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Hand sanitizer is a gel in its bottle, but when I pour it on my palm, it turns into a free flowing liquid. Why does that happen?

It doesn't turn to a liquid when I pour it on my floor or my table. It doesn't seem to be related to heat, as I tried it also with a hot surface.

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    $\begingroup$ have you tried wetting it on fruit or a similar organic surface? $\endgroup$
    – New Alexandria
    Oct 4 '12 at 4:53
  • $\begingroup$ Here is my experiment that I performed in 2017 when I was 16. youtu.be/PC3Lv1KuopM The answer to OP's question is that salt on your hands. If you want to go through the whole process of finding out why salt does this too hand sanitizer is up to you, but now we know it's the salt on your hands from sweating. $\endgroup$
    – Dean
    Jul 17 '21 at 0:23
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A little research and perhaps a small experiment might help you figure something like this out on your own. First, have you tried wikipedia? A small mention of research can go a long way. Describing what product you are interested in never hurts either. Is the polyacrylic acid, a dispersant in this substance? That material can absorb water effectively, so here is what I would do:

Take a very small amount of water and smear it on the same surface that it does not "turn into a free flowing liquid" and see if it does with the addition of water. If it works, you might conclude that your alcohol-based rubs use a suspension material that when contacted with a little water, such as from your palms, will cause the gel to have just enough water absorbed inside to decrease its viscosity.

Perhaps you will like to share the results of your experiment.

EDIT: OP seems to be observing a gel demonstrating Thixotropy, this I have determined from a quick google search

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  • $\begingroup$ I tried, and it remains a gel even with water smeared on the surface. But I think polyacrylic acid would make the sanitizer more gel-like rather than liquid, because it is a powder and absorbs water to become a gel. $\endgroup$
    – duci9y
    Sep 12 '12 at 8:36
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    $\begingroup$ @duci9y Okay well, the only other explanation is thixotropy. I was under the assumption that you were simply putting it on your hand and doing nothing else. Simply rubbing the gel into your hand or allowing the oxygen or gas inside to escape (if any) will cause it to change significantly. The gas inside is used to suspend the materials, such as you would notice in your soap. $\endgroup$
    – Leonardo
    Sep 16 '12 at 14:10
  • $\begingroup$ @duci9y It is due to Thixotropy. I have attended a workshop in which they talked about thixotropy of commercial products (such as tootpaste) and how to measure it $\endgroup$
    – Eka
    Oct 18 '12 at 15:50
  • $\begingroup$ Thanks a lot an sorry for responding so late. Thixotropy seems the most plausible theory. It is incredible that there is so little research on these fluids. $\endgroup$
    – duci9y
    Nov 25 '12 at 20:07
  • $\begingroup$ If it is Thixotropy (shear thinning) then you could see if it is reversible. Shake the closed bottle vigorously, see if it turns into a liquid. The let the bottle stand for say an hour, and see if it has turned back into a gel. You could also compare to putting a large blob gel on your hand, move it into a liquid, then scrape into a small dish cover it (with plastic wrap, to stop evaporation) and leave it and compare after an hr. $\endgroup$
    – DarcyThomas
    Jun 2 '15 at 20:16
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GermX thins when it hits your hands because of the sodium ions present in your hands. The polymer used in GermX is a polyelectrolyte - in this case it is poly(acyrlic acid) that has been neutralized with a base made with a large cation. The large cation is too large to effectively separate and quench the electrical repulsion between the neighboring negative charges on the acrylate monomers. When sodium ions are available, like when it hits your hands or when you add salt water, the smaller sodium ions may exchange with the large cations and collapse the polyelectrolyte. This collapse is thermodynamically favorable both in terms of enthalpy and entropy change. The answers using rheology to back up their claims are erroneous.

Source: PhD Polymer Science and Engineering, and my professor, who patented this technology.

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    $\begingroup$ The references you provide are flimsy at best. It would be better to link to or cite verifiable sources, maybe mention who is "your professor". $\endgroup$
    – Buck Thorn ♦
    Jul 17 '21 at 7:36

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