Self-healing Superhydrophobic Materials Showing Quick Damage Recovery and Longterm Durability Liming Wang, Chihiro Urata, Tomoya Sato, Matt W. England, Atsushi Hozumi* National Institute of Advanced Industrial Science and Technology (AIST), 2266-98, Anagahora, Shimoshidami, Moriyama, Nagoya 463-8560, Japan *Email: a.hozumi@aist.go.jp
Figure S1. Changes in water advancing CA (θ A, ), receding CA (θ R, ), and CAH ( θ, ) of the samples as a function of ratios of SO to PDMS. The ratio of TCPS to PDMS precursors was fixed at 20 vol% (A) and 30 vol% (B), respectively. Figure S2. SEM images of Samples C (images A and B) and D (images D and E) prepared from mixtures of TCPS, SO, and PDMS at ratios of 50:150:100 and 100:200:100 vol%, respectively. Water droplet profiles in advancing and receding modes on Samples C (image C) and D (image F), respectively. Water droplets are around 4-6 µl.
Figure S3. (A) PDMS-SO/TCPS (the ratio of SO/PDMS was 200 vol%) with a hollow structure prepared with a knife. No obvious SO aggregation was observed in this region. (B) PDMS-SO (left) and PDMS-SO/TCPS (right) (both the ratio of SO/PDMS was 70 vol%) placed on the tissue papers. (C) After leaving these samples for 3 days, leaching of SO was only observed for the PDMS-SO (weight loss to the infused SO was about 0.8 %). Scale bars are 1.0 cm. Although the amount of SO was twice as much as that PDMS by volume, SO did not leach out into a hollow area on the sample. This demonstrates that SO was firmly stabilized within the PDMS network due to high affinity between the SO and the PDMS (Figure S3A). Silicone micro/nanograss was only formed on the topmost region of the sample, due to the hydrolysis and polycondensation reactions of the gradually leached out TCPS with moisture in air. After leaving PDMS-SO and PDMS-SO/TCPS on soft tissue papers for 3 days, no marked weight loss of SO was observed for the PDMS-SO/TCPS with silicone micro/nanograss on the surface (Figure S3B, C). This clearly suggests that the micro/nanograss-structures could effectively prevent SO migration/extraction through the sample surface, which must significantly contribute to long-term stable preservation of SO.
Figure S4. SEM images of damaged flat PDMS after plasma irradiation. (A) PDMS-SO (the ratio of SO/PDMS was 70 vol%) after plasma irradiation for 30 minutes. (B) PDMS-SO (the ratio of SO/PDMS was 150 vol%) after plasma irradiation for 30 minutes. (C) Same sample of image (A) after plasma irradiation for 12 hours. Compared with PDMS-SO/TCPS with silicone micro/nanograss, a number of cracks were observed on the flat PDMS-SO (the SO/PDMS ratio was 70 vol%) after 30 minutes of plasma irradiation. These cracks were much larger (15 µm in width) than those on the PDMS-SO/TCPS after 24 hours of plasma irradiation (Figure S4A). The cracking on the PDMS surfaces triggered by plasma has been frequently observed due to the formation of fully oxidized SiO x (x=3 or 4) layers having high tensile stress on the surfaces. [1] With increasing the amount of SO in the PDMS matrices (the ratio of SO/PDMS was increased to 150 vol%), the cracks with widths up to tens of µm appeared, while the number of cracks and their depth decreased. Moreover, formation of wrinkles was clearly observed on the surface after 30 minutes of plasma irradiation (Figure S4B). These results clearly showed that the infused SO played an important role in the final surface structures of our samples after plasma irradiation. Extending plasma irradiation to 12 hours led to the formation of many cracks on the PDMS-SO sample (the SO/PDMS ratio was 70 vol%) (Figure S4C). Figure S5. SEM images of (A, B) insular and (C) microvoid areas on Sample A after plasma irradiation for 12 hours. Dense cracks were observed on the insular areas. Large cracks with widths up to ~10 µm were distributed through island-like areas on Sample A (TCPS:SO:PDMS of 20:70:100 vol%) after 12 hours of plasma exposure, while such cracks were hardly observed on the surrounding
areas (Figure S5A). These cracks on the island-like areas were formed during extended plasma irradiation instead of during TCPS reactions on the surface, because there was no silicone micro/nanograss grown inside these cracks (Figure S5B). By contrast, silicone micro/nanograss was observed inside the microvoids, which were formed during the TCPS reactions (Figure S5C). The island-like areas are barely laterally connected to the surrounding areas, thus continuous migration/supply of SO to the island-like areas is possibly diminished. This could lead to highly crosslinked oxidized topmost layers on these areas, which results in high surface moduli and consequently heavily cracked areas. Figure S6. (A, B) SEM images of Sample A after 5 cycles of repeated damage-healing processes (total plasma irradiation for 24 hours). Surface cracks were small with widths generally less than 1 µm in width, and no obvious differences of surface cracks have been observed between the samples damaged by continuous and repeated plasma irradiation for 24 hours.
