Smart Superhydrophobic Surfaces with Reversible Thermochromism for On-Demand Photothermal Anti-Icing
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThis paper reports the preparation of anti-icing coatings. PDMS-based coatings were fabricated by using laser ablation. Furthermore, thermochromic capsules were incorporated into these coatings to benefit from the photothermal effect while preventing the overheating problem. The idea is good. An elegant approach was developed. The following minor issues need to be considered:
1) What is the thickness of the coatings?
2)The abrasion test should be clearly explained in the Materials and Methods section.
3) If available, more data on the capsules should be supplied (origin, particle size, etc.).
Comments on the Quality of English LanguageThe level of English needs to be increased. Please check all the chemical formulas (-CH3).
Author Response
Thank you for your positive and helpful recommendations about our work. We have carefully revised the Manuscript based on the suggestions. We will be happy to further revise it if there are still questions or suggestions. For the comments as you mentioned, we have made replies one by one as follows:
1) What is the thickness of the coatings?
Response: We sincerely thank the reviewer for pointing out this missing information. To address this, we utilized SEM to measure the cross-sectional thickness of the S-PDMS/TC coating and added the corresponding data and image (Figure 2e) to the manuscript. Specifically, the detailed modifications in the revised manuscript (Section 3.2) are as follows:
"To characterize the thickness of the S-PDMS/TC coating, we used SEM to examine the cross-section of the structure, and the thickness was found to be 264 μm (Figure 2e). "
2) The abrasion test should be clearly explained in the Materials and Methods section.
Response:
We appreciate this highly constructive suggestion. We have comprehensively rewritten the protocol for the sandpaper abrasion test to clearly specify the experimental conditions, including the sandpaper grit, applied load, and sample orientation. Specifically, the detailed modifications in the revised manuscript (Section 2.5) are as follows:
"Mechanical Durability (Sandpaper Abrasion Test): To evaluate the mechanical robustness of the coating, a standard sandpaper abrasion test was conducted. The prepared S-PDMS/TC sample was placed face-down on a piece of 1000-grit sandpaper. A 100 g weight was applied to the back of the sample to ensure uniform contact pressure. The sample was then dragged longitudinally across the sandpaper for a distance of 20 cm, which was defined as one abrasion cycle."
3) If available, more data on the capsules should be supplied (origin, particle size, etc.).
Response:
We thank the reviewer for this valuable advice. We have confirmed the specifications with the supplier and added the physical parameters (average particle size of 3–5 μm), the origin, and the ternary chemical composition of the thermochromic capsules to the manuscript. Specifically, the detailed modifications in the revised manuscript (Section 2.1) are as follows:
"Thermochromic microcapsules (TC) with a nominal color transition temperature of ~5 °C and an average particle size of 3–5 μm were purchased from Wuhan Lanabai Pharmaceutical Chemical Co., Ltd. (Wuhan, China). These commercial microcapsules consist of a protective polymer shell encapsulating a classic ternary thermochromic core: a fluoran-based leuco dye (2-Anilino-6-dibutylamino-3-methylfluoran, ODB-2) as the color former, Bisphenol A (BPA) as the proton-donating color developer, and a specific aliphatic co-solvent tailored to dictate the ~5 °C phase transition."
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThe study proposes the development of a smart superhydrophobic polydimethylsiloxane (PDMS) with a thermochromic encapsulated surface (S-PDMS/TC). The PDMS/TC composite and S-PDMS/TC were characterized. The S-PDMS/TC surface utilizes a mechanism for increased wear resistance and spontaneous dewetting behavior during ice melting, as a strategy for frost protection, balancing defrosting efficiency with thermal management.
The smart superhydrophobic surface was developed and characterized for potential application in anti-freeze photothermal systems. The manuscript can be considered for publication in the journal Coatings. The final version should incorporate the modifications suggested below:
- Abbreviations need to be defined the first time they are used (see, for example, PDMS).
- The performance of smart superhydrophobic surfaces developed (such as icing delay time, surface color, and ice adhesion strength) could be compared with other nature-inspired superhydrophobic surfaces.
