concentration, R- constant, T- temperature). I have to ask again: where did you take your definitions? Did you invent these definitions? If you know the source, please cite!Agor153 (talk) 01:41, 21 April 2014 (UTC)

Dear Agor153, thank you for your comments, and you raise some good points. I think if we can engage in a conversation about this topic, then we (along with other contributors), can help to craft this page into a resource that can help students (of all ages).

One of the major problems students face is that the term "diffusion" is often defined differently in each class they attend (e.g., biology, physics). If you think of when a student first encounters diffusion in school (probably at age 14 or 15 in introductory classes about biology, chemistry or physics), they often turn to Wiki to research the topic. Unfortunately, I'm not sure that encountering a theoretical physics lesson is the easiest way for them to begin to understand diffusion.

The definition I gave is that most commonly given in biology textbooks. For example:

http://www.ncbi.nlm.nih.gov/books/NBK21626/

http://www.ncbi.nlm.nih.gov/books/NBK21052/def-item/A5076/

http://www.ncbi.nlm.nih.gov/books/NBK9847/

and is consistent with the definition provided by resources commonly used by students to research the definitions of words, such as Merriam-Webster (a : the process whereby particles of liquids, gases, or solids intermingle as the result of their spontaneous movement caused by thermal agitation and in dissolved substances move from a region of higher to one of lower concentration), and Oxford Dictionaries (The spreading of something more widely).

The problem, as I see it, is that science disciplines often work in silo'd environments that develop their own, discipline-specific, definitions and usage of concepts. The challenge is to work together to develop a resource through which students can build their understanding of diffusion - including the discrepancies between disciplines. As a starting point, I suggest a discussion of common ground and deviations from this common ground to see if this can be achieved.

As a physiologist, my research involves real world systems that have many variables, and I have to introduce concepts from a very basic level and then add complexity. I have little knowledge of theoretical physics, so please treat me like a 14 year old student when introducing concepts (seriously!). I sincerely hope that other Wiki users will join in this conversation to help refine this Wiki page. Diffusion was identified by the American Medical Association as the most important concept for students wishing to enter biomedical careers to master. Perhaps the page can be structured to have a basic introduction and then introduce more complexity down the page such that student learning is appropriately scaffolded at each stage?

Starting points, let's try and find common ground:

Definition of Diffusion: Since the word diffusion is derived from the Latin word meaning to "spread out", then this would confine the correct use of the word "diffusion" to processes involving something moving from an area of high concentration to an area of lower concentration. Do you agree? (Let's leave the concentration vs pressure discussion until later). I would contend that any use of the word diffusion in contexts that do not involve the "spreading out" of an object (ion, molecule, idea, etc.) would be incorrect. Does theoretical physics use this term differently? (remember - treat me as a 14 year old student!)

"Diffusion": In biology, this is defined as the one I have placed into the Wiki page, and is confined to the movement of ions and molecules over short distances (e.g., across a cell membrane). Diffusion occurs because of the random motion of ions/molecules which results in the spreading out the ion/molecule, and "ceases" once the concentrations in the regions to and from which the substance is diffusing are equal (when an equilibrium is reached). The ions/molecules still move randomly, but the movement of ions/molecules in either direction is equal, so it said that there is no "net movement" of ions/molecules once an equilibrium is reached. Since the ions/molecules are still in motion, yet there is no net movement, this is referred to as being a "dynamic equilibrium". From your initial remarks, you mentioned a situation in which substances move from an area of low concentration to high (Phrase Transition?). From a perspective of the definition of the word (spreading out), the use of the term "diffusion" in this context would appear to be inappropriate. How would you modify the definition to make it a) accessible to students, and b) correct within the realm of theoretical physics?

In situations considered to be "bulk flow", this usually involves the movement of "mixtures" (e.g., air as a mixture of gases, blood as a mixture of water, marcomolecules and solutes) on a comparatively "macro" scale: air moving in and out of the body, blood moving around the body. Thus, "bulk flow" (caveat - as I and many other phsyiologists use the term) is due to differences in pressure. There are many other factors, but the difference between diffusion and bulk flow is first introduced as being differentiated by concentration gradients and pressure gradients, respectively.

