Kripa Kiran Varanasi

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

EXPERIMENTAL INVESTIGATION OF MICRO/NANO HEAT PIPE WICK STRUCTURES

The performance of electronic devices is limited by the capability to remove heat from these devices. A heat pipe is a device to facilitate heat transport that has seen increased usage to address this challenge. A heat pipe is a two-phase heat transfer device capable of transporting heat with minimal temperature gradient. An important component of a heat pipe is the wick structure, which transports the condensate from the condenser to the evaporator. The requirements for high heat transport capability and high resilience to external accelerations leads to the necessity of a design trade off in the wick geometry. This makes the wick performance a critical parameter in the design of heat pipes. The present study investigates experimental methods of testing capillary performance of wick structures ranging from micro- to nano-scales. These techniques will facilitate a pathway to the development of nano-engineered wick structures for high performance heat pipes.Copyright

DAMPING FLEXURE MECHANISMS USING LOW-DENSITY, LOW-WAVE-SPEED MEDIA

Lightly damped poles and zeros in the response of flexure-based servomechanisms often limit their dynamic performance. In this paper, we measure the frequency response of a single-and a double-parallelogram flexure stage coupled to low-density, low-wave-speed foams in various configurations, and find that addition of the foam yields relatively high damping of in-plane, out-of-plane, and higher-order resonances. At frequencies high enough for waves to propagate into the foam, strong interactions between the foam and flexure structure occur, giving rise to a great deal of damping. This is a promising method for improvement of the dynamic performance of positioning and constraint systems that employ flexures.Copyright

Vibration Damping Using Low-Wave-Speed Media: Complex Wavenumber and Energy Approximations

Significant damping of structural vibration can be attained by coupling to the structure a low-density medium (such as a powder or foam) in which the speed of sound propagation is relatively low. We describe a set of experiments in which flexural vibration of aluminum beams over a broad frequency range is damped by introduction of a layer of lossy low-wave-speed foam. At frequencies high enough to set up standing waves through the thickness of the foam, loss factors as high as 0.05 can be obtained with a foam layer whose mass is 3.9% of that of the beam. In our prior studies [1,2], we modeled the foam as a continuum in which waves of dilatation and distortion can propagate and obtained approximate solutions for the frequency response of the system by means of a modal expansion. However, these modal expansion models are cumbersome for design and quick calculation. In this paper, we develop a simple approximation for the system loss factor based on the complex wavenumber associated with wave propagation, and find that the damping estimates are in close agreement with measured responses and those predicted by modal expansion methods and strain energy approximations. Finally, we extend this approach to longitudinal vibration in a bar coupled to foam and obtain estimates for the system loss factor.Copyright