Droplet Impact Physics of Ammonium Dinitramide (ADN)-Based Monopropellants on Solid Substrates of Varying Roughness

Abstract

This study explores the impact dynamics and morphological evolution of ADN-based green liquid propellant droplets on flat, non-heated surfaces with systematically varied surface roughness Ra from 0.015 to 2.166 µm. Using high-speed imaging, the droplet interactions were captured across three Weber numbers (We = 46.29, 104.15, and 186.15), corresponding to the impact velocities of 1, 1.5, and 2 m/s for the 2 mm diameter droplets. The spreading behaviour was quantified through time-resolved measurements of the spreading ratio β, while morphological features, such as lamella expansion, rim formation, and contact line stability, were evaluated. Results reveal that surface roughness critically controls the maximum spreading, retraction rate, and energy dissipation. The maximum spreading ratio β_max was found to scale with the Weber number, with rapid retraction observed. A curve fitting analysis was performed for this scaling relationship, aligning well with classical inertial-capillary dynamics. Moderately rough surfaces (Ra = 0.2915 µm) enhanced spreading due to optimal capillary attachment, but beyond the roughness Ra = 0.318 µm, micro texture-induced damping suppressed further spreading, reduced β–We sensitivity, and halted retraction. Compared to conventional fluids, ADN droplets exhibited higher maximum spreading on smooth substrates and sharper saturation on rough ones.