A high efficiency heat sink for the cooling of microelectronic devices involves a phase change from liquid fluid to fluid vapor with a vapor quality of 100%. The liquid fluid is provided to an active area that contains fins having micrometer dimension that support a membrane that is nanoporous. The membrane is effectively impermeable to liquid fluid but permeable to fluid vapor. The heat sink provides very high heat flux and coefficient of heat transfer at low mass flux over a broad range of surface superheat temperatures. The heat sink can be constructed of equi-spaced posts that separate liquid microchannels from vapor microchannels that are connected through capillary forced valves formed between adjacent equi-spaced posts.