Directional transport of burning droplets on heated concentric microgroove surfaces with gradient width

Controlling the ordered motion of fuel droplets is critical for enhancing energy utilization efficiency and improving fire safety management. This study investigates the directional motion regulation of burning fuel droplets on hot concentric microgroove substrates with gradient width. Results show that n-heptane droplets exhibit bidirectional motion, moving rightward (away from the curvature center of the substrate) under transition boiling and leftward (toward the curvature center) under film boiling, while ethanol droplets maintain unidirectional rightward motion regardless of boiling state. Quantitative analysis reveals the variation laws of key parameters (left/right contact line lengths $L_\text{c}$, maximum spreading diameter $D_\text{max}$, and lateral displacement $\Delta l$) with respect to off-center distance $r$ and impact Weber number $We$. Specifically, the lateral displacement $\Delta l$ of n-heptane increases with off-center distance $r$ in transition boiling but decreases in film boiling; in contrast, ethanol shows a non-monotonic trend with off-center distance under both boiling states. The maximum spreading diameter $D_\text{max}$ follows a scaling law with Weber number, i.e., $D_\text{max} \sim We^{1/4}$, which is independent of off-center distance $r$. The underlying mechanism is elucidated: asymmetric wettability and explosive boiling drive motion under transition boiling, whereas partial capillary penetration/emptying processes induced by gradient width in microgrooves govern directional behavior under film boiling. These findings provide a theoretical basis and practical guidance for controlling the ordered motion of burning droplets.