A Parametric Study of Droplet Directional Detachment on Inverted Wedge Patterns with Wettability Contrast

Constructing inverted wedge-shaped hydrophilic channels with a small apex angle on surfaces with wettability patterns is an effective strategy to promote efficient and complete droplet detachment, which is crucial for applications such as condensation heat transfer and self-cleaning. However, a comprehensive understanding of how wedge geometry parameters affect droplet dynamics has not been established. In this study, we systematically investigate the dynamics of droplet formation and detachment within inverted wedge-shaped superhydrophilic channels fabricated by laser etching on hydrophobic or superhydrophobic substrates. Four distinct droplet detachment mechanisms are revealed. Our results indicate that, within the experimental parameters tested, a slender channel geometry-featuring a narrow upper base, a minimized lower base, and sufficient height-combined with a superhydrophobic substrate, promotes high-position droplet formation, extends the droplet sliding distance, and significantly reduces resistance. This synergy leads to the most efficient detachment mechanism: inertia-driven direct shedding. For the tested configurations, the C1.2/0/40 channel achieved the highest recorded detachment frequency of 318 min^-1 at a flow rate of 0.5 mL/min. Furthermore, droplet rebound at the channel tip is observed in some configurations, where two to three droplets must form sequentially and coalesce to trigger a single detachment event. This work provides actionable geometric design strategies for engineering surfaces capable of directional and highly efficient droplet detachment.