Ultrahigh evaporative heat transfer measured locally in submicron water films

Abstract

Thin film evaporation is a widely-used thermal management solution for micro/nano-devices with high energy densities. Local measurements of the evaporation rate at a liquid-vapor interface, however, are limited. We present a continuous profile of the evaporation heat transfer coefficient (h_{text {evap}}) in the submicron thin film region of a water meniscus obtained through local measurements interpreted by a machine learned surrogate of the physical system. Frequency domain thermoreflectance (FDTR), a non-contact laser-based method with micrometer lateral resolution, is used to induce and measure the meniscus evaporation. A neural network is then trained using finite element simulations to extract the h_{text {evap}} profile from the FDTR data. For a substrate superheat of 20 K, the maximum h_{text {evap}} is 1.0_{-0.3}^{+0.5} MW/text {m}^2-K at a film thickness of 15_{-3}^{+29} nm. This ultrahigh h_{text {evap}} value is two orders of magnitude larger than the heat transfer coefficient for single-phase forced convection or evaporation from a bulk liquid. Under the assumption of constant wall temperature, our profiles of h_{text {evap}} and meniscus thickness suggest that 62% of the heat transfer comes from the region lying 0.1–1 μm from the meniscus edge, whereas just 29% comes from the next 100 μm.