Talk:WikiJournal Preprints/Surface tension

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Author: Karl Hahn[i], et al.

Hahn, K; et al..

This article has been declined for publication by the WikiJournal of Science. It is archived here as a record. Discussion can be viewed below.

Plagiarism check

From WMF copyvio tool: Matching material in six other websites were flagged. An attempt to assess the nature of these flags is underway. I strongly suspect that the following articles were copied from the Wikipedia article

The following flags do not indicate any plagiarism on the part of Wikipedia:

My guess is that the large number of flags is due to the fact that surface tension is often taught at the pre-college level. Many of the concepts associated with surface tension are at a level appropriate for such education. Do not hesitate to add comments below.--Guy vandegrift (discusscontribs) 20:04, 19 July 2018 (UTC)

First review

Review by José Bico (ESPCI, Paris) ,
This review was submitted on , and refers to this previous version of the article

I appreciate the efforts to write a wikipage on surface tension. The curent version of the page is encouraging although many points could be improved. Some of my remarks concern the structure of the document, some other remarks are limited to minor points. Please find below the different comments that originated from a linear reading of the document.

1. "Surface tension is the elastic tendency of a fluid surface” -> I find this statement misleading: surface tension is not limited to fluids but is found in solids as well. The sentence also sounds grammatically awkward to me. Here is an attempt at an alternate definition: Surface tension is a mechanical tension experienced at the interface between two phases (e.g. water / air, oil / water, metal / water). The physical origin of surface tension is the attraction between the molecules composing each phase.
2. I would leave consequences (e.g. water striders) of surface tension for later.
3. I think that talking about adhesion is confusing. Just say "At liquid–air interfaces, surface tension results from the greater attraction of liquid molecules to each other than to the molecules in the air”.
4. I do not like much the analogy with a stretched elastic membrane. Although there is indeed some analogous behavior in terms of stretching, surface tension cannot sustain any shear in contrast with an elastic membrane.
5. "Surface tension is an important factor in the phenomenon of capillarity.” -> should come later when discussing the consequences of surface tension (with floating insects).
6. The discussion of the dimensions should come before the discussion on the specific value for water.
7. The discussion about surface stress in condensed mater, can also be a tricky issue. Basically, surface tension and surface energy are synonymous. However, surface stress is the sum of surface tension with the partial derivative of surface tension with respect to the strain. Of course, this additional term vanishes in the case of a liquid phase.
8. The discussion with the unbalanced forces leading to a pressure is delicate. Indeed one may think that since the surface molecules are attracted towards the bulk, the pressure they experience should always be negative independently from the curvature of the interface. When teaching these effects, I usually describe the system in terms of surface energy. Creating an interface costs the energy of the attractive links that are cut in two when slicing through the bulk. Then I would have the discussion about minimizing the energy, instead of deriving forces. I then get the pressure by comparing the work corresponding to the increase by dV at a pressure P with the variation of surface energy gamma dS (as it is done letter in the “themodynamics of soap bubbles”). In any case, this discussion should come later as it is not really a “cause”.
9. "The molecules at the surface do not have the same molecules on all sides” -> the same repartition of molecules ?
10. "In the absence of other forces, including gravity, drops of virtually all liquids would be approximately spherical” -> they would be exactly
11. Section “Effects" should come before “Methods of measurements” as the methods are consequences of the effects. Actually two sections are called effects… I guess both sections could be merged.
12. The “Surfactants” section is misleading. First, I would present a typical sketch of a surfactant molecule (hydrophilic head / hydrophobic tail). Such amphiphilic molecules tend to adsorb at interfaces (although in most cases there is a balance between adsorbed surfactant and surfactants in solution). Adsorbed surfactants behave as a 2D gas at the interface, which results into a 2D pressure. This pressure thus tends do decrease the tension at the interface (the surfactants "would be happy” to spread in a wider surface). A simple experiment to show this effect consist in depositing a droplet of dishwashing liquid on a bath of water covered with ground pepper: https://www.youtube.com/watch?v=ho0o7H6dXSU
