EXPERIMENTAL STUDY ON DIRECTIONAL MOTION OF WATER DROPLETS IMPACTING ON A HOT CONCENTRIC MICROGROOVE SUBSTRATE

Controlling the ordered motion of droplets is of great engineering significance for improving the thermal management efficiency of industrial heat exchange equipment and ensuring the safe operation of systems. To address the challenge of directional droplet transport in high-temperature environments, this study experimentally investigates the dynamic behavior of water droplets impacting a high-temperature concentric microgroove substrate with gradient groove widths. The droplet dynamic characteristics and driving mechanisms under both transition boiling (250 °C) and film boiling (350 °C) regimes are systematically analyzed. The results indicate that, in both boiling states, water droplets exhibit stable unidirectional transport behaviors and always move leftward toward the curvature center of the substrate. Key normalized motion parameters, including the solid-liquid contact line length, maximum spreading factor, and lateral displacement ratio, are quantitatively characterized to reveal their evolutionary laws with off-center distance (i.e., the impact position relative to the curvature center of the substrate) and Weber number. The maximum spreading factor in both boiling states is found to be primarily dominated by the Weber number, while the effect of off-center distance is insignificant. The lateral displacement ratio shows a trend of first increasing and then stabilizing with off-center distance under film boiling, whereas it first increases and then decreases under transition boiling, with both reaching their maximum values at half length of the radius of the microgroove substrate. Furthermore, the intrinsic directional motion mechanisms are elucidated. For transition boiling, the directional take-off is co-driven by the asymmetric wettability and Janus thermal state induced by gradient microgroove widths. For film boiling, asymmetric contraction and capillary penetration induced by the microgroove structure generate a resultant force directed toward the curvature center, thereby driving the continuous directional motion of the droplets. These findings provide experimental evidence and theoretical support for the precise regulation of ordered droplet motion in high-temperature thermal management systems.