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Tunable Nanostructured Surfaces
		The ability to actively control and tune surface properties between hydrophobic and hydrophilic states is important for a variety of microfluidic and lab-on-a chip applications. For example, in microfluidic systems, mass diffusion often limits the rate of the biochemical reactions. We have investigated the use of electrically tunable superhydrophobic nanostructured surfaces to achieve rapid micromixing. We electrowet the droplet to a wetted state with the use of an electrical field and bring the droplet back to an unwetted state with heat, to induce fluid mixing in a droplet. This concept has been demonstrated qualitatively using particle diagnostics and quantitatively with a DNA hybridization assay and enzyme-linked immunsorbent assay (ELISA).  The use of nanostructured superhydrophobic surfaces offers a promising method for the active control and acceleration of biochemical reactions in micro total analysis systems.
References: 1. T.N. Krupenkin, J.A. Taylor, E.N. Wang, P. Kolodner, M. Hodes, T. Salamon, "Reversible Wetting-Dewetting Transitions on Electrically Tunable Superhydrophobic Nanostructured Surfaces, Langmuir, 23(17) 9128-9133. 2. E.N. Wang, M. Bucaro, J.A.Taylor, P.R. Kolodner, J. Aizenberg, T.N. Krupenkin, "Droplet Mixing Using Electrically Tunable Superhydrophobic Nanostructured Surfaces," submitted 2007
Two-phase Microchannel Cooling
	Thermal management is the bottleneck for a variety of applications such as solar cells, high performance microprocessors, and high power laser diodes ranging from power levels of hundreds to thousands of Watts.  One promising solution is two-phase microchannel cooling because they offer compact designs where latent heat during phase-change can be used to transfer and carry high heat fluxes.  Challenges, however, arise due to flow instabilities during liquid-vapor phase-change and the associated large pressure drops in microchannel geometries.  These instabilities belong to a different regime than their macroscale counterparts, and demand a comprehensive understanding of boiling in microchannels.  Towards this goal, we fabricated silicon-based microchannels with integrated sensors to determine temperature and flow characteristics.  To understand bubble dynamics during incipient boiling, optical diagnostics (i.e., micron-resolution particle image velocimetry (µPIV)) were combined with computational models to determine important parameters, such as geometry, growth rate, and forces associated with bubble growth and departure.  The insights obtained by such studies are critical steps towards the development of optimized two-phase microchannel heat sinks.
References: 1. E.N. Wang, S. Devasenathipathy, H. Lin, C.H. Hidrovo, J.G. Santiago, K.E. Goodson, T.W. Kenny. “A Hybrid Method for Bubble Reconstruction in Two-Phase Microchannels,” Experiments in Fluids, 2006, 40(6), p.847-858 2. L. Zhang, E.N. Wang, K.E. Goodson, T.W. Kenny.  “Phase Change Phenomena in Silicon Microchannels,” International Journal of Heat and Mass Transfer, 2005, 48(8), p.1572-1582

Microjet Impingement Cooling
	An alternative to microchannel heat sinks are microjets, where high velocity water streams directly impinge onto the hot surface.  This method offers several potential advantages, such as high heat transfer coefficients from the thin liquid boundary layer and uniform cooling with jet arrays.  During boiling, microjets can minimize variations in saturation temperature and increase system stability.  To study and optimize microjet structures, single jets and multi-jet arrays were fabricated and tested on custom heater structures with integrated temperature sensors.  Models were developed to predict temperature profiles of single microjets.  Flow visualizations were performed with white light microscopy and µPIV.  The results obtained from these studies suggest that microjets are a promising solution for large heat loads, but careful optimization of flow rates and jet orifice diameters, as well as the design for fluidic recovery are needed.
References: 1. E.N. Wang, L. Zhang, L. Jiang, J.-M. Koo, J.G. Maveety, E.A. Sanchez, K.E. Goodson, T.W. Kenny.  “Micromachined Jets for Liquid Impingement Cooling of VLSI Chips,” Journal of MicroElectroMechanical Systems, 2004, 13(5), p.833-842 2. E.N. Wang, J.G. Santiago, K.E. Goodson, T.W. Kenny.  “Microjet Impingement Cooling with Phase Change,” Proceedings of the ASME International Mechanical Engineering Congress & Exposition, Anaheim, CA, November 13-19, 2004, IMECE2004-62176

Last Updated 01.08