Foaming soap dispensers for both home and industrial use are becoming
quite popular. Why is foaming soap a superior to liquid soap for
cleaning hands? Your explanation should competently explain the
relationship between foam and tensegrity. If possible, please quantify
the advantage: how much soap is needed to wash your hands when it
is foamed and when it's not? How much water? For washing the same
amount of time, how much more effecctive is foaming soap at
killing bacteria, etc.

Hi,

This is a completely unofficial answer, but I believe this is simply a
marketing strategy.  Foaming soap requires "less product," but can be
sold for more money.  This allows the manufacturing to reduce quantity
and cost and increase revenue.  I am unaware of the foaming action to
be more effective than liquid soap; more important is probably washing
time, friction, and the use of warm water and proper rising.  Hope
this helps.

Tom

Thanks for your comment. From my anecdotal experience, it's quite
clear that foamed soap is more effective than soap in a liquid form. I
have experimented with one of the Sudz soaps from Kiss My Face; one
squirt from the dispenser is very effective at cleaning my hands -- it
seems to be about 10x less soap than I use with a pump dispenser. Even
more striking is now well this foam works in cleaning my glasses -- it
is very thorough in removing the grease from my lenses.

The foaming process happens in the dispenser. AFAIK, many "normal"
liquid soaps can be used in the dispenser. Dial sells foaming soap
refills that are comparable to the cost of other soap refills. To coin
a phrase or two, I think this bursts the bubble that manufacturers are
artificially inflating the price of foam'd soap.

There's something that happens energetically when soap is foamed.
Note: foam is not a stable state for the soap; it will rapidly break
down to bigger bubbles if left in the open. The site
http://www.cockeyed.com/inside/foamsoap/foamsoap.html shows the foam life-cycle.

--phil

Very interesting.  I had no idea... I'll be curious to the answer... thanks.

It seems the only difference between foamed and unfoamed soap (if you
are using the same product) is the form it takes.  Thus, the increase
in effectiveness is almost definitely a result of the foaming action.

The answer to this problem seems to have several possible components. 
The most obvious is that the surface area of a "foamed" soap is much,
much larger than the same quantity of soap in its "unfoamed" form.  As
the surface area increases, so does the interaction between the soap
and whatever it is you are trying to wash away.  This increased amount
of interaction allows the soap to more quickly solvate (which is how
soap works) this unwanted material.

Another, more much complicated possibility is related to fluid
mechanics.  The fluid in a bubble is moving much faster than it is in
a standing form.  This increase in velocity helps the two substances
(the soap and the grease, for example) to mix more quickly, and thus
it solvates faster.

You can look here for a summary of this "turbulent mixing":
http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.fluid.36.050802.122015

Quantitative measures of these types of reactions are really tough,
and might even be beyond the reach of modern computers to simulate. 
I'm not sure how you might get an estimate of your fluid soap to
foamed soap ratio except by experimentation.

Hope this helps,
Jason

Thanks, Jason. That is a very helpful response. I will persue the
research article you mentioned.

There is something counterintuitive in my thinking about surface area.
I understand that the foam itself has a very large fractal-like
surface area. But it would seem as if there would be the same surface
size of contact if you completely dipped your hands in liquid soap.
OTOH, liquid soap on your hands doesn't seem to solvate the
dirt/grease; one must fist lather up the soap before it becomes
effective.

In short, I don't know how the concept of surface area works in this context.

--phil

Perhaps with liquid soap, much of it is wasted, not coming in contact
with the dirt.  Maybe if one used less liquid soap and more effort to
foam it, this would be overcome, but most of us just take a squirt and
rinse the soap off under running water, adequate  - and wasteful -  in
most cases; we aren't surgeons.