Conversely, in presence of high concentration of DTAB (Cs = 1 mM), a disordered deposit with particles spread over the whole substrate and a local gel-like structure was obtained (Fig. ). This was due to the adsorption of cationic surfactant on the oppositely charged particle surface strongly decreasing the interparticle electrostatic repulsion potential and thus hampering ordered arrangement. Using a low but non-zero concentration of cationic surfactant, typically of the order of a few µM for DTAB (i.e., around 10 -3 CMC where CMC is the critical micelle concentration), was thus necessary to enable the adsorption of the charged particles at the interface and the deposition of an ordered monolayer. We however noticed that a small multilayered structure could be seen in the centre of some deposits, especially those involving smaller particles (Figs. ). We attribute this local multilayer formation to particles tardively reaching the fluid interface where they could not adsorb due to the already formed monolayer, especially at its centre where the compression forces from the surrounded adsorbed particles were at maximum. Obtaining perfect monolayers without central accumulation was thus possible but only with sufficiently large particles made of silica (dp = 2.4 µm) and with even higher diameters for those made of a lower density material such as polystyrene (dp = 5.1 µm, Fig. ). In an additional control experiment, the typical surfactant/particle mixture (dp = 560 nm, Cp = 0.05 mg/mL, Cs = 10 μM) was dried in upright configuration (Fig. ), thus removing the gravity-driven transport of particles and their strong confinement at the a/w interface.