Capillary Wicking in Hierarchically Textured Copper Nanowire Arrays.

Capillary wicking through homogeneous porous media remains challenging to simultaneously optimize due to the unique transport phenomena that occur at different length scales. This challenge may be overcome by introducing hierarchical porous media, which combine tailored morphologies across multiple length scales to design for the individual transport mechanisms. Here, we fabricate hierarchical nanowire arrays consisting of vertically aligned copper nanowires (∼100 to 1000 nm length scale) decorated with dense copper oxide nanostructures (∼10 to 100 nm length scale) to create unique property sets that include a large specific surface area, high rates of fluid delivery, and the structural flexibility of vertical arrays. These hierarchical nanowire arrays possess enhanced capillary wicking ( K/ Reff = 0.004-0.023 µm) by utilizing hemispreading and are advantageous as evaporation surfaces. With the advent and acceleration of flexible electronics technologies, we measure the capillary properties of our freestanding hierarchical nanowire arrays installed on curved surfaces and observe comparable fluid delivery to flat arrays, showing the difference of 10-20%. The degree of effective inter-nanowire pore and porosity is shown to govern the capillary performance parameters, thereby this study provides the design strategy for capillary wicking materials with unique and tailored combinations of thermofluidic properties.

Controlled Wetting Properties through Heterogeneous Surfaces Containing Two-level Nanofeatures.

Addressing the direct control of surface wettability has been a significant challenge for a variety of applications from self-cleaning surfaces to phase-change applications. Surface wettability has been traditionally modulated by installing surface nanostructures or changing their chemistry. Among numerous nanofabrication efforts, the chemical oxidation method is considered a promising approach because it allows cost-effective, quick, and direct control of the morphologies and chemical compositions of the grown nanofeatures. Despite the wide applicability of the surface oxidation method, the precise control of wetting behaviors through the growth of nanostructures has yet to be addressed. Here, we investigate the wetting characteristics of heterogeneous surfaces that contain two-level features (i.e., nanograsses and nanoflowers) with different petal shapes and structural chemistry. The difference in growth rates between nanograsses and nanoflowers creates a time-evolving morphology that can be classified by grass-dominated or flower-dominated regimes, which induces a wide range of water contact angles from 120 to 20°. The following study systematically quantifies the structural details and chemistry of nanostructures associated with their wetting characteristics. This investigation of heterogeneous surfaces will pave the way for selective growth of copper nanostructures and thus a direct control of surface wetting properties for use in future copper-based thermal applications.