Abstract: A method and an apparatus are described to prepare monolayers of particles (or molecules) (3), realizing the steps of: injecting a liquid film (2) with particles (3) dispersed on the external surface of a rotary member (1); adjusting the surface charge density of the particles (3) by injecting adsorption reagents (10), carrying particles (3) placed on film (2) surface so that they form a substantially uniform monolayer (5); putting the monolayer (5) on the liquid film surface (2) in contact with a substrate (7); and advancing the rotary member (1) longitudinally with respect to the substrate (7), in both directions, so that the monolayer (5) is detached from the liquid film (2) and attached to the substrate (7).