Revisiting the fabrication of superhydrophobic aluminum surfaces and their use as soft substrates for droplet manipulation
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Superhydrophobic aluminum (Al) surfaces are desired in many applications and have been widely studied, but some fundamental issues in their fabrication are not yet fully understood. In this study, commonly used HCl solution and organic acid/sulfur reagents were employed for clarifying the phenomena in etching and composition modification. An unstable layer was revealed after HCl etching, which was removable with water flushing, and this layer, consisting of scattered AlCl3/Al2O3 particles surrounded by Al2O3 plates with cracks, caused changes in surface appearance from bright to dark. A complete change in surface color corresponded to sufficient etching, and hence, this visible phenomenon could be used to confirm the shortest etching time for achieving desirable superhydrophobicity. Composition modification with dodecanethiol and myristic acid also modified the microstructure through a slight etching mechanism and a synergistic effect was found with the use of their mixture, which led to the finest grains and most abundant carbon chains on the final surfaces. Consequently, the best superhydrophobicity was revealed in static and dynamic experiments with droplets. Finally, using the advantage of flexibility, a superhydrophobic Al sheet was demonstrated to be suitable for subjecting droplets to controlled rolling and coalescence, which could be meaningful for many droplet-related studies.
This work was financially supported by the National Natural Science Foundation of China (Grant No. 51672224), Shaanxi Provincial Natural Science Foundation (Grant No. 2018JM1002), the Fundamental Research Funds for the Central Universities (Grant No. 3102018zy037), and Opening Project of Shanghai Key Laboratory of Special Artificial Microstructure Materials and Technology (Grant No. ammt2017A-1).
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