Author Response
Thank you for your positive and helpful recommendations about our work. We have carefully revised the Manuscript based on the suggestions. We will be happy to further revise it if there are still questions or suggestions. For the comments as you mentioned, we have made replies one by one as follows:
1) Abbreviations need to be defined the first time they are used (see, for example, PDMS).
Response:
We thank the reviewer for pointing out this oversight. We have carefully reviewed the entire manuscript and ensured that all abbreviations, including PDMS, TC, and CA, are explicitly defined upon their first appearance in both the Abstract and the Introduction. Specifically, the detailed modifications in the revised manuscript (Abstract & Section 1) are as follows:
In Abstract: "To address this limitation, we developed a smart superhydrophobic polydimethylsiloxane (PDMS) surface embedded with thermochromic capsules (TC) (S-PDMS/TC) featuring reversible thermochromic capability via a facile combination of spin-coating and femtosecond laser ablation."
In Section 1: "Herein, we propose a smart superhydrophobic polydimethylsiloxane (PDMS) surface embedded with thermochromic capsules (TC) (S-PDMS/TC) featuring adaptive thermochromic capability, fabricated via a facile combination of spin-coating and femtosecond laser ablation. By embedding TC into a PDMS matrix and constructing a hierarchical micro-grid structure, the resulting surface achieves both excellent superhydrophobicity and mechanical durability. "
2) The performance of smart superhydrophobic surfaces developed (such as icing delay time, surface color, and ice adhesion strength) could be compared with other nature-inspired superhydrophobic surfaces.
Response:
We are grateful for this insightful suggestion. We have introduced a conceptual classification map (Figure 3j) and added a comprehensive comparative discussion benchmarking our work against recently reported passive icephobic surfaces and static photothermal coatings. Specifically, the detailed modifications in the revised manuscript (Section 3.3) are as follows:
"To comprehensively evaluate the rational design of our smart surface, its performance was benchmarked against recently reported state-of-the-art anti-icing coatings in a conceptual classification map (Figure 3j)... Uniquely, our S-PDMS/TC coating occupies the exclusive optimal quadrant: Smart Photothermal coatings, simultaneously achieving highly competitive icephobicity well below the efficient icephobic threshold while pioneering smart, adaptive thermal management to resolve this critical summer overheating design bottleneck."
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for AuthorsIn this work, the authors report a superhydrophobic surface (S-PDMS/TC) featuring reversible thermochromic capability via a facile combination of spin-coating and femtosecond laser ablation. The resulting hierarchical micro-grid structure acts as a sacrificial layer, shielding fragile nanostructures against mechanical abrasion, while endowing the surface with robust superhydrophobicity (CA > 155°). S-PDMS/TC exhibits an adaptive color transition from pale yellow to deep black when the temperature drops below 5°C. This response enables on-demand photothermal enhancement, boosting solar absorption in freezing environments while minimizing heat absorption at room temperature. Consequently, S-PDMS/TC demonstrates anti-icing performance, extending the freezing time to 310 s and reducing ice adhesion strength to 40.4 kPa. Notably, during photothermal de-icing, the meltwater exhibits spontaneous dewetting behavior driven by the replenishment of the air cushion, effectively preventing secondary icing.
This work is an incremental advancement in terms of materials design with no new chemistry or mechanism discussed. The authors should provide mechanistic explanantions to support the results. The photothermal performance should be compared to existing literature to back the claim of superior performance. Icing-anti-icing cyles are missing. Physical and chemical durability is not well explored. The work lacks appropriate control experiments. Contact angle hysteresis should be provided for superhydrophobic surfaces over roll off angles. Overall, the quality of work is poor with lack of in-depth experimentation and explanations.
Author Response
Thank you for your positive and helpful recommendations about our work. We have carefully revised the Manuscript based on the suggestions. We will be happy to further revise it if there are still questions or suggestions. For the comments as you mentioned, we have made replies one by one as follows:
1) This work is an incremental advancement in terms of materials design with no new chemistry or mechanism discussed. The authors should provide mechanistic explanantions to support the results.