"Gradient". I'm very interested to read your definition of a gradient. The most common misconception that I encounter is when students confuse "concentration differences" with "concentration gradients". The rate of diffusion (time taken to reach an equilibrium) is governed by the concentration difference and the diffusion distance (the distance between the the regions from and to which the substance is diffusing). The concentration gradient is a function of the concentration difference and diffusion distance - as plotted on a graph - and the gradient (slope) is the change in concentration over that distance (as illustrated on the wiki page for gradient). Please let me know how this definition is incorrect from your perspective?

"Random Walk/Motion". Another common misconception is that individual ions/molecules move randomly. This motion is often illustrated by a single substance (e.g., ion) moving randomly in isolation, which doesn't convey the real world. In Brown's original experiments, the movement of pollen within water appeared random, but the motion is due to collisions with the molecules within water - I have added an animation to try and help students understand this concept - is this helpful or is it misleading from your perspective? From my naive perspective, if an atom was placed into a vaccuum (with zero gravity) with kinetic energy, then it would move in a straight line until it encountered another object. As such, I think that saying atoms, ions and molecules move randomly without the caveat of why (collisions with other ions etc.) is a little misleading - would you agree?

I look forward to your comments, though I may not be able to respond quickly due to work commitments, so please edit the Wiki page as you see fit, and hopefully we (and others) can work towards a good resource for students. CogentEducator (talk) 19:21, 21 April 2014 (UTC)

I have no idea what kind of diffusion you guys are arguing about. THE ARTICLE I WANTED IS CALLED Molecular diffusion Mrdthree (talk) 10:21, 9 April 2016 (UTC)

There is such a thing called "uphill diffusion" where molecules diffuse from a region of low concentration towards a region of high concentration (e.g. inside a miscibility gap). The fact that diffusion is driven by chemical potential gradient rather than simple concentration gradient is a well-accepted fact in the scientific community. Please do not revert back to the incorrect (or at least incomplete) definition. -- from a frustrated engineering professor — Preceding unsigned comment added by 108.26.177.121 (talk) 04:07, 15 February 2018 (UTC)

What's the difference between Molecular Diffusion and Diffusion?

I noticed that there is another Wikipedia article that talks about molecular diffusion. How is diffusion different from molecular diffusion? From reading that article: https://en.wikipedia.org/wiki/Molecular_diffusion , it seems like molecular diffusion and diffusion are synonyms. But, is it actually the case? Andrea.ns1005 (talk) 02:16, 8 March 2020 (UTC)

If you read both articles you will see that they are essentially the same, except that molecular diffusion refers specifically to the diffusion of particles in a fluid; the diffusion article is more general.—Anita5192 (talk) 02:39, 8 March 2020 (UTC)

Request for information on 'diffuion' in a diffuser/stator that converts kinetic energy to static pressure rise.

Any information explaining what 'diffusion' means when the term is used for flows in diffuser/stator to explain the rise in static pressure would be really helpful, as googling the term brings up the application of the term without explaining it. Thank you. — Preceding unsigned comment added by 49.206.9.1 (talk) 08:30, 23 May 2020 (UTC)

is vinegar high concentration and egg low concentration

— Preceding unsigned comment added by 2601:2c3:4201:d70:6dfc:c22a:45fc:e601 (talk) 07:00, 11 July 2021 (UTC)

is egg high concentration and syrup low concentration

— Preceding unsigned comment added by 2601:2c3:4201:d70:6dfc:c22a:45fc:e601 (talk) 07:00, 11 July 2021 (UTC)

why does food coloring or ink spread faster in hot water

— Preceding unsigned comment added by 2601:2c3:4201:d70:6dfc:c22a:45fc:e601 (talk) 07:00, 11 July 2021 (UTC)

as hot water increases the kinetic energy of particle and heat energy is directly proportional to diffusion. 103.195.249.11 (talk) 12:09, 23 June 2024 (UTC)

Wiki Education Foundation-supported course assignment

This article was the subject of a Wiki Education Foundation-supported course assignment, between 25 February 2020 and 8 May 2020. Further details are available on the course page. Student editor(s): Kristinetole.

Above undated message substituted from Template:Dashboard.wikiedu.org assignment by PrimeBOT (talk) 19:29, 16 January 2022 (UTC)

Some serious errors

I just read through this article to see if I had something to contribute, and was slightly sad/shocked to find both serious and less serious, but significant, errors that need fixing. They are more than I have time to fix myself at the moment, so I was thinking I would bring some attention to them here.

