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I am curious if the main cleaning effect of soap comes from its foam (or, maybe, it is just a by-product).
Here are some points, that might clarify what I mean:

  1. Does foam sort of "carry" dirt and germs away from the surface?
  2. If it is not the "main motor", which makes the soap function: is it possible to make a soap which will still be as effective without it producing any foam?
  3. What percent of the cleaning work, if any, does foam undertakes (i.e. how effective would soap be against germs and dirt without its foaming ability)?
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Foam is a side effect of using a tensio-active agents (though some tensio-active agents are engineered to produce as little foam as possible, e.g. in washing machines). In fact, you could consider that, if soap has nothing better to do, it will form foam. This answers some of your questions: foam does not (really) carry away dirt and its percentage of efficiency in the whole process is low.

Molecules of soap are made of 2 parts: a hydrophobic tail (which loves fat, grease, lipids, whatever you call them) and a hydrophilic head which loves water, and hate lipids.

When the hydrophobic tails are busy linking to a greasy stain or grease on your hands, they no longer participate into forming bubbles/foam.

The easiest way to verify that is to pour a little bit of oil in your hands and rub them. Wash your hands with soap a first time: there will be almost no foam. Now rinse your hands and wash them again: there should be much more foam!

Note that it does not mean that your hands are cleaner, regarding bacteria, but just that they are less greasy!

If you want to remove bacteria, rub your hands with soap for at least 30 seconds. So-called "anti-bacterial" soaps do not achieve better and promote bacterial resistance.

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    $\begingroup$ Anti-bacterial soaps are better at killing bacteria. It's just that normal soap is already amazing at killing bacteria, and humans are resistant enough that the difference is essentially non-existent for a normal healthy human. It's certainly very noticeable where people aren't healthy. $\endgroup$
    – Luaan
    Apr 21, 2019 at 15:31
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    $\begingroup$ @Luaan Normal soap doesn't "kill" bacteria at all. Rather, the surfactants in soap allow the bacteria to be washed away. This is why hand-washing technique is also so important, and why water alone (no soap) is still surprisingly effective when no other options are available. If you want a biocidal effect, you need an anti-bacterial soap. But, as pointed out here, the efficacy levels are not significantly higher (although they do increase the longer you rub the soap around, unlike the surfactant effect, which peaks after a certain minimal amount of time), and they promote resistance. $\endgroup$
    – Cody Gray - on strike
    Apr 21, 2019 at 16:47
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The foaming action of soap was put there as a sort of "indicator" that something is happening. It's a marketing strategy because people are ignorant about how soap works. The bubbles are just air pockets and have no effect on cleanliness.

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    $\begingroup$ This is not true for "normal" soap, but is true for some "detergents". $\endgroup$
    – Hot Licks
    Apr 21, 2019 at 19:08
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    $\begingroup$ Actually, it is. You can make some soap yourself and it'll only foam slightly, if you don't add foaming agent like producers do. $\endgroup$
    – Mithoron
    Apr 21, 2019 at 22:05
  • $\begingroup$ Made soap yesterday and have been making soap for personal use for several years. I use the standard oil/lard process. This soap foams quite well, in my experience. The bubbles seem finer, and the froth seems "creamier". $\endgroup$
    – Tim Nevins
    Apr 22, 2019 at 16:10

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