Rapid bubble growth within 200 μs in flat microchannels under “hot spot” heating conditions at ultra-high heat fluxes

•The bubble nucleation and growth in flat microchannels at localized ultra-high heat fluxes were studied.•The time duration of bubble growth was less than 200 μs.•A linear growth rate of about 20 m/s was evidenced at about the first half of bubble growth stage.•The short micro-pin fin arrays lowered the threshold heat flux and onset temperature of bubble nucleation.