Evolution of Electrosprayed Particles at a Static Air-Water Interface on Multiple Time Scales.

HYPOTHESIS: Electrospray deposition offers a field-driven method for efficiently delivering charged nano/microparticles to fluid interfaces with minimal disturbance. At an air-water interface, the long-term arrangement and mobility of colloidal particles is expected to depend on electrostatic repulsion, particle composition, and environmental factors such as ionic strength and surface contamination. These competing effects may give rise to time-dependent transitions that are not accessible in quasi-equilibrium systems.

EXPERIMENTS: We created a custom fluidic device to generate geometrically stable air-water interfaces via continuous liquid infusion and image-based feedback control. Combined with electrospray interfacial targeting, this platform enabled sustained observation of charged colloidal assembly and dynamics at a fluid interface under non-equilibrium conditions. Particle motion was captured over several hours, and the salt concentration in the aqueous subphase was modulated to establish the different assembly structure regimes.

FINDINGS: Interfacial order progressively degrades over time (> 5 h) due to charge relaxation, a process that is further accelerated by external ionization. Counterintuitively, adding salt to the subphase enhances hexagonal ordering immediately after deposition, revealing a nontrivial interplay between electrostatic screening and interfacial structure. At intermediate time scales (2-3h), we observe spatially heterogeneous diffusion among neighboring particles, which is attributed to localized surface contamination. These results reveal how evolving electrostatic charge drives interfacial self-assembly and long-term colloidal dynamics at liquid interfaces.