The cleansing action in any cleanser comes from the compounds called "surfactants". They are molecules with two different ends: one end loves oil, and the other one loves water. Because of this property, these molecules get attracted to oil on one side, and also pull towards water on the other - and this is how you remove dirt and oil from your skin when you use a cleanser, and then rinse it off with water. A physical action of whipping the surfactants off together with a bit of water present in the product achieves the same effect - this is how micellar waters work (we still recommend to rinse micellar water off with water to make sure no surfactants remain on the skin to reduce the risk of irritation).
Not all surfactants produce foam. The ability of a surfactant to produce foam has nothing to do with its cleansing abilities (as in your skin can get perfectly clean without any foam whatsoever). It is just so happened that the first surfactant that was readily available for skin cleansing (soap) creates foam, and this is how we've learned to associate foam with cleanliness.
Ok, so foam is not needed for a good cleansing, but is it bad for the skin or is it a harmless add-on to make the cleansing experience more fun?
Unfortunately, the foaming fun doesn't come for free to our skin. On average, a foaming cleanser is likely to be more irritating to our skin compared to a non-foaming cleanser for two reasons:
Surfactants that produce foam tend to be more alkaline than non-foaming surfactants. Too alkaline cleansers mess up the natural pH of our skin surface and this can lead to irritation.
Formulators need to use a higher total concentration of surfactants in the product to create a nice foaming experience - which again can make the product too alkaline or too harsh in terms of its cleansing power.
The bottom line: yes, foaming cleansers are not the best choice for a gentle cleanse. At the same time, cosmetic formulators and ingredient suppliers are working hard on creating more gentle foaming agents and formulations, so the industry might surprise us soon with truly gentle bubbling products.
Photo by Rodion Kutsaev