Graphene-mediated suppression of Leidenfrost effect for droplets on an inclined surface

The ability to delay the Leidenfrost effect is desirable in many engineering applications operating at a temperature above the Leidenfrost point. Unlike previous studies that frequently demonstrate the suppression of the Leidenfrost effect via a free-falling droplet on the horizontally-aligned heated-surfaces, here, we investigate the suppression of the Leidenfrost effect for a droplet that rolls on an inclined heated-surface coated with graphene for cooling enhancement. With graphene-coated surfaces, we observe the suppression of the Leidenfrost effect at a temperature up to 290 °C, representing more than 100 °C increase of Leidenfrost point as compared to that on an uncoated surface. This can be attributed to the unique rapid water permeation characteristics of graphene that enables vapor to escape through the nanostructure, obstructing the formation of a continuous vapor layer. Specifically, when a droplet rolls onto the graphene-coated surface, we observe three interesting regimes: Regime-I (contact boiling), Regime-II (intermittent contact boiling), and Regime-III (transient contact boiling). In Regime-I, the droplet in full contact and pinned on the graphene-coated surface, leading to a largest reduction in surface temperature. In Regime-II, the droplet in partial contact with the graphene-coated surface and subsequently bounced in the reversed direction. Finally, in Regime-III, the droplet momentarily in contact with the graphene-coated surface and quickly bounced in the forward direction. Overall, with the graphene-coated surface, we observe an up to 64 °C reduction in the temperature of the heated surface.