Hydrophilicity degradation and steam-induced rewetting during capillary-fed boiling

Abstract

Capillary-fed evaporation/boiling is a crucial process that occurs within two-phase heat transfer devices. Herein, we delve into the capillary-fed boiling performance of a grooved wick with an upper width of 200 μm and a depth of 150 μm fabricated by ultrafast laser micromachining and observe a substantially 51.8% reduction in critical heat flux (CHF), from 145.0 ± 3.3 W/cm2 to 70.1 ± 2.9 W/cm2, following five boiling cycles. Comprehensive experimental investigations are conducted to gain insights into the underlying mechanisms. Our analyses confirm that the diminished CHF arises from the hydrophilicity degradation of the evaporator, attributed to the surface adsorption of airborne organics after the dry-out. The superhydrophilic evaporator becomes highly hydrophobic with static contact angles (CAs) greater than 140° after five boiling cycles. Liquid wicking in subsequent boiling tests initiates through a steam-induced rewetting process. Moreover, we present compelling evidence that the undesired hydrophilicity degradation is effectively mitigated on nanoporous surfaces. Consequently, a microgroove-nanoparticle composite wick successfully retains its original boiling performance during repeated boiling tests. These findings enhance the understanding of capillary-fed boiling performance and offer promising avenues for the design of high-performance wicks tailored for ultrathin two-phase heat transfer devices.