Liquid cooling of a hot spot using a superhydrophilic nanoporous surface

The performance of thin-film evaporative cooling for near-junction thermal management was investigated. A liquid delivery system capable of delivering water in small volumes ranging 20~75 nl at frequencies of up to 600 Hz was established. On one side of the silicon chip, a resistive heating layer of 2 mm × 2 mm was fabricated to emulate the high heat flux hot-spot, and on the other side a superhydrophilic nanoporous coating (SHNC) was applied over an area of 10 mm × 10 mm. With the aid of the nanoporous coating, delivered droplets spread into thin films of thicknesses less than 10 μm. With this system, evaporative tests were conducted in ambient in an effort to maximize dryout heat flux and evaporative heat transfer coefficient. During the tests, heat flux at the hot spot was varied to values above 1000 W/cm². Water was delivered at either given constant frequency (constant mass flow rate) or programmed variations of frequency (variable mass flow rate), for a given nanoliter dose volume. Heat flux and hot spot surface temperatures were recorded upon reaching steady state at each applied heat flux increment. A mixed mode of cooling consisting of simultaneous thin-film evaporation and boiling was observed. Relative to bare silicon surface, dryout heat flux of the SHNC surface was found to increase by ~5 times at 500~600 Hz.