Preparation of Transparent Sandwich-like Superhydrophobic Coating on Glass with High Stability and Self-Cleaning Properties
Round 1
Reviewer 1 Report
Qiang Li and co-workers investigate in their manuscript "Preparation of transparent sandwich-like ... " a new superhydrophobic coating on glass with possible applications in imaging for coal mining.
It's not clear to me what exactly is new with these coatings. The authors should maybe add a few sentences about that in the introduction. However, the emphasize of the paper lies on optimization and testing of the coating for coal mining. This could also be transferred to other possible applications. Therefore, the topic will for sure be of interest for the readers of "Coatings". The characterization of the layers seems scientifically sound, even so some of the characterization methods such as the imaging done for Figure 4 should be described in more detail. The authors should also add some more lines on Wenzel and Cassie states and why the authors think that they describe their system.
Therefore, and because of the shortcomings (see the list below), I recommend that the paper might be published after minor revision.
Some more points:
- Lines 91 - 97: delete.
- Line 101: It is unclear what the authors mean with "KAT-245 et al."?
- Lines 151 - 153: delete.
- Line 163 (and several other occasions): it is "Wenzel state", not "wenzel state".
- Figure 3: The figure caption is missing
- Figure 4 caption: "primer amount", not "primer amoun".
- Figure 4: It is not clear what kind of images the third and fourth column are. Confocal optical microscope images in column 3? SEM in column 4?
- Line 196 (and several other occasions): It's "Cassie", not "cassize" or "cassie".
- Figure 8: A water droplet is emphasized in (c), but it is not discussed in the text.
- Formula (1): g is not defined.
- Line 310: It is stated that in Fig. 9(b) "the CAs and SAs evolution" is presented. However, only values for CA are shown.
- Line 325: The authors should make clear what they mean with "single adhesive layer".
Author Response
Reply to Reviewer’s Suggestions
Dear Reviewer:
We appreciate your comments concerning our manuscript entitled “Preparation of transparent sandwich-like superhydrophobic coating on glass with high stability and self-cleaning properties” (Manuscript ID: coatings-1570609). Those comments are all valuable and very helpful for revising and improving our manuscript. We have studied the comments carefully and made correction which we hope to meet with approvals. All revised portions are marked in red in the revised manuscript. The main corrections in the revised manuscript and the responses to the reviewer’s comments are as followed:
The Answers to all the queries.
Q1. Lines 91 - 97: delete.
Response: According to the reviewer’s suggestion, we have deleted Lines 91 – 97.
Q2. Line 101: It is unclear what the authors mean with "KAT-245 et al."?
Response: Thank you for your patience and comments. KAT-245 is one of the silane coupling agent. We have supplemented the introduction in the revised manuscript and marked in red.
Q3. Lines 151 - 153: delete.
Response: According to the reviewer’s suggestion, we have deleted Lines 151 – 153.
Q4. Line 163 (and several other occasions): it is "Wenzel state", not "wenzel state".
Response: We are sorry for our careless. All statements of "wenzel state" have been changed to "Wenzel state".
Q5. Figure 3: The figure caption is missing.
Response: We are sorry for our careless. The figure caption “Superhydrophobicity of the coated glass toward different aqueous solutions.” has been supplemented and marked in red.
Q6. Figure 4 caption: "primer amount", not "primer amoun".
Response: We are sorry for our careless. The mistake has been corrected in the revised manuscript and marked in red.
Q7. Figure 4: It is not clear what kind of images the third and fourth column are. Confocal optical microscope images in column 3? SEM in column 4?
Response: Thanks a lot for the reviewer’s meaningful comments. In order to clarify these images, we have revised the manuscript as followed and marked in red, “The coating peak height distribution(Column 1 and 2, PS50 surface profilometer), 3D profile (Column 3, LEXT OLS 4000 laser confocal microscope) and surface morphology (Column 4, LEXT OLS 4000 laser confocal microscope) with different primer content were presented in Fig. 4.”
Q8. Line 196 (and several other occasions): It's "Cassie", not "Cassize" or "cassie".
Response: We are sorry for our careless, and all statements of "Cassize" or "cassie" have been changed to "Cassie" in the revised manuscript.
Q9. Figure 8: A water droplet is emphasized in (c), but it is not discussed in the text.