Diffusion in the context of different disciplines : In chemistry and materials science, diffusion refers to the movement of fluid molecules in porous solids. In both chemistry and materials science, most diffusion processes of interest do not concern porous media. Diffusion in porous media is a small subset of the situations in which diffusion is relevant to these fields. (misleading)

Onsager's equations for multicomponent diffusion and thermodiffusion:

1) It should be stressed that the separate diffusion equations describe the mixing or mass transport without bulk motion. This is wrong, the diffusion equations in the relevant context describe the mass transport relative to some bulk velocity, meaning that the fluxes do not contribute to the bulk velocity.

2) Therefore, the terms with variation of the total pressure are neglected. It is possible for diffusion of small admixtures and for small gradients. The Onsager relations, and the non-equilibrium formalism has been shown to hold inside the front of shockwaves and across surfaces. Which are systems decidedly far outside equilibrium, and with large gradients. (misleading)

Nondiagonal diffusion must be nonlinear

A single article is cited as the source for this section. The authors of said article have forgotten that the gradients in a system are constrained by the Gibbs-Duhem equation. If one applies the Gibbs-Duhem equation to their argument, it falls apart. An important part of non-equilibrium thermodynamics concerns exactly the point of selecting appropriate independent force-flux pairs. What the cited article shows is that if one treats a dependent set of forces as an independent one, unphysical results will follow.

I'll start cleaning this up once I have time, unless someone strongly disagrees, in which case we can discuss it here first :) ItsBigBoat (talk) 15:36, 15 December 2023 (UTC)

I applied a quick fix for the porous solids issue.

The reference in nonlinear issue has over a 100 citations. Therefore we will need a source that contradicts or corrects that source before removing the section. Maybe some of the citations do so, I did not dig in. Johnjbarton (talk) 16:42, 15 December 2023 (UTC)

Thanks for the quick fix!

Regarding the nonlinear issue: I'll start looking through the references, however I have a feeling that there will be none addressing the issue. The reason I think so is because "Article XYZ has an elementary error and should never have been published" is not something that will be published. Also, the article is in a mathematics journal, written by mathematicians, who cannot be expected to be aware of all the physical constraints of the systems the are talking about.

My point is: There are hundreds (at least) of articles out there whose results, without citing the mentioned article, directly contradict what the article says. Even elementary, classical books in irreversible thermodynamics will directly contradict or disprove what this article claims. Nobody serious in the field would spend time citing this article with the purpose of disproving it, because the argument is so fundamentally flawed. 2A05:9CC3:76:A975:ADAA:40A5:39B9:8330 (talk) 01:52, 16 December 2023 (UTC)

Regarding the Onsager-model sentences:

In #1 the current text and your version sound the same to me. "..without bulk motion" vs "...relative to some bulk velocity."

In #2 the article seems to be extrapolating from constraints on the theory ("variation of the total pressure are neglected") to assumptions about applicability that may have been shown not to be needed. A paragraph with references for "...systems decidedly far outside equilibrium, and with large gradients" would be great. Johnjbarton (talk) 17:13, 16 December 2023 (UTC)

I can agree that the first sentence may be ambiguous enough that it can be interpreted as being correct, but the way I read it, the current sentence is stating that the equations apply in the absence of bulk motion, while the correct statement would be that they apply also in a system with a bulk velocity, but that they describe fluxes relative to that velocity, and that these fluxes (by design) cannot contribute to a net bulk flow.

When I type it out I can see that I'm not formulating it very concisely either, so if you have suggestions on how to formulate that in a good way I'm all ears :) On the same note, I think it would be nice to add a section on how diffusion is always defined relative to a reference frame (bulk velocity), as the change in diffusion coefficients with changing reference frames is subtlety that I know that even many people in research are not especially familiar with.

For #2, I can add a paragraph with some references. Theres been quite a lot of development in the field the past ≈20-30 years, and especially the assumptions around local equilibrium and linear response have been shown to hold remarkably well. Then theres the issue that non-equilibrium thermodynamics is somewhat of a niche field, so many of the preconceptions people had in the 1980's, for example that it only applies near equilibrium, are still sticking around. ItsBigBoat (talk) 19:41, 16 December 2023 (UTC)

Basic models of diffusion

I think the section "Basic models of diffusion" would be much clearer if the subsections were named by the model discussed and that model started the section. Johnjbarton (talk) 18:14, 17 December 2023 (UTC)

Ok I WP:Boldly changed the section names.

The "Diffusion in Physics" now stands out even more. IDK what it means. Johnjbarton (talk) 19:07, 17 December 2023 (UTC)