  Nanoporous structures including single nanopores and nanoporous membranes
have been utilized as a platform to study fundamental liquid-vapor phase change
heat transfer (PCHT) processes as well as a promising candidate for high flux
heat dissipation. Both thin film boiling and evaporation through nanoporous
structures have been demonstrated to achieve high heat flux, but they are
usually considered two mutually exclusive regimes operated under vastly
different conditions, and the factors dictating how close the PCHT process is
to the kinetic limit are elusive. In this work, we utilized a unique transition
between thin film boiling and evaporation through nanoporous membranes to
clarify the factors determining the heat flux and heat transfer coefficient
(HTC) with respect to the kinetic limit conditions. We unambiguously showed the
controllable transition from boiling to evaporation, when the liquid receded
into the nanopores and provided additional driving force from capillary pumping
sustained in the nanoscale pores. We showed that this transition is universal
and can be understood from a simple fluid transport model for all the four
types of fluids we studied, which cover a wide span of surface tension (water,
ethanol, isopropanol (IPA), FC-72). More importantly, PCHT conditions at the
transition points between boiling and evaporation were close to those of the
kinetic limit of all these fluids. However, further increase of the heat flux
beyond the transition points led to decreasing HTC and deviation from the
kinetic limit, which can be attributed to the increasing vapor resistance in
the vapor space and inside the nanopores. This increasing vapor resistance was
also confirmed by experiments on IPA with different vapor pressures.