Response:
We deeply appreciate the reviewer's rigorous evaluation. To significantly strengthen the academic depth of our study, we have provided comprehensive mechanistic explanations from both physical and chemical perspectives to support our results. First, we elaborated on the physical anti-icing mechanism related to the dynamic heat transfer and the "air cushion" effect (Figure 3b). Second, we provided an in-depth chemical mechanistic explanation of the reversible thermochromism and its underlying structural evolution. Specifically, the detailed modifications in the revised manuscript (Sections 3.3 and 3.4) are as follows:
In Section 3.3 (Physical Mechanism): "Figure 3b illustrates the mechanism underlying this delayed freezing. At room temperature, the droplet maintains a stable Cassie-Baxter state on the S-PDMS/TC surface, supported by the hierarchical micro/nano-structures [24,25]. This configuration traps distinct air pockets at the solid-liquid interface. These air pockets serve as effective thermal insulators, significantly hindering heat transfer from the cold substrate to the droplet, thereby retarding nucleation and freezing [26]. As the temperature decreases, although partial penetration of the droplet into the micro-texture may occur, the trapped air tends to migrate upward and accumulate at the droplet's apex, continuing to provide thermal resistance until the droplet is fully frozen. "
In Section 3.4 (Chemical Mechanism):"Fundamentally, this macroscopic reversible transition originates from the syner-gistic chemical interactions within the ternary core of the embedded TC microcapsules, which comprises a fluoran-based leuco dye (ODB-2) as the color former, Bisphenol A (BPA) as the proton-donating developer, and a phase-change co-solvent[48]. At ambi-ent temperatures (> 5 °C), the molten liquid co-solvent spatially isolates ODB-2 from BPA, maintaining the lactone ring of ODB-2 in a closed, sp3-hybridized state that lacks an extended π-conjugated system, thereby rendering the microcapsules pale yellow to minimize undesired solar absorption. However, upon exposure to freezing conditions (< 5 °C), the rapid crystallization of the co-solvent induces localized phase separation, forcing the BPA molecules into highly aggregated proximity with ODB-2. Driven by this thermodynamic solidification, the phenolic hydroxyl groups of BPA aggressively donate protons to cleave the lactone ring, transforming the central carbon of ODB-2 into an sp2-hybridized state[49]. This critical structural evolution establishes a massive, highly delocalized π-conjugated framework across the molecule, rapidly shifting the microcapsules to a deep black state that enables strong broadband solar absorption and subsequent non-radiative relaxation for highly efficient, on-demand photothermal anti-icing. "
2) The photothermal performance should be compared to existing literature to back the claim of superior performance. Icing-anti-icing cyles are missing.
Response: We thank the reviewer for these important points. We have added the performance comparison in Figure 3j and conducted new cyclic icing/deicing tests over 10 consecutive cycles, presenting the data in Figure 3i. Specifically, the detailed modifications regarding the cycles in the revised manuscript (Section 3.3) are as follows
"To evaluate the long-term reliability of the anti-icing performance, cyclic icing/deicing tests were conducted on the S-PDMS/TC coating, as depicted in Figure 3i. Throughout 10 consecutive cycles, the ice adhesion strength remained remarkably stable at 40.4 ± 5 kPa, exhibiting no obvious signs of deterioration. Simultaneously, the icing delay time consistently maintained a high level of 310 ± 20 s without any observable decreasing trend."
"To comprehensively evaluate the rational design of our smart surface, its perfor-mance was benchmarked against recently reported state-of-the-art anti-icing coatings in a conceptual classification map (Figure 3j). While achieving icephobic performance (initial ice adhesion strength < 100 kPa) is the universal objective for all comparative coatings, Figure 3j elucidates a critical limitation in current strategies. As shown in the left and middle regions, conventional passive icephobic surfaces [35-40] rely solely on chemical/structural modifications, leading to performance stagnation with many points above the 100 kPa threshold. Static photothermal surfaces [41-46] effectively lower ice adhesion via heat generation but remain locked into a static thermal mode, and the unmanaged excessive heating risks are inherent in these static designs under high-irradiance, non-freezing conditions. Uniquely, our S-PDMS/TC coating occupies the exclusive optimal quadrant: Smart Photothermal coatings, simultaneously achieving highly competitive icephobicity well below the efficient icephobic threshold while pioneering smart, adaptive thermal management to resolve this critical summer over-heating design bottleneck[47]. "
3) Physical and chemical durability is not well explored. The work lacks appropriate control experiments.
