The rapid increase of heat flux in high performance electronic devices has necessitated the development of high capacity thermal management techniques that can support extremely high heat transfer rates. Flow boiling in microgap is very promising for this purpose due to its high heat transfer rate and ease of fabrication. However, the effects of microgap dimension on heat transfer and pressure drop characteristics along with flow visualization have not been investigated extensively. This paper focuses on flow boiling experiments of deionized water in silicon microgap heat sink for ten different microgap dimensions from a range of 80 mu m-1000 mu m to determine the most effective and efficient range of microgap dimensions based on heat transfer and pressure drop performance. High speed flow visualization is conducted simultaneously along with experiments to illustrate the bubble characteristics in the boiling flow in microgap. The results of this study show that confinement in flow boiling occurs for microgap sizes 500 mu m and below and confined slug/annular flow is the main dominant regime whereas physical confinement does not occur for microgap sizes 700 mu m and above and bubbly flow is the dominant flow regime. The microgap is ineffective below 100 mu m as partial dryout strikes very early and the wall temperature is much higher for a fixed heat flux as microgap size increases above 500 mu m. In addition, results show that pressure drop and pressure fluctuation decrease with the increases of gap size whereas wall temperature and wall temperature fluctuation increase with the increases of gap size. A strong dependence of heat transfer coefficient on microgap sizes is observed for microgap sizes 500 mu m and below. However, the heat transfer coefficient is independent of microgap size for microgap sizes 700 mu m and above. (C) 2012 Elsevier Ltd. All rights reserved.