Fabrication of Zinc Substrate Encapsulated by Fluoropolyurethane and Its Drag-Reduction Enhancement by Chemical Etching

Round 1

Reviewer 1 Report

The manuscript reports an experimental study on the fabrication and characterization of zinc substrates modified by a combination of etching (with copper ion + nitric acid) and spray coating (with fluoro-polyurethane). The preparation of the coating chemical is also reported. The fabricated surfaces are characterized in terms of FTIR, profilometry, SEM, static wetting, stability of surface wetting after immersion in acidic, alkaline, and saline environments,  and micro-channel flow test with de-ionized water to measure pressure drop.

1. The main goal, as clarified in the title and introduction, is to make superhydrophobic surface on zinc capable of drag reduction for flow over the surface. However, the results show that the drag reduction of the hydrophobic and superhydrophobic surfaces is only marginal (around 3-4%), relative to the un-etched/un-coated zinc surface. There is no explanation of the uncertainty associated with the measurement of pressure drop, flow rate, and calculation of Reynolds number based on the measured variables.
2. Secondly, the superhydrophilic surface shows a much higher drag reduction compared to the hydrophobic and superhydrophobic surfaces. This is physically puzzling. The authors do not provide any reason for this observation. Moreover, this is not the focus of this study and, as the authors state, this is a topic of another study (line 258). But the discussion is mixed with (super-)hydrophobic and superhydrophilic surfaces. Hence I do not see how the purpose of the current study is achieved in this manuscript.
3. I do not see anything new in terms of the procedure for fabrication of the superhydrophobic surface. Preparation of a micro-nano-structure followed by a low surface energy coating of the surface is a commonly followed procedure reported in many studies (see the review article DOI:10.1016/j.cocis.2006.06.002).

The presentation of the study, in general, is very poor and is full of major and minor mistakes. There are many portions in the manuscript where the discussion is hard to follow due to lack of definition of terms or poor use of language or a combination of both. Some instances are listed below.

1. Reynolds number is not defined anywhere in the manuscript.
2. References are not displayed properly throughout the manuscript. 
3. Lines 19-20: What do the authors mean by 'surface reliability'?
4. What do the authors mean by 'non-standard design micro-channel'?
5. Lines 44-46: No references are cited to support the statements here.
6. Lines 109, 122: The abbreviations MF-APS and NCO have to be defined when it is first used.
7. Lines 117-120: From these lines, it is not clear which surfaces are the test surfaces and which ones are the 'control groups'.
8. Line 132: "..exposed to.." -- do the authors mean "...immersed in..."? Please clarify!
9. Line 144: ":..under the dimension of microchannel...and Reynolds number regions..." -- What does this mean?
10. The dimensions of the flow channel used in the test are not mentioned in the text anywhere.
11. Figure S2: The diameter of the inlet to the channel is marked as 10 m -- is this correct? What are the surfaces/walls of the micro-channel with which liquid gets in contact during the flow inside the micro-channel?
12. What is the critical Reynolds number for flow through a micro-channel at which the flow transitions from laminar to turbulent?  What is the range of Re in the test? How does this critical Reynolds number compare with the range of Re tested here? Include these information, citing appropriate literature, in Sec. 2.4.
13. Figure 2: Is the FTIR done on the fluoro-polyurethane before spraying it on the surface? If yes, how does this ensure that the composition is similar on the sprayed surface?
14. Line 154: The wave number 2332 cm^-1 is mentioned as 2232 cm^-1 in Fig. 2. Make appropriate corrections.
15. Lines 159-166: Figure 3 does not show the effect of etching time on roughness and morphology of the surface -- but this is discussed in the text here while referring to Fig. 3. Check and correct either the figure or the discussion here!
16. Lines 171-174: The caption does not reflect the panels in the figure. Panels g and h are missing in the caption. Perhaps, the description of panels c and d are wrong too -- they do not seem to represent a rough etched surface. Check and correct!
17. What is the difference between 'fluoro-polyurethane' and 'modified polyurethane'? These are used interchangeably, and it is confusing. Please be consistent in the definition and usage of terms and phrases throughout the manuscript.
18. Lines 177-184: There is no discussion of the surfaces sprayed with fluoro-polyurethane, shown in Fig. 4, in the text.
19. Line 179: The image corresponding to etching with 'simple dilute nitric acid' is not in Fig. 4. In addition, while discussing surfaces in lines 177-184, it would be better to refer to the particular figure panel so that the readers can follow the discussion.
20. Figure 5: Show the measurement of CA in these images.
21. Lines 203-204: These values of the surface energy are higher than the typical range of surface energy of perfluoroalkane mentioned in the introduction (lines 53-54). What is the reason behind this?
22. Lines 210-212: Here suddenly the discussion shifts to hydrophilicity and superhydrophilicity -- how is this relevant here? What is 'polyurea' coating? Make changes and clarify this discussion.
23. Line 215: Mention the range of mass or volume flow rate tested in this study. Which practical applications are aimed at with the range of flow rates tested in this study?
24. What is the uncertainty in the estimation of the volume flow rate as the slope of v vs t graph? Some details on this calculation procedure showing v vs t graphs should be provided in the manuscript, at least as Supplementary material.
25. Line 221: Briefly show the calculations in the manuscript, based on which the microchannel height was 'determined'.
26. Line 225: "...the pressure drops of pcfrfrefthe these..." -- correct the typo.
27. Figure 7: Why does the superhydrophilic etched surface show a lower pressure drop than the superhydrophobic etched and sprayed surface? 
28. Figure 7, Y-axis: Change 'Mpa' to 'MPa'.
29. Drag reduction, drag reduction rate, relative drag reduction rate -- these are used interchangeably. Please be consistent in the usage of terms.
30. How was such a high precision (~ 0.01 microns) in the adjustment of the micro-channel height achieved? What is the positioning accuracy of the screws used? What type of screws are these?
31. Lines 271-273: No information is given on the details of these measurements.

