Talk:Surface tension

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[edit] Miscelaneous comments

Good topic heading, maybe should be applied to the majority of discussion pages. What I want to say is, why on earth is there a link to the U.K. in the first paragraph of an article on "Surface Tension"?

Wikipedia is already completely saturated with obscure and unnecessary references and links to this 'country', making me suspect that the site is run by British nationalists, but putting it in the opening paragraph of an article on Surface tension, moreover as a hyperlink, is doubly inappropriate.

What do we think guys? Can we change it? —Preceding unsigned comment added by 87.254.69.101 (talk) 15:14, 18 March 2008 (UTC)

[edit] Conversation between Robinh and Derek

  • Hello folks (newbie here).

I put a provocative statement in the main entry, to maybe get some discussion going.

The problem is that surface tension can't simply be explained by a "net inward force" argument, which is absurd. A net inward force would cause molecules on the surface to accelerate inwards: and bulk liquid would collapse, contrary to observation.

One theory states that molecules on the surface _do_ accelerate inwards, and sort of bounce back. In this view, the surface region is constantly in motion, and much as grains of sand on a rattling surface are.

But this can't be right either, because molecules in the next layer (ie just a bit deeper than the surface proper must experience a net force _out_ of the bulk. And this doesn't happen either.

My personal opinion is that surface tension is too complicated to be explained by classical mechanics of the highschool type, and any attempt to do so just confuses matters.

Real water is essentially quantum; even in the classical view, molecules have a dipole moment and it's not clear whether this moment is oriented into the liquid, out of the liquid, or parallel to the surface.

Just my two cents....comments please! —Preceding unsigned comment added by 195.92.67.68 (talk • contribs) 03:00, December 12, 2003

  • [sorry, wasn't logged in. I've just discovered the four tildes thing.
best
Robinh 21:33, 11 Dec 2003 (UTC)
  • The "net inward force" idea is, as stated above, wrong.
One way to see how surface tension works is to see that molecules at the surface, being in the potential wells of fewer neighbours than molecules inside the liquid, have higher energy. They will tend to move to a position of lower energy. This process will stop when equilibrium has been reached. Then it is seen that the density of molecules in the surface layer is very slightly lower than in the bulk of the liquid, because some of them moved. They are therefore further apart than the equilibrium position, and that means that there is an attractive force parallel to the surface, between any two molecules. They do not move, though, because they are pulled from each side. This is the cause of surface tension: far from there being a "surface film", there is a very slight reduction in the number of molecules, in a very thin layer, only a very few molecules thick.
Derek Locke - February 6th 2004. —Preceding unsigned comment added by 82.33..28.179 (talk • contribs) 06:27, February 7, 2004
  • Hi Derek
Thanks for your comments. I don't want to be overly confrontational here, but it does seem to me that, if there is a "very slight reduction in the number of molecules" in the surface layer, then these molecules will accelerate towards one another and collapse. And they don't.
(or do they? I admit to being 100% baffled by surface tension!)
Basically, I don't think that the surface layer of a liquid is in any sort of static equilibrium (except possibly statistical equilibrium) so one cannot apply equilibrium analysis to the situation (for example, you say above that "some of them moved". My idea of molecules in a surface layer is that they are all vibrating and rotating about like crazy!). But I could be wrong.
I don't want to be critical of your arguments above, but I would say that using the phrase "tends to move" is not helpful here. Surely the correct concept would be acceleration?
I would also suggest that a force is _by definition_ a gradient of potential energy. Your arguments would suggest that an inwards force does indeed exist!
best wishes
Robinh 22:21, 7 Feb 2004 (UTC)
  • Hello Robinh, Thank you for your comments.
I have calculated the reduction in density at the surface. The whole substance is in equilibrium, though subject to thermal motion. Change in density does not always imply any acceleration. For example, in a beam that sags under gravity, the density is a little greater at the top than the bottom. It is in fact the change in density that produces the forces that allow and equilibrium to be found. In a beam, we don't usually refer to "density": we refer to "strain". So the whole point of the less dense surface of the liquid is that the density variation is just that which is needed to compensate for the fact that the surface molecules have fewer neighbours than inner ones do.
I have not seen this "explanation" elsewhere, but it is the only one I have seen that provides a reason for a tension parallel to the surface.
If an inward force exists, it only does so during the minute time that the system needs to reach equilibrium. After that, there are no outward or inward forces, or there would be a flow.
Best regards, Derek Locke _________—Preceding unsigned comment added by 82.33.28.179 (talk • contribs) 02:23, February 13, 2004
  • Hi Derek
Very interesting comments above. I like your "beam" analogy. I will have to think about this. I guess my difficulty is that I imagine water molecules to be like little gas molecules, unable to interact except during collisions.
My image cannot be right if your beam analogy holds up (!).
In the beam, one can only understand the situation by assuming local isotropy and local homogeneity (on a molecular scale). Neither of these assumptions are true in the surface layer of water. This might or might not be important, but it seems to me that we should be clear about whether or not we assume them.
Maybe the molecules in the rarefied regions are moving about faster; would this make sense?
best wishes
Robin
Robinh 22:02, 12 Feb 2004 (UTC)
  • Hello Robin,
I believe that in a liquid, each molecule is always interacting with its nearest neighbours. The mean speed of the molecules depends only on the temperature and not on the local density, though it is possible that "temperature" has little meaning if we consider a region that is too small. This is the case because temperature is a property of bulk matter, or at least of samples big enough that the mean energy is constant at the accuracy with which we can measure.Regards Derek ____________—Preceding unsigned comment added by 164.11.204.245 (talk • contribs) 18:01, February 17, 2004

