You actually have to do surprisingly little! Simply running a pipe
containing liquid soap into a pipe containing the running water is
enough (use a "tee"). But -- and here's the hard part, the liquid soap
has to be at a higher pressure than the water in the pipe. Otherwise
the water will push the soap back up into the soap container. A vacuum
will not pull the soap -- rather, pressure elsewhere pushes the soap to
fill a vacuum.
It appears as if the important features of the design are:
1) Making soap flow into the water when the water is flowing.
2) Keeping the soap from flowing when the water is shut off.
3) Keeping the water from pushing into the soap container.
It is easiest to build and refill the soap container if it is not
under high water pressure all the time but is open to the air. You may
want to mix the soap and water at a point where the water pressure is
low, then, downstream of the valve used to turn the water on and off.
Then problem number 2 comes up -- you may need two valves hooked
together -- one for the soap and one for the water, or one downstream
of the mixing point, to keep soap from running out when the water is
You can use gravity to your advantage to adjust the pressure of
the soap at the mixing point. Putting the soap container higher will
increase the pressure lower down.
You can restrict the rate at which the soap flows into the water
by getting a "tee" with a very small side pipe, or by having an
adjustable valve on the soap line before the mixing point. You can also
get "one-way" valves which allow fluid to flow in one direction only,
to prevent water from flowing back up into the soap container.
Good luck with your invention!
(republished on 07/19/06)