Collaborative mechanisms boost the nanoscale boiling heat transfer at functionalized gold surfaces

Abstract

Liquid-vapor phase change heat transfer, i.e., boiling heat transfer, has been demonstrated to be an effective thermal management strategy for high-power electronics and power generators. Improving the boiling heat transfer performance can largely increase the energy conversion efficiency in these corresponding devices. In this paper, we demonstrate that the boiling heat transfer coefficient (HTC) at functionalized Au surfaces can be improved 3.3 times at most by introducing functionalization groups (FGs) to Au surfaces. Our molecular dynamics simulations show that the increase of HTC is resulting from the high thermal conductance across functionalized Au/water interfaces and the strong interaction between FGs and water molecules. The high interfacial thermal conductance of functionalized Au/water interfaces stems from the strong bonding between the functionalized Au surface and water, and the strong vibrational coupling at 0∼4 THz between Au and FGs. The strong interaction between FGs and water molecules comes from their mutual adhesions including van der Waals and electrostatic interactions, and bridging effects. For the FGs without electronegative atoms (e.g., -CH3 FGs), FGs will facilitate thermal energy transfer via their van der Waals interactions with water molecules. When FGs with electronegative atoms (e.g., -CF3, -OH, and -COOH FGs) are introduced, hydrogen bonds will form due to their electrostatic interactions which benefit the thermal energy exchange between FGs and water molecules. Meanwhile, the vibrations for the FGs with -CF2 and -CF3 terminal groups are coupled with Au at low frequencies of 0∼4 THz and water molecules at middle frequencies of 6∼8 THz. The FGs can then bridge the thermal energy from Au to water through the dual vibrational couplings. The Au-CF2(COOH) FGs surface, which is designed to include all the mechanisms mentioned above, can therefore increase the boiling HTC 3.3 times compared to that of the planar Au surface. Our results here provide insights into the design of surfaces with high boiling heat transfer performances using chemical FGs.