13. However, contrary to what is usually stated (even in textbooks), the decrease of surface tension does not play a major role in the stabilization of soap films or emulsions (this is just a decrease of a factor 3). Surfactant molecules are actually used to prevent two similar interfaces to get to close and to merge. Electrostatic charges, or steric forces induce a repulsion between emulsion droplets covered with surfactant. In the case of soap film two liquid/air interfaces are facing each other. The hydrophilic heads (usually ionic) repel each other which limits the thinning of the soap film. Actually, soap films are even more complex. One can make giant soap bubbles or films. Such object have to resist over their own weight. For instance, a soap film maintained vertically experiences a stronger tension at the top than at the bottom. This situation is only possible if there is a possibility for a gradient in surface tension (which is the case with surfactants).
14. I think that the discussion on “Contact angles" should come before “Floating objects” since this notion is required to understand capillary forces. In terms of outlines, I would first go to isolated interfaces (involving a single surface tension between 2 phases) and then to a liquid in contact with a solid (or with another liquid), which involves 3 phases and thus 3 interfacial tensions). One can then define the contact angle of a droplet on a surface (which can constitute a separate article with more features such as hysteresis, surperhydrophobic surfaces and so).
15. Showing the barometer is also misleading. Indeed the effect of Laplace pressure is negligible in comparison with atmospheric pressure for the diameters used in standard barometers. I guess the idea was to show the shape of the meniscus, but this can be done by looking at the meniscus along a wall.
16. Concerning the break-up of streams, I would say that there are 2 main radii of curvatures, one along the longitudinal direction (as sketched), which provides a stabilizing effect and one in the radial direction, which is, conversely, destabilizing.
17. There is a whole discussion for the evolution of surface tension of water with temperature. Then comes a discussion for salty water where a different relation for pure water is presented (from IAPWS). This relation should be presented in the section discussing temperature (which could be simplified by the way).
18. The images are not called in the text.
19. Figure 1: I would add: Water droplet lying on a hydrophobic damask. I think there is a mistake: "floating below” -> flowing through?
20. Figure 3 and 4: Three very different effects are described and some effects are in common with what is described in Figure 4: 3A <-> 4A, 3C <-> 4B, 4D in all these exemples surface tension tends to minimize the surface by making droplets.
21. 3B is a nice illustration of the tension but should go with Figure 5.
22. 3D " Separation of oil and water (in this case, water and liquid wax) is caused by a tension in the surface between dissimilar liquids. This type of surface tension is called "interface tension", but its chemistry is the same.” -> I do not agree, surface tension is a consequence of the non-miscibility of both phases. Actually, the notion of interfacial tension would be included in my proposition for a definition of surface tension. In the present case, surface tension is responsible for the rounded shapes of the lava blobs.
23. 4E this is a consequence of surface tension gradient induced by evaporation. I would skip it for now (actually we do not see so well the “tears”).
24. Figure 7 The arrow Fw should start at the center of mass. Both arrows Fs should start at the contact line. We do not see the contact angle on the sketch.
25. Figure 8 It would be interesting to show minimal surfaces when discussing Laplace’s law, with more exemples.
26. Figure 10 If it is a pendant drop, there is no need of a goniometer. Actually I do not find this illustration very useful. What would be nicer would be a close-up view of a droplet with a superimposed fit of the shape calculated by solving Laplace equation.
27. Figure 17 I think it is interesting to get the variation of surface tension of water with the temperature, but why benzene?

Sylvain Ribault (discusscontribs) 21:24, 9 August 2018 (UTC)

Second review

Review by Derek Chan (University of Melbourne) ,
This review was submitted on , and refers to this previous version of the article

One of the points of differentiation between Wikipedia and WikiJournal of Science (Wiki J. Sci.) is that articles in the latter are meant to be of a standard that can be counted as a "bona fide scholarly publication" and that "The journal targets a broad population spanning from advanced researchers and professionals to students and laypersons, wherein the latter can get quick explanations of advanced terms". (See https://en.wikiversity.org/wiki/WikiJournal_of_Science/About) As such a standard of rigour and precision in language should be expected. Topics treated in Wiki J. Sci. should provide accurate physical insight that would serve as a solid foundation for non-specialists to explore more deeply into the field and facilitate access to more technical treatment in textbooks or journal publications.

