Novel Applications of High-Voltage Electrostatic Flocking — Hydrodynamic Drag Reduction in Shipping

In 2018, Zheng et al. utilized electrostatic flocking technology for the first time to fabricate a Salvinia-like structure, successfully mimicking a hairy and elastic surface.

The Salvinia structure is a superhydrophobic structure inspired by the surface of Salvinia natans leaves. The upper surface of these leaves is covered with complex multicellular hairs, where four hairs cluster together and connect at their tips, forming a whisker-like three-dimensional structure (Figure a). [95] This structure exhibits remarkable superhydrophobicity and excellent gas retention capability, [96,97] showing broad application potential in oil-water separation, [98] drag reduction and noise suppression, [99,100] thermal insulation, [101] as well as waterproofing and anti-fouling. [102]

Although the structure fabricated via electrostatic flocking did not fully replicate the whisker-like tip morphology or the hydrophilic patches, it still achieved the longest recorded gas retention time of 530–610 hours. [95] The hydrophobic surface enhanced by electrostatic flocking forms a stable air layer at the water-contact interface, resulting in a silver mirror effect (Figure b). The air layer created by the Salvinia structure significantly reduces frictional resistance between the material and liquids, effectively enabling drag and noise reduction. In experiments conducted by Zhang et al., a maximum drag reduction efficiency of 28% was achieved (Figures c, d). [58] This finding highlights the potential of Salvinia-like structures for reducing resistance in electrohydrodynamic applications, offering a promising avenue for further research and optimization.

a) Salvinia molesta; reprinted with permission. [95] Copyright ©2018 Royal Society of Chemistry.
b) Silver-like sheen observed underwater due to retained air layer on the surface, with cross-sectional SEM image; reprinted with permission. [95] Copyright ©2018 Royal Society of Chemistry.
c) Torque measurement of drag reduction performance of the sample; reprinted with permission. [58] Copyright 2022, Published by Elsevier Ltd.
d) Drag reduction rate of the sample's drag-reducing performance; reprinted with permission. [58] Copyright 2022, Published by Elsevier Ltd.

References
[58] L. Zhang, Z. Huang, W. Cai, X. Xue, X. Min, H. Zhang, Z. Zhang, Prog. Org. Coat. 2023, *174*.
[95] Y. Zheng, X. Zhou, Z. Xing, T. Tu, RSC Adv. 2018, *8*.
[96] M. Moosmann, T. Schimmel, W. Barthlott, M. Mail, Beilstein J. Nanotechnol. 2017.
[97] W. Barthlott, T. Schimmel, S. Wiersch, K. Koch, M. Brede, M. Barczewski, S. Walheim, A. Weis, A. Kaltenmaier, A. Leder, H. F. Bohn, Adv. Mater. 2010, *22*.
[98] Y. Yang, X. Li, X. Zheng, Z. Chen, Q. Zhou, Y. Chen, Adv. Mater. 2018, *30*.
[99] J. Busch, W. Barthlott, M. Brede, W. Terlau, M. Mail, Philos. Trans. R. Soc. A 2019, *377*.
[100] M. J. Mayser, H. F. Bohn, M. Reker, W. Barthlott, Beilstein J. Nanotechnol. 2014, *5*.
[101] Y. Zheng, X. Zhou, Z. Xing, T. Tu, J. Res. 2019, *89*.
[102] Y. Xiang, S. Huang, T. Huang, A. Dong, D. Cao, H. Li, Y. Xue, P. Lv, H. Duan, Proc. Natl. Acad. Sci. U.S.A. 2020, *117*.