Keeping all these aspects in mind, I suggest that the manuscript cannot be accepted for publication in Coatings.

Author Response

Please find the attached word file for a detailed reply.

Author Response File: Author Response.pdf

Reviewer 2 Report

After carefully, reviewing the manuscript titled “Fabrication of Zinc Substrate Encapsulated by Fluoro-polyurethane and its Drag-reduction Enhancement by Chemical Etching” it is advised to be accepted for published as is. In recent times, there is a renewed interest in the advanced and efficient fabrication techniques in the field to nanoscale science and device engineering through nanostructure from chemical etching process. Hence, the proposed method is of overwhelming interest to the scientific community together from the industry and academia as to find ways to scaling and enhancing efficiency of various nanoscale devices and systems through various research and development advancement. Study like the ones presented by author in this work add value to the scientific community and could be helpful for the general reference for the future work. In general, the manuscript is sound with scientific quality.

Author Response

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Reviewer 3 Report

I read the work by Yuanzhe Li et al. It is a very interesting paper. I suggest it is published after addressing the following comments:

1. Please upload a version without any trackchanges
2. Check again spelling and grammar, I have found several mistakes throughout the the text.
3. Literature. Please add and discuss the following highly relevant references: P Sarkiris, K Ellinas, D Gkiolas, D Mathioulakis, E Gogolides, Advanced Functional Materials 29 (35), 1902905, K Ellinas, A Tserepi, E Gogolides, Microfluidics and nanofluidics 17 (3), 489-498, Gao N, Geyer F, Pilat DW, et al. Nat Phys. 2018;14(2):191-196. Also add some review papers about durable superhydrophobic surfaces, an example is the following: Advances in colloid and interface science 250, 132-157. Inside you can find several examples of the literature about durable surfaces which you can use to compare your surface durability (i.e. the data presented in figure 7)

    4. Figure 9, again compare your findings with the following works: Microfluidics and nanofluidics 14 (1-2), 247-255, Physics of Fluids 20, 113601 (2008) and from other works from C. D. Meinhart. You should add a discussion about what is known to date and which is your contribution/addition on this scientific subject.

Author Response

Please find the attached word file for the update.

