Advanced microchannels heat sinks featuring enhanced flow boiling performance offer attractive solutions for the efficient cooling of high-heat-flux devices. In this study, a type of copper microchannels with unique Omega-shaped reentrant configurations was developed for heat sink cooling. The comparisons of flow boiling performance between this reentrant microchannels (REEM) and conventional rectangular microchannels (RECM) with the same hydraulic diameter were conducted to explore the microchannel shape effects and feasibility of enhancement by such reentrant shape. Comprehensive comparative experiments with two coolants, i.e., deionized water and ethanol, were performed at inlet subcooling of 10 and 40 degrees C, and mass fluxes of 200-300 kg/m(2) s. Test results show that the reentrant microchannels present significant augmentation in two-phase heat transfer in the large inlet subcooling cases and moderate to high heat fluxes, as their unique configurations play a significant role on the two-phase flow behaviors. Besides, a reduction of two-phase pressure drop and mitigation of two-phase flow instabilities were also reached for reentrant microchannels. The above encouraging results highlight the merits of the unique design of reentrant shapes on flow boiling enhancement of microchannel heat sinks. (C) 2014 Elsevier Ltd. All rights reserved.