Understanding the influence of surface characteristics on flow boiling heat transfer behavior in microgap is necessary to enhance the performance of microgap heat sink. The influences of surface roughness on flow boiling heat transfer, pressure drop and instability in microgap heat sink are experimentally investigated. Flow boiling experiments are conducted over silicon microgap heat sink of three different microgap dimensions namely 500 mu m, 300 mu m and 200 mu m. The original silicon surface of surface roughness, R-a = 0.6 mu m is modified to R-a = 1.0 mu m and 1.6 mu m to examine the effect of surface finish. These studies are carried out with the inlet deionized water temperatures 91 degrees C at two different mass fluxes, G = 390 kg/m(2) s and 650 kg/m(2) s and imposed effective heat flux, q(eff)'' ranging from 0 W/cm(2) to 85 W/cm(2). High speed flow visualizations are conducted simultaneously along with experiments to explore the bubble behavior in microgap heat sink. The results of this study show that bubble nucleation site density as well as heat transfer coefficient increases with the increase of surface roughness and pressure drop is independent of surface roughness in microgap heat sink. Moreover, rougher surface maintains lower and uniform wall temperature over the heated surface. However, surface roughness has an adverse effect on the inlet pressure instability and inlet pressure fluctuation increases with increasing surface roughness at larger microgap heat sink. (c) 2013 Elsevier Ltd. All rights reserved.