Microscale sets a fundamental limit to heat transfer

Classical heat transfer relations, claiming continual increase of cooling with downscaling, contradict previous experimental findings. We show that the reemergence of viscous dissipation at the microscale breaks heat transfer's symmetry and heat flux independence – the first such analysis for microjets. We also find that all bounded flows have maximal attainable cooling rates, corresponding to scale-down limits, given by a critical value of a new dimensionless group. While macro- and micro- dissipation are similar, their parameter ranges differ with jets introducing additional dependencies. This new micro-cooling performance envelope curbs the ongoing trend of flow miniaturization.