Figure 3
From: Radial Wettable Gradient of Hot Surface to Control Droplets Movement in Directions
Schematic diagrams for the mechanism of directional droplet motion on over-heated wettable gradient surface.
a) The directional spreading and evaporation behaviors of droplet on wettable gradient surface. The droplet initially spreads directionally along wettable gradient (Frame 2, 3). With lower superheat, the evaporation velocity of droplet at relative hydrophobic site is larger. The less wettable side shrinks and the more wettable side is pinned until the whole droplet disappears, exhibiting directional evaporation (Frame 4, 5). b) The directional sliding behaviors of droplets on wettable gradient surface with gentle film boiling mode. At higher superheat, the decrease of waiting time induces a larger bubble emission frequency and the generated bubbles are easier to merge to vapor blanket (Frame 2). It induces the droplet in a ball shape (Frame 3). More bubbles at the hydrophobic site induce vapor pressure gradient. Droplets slide off the graphite plate quickly under effects of pressure and gravity (Frame 4).