Question from another newbie, (with only the smallest education in physics):How does one break the surface tension of water without resorting to the use of surfactants? My problem is to remove the water from a polymer surface at no more than 135 degrees Fahrenheit, using forced air, vacuum, or any other economical method. This is to be accomplished on a complex surface on the inside of a cube(excluding the bottom face, which will be attached at a later time).—Preceding unsigned comment added by 66.248.123.78 (talk • contribs) 10:28, March 25, 2004

If you wish to merely remove the water, you can easily convert it to a gas state by reducing pressure (vacuum will do it) - or you can use it's adhesive properties (capilliary action) to get rid of it - by using a tissue or something like that! (20040302)

robin and derek,

I feel that the molecules on the surface have a velocity only in the direction parallel to the surface. They dont move into the surface.they resist moving down. So it appears that there is tension on the surface. The net downward force is balanced by the liquid's resistance to compression. This resistance to compression is like the normal contact force, there is no fixed value. If we place a small object (small enough such that it doesnt displace any liquid)it will exert more force due to which surface expands (depression) the molecules on the surface still stay on the surface keeping the object on the surface. —Preceding unsigned comment added by 202.153.37.148 (talk • contribs) 11:14, February 18, 2005

Hi there - Surface tension seems to be a matter still under discussion however i would like to ask if it is possible to measure surface tension using a variety of methods ? Is the apparatus accurate and is it affordable - i wish to use this topic as a physics project.

Thanks for any help. —Preceding unsigned comment added by 193.62.43.210 (talk • contribs) 16:22, August 19, 2005

[edit] Photo conversation

That flower photo looks more like a buoyancy effect. davidzuccaro 10:56, 5 October 2005 (UTC)

Its my photo. Clearly the reasonb the flower floats is buoyancy. Clearly the reason the water doesn't flow over the top is surface tension. William M. Connolley 15:04, 5 October 2005 (UTC).

Hi guys,

I added a link to the Tolman length since surface tension changes with curvature; this was missing in the original article.

Regards, Joris —Preceding unsigned comment added by Joriskuipers (talk • contribs) 22:50, January 4, 2006

[edit] The short story

Isn't there a Surface Tension short story about spacefaring humans who land in some planet and leave some altered beings with a "human spark"? These humans live in a pond and eventually evolve a technology that enables them to break the surface tension and to do interpondary travels, clearly a parallel of escape velocity and interplanetary travel.


Hi. Indeed there is. I think it was called "seedling stars" by James Blish. I have a copy somewhere...

best wishes

Robinh 15:56, 7 March 2006 (UTC)


[edit] Mistake in article

The article contains the paragraph:

Surface tension, measured in newtons per meter (N·m-1), is represented by the symbol σ or γ or T and is defined as the force along a line of unit length perpendicular to the surface, or work done per unit area.

