Hot Embossing for Whole Teflon Superhydrophobic Surfaces

Abstract

In this paper, we report a simple fabrication process of whole Teflon superhydrophobic surfaces, featuring high-aspect-ratio (>20) nanowire structures, using a hot embossing process. An anodic aluminum oxide (AAO) membrane is used as the embossing mold for the fabrication of high-aspect-ratio nanowires directly on a Teflon substrate. First, high-aspect-ratio nanowire structures of Teflon are formed by pressing a fluorinated ethylene propylene (FEP) sheet onto a heated AAO membrane at 340 • C, which is above the melting point of FEP. Experimental results show that the heating time and aspect ratios of nanopores in the AAO mold are critical to the fidelity of the hot embossed nanowire structures. It has also been found that during the de-molding step, a large adhesive force between the AAO mold and the molded FEP greatly prolongs the length of nanowires. Contact angle measurements indicate that Teflon nanowires make the surface superhydrophobic. The reliability and robustness of superhydrophobicity is verified by a long-term (~6.5 h) underwater turbulent channel flow test. After the first step of hot-embossing the Teflon nanowires, microstructures are further superimposed by repeating the hot embossing process, but this time with microstructured silicon substrates as micromolds and at a temperature lower than the melting temperature of the FEP. The results indicate that the hot embossing process is also an effective way to fabricate hierarchical micro/nanostructures of whole Teflon, which can be useful for applications of Teflon material, such as superhydrophobic surfaces.

Superhydrophobic Drag Reduction for Turbulent Flows in Open Water

“…Although the current study was performed with a 4 × 7 cm 2 SHPo sample on a 13 foot boat for practical reasons, similar SHPo surfaces in a large area would benefit a wide range of watercrafts, from surfboards to vessels, by increasing the top speed or saving propulsion energy. Mass manufacturing of the SHPo surfaces in this study is feasible and under development [42,43], although technological and economical challenges are abundant. Finally, it is important to note that SHPo drag reduction can be considered for a wide range of applications, whenever there is a need to apply the technology for friction reduction of liquid flows, including pipe flows.…”

“…Finally, 2 wt % Teflon AF 1600 (DuPont) is spin coated on the sample and baked in two stages (165°C for 15 min and 330°C for 10 min, following the manufacturer's specifications) to a thickness around 200 nm [41]. Additional submicron details and fabrication techniques of microtrenches with re-entrant top corners [42,43] are beyond the current scope.…”

Superhydrophobic drag reduction in high-speed towing tank

“…Moreover, by dividing the microtrenches into two parts in length, the SHPo surface has shown improved plastron retainment and enhanced drag reduction, making the trench length an important parameter to consider for large-area applications in the future. Although silicon-based SHPo surface samples were used in this study, whole-Teflon surfaces of a similar geometry are feasible using the hot embossing process, as demonstrated in Xu, Liu & Kim (2020c) and Li et al (2018), which is scalable for mass manufacturing.…”

“…Although silicon-based SHPo surface samples were used in this study, whole-Teflon surfaces of a similar geometry are feasible using the hot embossing process, as demonstrated in Xu, Liu & Kim (2020 c ) and Li et al. (2018), which is scalable for mass manufacturing.…”

Superhydrophobic drag reduction in turbulent flows: a critical review

“…While there have been many approaches, most methods employed to fabricate the scalable SHPo surface were limited to random-roughness morphology (Xue et al 2010). However, there are on-going efforts that show a potential for mass production of largearea SHPo surfaces with organized-roughness morphology, including roll-to-roll imprinting (Ahn and Guo 2009;Dong et al 2018) and hot embossing (Li et al 2018;Wang et al 2020;Xu et al 2020c). In particular, Xu et al (2020c) have developed a one-step hot-embossing process to fabricate the self-powered SHPo surfaces out of a Teflon FEP film, which can restore the depleted plastron autonomously in a self-limiting and self-powered fashion.…”