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Boiling in microchannel heat sinks is attractive for high-performance electronics cooling due to the high heat transfer rates that can be achieved. However, the physics of flow boiling in microchannels, the flow patterns present, and the effect of microchannel size on the boiling regimes have not been investigated extensively, particularly with dielectric fluids. In the present work, experiments are conducted with a perfluorinated dielectric fluid, Fluorinert FC-77, to investigate the effect of channel size and mass flux (250 to 1600 kg/m2s) on microchannel flow boiling regimes by means of high-speed photography. Seven different silicon test pieces with parallel microchannels of widths ranging from 100 to 5850 mum, all with a depth of 400 mum, are considered. Flow visualizations are performed with a high-speed digital video camera while local measurements of the heat transfer coefficient and pressure drop are simultaneously obtained. The visualizations show that flow regimes in microchannels of width 400 mum and larger are similar, while those in the 100 mum wide microchannels are distinctly different. Also, unlike the 100 mum wide microchannels, in which bubble nucleation at the walls is suppressed at a relatively low heat flux, nucleate boiling is dominant over a wide range of heat flux for microchannels of width 400 mum and larger.