Two-phase microgap channel cooling concept has been recently proposed for cooling the heat sources directly in application of electronic devices thermal management. This concept is relatively new and more research should be carried out systematically to investigate the size effects of microgap channel on heat transfer and pressure drop mechanisms. In this study, local flow boiling phenomenon in different microgap sizes has been investigated experimentally. Experiments are performed in silicon based microgap heat sink having microgap of depth 190 mu m, 285 mu m and 381 mu m, using deionized water with inlet temperature of 86 degrees C. The effects of mass flux and heat flux on heat transfer coefficient and pressure drop characteristics are examined by using three different mass fluxes 420 kg/m(2) s, 690 kg/m(2) s and 970 kg/m(2) s and effective heat flux varying from 0 to 110 W/cm(2). An array of integrated micro-temperature sensors are used in this study to obtain the local temperatures and subsequently local heat transfer coefficients are determined. Apart from these experimental investigations, simultaneous high speed visualizations are conducted to observe and explore the mechanism of flow boiling in microgap channel. The results of this study show that flow boiling heat transfer coefficient is dependent on gap size, and the lower the gap size, higher the heat transfer coefficient. Moreover, it has been observed that confined slug and annular boiling are the dominant heat transfer mechanisms in microgap channels after the onset of nucleate boiling. Hence, local heat transfer coefficient increases significantly because of thin film evaporation during confined boiling at high heat flux. This study also evaluates the effectiveness of microgap cooling technology, to eliminate temperature gradient and hotspots. (C) 2012 Elsevier Ltd. All rights reserved.