Non-foaming surfactant to add to water I'm looking for a surfactant that I can add to water to decrease the surface tension by 20 to 40%.  I'm working on a new kind of mixing device that is so designed such that it mixes relatively large volumes of two liquids and cannot be made small enough for laboratory scale.  So flow on the order of 1.5 to 2.5 liters/min.  I would like to be able to run my experiments for a number of minutes, so the estimate is 10 gallons for each hypothetical liquid (total amount would be 20 gallons for an experiment that would give me about 20 minutes to adjust parameters, take photographs, etc.).  Adding something like isopropyl alcohol can get expensive very fast (10% by weight in water lowers the surface tension to about 41 where pure water is 72).
Very important:  The surfactant needs to be non-foaming as release of gas will be problematic for these tests.
Thanks for any thoughts.
Conceptual thoughts about mixing two immiscible liquids (not on surface tension):

A reflux condenser used in the ochem lab equally has a circulation of about 2 L/min water (reference from an advertisement, which doesn't look too much off-range)

After the two liquids are mixed, what application will follow?  Do you target cleaning an engine, or deployment for food processing?  What are intended storage conditions once the two are mixed (e.g., time of storage / transport, temperature of storage / use)?  Altogether with the other liquid you want to mix with water, this may narrow your choice of the surfactant.

Mixing the two continuously may be achieved with a tube reactor with two inlets, one outlet, and a screw-shaped inner geometry

(reference, section «Liquid-Liquid &amp; Liquid-Gas Mixers»)
Depending on parameters of your liquids (e.g., density, viscosity, flow rate, key word Reynolds number) you may enter the regime of turbulent flow until you divide the liquid into tiny droplets which increases the surface accessible for mixing.  The addition of surfactant may help to achieve the intended mixing, too.

(loc. cit.)

The formation of foam however is the result of an entrapment of a gas.  Whipping cream with a whisk for example intentionally aerates the cream.  Liquids like water may boil (i.e., generation of gas) because of sudden pressure changes while passing a propeller blade at a flow rate too high (cavitation), too.
Thus, mixing water and your other liquid in a tube reactor like the one mentioned above, in absence of air and at the right hydrodynamic state may prevent the formation of foam.  (You mau fight foam by ultrasound (example), though.)