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Background

Some years ago I was studying "Fundamentos de biología" (Biology fundamentals) and learned how the lipids create a bilayer due to the water repulsion. Some time later I learned that this was called the hydrophobic effect and occurred in some other cellular materials.

But something else was in my mind back then. I had already read in wikipedia about a particular effect, the Casimir effect and felt like they were similar, but in completely different fields and medium.

Similarity

When there's a material immersed in the medium, the entropy of the medium is reduced. Therefore, if there are two materials, there's a force that bonds them together as the total amount of reduced entropy also depends upon the total surface [and closed systems tend to increase the entropy, not reduce it].

Note: In the English Casimir Effect's wikipedia page there's no allusion to entropy; nor there's in the Spanish one, but only in the Spanish one there's a couple of notes that strongly suggest that the entropy is reduced with the two plates.

Questions

1.a Are they actually the same "kind" of effect but in completely different areas or I'm interpreting them wrong?

1.b Is there any other similar effect to these?

2.a If the material affecting the Casimir effect was liquid, wouldn't it act similar to the lipid bilayer, forming an sphere?

2.b If so, wouldn't be a good way to measure the Casimir effect to rotate this small sphere at certain velocity and see the expected deformation vs the one that occurs taking into account the Casimir effect?

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1 Answer 1

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I don't think the formation of lipid bilayers is analogous to the Casimir effect because it's a surface energy effect. However there is an effect called depletion flocculation that is very closely analogous. This happens in a polymer solution when surfaces approach more closely than the size of the polymer molecule. The polymer is excluded from the region between the surfaces and this creates a difference in the chemical potentials in the solution between the surface and the bulk solution. The result is an attractive force between the surfaces. It seems to me that this is closely analogous to the exclusion of long wavelength modes in the Casimir effect.

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  • $\begingroup$ Great answer, +1. I've always found the link between the Casimir effect and the depletion interaction fascinating. Do you know if it is possible to derive the Casimir force from entropic considerations alone? $\endgroup$
    – Mark Mitchison
    Commented Jan 25, 2013 at 7:46
  • $\begingroup$ @MarkMitchison: I have absolutely no idea if you could calculate the Casimir force just from entropy: sorry! $\endgroup$
    – John Rennie
    Commented Jan 25, 2013 at 8:04
  • $\begingroup$ I thought about lipid bilayers because that's what I was studying in the first place, but then thought that it would be more similar without the hydrophilic heads. Which is again really similar to the Depletion force and Casimir effect for what I could understand in the link, right? (non-native so a couple of things in the link were not clear for me) $\endgroup$
    – Francisco Presencia
    Commented Jan 25, 2013 at 13:59
  • $\begingroup$ I think the main similarity between the depletion and hydrophobic interactions is that they are entropic forces, arising because the system can minimise its free energy by maximising configurational entropy. But most macroscopic forces are also entropic, for example the forced exerted by an ideal gas on the walls of the container is an entropic force. $\endgroup$
    – Mark Mitchison
    Commented Jan 25, 2013 at 14:51

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