A slot distributor lays a film onto a vertical or near-vertical face and the water stays in contact with it all the way to the trough. Because nothing is ever in free air, it is quiet, it loses very little to evaporation, and it is the only vertical effect that behaves in wind — we have run film walls at 8 m/s with no measurable drift.
The surface is the effect. On honed stone the film is even and matt; on polished granite it beads and breaks into rivulets within a metre; on rough concrete it disappears into the texture and simply looks like a damp wall. We wet-test a 1 m² sample of the actual specified material before the wall is signed off, every time, because you cannot predict this from a data sheet and the architect’s stone selection is usually made for dry appearance.
Its enemy is mineral staining. A film wall evaporates slowly and continuously across a large area, which concentrates whatever is in the water on the face. In hard water we soften the make-up supply and we choose a stone that hides carbonate — a pale limestone forgives it, a dark basalt records every month of operation as a white tide line.