The present article introduces and discusses topics associated "surface tension". However, key basic physical concepts have been presented in rather confusing ways and in some instances the ideas are incorrect. Therefore, this article is not recommended as a sound introduction to physical ideas that underpin surface tension, surface energy and cognate interfacial phenomena. It will confuse rather then enlighten readers new to the field. Below are a few examples to substantiate this assessment.

1. Firstly, an introduction to interfacial phenomenon should start with the surface energy (energy per unit area of an interface) rather than the surface tension (force per unit length associated with an interface). The principle of minimisation of surface energy then provides the fundamental underpinning concept to analyse and interpret different physical problems. The article pointed out correctly that both the surface energy and the surface tension have the same numerical value, however, the surface energy is a scalar quantity and the surface tension (identified as a force) is a vectorial quantity, though with a magnitude that is the same as the surface energy. Whereas descriptions based on the surface tension allow one to draw figures with evocative arrows in simple examples, but even so, some of the explanations examples given here are not correct (see below). Furthermore, it is difficult if not impossible to apply surface tension concepts to more complex problems such as wicking of porous material.
2. The Abstract of the article started by confusing elasticity and surface tension/surface energy. If the area of an elastic surface is increased from ${\displaystyle A_{0}}$ to ${\displaystyle A}$, the change in energy is: ${\displaystyle {\frac {1}{2}}K(A-A_{0})^{2}}$, analogous to the energy required to stretch a Hookean spring from length ${\displaystyle L_{0}}$ to ${\displaystyle L}$: ${\displaystyle {\frac {1}{2}}k(L-L_{0})^{2}}$ where ${\displaystyle K}$ and ${\displaystyle k}$ are elastic constants. The point to note is that the energy change in an elastic surface is proportional to the square of the change in area. For a fluid interface with surface energy ${\displaystyle \gamma }$ the energy change in increasing the surface area from ${\displaystyle A_{0}}$ to ${\displaystyle A}$ is ${\displaystyle \gamma (A-A_{0})}$ - it is proportional to the first power in the change in area. So for an elastic surface, the energy change has the same sign for an increase or decrease in the area, whereas for a fluid interface, the sign of the energy change can be of either sign depending on whether there is an increase or decrease in area. Thus physically elastic surface energy and interfacial energy are qualitatively different.
3. The description of the physics in the section "Causes" about unbalance forces on molecules near an interface creating "internal pressure" is misleading. Molecules at flat fluid/vapour interface certainly experience unbalanced forces, but there is no pressure difference between the fluid phase and the vapour phase because the interface is flat or has zero curvature. In the article, there appears to confusion between the pressure in a fluid and the pressure difference (namely the Laplace pressure) associated with a curved interface.
4. The third paragraph in the section "Causes" is very confusing: mixing smoothness and minimal surface whereby the statement that "any curvature in the surface shape results in greater area" is clearly not correct. A body of liquid in the absence of external forces will minimise it surface energy by forming a sphere that is a surface with a constant curvature.
5. The discussion about balancing the horizontal component of the force vectors in Fig. 6 - “the horizontal components of ${\displaystyle f_{la}}$ is cancelled by the adhesive force ${\displaystyle f_{A}}$” - is not correct. The correct explanation is that the horizontal components of ${\displaystyle f_{la}}$ is balanced in this case by the rigid surface. Just consider a drops of liquid floating on another liquid for which the Neumann force triangle condition has to hold in which one has to balance forces in two perpendicular directions.

Other problematic issues are raised by the first reviewer.

My recommendation is to not to publish the submission in its present form.

Sylvain Ribault (discusscontribs) 20:55, 17 August 2018 (UTC)