Author Response File: Author Response.pdf

Reviewer 4 Report

The present manuscript by Yuanzhe Li, Zhe Cui, Qiucheng Zhu, Srikanth Narasimalu, and ZhiLi Dong, entitled "Fabrication of Zinc Substrate Encapsulated by Fluoro-polyurethane and its Drag-reduction Enhancement by Chemical Etching", describes the attempt to create a superhydrophobic zinc substrate by combining chemical etching and subsequent deposition of fluoro-polyurethane film. By the reviewer's estimation, the topic of this research is highly relevant. Despite the large number of publications, which highlight the enhancement of a material's hydrdophobicity, the reviewer hasn't found that such a study was ever conducted previously, thus the presented results are highly novel. The amount of research done is sufficient, and the applied experimental methods are relevant. However, this manuscript has also a number of drawbacks, which must be eliminated, before it may be considered for publishing. The reviewer's comments and recommendations are brought below.

1. Despite being generally understandable, English still needs editing. Some cases (not all) are specified below as the examples:
   a) lines 28-29: 'as this newly fabricated surface was placed', not 'as this newly fabricated surface being applied'.
   b) line 64: 'plant leaves, whose slipping phenomenon', not 'plant leaves with slipping phenomenon'.
   c) line 72: simply 'aluminium', not 'the aluminium'; 'limited number of metals' or 'limited number of metal alloys', not 'limited metal materials'.
   d) line 90: 'surface characteristics have', not 'surface characteristics has'.
   e) lines 103-104: 'zinc substrate was ... ultrasonically cleaned', not 'zinc substrate was ... conducted ultrasonic cleaning'.
   f) line 104: 'after being dried', not 'after dried'.
   g) lines 118-119: 'unetched zinc substrate, etched zinc substrate capped with fluoro-polyurethane and unetched zinc substrate capped with fluoro-polyurethane served', not 'unetched zinc substrate, etched zinc substrate capped with fluoro-polyurethane and unetched zinc substrate capped with fluoro-polyurethane were served'.
   i) line 129: 'surface energy', not 'Surface energy'.
   j) line 134: what [citation] "... was measured periodically as well ..."?
   k) line 137: simply 'shown in Fig. S2', not 'as shown in Fig. S2'.
   l) line 140: simply 'pressure difference', not 'different pressure difference'.
   m) line 142: 'in comparison with', not 'in comparison of with'.
2. References are missing in lines 66-68, 114, 249-250.
3. All the statements, which are given in the chapters 'Introduction' and 'Resutls and discussion', must be supported either by the resutls of the study or references to correspondent works, which isn't always done. For example, the statement about better properties of silicone-polyurethane copolymer in comparison with polyurethane homopolymer, as well as the statement about the further positive influence of polyamino functional groups to the properties of the above-mentioned copolymer (Introduction, paragraph 2) must be provided with (a) reference(s) to correspondent literature source(s).
4. Some places in the manuscript must be formulated in a more clear way. The exact cases are as follows:
   a) line 45: what is meant under [citation] "... poor surface properties ..."?
   b) lines 68-69: [citation] "... the strong acids and alkali solution ... used for chemical etching ...", as well as commercial applications should be precicely specified.
   c) line 75: [citation] "In previous attempts ..." Which attempts?
   d) lines 76-77: [citation] "... change of surface materials as well as the modification using low surface-energy coatings ..." sounds as it would be the same process, but differently named; the reviewer assumes that authors meant modificaton of the surface of the material and deposition of a coating with a low surface energy at the surface of the material; however, this must be postulated more clearly.
   e) lines 77-79: it should be specified, how exactly [citation] "... spatial surface structures formed by chemical etching, which are usually poor and easy to wear, would also enhance the drag-reduction effect ..."; it also should be specified, what is meant under 'spatial surfaces are poor'. Moreover, currently it seems that this statement is the repetition of the idea, formulated in lines 62-63 and 68-69. If this is a repetitive statement, it must be omitted.
   f) lines 79-81: if the first method is chemical etching, what is the second method, whose combination with the first one [citation] "... could not only extend the original mechanical and chemical stability of the protective layer ...".
   g) line 80: why 'protective layer'? What is meant under 'surface-structure advantages'? Why is just flat surface mentioned?
   i) line 83: what is meant under 'stable surface' here?
   j) the last paragraph in chapter 'Introduction' should only present the goals of the research and (possibly, but not compulsorily) shortly summarize the findings. Therefore the reviewer recommends to omit the description of the preparation process of the hydrophobic surface (lines 84-89).
   k) lines 165-166: no evidence is actually given in the text to prove that [citation] "... as the corrosion time became longer, the morphology of the surface of the zinc substrate was basically unchanged ...". Thus either corresponding data should be provided, or this statement should be omitted.
   l) lines 265-267: it should be specified here, when exactly (i.e., at low or at high Reinolds number) [citation] "... the non-shear air-water interface was formed by the separated air ...", and if this statement refers to all of the studied surfaces (unetched zinc, unetched zinc with fluoro-polyurethane film on it, etched zinc, etched zinc with fluoro-polyurethane film on it), or to a definite surface type.
   m) line 268: was drag reduction meant here under 'friction factor'?
   n) line 270: how exactly would [citation] "... fluoro-polyurethane ... still protect its surface structure formed by chemical etching ..." (i.e., due to intrinsic hydrophobicity, or in another way)?
   o) line 277: slip effect (or drag reduction effect) should be the consequence of low surface energy, therefore there is no need to mention them together.
5. In addition to this, there is a number of issues in the present manuscript, which need to be altered or explained:
   a) lines 99-100: the reviewer recommends to mention the zinc substrate for the first in the next subchapter (2.2 Fabrication of drag-reduction substrate).
   b) lines 103-104: was ultrasonic cleaing carried out in deionized water or in another liquid?
   c) lines 110, 115: rpm/min must be recalculated to s^(-1) (rpm isn't a SI unit; by the way, rpm already means 'rounds per minute', therefore '/min' is unnecessary).
   d) lines 159-160: [citation] "In previous experiments, the zinc substrate structure etched by nitric acid with copper ions for 10 s and 60 s has been observed ...". Which experiments?
   e) Figure 4, lines 171-174: what do illustrations (g) and (h) show?
   f) Figure 7: the data for the unetched zinc with fluoro-polyurethane substrate seems to be missing.
   g) lines 285-286: how exactly may the fluoro-polyurethane film [citation] "... protect the surface from scratches and abrasion ..."?
6. Remarks about references.
   a) the number of up-to-date references (from 2017 or later years) may be at least 25% (currently it is 5%, or one out of 20).
   b) for the sake of style uniformity, the reviewer asks the authors to use either hyphen or en dash to designate the page ranges (currently both punctuation marks are used).

