Micelle

A surfactant molecule has two ends: a head that likes water and a tail that does not. In a dilute solution they crowd the surface, tails in the air. Add more and the surface fills up; past a threshold called the critical micelle concentration, the extra molecules solve the problem differently and cluster into spheres — tails tucked inside, heads facing out. That sphere is a micelle.

The inside of one is effectively an oil droplet a few nanometres across. Sebum, sunscreen filters and makeup pigments dissolve into it, and because the outside is water-loving the whole loaded package rinses or wipes away. From a bar of soap to a foaming gel, this is what cleansing is.

Micellar water is the same chemistry at a low surfactant load in plain water, which is why it lifts makeup without foaming, and why it leaves a residue when it is not rinsed off: the micelles stay on the skin along with everything they picked up.

The concept also explains a common route to irritation. A micelle does not distinguish between the oil you want gone and the intercellular lipids holding the barrier together, so a cleanser strong enough to strip a heavy sunscreen in one pass is strong enough to take structural lipids with it. How much it takes is decided by contact time and by which surfactants were chosen, not by how impressively it foams.