The role of the electrostatic double layer interactions in the formation of nanoparticle ring-like deposits at driven receding contact lines

In order to produce well-ordered structures via evaporation, it is essential to control the evaporation flux, solute concentration, interaction between the solute and the substrate, etc. During the drying of particle suspensions, the particle deposition process can be dictated by electrostatic and van der Waals forces. However, the complex physics involved in the drying of colloidal particle suspensions and the erratic contact line dynamics of evaporating sessile drops complicate the analysis of the problem. In this work, we propose a new methodology based on shrinking sessile drops to standardize the contact line dynamics of evaporating drops, but with no observable evaporation (macroscopic scale). We used a microinjector to decrease the drop volume through a small hole drilled in the substrate. Unlike drying drops, with our methodology the particle concentration in the drop bulk remained constant during the entire process and the macroevaporation was negligible. We probed the arrangement of nanoparticles at driven receding contact lines, with low capillary numbers and at time scales shorter than during free evaporation. The electrostatic double layer interactions were modified by diluting the nanoparticles in buffer solutions at different pH values. We also examined the impact of the wettability contrast between the substrate and the particle on the deposit morphology. We found that the ring-like deposits formed at driven contact lines might be suppressed with strongly interacting particles.