Nitin Bhate

Effect of surfaces on the size-dependent elastic state of nano-inhomogeneities

The effect of surface energies, strains, and stresses on the size-dependent elastic state of embedded inhomogeneities are investigated. At nanolength scales, due to the increasing surface-to-volume ratio, surface effects become important and induce a size dependency in the otherwise size-independent classical elasticity solutions. In this letter, closed-form expressions are derived for the elastic state of eigenstrained spherical inhomogeneities with surface effects using a variational formulation. Our results indicate that surface elasticity can significantly alter the fundamental nature of stress state at nanometer length scales. Additional applications of our work on nanostructures such as quantum dots, composites, etc. are implied.

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.

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.

Nonwetting of impinging droplets on textured surfaces

This paper studies the impinging droplets on superhydrophobic textured surfaces and proposes a design guideline for nonwetting surfaces under droplet impingement. A new wetting pressure, the effective water hammer pressure, is introduced in the study to clearly define wetting states for the impinging droplets. This approach establishes the design criteria for nonwetting surfaces to impinging droplets. For impingement speed higher than raindrop speed, the surfaces need to have sub-100-nm features to generate a large enough antiwetting pressure for the droplets to take a nonwetting state after impingement.

Design of Superhydrophobic Surfaces for Optimum Roll-Off and Droplet Impact Resistance

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.Copyright

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 degradations associated with oil sealing. This paper describes the benefits and development of the non-metallic brush seals for oil sealing application.Copyright

EVALUATION OF BRUSH SEAL PERFORMANCE FOR OIL SEALING APPLICATIONS

Oil sealing at high speeds is one of the major problems engineers should address in turbomachinery design. High temperatures faced in oil sumps in aircraft engines, and large seal sizes typical in land based turbine applications further complicate the problem. Labyrinth seals can overcome problems faced with carbon seals in high temperature and large size applications. On the other hand, use of labyrinth seals may result in high leakage rates leading to increased oil consumption, unintended oil contamination in some flow cavities, early oil degradation or even fires in some cases. Successful engine secondary flow path applications of brush seals lead to questions of their applicability for oil sealing. Because brush seals are contact seals, oil temperature rise and coking become major issues in addition to leakage performance. This paper presents an investigative study of brush seal leakage and coking performance using common lube oil. Both metallic and non-metallic prototypes have been tested under static and dynamic conditions. It has been concluded that properly designed brush seals can achieve lower leakage rates than labyrinth seals without causing coking problems.