This is incorrect. The force of surface tension is parallel to the surface and normal to the line along which it is measured. I will correct this later today. Karlhahn 16:49, 2 October 2006 (UTC)

[edit] Another mistake

I changed "taught thread" which doesn't make any sense, to "taut thread".

I think this article needs to be improved, although I'm no expert. Surface tension is something that we understand intuitively because of the way we see its effects every day in the meniscus of a cup off coffee, the cohesion of a soap bubble, and the bulge at the top of a column of mercury in a clinical thermometer or a mercury barometer. An encyclopaedia article should probably engage the reader in that way, but our current article seems to treat the subject as some bit of high physics for the initiated. Rosejpalmer 03:47, 5 October 2006 (UTC)

Could you email responses to me? I entered my email address but it wouldn't accept it. It's rosejpalmer at google mail. I will probably watch this page too. Is there a forum or mailing list for this stuff? Can people edit my posts? This is crazy. Rosejpalmer 03:47, 5 October 2006 (UTC)


[edit] New subsections added

I have added several subsections: "Water strider physics" "Liquid in a Vertical Tube" and "Pool of liquid on a nonadhesive surface". These are targeted to a less technical audience than the equation-laden sections that follow later on. Those later sections still need work to put more words to the equations. Without such explanations, the equations are of benefit only to those who are versed in mathematical physics. Karlhahn 03:46, 12 October 2006 (UTC)

[edit] More New Material

I've busied myself today expanding and reshaping this article. I guess I have to be a real nerd to be so captivated by the subject of surface tension that I would go to this much effort. I am beginning to think a supplementary page might be in order that would contain the heavy-duty math. I had started this effort with the goal in mind of making the article more accessible and engaging to non-math type readers. I fear I have not succeeded, since everything in physics always leads to math. So I am open to suggestions about keeping only the descriptive material on the main page and moving the math-content to a supplementary page. You can include your comments both here and at my user talk page. Karlhahn 21:58, 14 October 2006 (UTC)

I think your new aditions are great and I fully agree with splitting all the math and thermodynamics into a more technical article Knights who say ni 12:39, 15 October 2006 (UTC)

[edit] Equations

Several of the equations at the end of the page are shown in raw Tex. Also, why isn't MathML used when supported instead of inaccessible graphics? --Belg4mit 03:11, 19 October 2006 (UTC)

Use of TeX within the <math> TeX code </math> tags is what is recommended in the Wikipedia editing help guide for rendering math formulas. Sometimes the server-side engine that renders this stuff into graphic form is over-burdened, and it can fail to render (which apparently happened to you). When it does, just reload the page, and it usually works. As for the use of MathML, most peoples' browsers do not have the plug-in for this. Karlhahn 15:30, 19 October 2006 (UTC)

[edit] Error in deltaP table

i think the last column (10 nm) has a problem Openlander 07:05, 10 December 2006 (UTC)

What probem?--Siddhant 06:58, 6 October 2007 (UTC)

[edit] Some questions about the article on surface tension

In the diagram of the cross section of a needle floating on water, aren't the two forces fs fictitious? In the case of no wetting, wouldn't all the forces from the water on the needle be normal to the surface, ie along radii of the needle? And wouldn't the vector sum (integral) exactly balance the weight?

For the coin and the flower floating on the water, couldn't we call this effect buoyancy by means of a virtual boat, the boat being the volume below the main surface in which the water has been displaced? Arcbimedes principle holds exactly as with a real boat, the displaced volume being equal to the weight of the coin, flower, etc, the "boat" itself being massless.

As an explanation of surface tension, isn't the tension the result of the molecules, on average, being a little further apart (in directions within the surface) than the mean separation for the given temperature in the bulk liquid?

Brantacan 16:10, 4 February 2007 (UTC)

If you imagine the surface as being an elastic sheet, then the two diagonal forces are the forces on that sheet. If you allow that surface tension forces are parallel to the surface, then this is how the diagram must be. Similar diagrams are given in physics texts. With regard to a "virtual boat," hydrostatic forces in the depression do come into play to some degree, but if the object is denser than water, by definition, hydrostatic forces are insufficient to keep the object afloat. There must be a contribution from something else, and that something else is surface tension. Since the article is not about bouyancy, the hydrostatic forces on the coin are not discussed (even though they are present). Karl Hahn (T) (C) 19:08, 5 February 2007 (UTC)

[edit] surface tension

it is cool stuff to learn about — The preceding unsigned comment was added by 72.93.7.144 (talk) 23:54, 10 February 2007 (UTC).

