The effects of mass flux, heat flux and microgap configuration on flow boiling instability in expanding microgap heat sink are experimentally investigated. Characterizations of instabilities are performed for deionized water mass fluxes, G = 400-1000 kg/m(2) s, imposed effective heat flux q(eff)" ranging from 0 to 80 W/cm(2) for three different microgap configurations, straight microgap of depth 200 mu m and expanding microgap having same inlet depth 200 mu m with gradually increasing exit depth 300 mu m and 460 mu m. High speed flow visualizations are conducted simultaneously along with experiments to explore the bubble behavior in expanding microgap heat sink. Results show that two phase expanding microgap heat sink has novel potential to mitigate the flow instabilities and flow reversal issues as the generated vapor has room to expand downstream and also due to reducing shear force along the direction of expansion. Reduced share force is beneficial as it contributes to easing flow in the direction of expansion and thus, eliminates flow reversal and maintains stable flow boiling condition. In addition, inlet pressure oscillation increases gradually with the increase of mass flux and heat flux due to the extensive repetitive backward expansion of vapor slug. Whereas, wall temperature oscillation decreases with the decrease of mass flux and increase of heat flux due to the presence of stable annular flow regime at high heat flux and low mass flux condition before dryout approaches. (C) 2015 Elsevier Ltd. All rights reserved.