Strategic Multi-Level Wettability Gradient Distribution for Directional Migration of Droplets: Toward Microgravity Condensate Management

Abstract This study employs two-dimensional numerical simulation to characterize droplet migration and collision dynamics on unidirectional and symmetrical multi-stage wettability gradient (WG) surfaces under Earth (g₀), Martian (0.38g₀), and microgravity (10⁻⁶g₀). Parametric analysis reveals WG = 15° optimizes single-droplet migration time in microgravity, exhibiting minimal sensitivity to gravity reduction, while WG = 20° maximizes efficiency under g₀. Larger droplets (D = 3 mm) accelerate terrestrial transport but severely impede microgravity migration, where smaller droplets (D = 2 mm) excel. Peak velocity (> 0.3 m/s), governed by WG and D independent of gravity, dictates acceleration capability. Final equilibrium morphology mainly depends on WG. For dual droplets, microgravity collision requires WG ≥ 15° and D = 2 mm; lower gradients or larger diameters prevent collision. The results demonstrate that reduced gravity disrupts mirror-synchronized droplet motion observed under g₀, delaying collision initiation. These findings provide critical guidelines for designing passive capillary fluidic systems in variable-gravity environments, particularly space applications.