[edit] Failed "good article" nomination

This article failed good article nomination. This is how the article, as of April 1, 2007, compares against the six good article criteria:

1. Well written?: pass but see if more links can be incorperated into the text.
2. Factually accurate?: fail. Very little material is sourced, and citations are limited
3. Broad in coverage?: pass
4. Neutral point of view?: pass
5. Article stability? B- not stable enough
6. Images?: pass, though the pictures could be a little better organized.

When these issues are addressed, the article can be resubmitted for consideration. If you feel that this review is in error, feel free to take it to a GA review. Thank you for your work so far.

— Sefringle 20:55, 1 April 2007 (UTC)

[edit] Personifying surface tension

I am a bit disturbed by the anthropomorphism in the article. For instance "the liquid squeezes itself", "For the liquid to minimize its energy state", "... the surface will want to assume".

I am particulary bothered by "Without a single calculation, the soap-film solves the complex minimization equation on its own". I don't think we want to give the mental image of soap-film doing math. Do apples solve gravity equations when they fall?

I would like to see the article re-worked to eliminate such language. Instead of "the liquid squeezes itself", "the liquid is compressed by the molecular attraction until". Replace "For the liquid to minimize its energy state" with "In order to reach a minimal energy state, the surface area must be minimized by having the fewest number of boundary molecules". Replace "the surface will want to assume" with "the surface will assume".

Finally, replace the bit about the thinking soap-film with the observation that the answer to complex math problems can be discovered via empirical methods. Instead of calculating how long it takes an apple to fall, I can always just meausure it.

Hint, if you are tempted to use the pronoun "it", don't. Watch the verbs. If the liquid is "acting out" the verb, it is probably not what is wanted.

Tony 147.177.179.147 14:15, 16 April 2007 (UTC)

I understand your objections to use of the pathetic fallacy, but often it's useful to personify an object in explaining it, particularly in layman's terms. Saying that soap bubbles solve partial differential equations is stretching it, though. It's not like if we can't solve an equation, we just put it on a wire with soap film.--Loodog 00:44, 17 April 2007 (UTC)
Perhaps a better wording would be that the soap film "arrives at the solution to" rather than it "solves" the minimization. The fact does remain, however, that there are plenty of differential equation that can't be solved, but rather only approximated. Of course there are many physical objects that solve equations -- any time you apply heat to an object or any time you play a musical instrument are just two examples. But the soap film is outstanding in that the object actually takes on the exact shape of the solution function. Karl Hahn (T) (C) 02:24, 17 April 2007 (UTC)
Just because you are so clever dosen't mean that everyone else will understand it. Your point is valid, but this page should be understandable to people who aren't academics or whatever. —Preceding unsigned comment added by Alexcooldude (talk • contribs) 10:46, 27 November 2007 (UTC)

[edit] minimum?

It says minimum. Why not maximum, because the surface tension can be reduced only to a maximum amount. unsigned post by 70.132.11.140 at 05:02 on 19 April 2007

Surface tension is constant for a given liquid, so it is neither maximum nor minimum. Surface tension does, however, cause surface area to be minimized if there are no other influences. Karl Hahn (T) (C) 12:01, 19 April 2007 (UTC)

[edit] Rework based upon review results from April 2007

Since the last review I have reworked the article for better organization with particular attention to organization of the images. I have also added numerous references. The section on thermodynamics contained development of various equations -- I have pared that down to the equations themselves (readers interested in the development can seek the references), and I have tried to make the explanatory text in this section clearer and more concise. Overall I have tried to make the article flow from the general to the particulars as it progresses. I have added two new sections into which some material from other sections was merged. This addresses all the issues cited by the last reviewer except that of article stability. Of course, as a single editor I have no control over that, but I can only hope that the improved organization will lead to less radical edits by other.

