PMC

A superhydrophobic Nasturtium leaf that lets a cold drop bounce off of its surface traps a hot drop. (A) A cold water drop at temperatures impacts a Nasturtium leaf that is initially at ambient temperature . High-speed images show that the cold water drop spreads, retracts and leaves the surface at a finite time . Simultaneously, thermal images show that the drop is at the lower temperature than the leaf in this experiment; (B) a hot drop at temperature at the same impact condition sticks to a Nasturtium leaf that is initially at ambient temperature . Simultaneous thermographic images show a temperature map of the drop and substrate during impact. The dotted line shows the contact line between drop and leaf.

A schematic illustrating how a thermally responsive superhydrophobic material could act as a fuse to prevent water above a critical temperature from reaching a sensitive surface. (A) If the superhydrophobic surface that is at rapidly pinned drops above a critical temperature , then a water drop with temperature below (top) would bounce off of the surface onto the target, whereas a water drop with temperature above (bottom) would stick, protecting the target; (B) surface microtexture with lengthscale L can trap air beneath the drop (Cassie–Baxter state), enabling the drop to bounce; (C) If heat from the drop melts the microstructure, the drop could enter a Wenzel state and stick; (D) Alternatively, if liquid from the drop evaporates and condenses within the microstructure, the drop could also enter a Wenzel state and stick.

Illustration of different phases of drop-on-drop collisions and the subsequent outcomes. (A) Case I: (χ = 0) impacting drop bounces back and the sessile drops stays on the substrate; (B) case II: (χ = 0.08) impacting drop bounces back and the sessile drop lifts off from the substrate; (C) case III: (χ = 0.25) impacting drop stays on the substrate and the sessile drop lifts off; and (D) case IV: (χ = 0.625) impacting drop bounces back and sessile drop stays on the substrate. For all these cases, We = 1.5. The drop labels 1 and 2 are for the impacting and sessile drops, respectively. t* is the nondimensionalized time used for the numerical simulations and is given by t = t/t_γ, where t_γ is the inertial capillary time scale, . The absolute values of the normalized velocities vary between zero (white) and twice the inertial capillary velocity, (dark blue).

The two landing tasks. In drop landing (DL, landings without a subsequent jump), the subjects drop off a 30-cm high box and land on the force plates (a). During drop vertical jump (DVJ, landings with a subsequent jump), the subjects drop off the box and then jump immediately after landing (b)