Figure S7. (A) Schematic illustration of silicone micro/nanograss bent by prolonged plasma irradiation. (C, D) SEM images of the region marked in image (B) of Sample A after 24 hours of plasma irradiation. Silicone micro/nanograss on the whitened areas in Figure S7B seemed to be bent after prolonged plasma irradiation. The sample slightly whitened after 12 hours of plasma irradiation and became more obvious after 24 hours (Figure S7). SEM images showed that the original silicone micro/nanograss with upright postures were massively overwhelmed to the surface, probably due to the changes in surface moduli of the micro/nanograss. There was no obvious structural fracture on the micro/nanograss, indicating that the silicone micro/nanograss may have somewhat structural flexibility
Figure S8. Changes in water advancing CA (θ A, ), receding CA (θ R, ), and CAH ( θ, ) on Sample B (TCPS:SO:PDMS of 30:120:100 vol%) after plasma exposure for a) 30 minutes, b) 5 hours, c) 12 hours, and d) 24 hours as a function of aging time under ambient conditions. In spite of Sample B having large amount of SO in the PDMS matrices, there was no marked difference in the recovery kinetics after plasma exposure between Samples A and B, except for a 30 minutes plasma exposure. Superhydrophobicity of Sample B could be regenerated within 40 minutes after damaged by plasma exposure for 30 minutes. In contrast, Sample A was needed for 60 minutes to recover. Similar recovery kinetics was observed for both Samples A and B after longer plasma exposure. This is mostly likely result from the retardation effect of the thick oxidized silica layer formed on the surface by extended exposure on the SO migration to the outmost surface.
Figure S9. SEM images of Sample A after submersion in NaOH solution (ph = 14) for 3 hours. Silicone micro/nanograss on the surface were completely dissolved in the solution, leaving a bare bumpy surface (image (B) is the magnified one of image (A)). Figure S10. Scratch test demonstrated on Sample A. (A) A 200 g weight was placed on the top of the sample (~1 cm 2 ). A piece of 1500-mesh sandpaper was pulled out from the bottom. (B-D) SEM images of the damaged sample: (B) the top and (C, D) cross-sectional views. Although silicone micro/nanograss was partially removed by physical ablation, a heterogeneous porous layer still existed underneath the micro/nanograss. Such porous structures are considered to compensate for the loss of
roughness after physical scratches, and accelerate recovery kinetics of the damaged areas. Thus, surface wettability and self-healing properties of the damaged sample remained almost unchanged. Figure S11. Photographs of Samples 1, 2, 3, and 4 (corresponding to Samples A, B, C, and D, respectively in this paper) after outdoor/anti-weathering tests for over one year. The photos were taken after overnight heavy rainfall. In Nagoya (Japan), the average number of rainy days per year is about 114 days (~1600 mm precipitation) and total sunshine hours are ~2010 hours. [2] Even after over year of outdoor exposure (anti-weathering) testing (from 13 th June in 2016 to 22 nd June in 2017) demonstrated on a building roof (no shield on top or around the samples), excellent superhydrophobicity of Samples A (Figure S11B) and B remained unchanged, while Samples C and D (Figure S11C) slightly lost their water repellency. Hardness of Samples A and B with less amount of SO in the PDMS matrices are expected to be higher than those of Samples C and D, resulting in the better performance against rainfall. All samples could survive for prolonged anti-weathering testing (e.g., damages from prolonged ultraviolet (UV) irradiation and rainfall), as supported by the results from plasma exposure as well as 172 nm vacuum UV
(VUV) light exposure. We also noted that after overnight heavy rainfall, surface superhydrophobicity decreased to some extent, as sown in Figure S11. This has not been observed on samples submerged in water for overnight, indicating that the decrease in superhydrophobicity was probably due to the damage by kinetic energy of rainfall (speed of ~9 m s -1 ). Nevertheless, superhydrophobicity could be always recovered. Thus, we can conclude that our samples have extraordinary long-term anti-weathering/self-healing abilities, comparable to or even better than natural plants. Figure S12. (A, B) SEM images of the SO-free PDMS with uniform micropost arrays after plasma irradiation for 12 hours. (C) UV-Vis spectra of a glass slide with and without PDMS-SO (the ratio of SO/PDMS was 70 vol%) with micropost arrays, and identical sample of images (A) and (B). Besides large cracks formed on the SO-free with uniform micropost arrays (Figure S12A), numerous small cracks (tens of µm in length and less than 2 µm in width) were also observed on the surface (Figure S12B). Both PDMS having micropost arrays with and without addition of SO showed good transparency (transmittance in the visible wavelength range is about 80 %, Figure S12C). The characters behind the PDMS-SO with uniform micropost arrays were clearly visible (inset image in Figure S12C). References [1] a) Hillborg, H.; Ankner, J. F.; Gedde, U. W.; Smith, G. D.; Yasuda, H. K.; Wikström, K. Crosslinked Polydimethylsiloxane Exposed to Oxygen Plasma Studied by Neutron Reflectometry and Other Surface Specific Techniques. Polymer 2000, 41, 6851-6863. b) Nguyen, L.; Hang, M.; Wang, W. X.; Tian, Y.; Wang, L.; McCarthy, T. J.; Chen, W. Simple and Improved Approaches to Long-Lasting, Hydrophilic
Silicones Derived from Commercially Available Precursors. ACS Appl. Mater. Interfaces 2014, 6, 22876-22883. [2] World Weather and Climate Information; https://weather-and-climate.com/average-monthly-rainfall- Temperature-Sunshine-fahrenheit,Nagoya-jp,Japan (accessed Jun 23 rd, 2017).