Hierarchical Superhydrophobic Surfaces Resist Water Droplet Impact

URL to paper listed on conference siteIn this paper, we present static and dynamic wetting interactions of water droplets on a variety of superhydrophobic surfaces. For sessile droplets, wetting states were determined by measuring contact angles and comparing them to that obtained from equilibrium Cassie and Wenzel states. Surprisingly, we find that roll-off angles are minimized on surfaces expected to induce Wenzel-like wetting in equilibrium. We argue that droplets on these surfaces are metastable Cassie droplets whose internal Laplace pressure is insufficient to overcome the capillary pressure resulting from the energy barrier required to completely wet the posts. In the case of impacting droplets the water hammer and Bernoulli pressures must be compared with the capillary pressure. Experiments with impacting droplets using a high-speed camera and specific surface textures that can delineate various wetting regimes show very good agreement with this simple pressurebalance model. These studies show that hierarchical micronano surfaces are optimum for droplet impact resistance.GE Global Research Cente

Ultrafast and direct imprint of nanostructures

Abstract We report a method of one-step direct patterning of metallic nanostructures. In the method, termed laser assisted direct imprinting (LADI), the surface of a metal film on a substrate is melted by a single excimer laser pulse and subsequently imprinted within ∼100 ns using a transparent quartz mold, while the substrate is kept at a low temperature and in a solid phase. Using LADI, we imprinted gratings with ∼100 nm linewidth, 100 nm depth, and 200 nm pitch, as well as isolated mesas of ∼20 μm size, in Al, Au, Cu and Ni thin films. We found that the quartz mold was able to imprint metals even at temperatures higher than its melting point. The technique could be extended to other metals regardless of their ductility and hardness, and would find applications in photonic and plasmonic device production

Bio-Inspired Hierarchical Superhydrophobic Surfaces Resist Water Droplet Impact

We present fluid dynamic videos of droplet impact on three types of superhydrophobic surfaces to explicitly show the effect of surface texture on droplet impact resistance and recoil efficacy. We predict that droplets impacting on superhydrophobic surfaces can completely recoil when the surface-dependent capillary pressure PC resists the dynamic impact pressures such as droplet-dependent water hammer pressure PWH and Bernoulli pressure PB. We perform droplet impact experiments on three types of surfaces: (1) sparse microtextured hydrophobic surface with PC < PB < PWH that leads to extensive texture wetting (2) dense microtextured hydrophobic surface with PB < PC < PWH that leads to partial texture wetting (3) hierarchical surface with nanodendrites on microposts (mimics the lotus leaf structure) exhibits complete droplet recoil as the capillary pressure exceeds the water hammer and Bernoulli pressures (PB < PWH < PC). This fundamental understanding can aid surface design for droplet impact resistance for a variety of applications (for e.g., anti ice surfaces for aircraft engine applications)