Round 1

Reviewer 1 Report

The authors presented a short communication on the effect of laser irradiation on carbon soot film. In particular the effect on physicochemical properties is investigated. Morphological changes have been observed under laser irradiation, while optical transmittance and wettability are not modified by the laser.

The paper itself is well organized, however in my opinion some of the experimental detail are not clear and also the comparison with the existing  literature in not always appropriate.

I would recommend this paper to be published after minor revisions.

Here is a list of the point to be addressed prior publication:

• line 75-81: the whole description of the laser irradiation process is not clear to me.  I would assume the photon density to be at the highest at the lens focal point, but it seems not to be the case. Moreover the use of a Gaussian beam profile will not allow a uniform heating of the surface. Could you comment on that?
• line 93-94: I find the references 10 and 17 not fully appropriate for the statement in the text. Ref. 10 reports HRTEM analysis of laser irradiated soot, showing an altered soot nanostructure of primary partcles within the aggregates. Ref. 17 reports the effect of laser irradiation on the particles size distribution. Therefore, I suggest to use the word "similarly" instead of "identically" to avoid a direct comparison of results which are obtained in completely different experimental conditions (high energy pulsed laser) and shows something very different.
• line 110, Table 1. What is the power condition for this data? Is there any difference using 114 or 171 mW? Please specify which are the power and exposure time conditions.
• line 119, Figure 5: Again missing information on the experimental conditions. Power? exposure time?
• line 123: how is the increased temperature measured? Please give some details.
• line 125-128: in ref. 17 the authors talk about aggregates fragmentation, while in the present paper the aggregates are believed to merge. In my opinion the effect is quite different. I suggest to change " analogously" with "differently from".
• line 129: "the increased surface oxidation is attributed likely to sublimation". If the samples are at room temperature and the laser irradiation produced a temperature increase of about 400°, sublimation hardly occurs since soot sublimation happens at 4000 K. Probably more work is needed to address the origin of the increased surface oxidation. Please revised the discussion of this point.

Author Response

Response to Referees

Title                     Laser irradiation of super-nonwettable carbon soot coatings – physicochemical implications

Authors              Karekin D. Esmeryan*, Yulian I. Fedchenko, Georgi P. Yankov and Krassimir A. Temelkov

Manuscript ID     coatings-1047852   

We would like to sincerely thank the reviewers for their time to review our manuscript and for the provision of helpful and constructive comments, suggestions and recommendations. We did our very best to address them in the revised version of the manuscript. Revisions in the text related to the reviewers’ comments are highlighted in yellow. The responses are given below, each comment is shown in italic and the response shown in standard font.

Referee 1:

1) The authors presented a short communication on the effect of laser irradiation on carbon soot film. In particular the effect on physicochemical properties is investigated. Morphological changes have been observed under laser irradiation, while optical transmittance and wettability are not modified by the laser.

The paper itself is well organized, however in my opinion some of the experimental detail are not clear and also the comparison with the existing literature is not always appropriate.

I would recommend this paper to be published after minor revisions.

Here is a list of the point to be addressed prior publication:

• line 75-81:the whole description of the laser irradiation process is not clear to me.  I would assume the photon density to be at the highest at the lens focal point, but it seems not to be the case. Moreover the use of a Gaussian beam profile will not allow a uniform heating of the surface. Could you comment on that?

Response:

We thank the reviewer for his/her positive evaluation of our work and pointing some important weaknesses in the manuscript that definitely must be addressed prior to publication. He/she is correct that the Gaussian beam profile will not allow a uniform heating of the surface, but it is appropriate to warrant high homogeneity in the center of the spot. Furthermore, we have tried to find a suitable balance between the highest possible photon density for modification of the soot and the maximum width of the spot facilitating the subsequent wetting state analysis.

Change in Manuscript:

Additional information regarding the laser irradiation process is now available on Page 2, Lines 78-84 in the revised manuscript.

2) line 93-94: I find the references 10 and 17 not fully appropriate for the statement in the text. Ref. 10 reports HRTEM analysis of laser irradiated soot, showing an altered soot nanostructure of primary partcles within the aggregates. Ref. 17 reports the effect of laser irradiation on the particles size distribution. Therefore, I suggest to use the word "similarly" instead of "identically" to avoid a direct comparison of results which are obtained in completely different experimental conditions (high energy pulsed laser) and shows something very different.

Response:

We agree with the reviewer and the word “identically” was replaced by “similarly”.   

3) line 110, Table 1. What is the power condition for this data? Is there any difference using 114 or 171 mW? Please specify which are the power and exposure time conditions.

