In article .com,
"Bill J." wrote:

Does anyone know how something like that would be designed?

I'm guessing the soap dispenser is a two cylinder pump - one pumps soap
solution, the other pumps air. Perhaps the firefighters among us can
provide details on foam nozzles - I assume those suck air in by venturi
action. I used to use a film washer that did the same thing to inject
bubbles in the column using the force of the incoming water, and it
would foam very nicely if one dropped a drop of photo-flo in the intake.

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