Response: Thanks a lot for the reviewer’s careful reminding. In order to explain the pinning effect of the water droplet, we have revised the manuscript as followed and marked in red, “In contrast, as illustrated in Fig. 8(c), the SA increased sharply to above 10° after 100 minutes and the water droplet pinning effect occured, indicating that the air pockets entrapped in the micro/nano structure body was fractured. this phenomenon also demonstarted that the damage of acid corrosion to superhydrophobic surface seemed much severer than that of high temperature.”
Q10. Formula (1): g is not defined.
Response: According to the reviewer’s suggestion, g has been defined as the gravitational acceleration 9.8 m/s2 in the revised manuscript and marked in red.
Q11. Line 310: It is stated that in Fig. 9(b) "the CAs and SAs evolution" is presented. However, only values for CA are shown.
Response: We are sorry for our careless. As shown in Fig. 9(b), the SAs have exceeded 10° only after 0.5 m abrasion distance, which illustrated our coating could hardly remain the Cassie stated even under the protection of UV layer if abraded by sandpaper. Therefore, we neglected the discussion of the SAs evolution in the manuscript and tried our best to resolve this problem. Now, we have revised the expression in the revised manuscript and marked in red.
Q12. Line 325: The authors should make clear what they mean with "single adhesive layer".
Response: According to the reviewer’s suggestion, we have revised the expression of the revised manuscript as followed and marked in red, “Fig. 9(b) presented the CAs evolution with the increase of the sandpaper abrasion distance and compared the morphology of the sandwich-like structure and single adhesive layer.”
Author Response File: 
Author Response.pdf
Reviewer 2 Report
This article is interested in Preparation of transparent sandwich-like superhydrophobic coating on glass with high stability and self-cleaning properties. There are a lot of previous studies that investigate materials with self-cleaning properties. However, the fact that the current material has not reported previously, increased the originality of this article. The techniques used in this study are classical and commonly used.
The aim and novelty of this work should be stated at the end of the introduction section.
Page 2, lines 91-97 should be deleted.
Section 2.1 lines 99-103: no need to the chemicals with capital letters.
Page 3, lines 111: Please cite references for Fabrication process of the superhydrophobic coating and protective layer.
Page 4, lines 129: Please cite references for pre-treatment.
Page 4, lines 136: Please cite references for Fabrication of UV gel protective layer.
Page 4, Figure 3: Please revise the Figure caption.
Please always be consistent, use Figure in caption and text.
A table should be added to compare the properties of the material prepared in this study and other materials reported in literature for the same purpose.
Comments for author File: Comments.pdf
Author Response
Reply to Reviewer’s Suggestions
Dear Reviewer:
We appreciate your comments concerning our manuscript entitled “Preparation of transparent sandwich-like superhydrophobic coating on glass with high stability and self-cleaning properties” (Manuscript ID: coatings-1570609). Those comments are all valuable and very helpful for revising and improving our manuscript. We have studied the comments carefully and made correction which we hope to meet with approvals. All revised portions are marked in red in the revised manuscript. The main corrections in the revised manuscript and the responses to the reviewer’s comments are as followed:
The Answers to all the queries.
Q1. The aim and novelty of this work should be stated at the end of the introduction section.
Response: According to the reviewer’s suggestion, the aim and novelty of this work have been stated at the end of the introduction as followed and marked in red, “The success of preparation of a facile and cost-effective green and robust transparent sandwich-like superhydrophobic coating and the application for unmanned coaling mining can expand the application field of superhydrophobic technology.”
Q2. Page 2, lines 91-97 should be deleted.
Response: According to the reviewer’s suggestion, we have deleted Lines 91-97.
Q3. Section 2.1 lines 99-103: no need to the chemicals with capital letters.
Response:According to the reviewer’s suggestion, the spelling form of the chemicals have been revised and marked in red.
Q4. Page 3, lines 111: Please cite references for Fabrication process of the
superhydrophobic coating and protective layer.
Response: Thank you for your patience and comments. Because the preparation process of the sandwich-like structured superhydrophobic coating is related to that in the literature[23], we have cited it in the revised manuscript.
Q5. Page 4, lines 129: Please cite references for pre-treatment.
Response: According to the reviewer’s suggestion, we have cited the paper [24] about pre-treatment.
Q6. lines 136: Please cite references for Fabrication of UV gel protective layer.
Response: According to the reviewer’s suggestion, we have cited the literature entitled ”Corrosion Behavior and Mechanical Properties of a Nanocomposite Superhydrophobic Coating” about the fabrication of UV gel protective layer.
Q7. Page 4, Figure 3: Please revise the Figure caption.
