5 research outputs found
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)