No abstract

Microchannel surfaces are common to microfluidics, biofluidics, thermal management, and energy applications. Due to processing limitations for the majority of metallic materials, the majority of hyperfine microchannels used in microfluidics and thermo‐fluids are fabricated on non‐metallic substrates, for example, silicon and polydimethylsiloxane. Here, a technique to fabricate ultrasmall microchannels on arbitrary metallic materials is developed using photolithography in combination with electrochemical deposition. The technique is used to prepare copper microchannels and to investigate the pool boiling heat transfer performance with a focus on the three‐phase contact line dynamics. The hydrodynamics of nucleating bubbles during boiling are observed in situ using in‐liquid endoscopy. The results show that the variation of critical heat flux enhancement has a linear relationship with the contact line increase ratio. The scalable microchannel surfaces exhibit superior heat transfer performance with a maximum heat transfer coefficient) enhancement of 930% with ultra‐low wall superheat of 5 °C. This work not only develops a scalable manufacturing method to develop ultra‐small microchannels on metallic materials, it outlines design guidelines for structure optimization of pool boiling heat transfer for temperature sensitive applications, such as electronics thermal management.

No abstract

Micro‐ and nano‐structured surfaces having high wicking capability enable excellent liquid transport efficiency and have great promise in water desalination, atmospheric water harvesting, biomedical device development, and electronics thermal management applications. However, the poorly understood degeneration of surface wickability during exposure to air represents the main hindrance to societal application of structured surfaces. Here, the authors investigate wicking degeneration on structured surfaces and elucidate the importance of environmental volatile organic compound adsorption from air. Based on their developed mechanistic understanding, the authors design a highly scalable, cost‐effective, and hierarchical structure having both superior wicking capability and antidegeneration performance. Year‐long continuous surface wickability measurements demonstrate a 4100% higher surface wickability durability of this structure when compared to widely used single‐tier surface structures. Pool boiling tests coupled with in situ and in‐liquid optical microscopy are used to characterize the effect of wicking degradation on boiling heat transfer performance. This work demonstrates the previously unidentified coexistence of several dry areas underneath individual bubbles during boiling on highly wicking structured surfaces, resulting in significant augmentation of the three‐phase contact line length. In addition, this work outlines design guidelines for the fabrication of surface wicking structures having high performance and durability.