Response: We are sorry for our careless. The figure caption “Superhydrophobicity of the coated glass toward different aqueous solutions.” has been supplemented and marked in red.
Q8. Please always be consistent, use Figure in caption and text.
Response: We are sorry for our careless. We have carefully checked the manuscript and corrected our mistakes.
Q9. A table should be added to compare the properties of the material prepared in this study and other materials reported in literature for the same purpose.
Response: Thank you for your patience and comments. We have added a table relevant to the performance comparison of the superhydrophobic transparent coating prepared in this paper and other recently published literature, which are listed in the table below.
|
Materials |
Methods |
CAs (°) |
Transmittance (%) |
Stability |
|
Modified SiO2[31] |
Self-assembly process |
161.5 |
83.13 |
Excellent sand, water jet and corrosive media resistance. |
|
Fluorinated silica multi-walled carbon nanotubes[32] |
Spray-drying |
159 |
75 |
Low |
|
SiO2, PMMA[33] |
Sol-gel and Template-based method |
152 |
93 |
Low |
|
Silica nanowires[34] |
CVD |
158 |
89 |
Displayed antifogging properties. |
|
Silica nanoparticles, FAS[35] |
Alkaline etching |
150 |
79 |
Exhibited a good resistance to acidic and basic solutions over a wide range of pH values. |
|
Silicone oils, SiO2[36] |
LISS |
110 |
|
Highly hot water, pH liquids and scratch resistance. |
|
Silica glass[37] |
Femtosecond laser and modify |
150 |
92 |
Excellent sand and water jet resistance. |
|
SiO2 and COP[38] |
Dip-coating |
150 |
87 |
Excellent sand and water jet resistance. |
Author Response File: Author Response.pdf
Reviewer 3 Report
Brief Description of the Work
The aim of the work is to resolving the machine vision inferior visibility during the unmanned mining process. For this, the authors fabricated a superhydrophobic coating, prepared on the borosilicate glass, consisted of three layers - by adopting the layer-by-layer spraying ("sandwich-like") method:
- a primary layer, made by a body in micro/nano structure of neutral silicone sealant;
- an intermediate layer made by hydrophobic silica nanoparticles;
- a protective layer of ultraviolet (UV) gel.
Main Results Obtained
Tests on the fabricated superhydrophobic coating concerning the impact resistance, thermal stability, chemical stability and the abrasive wear characteristic have been performed, with the following findings:
1) due to the bonding effect of the silicone glass sealant and the protective effect of the UV gel protective, the prepared coating preserved stability and mechanical durability;
2) The coated glass showed much more efficient self-cleaning property with respect to the original glass;
3) The coated glass exhibited a super-hydrophobicity towards several aqueous solutions and had a high transparency to visible light of 80% with a primer mass content of 4% by weight.
General Considerations (GC)
GC1) The manuscript is clearly written and it was a pleasure to read it.
GC2) There are, however, two main gaps which, in my opinion, need to be filled:
- the manuscript is not sufficiently well framed within the works appeared in literature;
- potential difficulties in obtaining a transparent synthetic superhydrophobic (SH) coating are not clearly mentioned.
The questions/suggestions below aim to fill these gaps.
Questions/Suggestions (Q/S)
Q/S1) As mentioned, some well-known scientific and technical reasons which prevent the application of the SH surfaces also in unmanned mining process, have not sufficiently been mentioned in this work. For instance, as known, when a surface roughness is present on a substrate the actual area of the surface is larger than its plan area. In addition, when chemical heterogeneity is present on a flat surface, a contact angle hysteresis (CAH) value also exists. The main impact of these effects is the reduction of transparency due to the presence of surface roughness, and the droplet condensation in the pores of SH surfaces in the presence of water vapour. Since this work does not mention the role of the base contact angle, what interpretation do the authors give regarding their results? The authors are asked to discuss if it is possible to use the Wenzel and Cassie-Baxter equations by taking into account the issue of the so-called “cancellation of these equations” (ref., for instance, to L. Gao and T.J. McCarty, Langmuir, 23, 3762−3765 (2007)).
Q/S2) We may object that the method proposed by the authors is quite complex since it involves three different multistep approaches and severe conditions, and require the use of specialised instruments and chemicals. In addition, the proposed method seems to be applicable only to the production of very small SH areas or some specific materials. Indeed, when a specific patterned surface roughness is formed on a small, low-surface-energy material by the deep reactive ion etching technique, it is usually very difficult to apply it to large areas. The authors are asked to dispel these possible objections.
