"Effective heat transfer in a an LCS requires the cooling fluid to be in contact with as much surface area of the TIM as possible. In micro-heat exchangers, the nondimensional heat transport rate, as expressed by its Nusselt number, is a constant quantitythe heat-transfer coefficient is inversely proportional to the hydraulic diameter.
Heat exchangers with high surface-to-volume-ratio microstructures (HSVRMs) through which the liquid flows should, therefore, provide significantly enhanced heat transfer. Fabrication of heat exchangers with reliable and efficient HSVRMs is therefore extremely critical.
Use of silicon microchannels is one of the commonly cited heat collector structures in liquid-cooling systems. However, low thermal conductivity of silicon makes it a less effective heat exchanger material.
Metallic materials with high thermal conductivity are preferable for more effective heat transfer. High-aspect-ratio microchannels and micropillars electro-formed by LIGA or thick photoresist are some of the advanced methods of fabricating metallic HSVR structures.
Three-dimensional nanoramified porous metal deposits is another electrochemically formed structure that provides a very high surface-to-volume ratio and is ideally suited for such applications. This structure can be fabricated by a technique that involves electro-deposition accompanied by hydrogen evolution."
FULL STORY @ ELECTRONICPRODUCTS (http://www2.electronicproducts.com/Liquid_cooling_targets_advanced_microelectronics-article-farr_cooligy_may2008-html.aspx)