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

Non-metallic brush seals for gas turbine bearings

A non-metallic brush seal has been developed as an oil seal for use in turbomachinary. Traditionally labyrinth-type seals with larger clearances have been used in such applications. Labyrinth seals have higher leakage rates and can undergo excessive wear in case of rotor instability. Brush seals reduce leakage by up to an order of magnitude and provide compliance against rotor instabilities. Brush seals are compact and are much less prone to degradation associated with oil sealing. This paper describes the benefits and development of the nonmetallic brush seals for oil sealing application

Process dynamics laboratory: orientation, protocol and design methodology

by Mukund Hari Divekar and Nitin Bhat

Heat transfer laboratory:orientation, protocol and design methodology

by Mukund H. Divekar and Nitin V. Bhat

Heat transfer laboratory:orientation, protocol and design methodology

by Mukund H. Divekar and Nitin V. Bhat

IMECE2008-67808 DESIGN OF SUPERHYDROPHOBIC SURFACES FOR OPTIMUM ROLL-OFF AND DROPLET IMPACT RESISTANCE

ABSTRACT We study the wetting behavior of water droplets on superhydrophobic arrays of lithographically fabricated square posts. To determine the droplet wetting state, we measure static contact angles and compare the results to predictions for 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 energy barrier required to completely wet the posts. These metastable Cassie droplets show superior roll-off properties because the effective length of the contact line that is pinned to the surface is reduced. We develop a model that can predict the transition between the metastable Cassie and Wenzel regimes by comparing the Laplace pressure of the drop to the capillary pressure associated with the wetting energy barrier of the textured surface. In the case of impacting droplets the water hammer and Bernoulli pressures must be compared with the capillary pressure. Experiments with impacting droplets show very good agreement with this simple pressure-balance model. Together these models can be used to optimize texture design for droplet-shedding and droplet-impact resistant surfaces

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)

Spatial control in the heterogeneous nucleation of water

Heterogeneous nucleation of water plays an important role in a wide range of natural and industrial processes. Though heterogeneous nucleation of water is ubiquitous and an everyday experience, spatial control of this important phenomenon is extremely difficult. Here we show for the first time that spatial control in the heterogeneous nucleation of water can be achieved by manipulating the local nucleation energy barrier and nucleation rate via the modification of the local intrinsic wettability of a surface. Such ability to control water nucleation could address the condensation-related limitations of superhydrophobic surfaces and has implications for efficiency enhancements in energy and desalination systems.General Electric Company (Nanotechnology Program)Massachusetts Institute of Technology (d’Arbeloff Career Development Chair

Frost formation and ice adhesion on superhydrophobic surfaces

We study frost formation and its impact on icephobic properties of superhydrophobic surfaces. Using an environmental scanning electron microscope, we show that frost nucleation occurs indiscriminately on superhydrophobic textures without any particular spatial preference. Ice adhesion measurements on superhydrophobic surfaces susceptible to frost formation show increased adhesion over smooth surfaces with a strong linear trend with the total surface area. These studies indicate that frost formation significantly compromises the icephobic properties of superhydrophobic surfaces and poses serious limitations to the use of superhydrophobic surfaces as icephobic surface treatments for both on-ground and in-flight applications.GE Global Research Center (Nanotechnology Advanced Technology Program)d'Arbeloff Fund for Excellence in EducationReed Awar