This paper presents a detailed experimental investigation of bubble dynamics during flow boiling in two heat sinks - one with straight microchannels and the other with wavy microchannels. Straight microchannel heat sink contains 22 straight parallel microchannels of 500 µm (width) × 500 µm (height) × 50 mm (length) and wavy microchannel heat sink contains 13 wavy parallel microchannels of 500 µm (width) × 500 µm (height) × 50mm (length). Ethanol and acetone have been used as the working fluids and investigation has been made for coolant mass flow rates ranging from 7.89×10^-3 kg/s to 4.46×10^-2 kg/s and heat fluxes ranging from 3×10⁴ W/m² to 6.23×10⁴ W/m². Flow visualization has been performed to analyse the bubble growth, their coalescence, and movement in each heat sink. A comparative study of bubble growth, bubble velocity, and bubble aspect ratio in both heat sinks has been presented. The results of the study demonstrate that bubble dynamics in the wavy microchannels are different from that of the straight microchannels. Comprehensive flow regime maps are also presented for both the straight and wavy microchannels indicating various flow regimes and flow reversal.