Author Response

Please find the attached word file for a detailed reply.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors seem to have responded satisfactorily to some of the review comments. I appreciate that much attention is paid on the materials aspect of the coating, which is only a part of this study. However, the ability of the coating to reduce drag, which is in the title of the study, is clearly a fluid mechanical and interfacial phenomenon. In this context, some basic features associated with the drag reduction experiments/data have to explained clearly -- for example, the uncertainty in experimental measurements and calculations of pressure drop, volume flow rate, micro-channel height, and Reynolds number -- without which the experimental data on drag reduction is incomplete. It is with this in mind that the comments in the previous review were provided. Unfortunately, the authors do not seem to have addressed this.

For example, the authors state in response to comment #1: "..around 3-4% drag reduction effect in microchannel was considered a relative high rate in most microchannel research especially for the polymer coatings..." -- without giving any reference to support this claim that 3-4% is relatively high. Furthermore, the point of this comment was to request the authors to provide some information on the uncertainty/error associated with the experimental data. This is clearly not addressed in the response.

The authors state in their response that dissolved gas and surface texture could be possible reasons for the observed drag reduction on superhydrophilic surface ("...another paper is under preparation for the experimental and modelling of the super hydrophilic chemical etched surface has drag-reduction effect. It is true and repeatable, which is due to the dissolved gas and surface texture."). It is not at all clear why the same reasons are not applicable for the superhydrophobic surface -- surface texture of both the superhydrophilic and superhydrophobic surfaces are quite similar (as shown in Figs. 4 and 5), and dissolved gas (say, air) would have higher affinity to stick to the superhydrophobic surface than to the superhydrophilic surface. These aspects have to be addressed/clarified.

Minor comment: Reynolds number is defined using a 'feature length' d. Is the hydraulic diameter (equivalent diameter) W' used as the 'feature length' in calculating Re? This should be clarified in the manuscript.

Author Response

Please refer to the attached word for detailed info.

Author Response File: Author Response.pdf