Surfaces with contact angle hysteresis below 5° and static water contact angle above 150° are known as superhydrophobic surfaces. Due to their high wettability, they are widely used in numerous applications, including oil-water separation and self-cleaning. Most importantly, superhydrophobic surfaces (SHSs) have been extensively researched due to their promising corrosion protection applications. They have demonstrated remarkable corrosion protection for various metals and their alloys, both in aqueous and atmospheric environments, and are more effective in suppressing the entry of corrosion media than most coating technologies. Generally, the corrosion protection of SHS in an atmospheric environment is based on the lotus effect induced by external forces like gravity, even though SHSs with no external forces can still provide remarkable corrosion protection. Unfortunately, the lotus effect increases the susceptibility of the surface droplets to external stimuli, which do not only deteriorate their corrosion resistance but also limit their applications in atmospheric corrosion protection.
Previous findings revealed that coalescence-induced droplet self-jumping behaviors on SHSs exhibit unique features that enhance their performances in different applications. Theoretically, this phenomenon enables effective removal of the corrosive media and can thus be a possible atmospheric corrosion protection mechanism for SHSs. To date, many factors believed to influence the occurrence of this behavior, such as the droplet size and surface microstructure, have been extensively studied. Nevertheless, the effects of surface energy are sparsely studied despite its impact on the stability and wettability properties of SHSs, and its potential influence on self-jumping behavior.
On this account, Chinese Academy of Sciences researchers: Ms. Xiaohan Liu, Dr. Peng Wang, Prof. Dun Zhang and Dr. Xiaotong Chen investigated the atmospheric protection mechanism and performance of SHS based on coalescence-induced droplet self-jumping behavior. In particular, two different SHSs, one favoring and the other not favoring the coalescence-induced droplet self-jumping mechanism were prepared by regulating the surface energy. The authors also studied the dynamic behaviors of the droplets and coalescence process of the two SHSs during condensation and modelled the correlation between the self-jumping phenomenon and the SHS’s surface energy from the energy perspective. Based on impedance spectroscopy testing, the performance of the two surfaces against atmospheric corrosion was evaluated and compared. The work is currently published in the journal, ACS Applied Materials and Interfaces.
Results showed that surface energy plays a key role in regulating the coalescence-induced droplet self-jumping mechanism by effectively controlling the adhesion energy between the liquid and solid interface. The authors further observed that superhydrophobic surfaces with low surfaces energy reduce solid-liquid interface contact area and adhesion property, thus decreasing the dissipating energy induced through adhesion. This enabled the droplet to gain more energy for promoting the droplet self-jumping. Moreover, the SHS favoring coalescence-induced droplet self-jumping exhibited superior corrosion protection performance. The high performance was attributed to its ability to reduce the liquid-solid contact area by either enhancing the contact mode transformation between the surface and the droplets or decreasing the droplet coverage.
In summary, the study proposed an atmospheric corrosion protection mechanism for superhydrophobic surfaces based on coalescence-induced droplet self-jumping behaviour. This is a novel route for superhydrophobic surfaces to realize corrosion protection performance. Most importantly, the surface energy influence on droplet self-jumping was analyzed, revealing the importance of surface energy in enhancing the corrosion performance of SHSs. Accordingly, the study provided an in-depth understanding of the coalescence-induced droplet self-jumping mechanism and its potential role in improving the anticorrosion performance of SHSs. It would be a milestone for the application of superhydrophobic surface in atmospheric corrosion protection. In a statement to Advances in Engineering, the authors noted that the study would guide the development of effective SHS-based corrosion protection methods.
Liu, X., Wang, P., Zhang, D., & Chen, X. (2021). Atmospheric Corrosion Protection Performance and Mechanism of Superhydrophobic Surface Based on Coalescence-Induced Droplet Self-Jumping Behavior. ACS Applied Materials & Interfaces, 13(21), 25438-25450.