Wetted area of droplet impact onto micropillar arrays with various pillar geometries

This study utilizes the volume of fluid method and dynamic contact angle model to numerically investigate the wetted area dynamics during droplet impact on square, cylindrical, and triangular pillar arrays. User-defined functions are implemented to real-time monitor wetted areas on the array substrate, pillar side surfaces, and pillar top surfaces with dimensionless parameters like the wetted area ratio (maximum to steady-state wetted area ratio) defined for analysis. The influences of Weber number (We) and static wall contact angle (θ) on wetted area are systematically examined. The results show that in triangular pillar arrays, the pinning effect restricts droplet spreading and retraction, yielding the smallest wetted area ratio. However, the side surface wetted area and steady-state total wetted area are the largest among the arrays studied. In square pillar arrays, the groove-driven effect dominates to produce the largest wetted area. Cylindrical pillar arrays, with smooth surface curvature facilitating flow division, exhibit the optimal bottom surface wetting and relatively rapid droplet retraction. Although the top surface wetted area is minimal, a pinning–depinning–repinning cycle occurs at low θ. The side surface wetted area constitutes the primary component of the total wetted area, accounting for the highest proportion. Micropillar geometry predominantly dictates the fractional distribution of wetted areas, with We and θ exerting weak influence. These findings furnish both quantitative foundations and engineering insights for designing hydrophilic microstructure surfaces.