Liquid-like surfaces (LLSs) are typically fully pobic surfaces formed by grafting highly flexible polymers onto smooth solid surfaces, and the molecular structure of the grafted polymers significantly affects key properties of LLSs, such as dewetting (the ability to repel substances) and durability (stability of repelling substances). Compared to superhydrophobic surfaces with micro-nanostructure fragility issues and liquid-infused surfaces with liquid lubricant loss issues, LLS exhibit stable repellent ability against various pollutants and are therefore widely studied in various fields such as biology, environment, and energy. However, most of the reported LLSs do not meet the requirements of practical applications, especially in terms of dewetting and durability.
Recently, the team of Wang Shutao/Meng Jingxin from the Technical Institute of Physics and Chemistry of the Chinese Academy of Sciences published a paper entitled "Liquid-Like Surfaces with Enhanced De-wettability and Durability: From Structural Designs to Potential Applications" in Advanced Materials (DOI:10.1002/adma.202407315). This review article provides a detailed overview of the development of LLSs from the perspective of molecular structure evolution, highlights recent research advances in improving the dewetting and durability of LLSs by optimizing their molecular structure design (linear, cyclical, cross-linked, and hybrid structures), discusses different applications based on LLSs (anti-adhesion/transport/condensation, anti-icing/scaling/waxing, and anti-biofouling), and finally looks forward to some of the challenges faced by the design, performance, functionality, and application of LLSs.
Over the past two decades, researchers have fabricated LLSs with enhanced dewetting and durability by designing different molecular structures (linear, cyclical, cross-linked, and hybrid) to meet the pressing needs of various applications (Figure 1). In general, LLLSs grafted with linear chains have better dewettability but less durability than LLSs grafted with ring, cross-linked, and hybrid structures. The poor durability of LLSs in grafted linear molecular chains can be attributed to three potential mechanisms: the breakage of the polymer chains themselves, the detachment of the polymer chains from the grafted surface, and the combination of both. The transition of the graft molecular structure from linear to cyclic and finally to a cross-linked network can mitigate the effects of polymer chain breakage, thereby effectively improving durability. In addition, the hybrid structure is used to increase the hardness of LLS, which in turn greatly improves the wear resistance and durability of LLS, thereby extending their service life. Therefore, as shown in Figure 1, the durability of LLSs grafted with these four structures gradually increases from left to right.
Figure 1. The history of LLSs with surface grafting linear chains, ring chains, cross-linked networks, and hybrid structures.
In order to effectively enhance the dewettability of LLSs, linear molecular chains with large bond lengths, bond angles and different electronegativity can be grafted. Linear PDMS chains with molecular weights between 2000 g mol-1 and 10000 g mol-1 or graft densities between 1 and 5 can be used to prepare LLSs with excellent dewetting. Cyclic molecular chains or cross-linked networks can replace common linear polymer chains to make relatively durable LLSs, and the addition of inorganics to organic molecular chains can produce LLSs with superior durability. The trade-off between excellent dewetting and durability needs to be chosen based on the actual application.
Figure 2. Effects of linear chain species, graft density and chain conformation on the performance of LLSCs.
Figure 3. Effect of molecular weight, end group and branching structure of linear molecular chains on the performance of LLSs.
Figure 4.High-performance LLSs obtained by a circular molecular chain design.
Figure 5. High-performance LLSs obtained through cross-linked network structure design.
Figure 6. High-performance LLSs obtained through hybrid structure design. Grafting LLSs with different molecular structures has its own advantages and limitations (Figure 7). The optimized LLSs with grafted linear molecular chains have ultra-low contact angle hysteresis (CAH) and roll angle (SA), but their durability still needs to be improved to meet the needs of various harsh application scenarios. Cross-linking and hybrid structures are effective ways to improve the mechanical wear resistance and durability of LLSs. In general, as durability increases, so does the CAH and SA of LLSs. Therefore, the trade-off between dehumidification and durability of LLSs must be overcome. LLSs with excellent dewetting (e.g., CAH < 5°) and high durability under a wide range of harsh conditions are ideal for practical applications, which can be achieved by designing innovative molecular structures.
Figure 7. Comparison of dewetting and durability of grafted LLSs with different molecular structures. In order to comprehensively study the durability of LLSs, researchers have selected different test methods to evaluate their performance in various harsh environments (Table 1). However, existing studies lack standard testing methods to assess the durability of LLS, making cross-comparison between studies difficult. Table 1 lists some of the standard test methods applicable to evaluating the durability of LLSs for researchers' reference. Table 1. Test methods for evaluating the durability of different types of LLSSs.
Figure 8. Outlook for the future development trend of LLSs. Cheng Xiaopeng, assistant researcher of Binzhou Weiqiao Institute of Advanced Technology of the Chinese Academy of Sciences, and Zhao Ran, a doctoral student from the Institute of Physics and Chemistry of the Chinese Academy of Sciences, are the co-first authors of the paper, and the corresponding authors are researcher Wang Shutao and researcher Meng Jingxin of the Institute of Physics and Chemistry of the Chinese Academy of Sciences. This paper was supported by the Strategic Pilot Project of the Chinese Academy of Sciences (XDB 0470201), the Beijing Natural Science Foundation (JQ23008), the Start-up Fund of Binzhou Weiqiao Institute of Advanced Technology of the Chinese Academy of Sciences, and the National Natural Science Foundation of China (22275203).
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Source: Frontiers of Polymer Science
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