I have subsequently resubmitted the article for review. Karl Hahn (T) (C) 15:07, 3 September 2007 (UTC)

[edit] GA Review

From the perspective of content, I think the article is very complete, and goes over the topic in much detail (maybe even a little too much). The referencing and citations is mostly there, though there are still a couple of gaps; at the least, editors should cite the source of where equations came from originally (don't place the citation on the equation itself, but on the reference to the equation in the text).

The major problem with this article in its present form is organization. The large number of fairly long and wordy section headers aren't doing any help to readers of the article; in fact, that's probably going to turn off many potential readers as "too technical". These should be simplified. This will be easy in some areas, for example, 'the cause of surface tension' can be reduced to 'causes' (the article title should not be repeated in section headers anyway, per WP:MSH). Some of the other section headers might be more difficult to shorten. It seems like several sections mainly consist of examples that explain some application of surface tension, and then provide the science and mathematical equations to back it up. Maybe the best solution here is to combine these example sections as subsections (3rd level headings) within a main section called 'examples', and place them near the end of the article. The text in the 'in everyday life' section might be used to provide some lead, introductory text to the new examples section.

The 'measuring methods' and 'thermodynamics' sections should ideally be located early in the article; at least before the examples. Change 'measuring methods' to 'methods of measurement'.

I think there's far too many images in the 'in everyday life' section. They seem to be clumped together and there's not really enough text, so it's hard to see the significance of each of these photos. Maybe these should be spread out a bit more; include some of these with some of the specific examples in other sections as they come up in the text?

I'm not really sure exactly where to place the 'contact angles' section; it seems out of place as-is. Maybe the physics definition section should be renamed and/or reorganized, and this information should fall under there? Or maybe reword the subsection title more as an example?

I would think that the physics definition section should probably appear as the very first major subsection, since a basic definition is essential to the article.

Hope this helps improve the article. I'll place this on hold at WP:GAC for about one week, so that these issues can be resolved. Cheers! Dr. Cash 02:02, 8 September 2007 (UTC)

Article has been promoted to GA status, having met all issues that were raised. Some additional references still should be added, but for the most part, it is well-cited and meets the GA criteria. Dr. Cash 03:51, 18 September 2007 (UTC)

[edit] Surface tension of vegetable oil?

What is the surface tension [N/m] of vegetable oils (like apricot seed oil) ..? (Oleum amygdalae persic, CAS 72869-69-3) Electron9 10:48, 10 September 2007 (UTC)

A quick Google search on just one example came up with [1], which has the surface tension of olive oil. I'm sure you can do the same with other common food-oils. Karl Hahn (T) (C) 17:02, 10 September 2007 (UTC)

[edit] Capillary action

Isnt capillary action an indirect consequence of surface tension? Because, surface tension causes the pressure difference on curvatures, and as a result of this pressure difference the liquid rises or falls in a capillary. This implies that capillary action is not due to surface tension but due to the pressure difference created by surface tension. Any comments?

--Siddhant 07:10, 6 October 2007 (UTC)

In all cases where surface tension affects the shape of the liquid, it is a consequence of pressure difference due to surface tension. That is what the Young-Laplace equation is all about. So if you say that capillary action is an indirect effect of surface tension, you would have to say the same thing about formation of drops, waterstriders' impressions on the water's surface, shape of the edge of a puddle, and just about every other effect documented in the article. Karl Hahn (T) (C) 09:07, 6 October 2007 (UTC)

In the case of drop formation, surface tension causes it to assume the spherical shape i.e. causes drop formation. The resulting pressure difference just keeps the drop from collapsing. In this case it is a bit contrasting:

Surface tension causes the pressure difference and the resulting pressure difference stops the drop to further collapse due to surface tension.

Is this reasoning correct? Any comments?--Siddhant 16:22, 7 October 2007 (UTC)

[edit] Missing table and misplaced barometer

0. Kudos.

I am very impressed by the diversity of topics and phenomena described. The article is written with great care, love and passion. Thanks!

Now couple critical remarks.

1. Missing table.

I got to this page when wanted to check the coefficient of surface tension for water. I remembered the number 72 - 73, but was not sure. Browsing through the article, I failed to find the value I was looking for. Tried searching for 72 and 73 - not there. Well, much later than I needed the number, I found it, buried in the Puddles on a surface section.
Suggestion: provide a table with gamma values for a few substances. Especially because a few less 'basic' tables are already there.