Response: We thank the reviewer for this comment. Regarding control experiments, we clarify that pristine aluminum (PAl) and non-lasered PDMS/TC coatings were systematically used as control groups throughout the study (Figures 2h, 3a, 3c, 4c). Regarding durability, we have now conducted extensive chemical immersion tests in acidic, alkaline, and salt solutions, and added the degradation mechanisms.Specifically, the detailed modifications regarding durability in the revised manuscript (Sections 2.5 & 3.3) are as follows:
In Section 2.5: "To assess the chemical stability of the S-PDMS/TC coating in harsh environments, the prepared samples were separately immersed in three corrosive aqueous solutions: an acidic solution (HCl, pH = 3), an alkaline solution (NaOH, pH = 11), and a salt solution (3.2 wt% NaCl). During the continuous immersion process, the samples were periodically extracted at 2-hour intervals, thoroughly rinsed with deionized water, and dried. Subsequently, the CA and ice adhesion strength were measured to monitor the dynamic degradation of its icephobic and hydrophobic properties over time. "
In Section 3.3: "To evaluate the environmental durability of the S-PDMS/TC coating in harsh outdoor scenarios, its chemical stability was systematically assessed through acid, al-kali, and salt resistance tests, as illustrated in Figure 3f–h. When immersed in an acidic solution (HCl, pH = 3), the CA and ice adhesion strength of the coating exhibited neg-ligible fluctuations regardless of the immersion time. This stability is attributed to the robust cross-linked PDMS network, which demonstrates excellent resistance to weak acids; protons are thermodynamically incapable of cleaving the stable Si–O–Si bonds at room temperature [30]. Furthermore, the superhydrophobic "air cushion" serves as a reliable physical barrier, effectively preventing the acidic solution from penetrating and corroding the vulnerable regions at the base of the micro-grid [31]. A similar ro-bust performance was observed during the salt resistance test (3.5 wt% NaCl). Both the CA and ice adhesion strength remained virtually unaffected, primarily because sodi-um and chloride ions lack the chemical capability to disrupt the stable siloxane back-bone [32].
In contrast, during the extreme alkali resistance test (Figure 3h, NaOH, pH = 11) the CA experienced a slight decrease from 155.2° to 152.3° after 24 h of immersion, accompanied by an increase in ice adhesion strength to 64.0 kPa. This localized degra-dation occurs because hydroxide ions can nucleophilically attack the Si–O–Si backbone of the PDMS matrix, leading to the cleavage of siloxane bonds and a partial reduction in intrinsic hydrophobicity [33]. Consequently, water droplets can partially penetrate the protective air cushion, seeping into the bottom of the micro-grid. Upon freezing, the penetrating ice forms a mechanical interlock with the micro-grid structures, re-sulting in an elevated ice adhesion strength [34]. "
4) Contact angle hysteresis should be provided for superhydrophobic surfaces over roll off angles.
Response:
We completely agree with the reviewer's professional assessment. Following your valuable suggestion, we have entirely re-measured the contact angle hysteresis (CAH) for all relevant samples, updated the corresponding figures (Figure 2a, Figure 2b, and Figure 3d), and systematically replaced all descriptions of "roll-off angle/sliding angle" with "contact angle hysteresis (CAH)" throughout the manuscript. Specifically, one of the representative modifications in the revised manuscript (Section 2.5) is as follows:
"After each predefined cycle, the contact angle (CA), contact angle hysteresis (CAH), and ice adhesion strength of the abraded surface were systematically measured to assess performance retention."
Author Response File: Author Response.pdf
Round 2
Reviewer 3 Report
Comments and Suggestions for AuthorsThe novelty of the work is still missing. But the authors have addressed all the other concerns raised. The manuscript may be considered for publication in the present form.