Q/S3) When light strikes a surface, it can be reflected or absorbed before transmission. Both the reflection and absorption of light are negative issues for the transparent surfaces. There are many drawbacks in the preparation methods of the transparent SH surfaces, which were reported in the literature. We may object that the authors' method involves a technique that is only suitable for laboratory scale studies with poor scale-up possibilities. The manuscript does not report convincing results in relation to the recoverability of surfaces against applied abrasion-induced wear. Thus, the long-term durability of transparent SH surfaces and their poor abrasion resistance may constitute a shortcoming of the authors' method. The authors are asked to comment this issue by dispelling this possible objection.
GC4) Water condensation on an SH coating may constitute another issue. When water vapour is condensed onto a cooled SH surface, the liquid water penetrates the microstructures to form sticky droplets on the substrate depending on the size scale of the roughness features, and the SH property is usually lost after the condensation of water. Authors are asked to discuss, very briefly, this issue in relation to their method.
GC5) The prevention of the accumulation of impurities and the limitations of the self-cleaning effect need to be discussed more in detail. The self-cleaning property tests of the SH surface are not described in a standardised way. More specifically, the actual range of particles that can be removed has not been specified in a systematic manner. Indeed, it is known that only receding contact angles dramatically decrease due to the presence of contamination, thus increasing the contact angle hysteresis which prevents the rolling behaviour of water droplets. Even in this case, the authors are asked to provide some comments about, in particular in relation to their method.
GC6) (It is not mandatory to answer this question)
In unmanned mining process, it is also important to investigate the anti-icing SH coatings. Indeed, the SH surfaces do not repel ice and water vapour and if an SH surface remains under a condensation condition (the solid temperature is below the dew point) then water vapour condensation will occur in the surface microstructures, imparting significant surface wetting and strong icing behaviour on this SH surface. In addition, if a droplet strikes an SH surface with sufficient kinetic energy, then the air pockets present on the surface can be easily removed and water can penetrate the rough structure, destroying the SH property. Have the authors investigated this aspect?
Conclusions
The theme dealt with in this work is highly topical; the number of publications in this field currently exceeds 13 500. As said the work is clearly written. However, some of the main issues that are currently under study have not been discussed or they are treated in a very fleeting way. In this sense, the work does not frame well within the works recently appeared in the literature. The authors are encouraged to make a further effort by taking into account the suggestions expressed above. In my opinion, this will not only avoid objections from the reader, but will also contribute to increase the solidity of the work significantly.
Author Response
Reply to Reviewer’s Suggestions
Dear Reviewer:
We appreciate your comments concerning our manuscript entitled “Preparation of transparent sandwich-like superhydrophobic coating on glass with high stability and self-cleaning properties” (Manuscript ID: coatings-1570609). Those comments are all valuable and very helpful for revising and improving our manuscript. We have studied the comments carefully and made correction which we hope to meet with approvals. All revised portions are marked in red in the revised manuscript. The main corrections in the revised manuscript and the responses to the reviewer’s comments are as followed:
The Answers to all the queries.
Q1. As mentioned, some well-known scientific and technical reasons which prevent the application of the SH surfaces also in unmanned mining process, have not sufficiently been mentioned in this work. For instance, as known, when a surface roughness is present on a substrate the actual area of the surface is larger than its plan area. In addition, when chemical heterogeneity is present on a flat surface, a contact angle hysteresis (CAH) value also exists. The main impact of these effects is the reduction of transparency due to the presence of surface roughness, and the droplet condensation in the pores of SH surfaces in the presence of water vapour. Since this work does not mention the role of the base contact angle, what interpretation do the authors give regarding their results? The authors are asked to discuss if it is possible to use the Wenzel and Cassie-Baxter equations by taking into account the issue of the so-called “cancellation of these equations” (ref., for instance, to L. Gao and T.J. McCarty, Langmuir, 23, 3762−3765 (2007)).
Response: Thanks a lot for the reviewer’s meaningful comments. We have carefully had a knowledge of the paper entitled “How Wenzel and Cassie Were Wrong” and discussed the effect of the original and coated glass surface roughness on the surface wettability. Meanwhile, we have also supplemented the relevant discussion as followed and marked in red, “Due to the low surface roughness of the original glass smooth surface, the water droplet would spread on it and exhibited a low CA below 30°. But when the primer mass content was 2wt%, the CA of the coated surface reached 149.6° and became highly hydrophobic with a typical Wenzel state due to the bonding effect between the primer and hydrophobic silica nanoparticles, which were beneficial for the improvement of surface wettability”.