2. Misplaced barometer

Section Liquid in a vertical tube starts with mentioning of a mercury barometer as an example of a meniscus. If one reads the article quickly, she may get an impression that the measurement of pressure somehow related to the surface tension. It is very confusing.
Suggestion: remove barometer from this section, for example move it to Puddles on a surface. In fact, if Liquid in a vertical tube will be moved after Puddles on a surface, it would be more logical and easier for a reader. Puddles on a surface is illustrated very well, it is easier to describe meniscus after this section.

Alex -- talk 07:35, 8 January 2008 (UTC)

[edit] Errors in Discussion on Contact Angle

Error 1: Young's Equation

The discussion on contact angle neglects the presence of the free energy of the solid-vapor surface, which also plays an important role in determining the contact angle. As a result this is inconsistent with the wiki entry on "Contact Angle"(http://en.wikipedia.org/wiki/Contact_angle). The equation "Gamma_ls=-Gamma_la*cos(theta)" should be Young's equation: "Gamma_sv-Gamma_sl=Gamma_la*cos(theta)".

Error 2: Getting Gamma_ls from knowing Gamma_la*cos(theta)

The claim in the second to last sentence that you can get Gamma_ls from knowing the contact angle a liquid with known Gamma_la is incorrect. Knowing the contact angle and the surface tension of the liquid is not sufficient to measure the solid-liquid free energy per unit area. A direct result of Young's equation is that you can only measure the difference in the free energies of the solid-vapor interface and the solid-liquid interface.

Error 3: Negative Surface tension

Also the last sentence of this section states that the liquid-solid surface tension (Gamma_ls) is negative when the contact angle is less than 90deg. Since surface tension is also the free energy per unit area, it is impossible for an interface to have a negative free energy and be in equilibrium. A negative surface tension would mean that an interface would want to maximize its total area in order to minimize energy, which would imply that the solid or liquid would break into its molecular constituents and become a gas (if Gamma_lv<0 or Gamma_sv<0) or would begin to solubilize (if Gamma_sl<0). This error results from not using Young's equation.

When the contact angle is less than 90deg this means: Gamm_la*cos(theta) > 0 -> Gamma_sv-Gamma_ls>0 or in words, the free energy of the solid-vapor surface is greater than the solid-liquid interface.


This is my first ever comment and I've no experience editing content. Can someone help fix these errors? Charile Foxtrot Bravo (talk) 21:46, 2 May 2008 (UTC)

I have updated the section as indicated above. I also updated the diagram accordingly. My intent when I wrote the original draft of that section was to keep it as simple as possible. I thought that including the solid-vapor term would be confusing to many readers. I invite others to comment on this issue. Let me know if I did the right thing here. Karl Hahn (T) (C) 00:31, 3 May 2008 (UTC)
One more comment -- I believe that a liquid/solid tension can be negative -- as it would be for say water and cellulose. Since the solid's surface area is limited, liquid in contact with a solid with which it has a negative γ the liquid has no opportunity to break up into molecule-sized pieces (that is a gas), as long as its tension with its vapor is positive. Karl Hahn (T) (C) 00:43, 3 May 2008 (UTC)

The changes you made look good. Thanks for your help on fixing this!

The free energy per unit surface area (or surface tension) between two surfaces cannot be negative and be in equilibrium. In the example of water and cellulose that you describe, if the energy per unit area were negative, the water will not turn into a vapor. The water would mix in with the cellulose and the cellulose would solubilize in the water.

Water does wet cellulose (contact angle less than 90deg) as well as many other solids (e.g. glass). All this means is that the solid-air surface tension is greater than the solid-water surface tension. The free energy of the system is lowered when more of the solid-air surface is covered with liquid. What prevents the water from completely covering the solid-air surface is the increase in water-air surface area, which increases the total energy of the system. The balance point, or equilibrium state, is achieved when the liquid makes a non-zero angle with the surface, which is what we can observe, and is called the contact angle. Charile Foxtrot Bravo (talk) 20:04, 3 May 2008 (UTC)