Transition to annular flow regime in microchannels is arguably one of the most complex phenomena in the flow boiling process. The instability of the vapor-liquid interface in this interstitial regime presents an intricate situation in which the interface pattern rapidly changes with the mass flow rate and surface heat flux. Although a few past studies have reported observing this regime, thermohydraulics of the process and flow and boundary conditions under which this transition occurs have remained largely unknown. The main obstacle in deciphering the physics of this process is lack of measurement tools to characterize hydrodynamics and thermal characteristics of this flow regime at microscales. The present study benefits from implementation of a novel test device that enables measuring the liquid film thickness and its rapid variations with micrometer and microseconds spatial and temporal resolutions. It is determined that each flow regime has a unique surface temperature signature that enables its clear distinction without need for high-speed visualization. Based on the dynamics of the flow, we identified that the transitional region is comprised of two regimes coalescing bubbles (CB) and semi-annular flow conditions. The difference between these two flow regimes emanates from motion of liquid film beneath the bubble.

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