Q2. We may object that the method proposed by the authors is quite complex since it involves three different multistep approaches and severe conditions, and require the use of specialised instruments and chemicals. In addition, the proposed method seems to be applicable only to the production of very small SH areas or some specific materials. Indeed, when a specific patterned surface roughness is formed on a small, low-surface-energy material by the deep reactive ion etching technique, it is usually very difficult to apply it to large areas. The authors are asked to dispel these possible objections.
Response: Thank you very much for the reviewer’s meaningful suggestion. Indeed, the aim of the layer-by-layer construction approach is to optimize the coating robustess and stability, and fully utilized the silicone sealant strong bonding effect and UV gel protective effect under the consideration of the transparency requirements and colloid mutual solubility with the dispersant. So the fabrication involved three different multistep approaches. Additionally, the hydrophobic silica nanoparticles, neutral silicone sealant and UV gel can be easily obtained and the spraying method is cost-effective and suitable for the automated large-scale production.
Q3. When light strikes a surface, it can be reflected or absorbed before transmission. Both the reflection and absorption of light are negative issues for the transparent surfaces. There are many drawbacks in the preparation methods of the transparent SH surfaces, which were reported in the literature. We may object that the authors' method involves a technique that is only suitable for laboratory scale studies with poor scale-up possibilities. The manuscript does not report convincing results in relation to the recoverability of surfaces against applied abrasion-induced wear. Thus, the long-term durability of transparent SH surfaces and their poor abrasion resistance may constitute a shortcoming of the authors' method. The authors are asked to comment this issue by dispelling this possible objection.
Response: Thank you for your patience and comments. During the process of conducting our work, we performed a series of durable and stable tests including the water droplet impact, hot water-boiling, stirring in acetic acid aqueous solution, and sandpaper abrasion. The results showed that the coating performed excellent properties to resist the mechanical impact, thermal effect and acid corrosion. In addition, we also designed and conducted the self-cleaning test of optical glass lens creatively under the dense-pulverized coal dust and water atomizing environment to verify the prepared superhydrophobic coating applicability. Under this circumstance, the floating pulverized coal dust can abrade the coating surface, and we repeated the experiment for 10 times, the optical glass lens still remained a clear visibility and self-cleaning ability. Therefore, the fabricated coating not only can be applied to the laboratory, but also has certain practical application value in unmanned coaling mining.
Q4. Water condensation on an SH coating may constitute another issue. When water vapour is condensed onto a cooled SH surface, the liquid water penetrates the microstructures to form sticky droplets on the substrate depending on the size scale of the roughness features, and the SH property is usually lost after the condensation of water. Authors are asked to discuss, very briefly, this issue in relation to their method.
Response: Thanks a lot for the reviewer’s meaningful comments. For the sandwich-like structure coating construction, we have found this issue and performed the condensation tests. However, due to the porous structures of the as-prepared coating, water vapour also nucleated at the surface and destroyed the superhydrophobic property. Therefore, we are taking our efforts to explore how to solve the problem, but we have not added the discussion of this part in this manuscript.
Q5. The prevention of the accumulation of impurities and the limitations of the self-cleaning effect need to be discussed more in detail. The self-cleaning property tests of the SH surface are not described in a standardised way. More specifically, the actual range of particles that can be removed has not been specified in a systematic manner. Indeed, it is known that only receding contact angles dramatically decrease due to the presence of contamination, thus increasing the contact angle hysteresis which prevents the rolling behaviour of water droplets. Even in this case, the authors are asked to provide some comments about, in particular in relation to their method.
Response: According to the reviewer’s suggestion, we noticed the deterioration effect of the pollutants cumulative on the SH coating performance due to the rough pore structure blockage. So, we repeated the self-cleaning tests for several times, and found that if the ultrasonic cleaning performed after each test, the SH coating can remained its excellent wetting and self-cleaning characteristics. And we have supplemented the expression of the revised manuscript as followed and marked in red, “Morevoer, we also repeated the experiments for 10 times, the optical glass lens still remained a clear visibility and self-cleaning ability if the ultrasonic cleaning can be performed after each test.”
Author Response File: Author Response.pdf
Round 2
Reviewer 3 Report
The authors answered point-to-point and satisfactorily to all the questions raised in my first report. In my opinion, this updated version is suitable to be published.
Author Response
Reply to Reviewer’s Suggestions
Thank you for your patience and the affirmation to our work.
