We next explored the physico-chemical determinants of this self-assembly mechanism. A key-component in our method is the adsorption of particles at the a/w interface to favour monolayer against multilayer assembly. This step was ensured by the small concentration of cationic surfactant used (Cs = 5-10 μM), which reduced electrostatic repulsion between the negatively charged particles and the a/w interface. To confirm the need for surfactant, a 5 µL drop containing particles (dp = 560 nm, Cp = 0.05 mg/mL) but no surfactant was left drying using the same method. In this case, the anionic particles did not adsorb at the a/w interface due to the electric potential barrier, resulting in a drop diameter continuously decreasing (Movie S5) and the formation of a thick multi-layer deposit (Fig. ). The deposit showed long-range ordering due to the progressive packing of highly charged particles but no monolayer was formed.