Response:

Thank you very much for this important remark. The increased soot oxidation is observed at laser power P = 171 mW and irradiation time of 350 s.

Change in Manuscript:

The information required by the reviewer is now available in Table 1 in the revised manuscript.

4) line 119, Figure 5: Again missing information on the experimental conditions. Power? exposure time?

Response:

We decided to measure the optical transmittance of the soot outside and inside the irradiated spot after laser irradiation with P = 171 mW for 350 s. The reason behind this decision is related to the largest effect of these particular experimental conditions on the morphology and oxidation of the soot. In other words, if one expects to see any differences in the optical properties of the material, perhaps he/she needs to examine the most affected parts of the coating.

Change in Manuscript:

The information required by the reviewer is now available in the caption of Figure 5 in the revised manuscript.  

5) line 123: how is the increased temperature measured? Please give some details.

Response:

The increased temperature is measured by performing laser-triggered paper ignition experiments. The ignition temperature of the paper is ~450 °C (https://books.google.bg/books?id=qa-I8QAOUL8C&pg=PA406&lpg=PA406&dq=flash+point+of+paper&source=web&ots=FjB5FslcKK&sig=Vlr2cc8M2-TWy3qkdwzGUeK-vnM&hl=en&sa=X&oi=book_result&ct=result&redir_esc=y#v=onepage&q=flash%20point%20of%20paper&f=false ), so since the laser was capable of igniting the paper after 5 s exposure time, we approximated its temperature to ~450 °C.

Change in Manuscript:

Details about the measurement of laser temperature is now available on Page 5, Lines 129-130 in the revised manuscript.

6) line 125-128: in ref. 17 the authors talk about aggregates fragmentation, while in the present paper the aggregates are believed to merge. In my opinion the effect is quite different. I suggest to change " analogously" with "differently from".

Response:

We agree with the reviewer and the word “analogously” was replaced by the phrase “differently from”.

7) line 129: "the increased surface oxidation is attributed likely to sublimation". If the samples are at room temperature and the laser irradiation produced a temperature increase of about 400°, sublimation hardly occurs since soot sublimation happens at 4000K. Probably more work is needed to address the origin of the increased surface oxidation. Please revised the discussion of this point.

Response:

We are very grateful for this particular comment. Indeed, the reviewer is correct that the soot sublimation occurs at 4000 K and definitely further systematic studies are needed to realize the origin of the increased soot oxidation. What might has happened is that the abundance of surface defects, associated with the amorphous nature of the soot and clearly identifiable by Raman spectroscopy (https://pubs.rsc.org/en/content/articlelanding/2016/RA/c6ra06436a#!divAbstract ), may accelerate the oxidative processes in the ambient environment, similarly to the observations described upon laser irradiation of carbon nanotube films.

Change in Manuscript:

We have made some relevant changes in the text, available on Page 5, Lines 135-136; Page 6, Lines 137-138 and Lines 145-146 in the revised manuscript, and cited one additional literature source.

Reviewer 2 Report

The article is devoted to the effect of laser irradiation of soot coating. The results of the first studies are presented. Changing the structure of surface agglomerates is shown to be dependent on the radiation power. It was found that for laser radiation with an energy of 405 nm (3 eV), a variation in the C and O elemental composition is observed, which indicates a partial oxidation of the surface layer of the soot coating, but its water-repellent properties stay unaltered.

Remarks:

1. It is necessary to quantitatively characterize the areas of different colors of a spot in Figure 2 (right image). Specify in the text, which area of this spot is shown in Figures 3 and 4.
2. The phrase on the line 96 ‘(qualitatively defined)’ looks as unfounded one. Please, indicate the method of quantification.
3. Table 1. Please, specify an accuracy of measuring the percentage of C and O elements.

Author Response

Response to Referees

Title                     Laser irradiation of super-nonwettable carbon soot coatings – physicochemical implications

Authors              Karekin D. Esmeryan*, Yulian I. Fedchenko, Georgi P. Yankov and Krassimir A. Temelkov

Manuscript ID     coatings-1047852   

We would like to sincerely thank the reviewers for their time to review our manuscript and for the provision of helpful and constructive comments, suggestions and recommendations. We did our very best to address them in the revised version of the manuscript. Revisions in the text related to the reviewers’ comments are highlighted in yellow. The responses are given below, each comment is shown in italic and the response shown in standard font.

Referee 2:

The article is devoted to the effect of laser irradiation of soot coating. The results of the first studies are presented. Changing the structure of surface agglomerates is shown to be dependent on the radiation power. It was found that for laser radiation with an energy of 405 nm (3 eV), a variation in the C and O elemental composition is observed, which indicates a partial oxidation of the surface layer of the soot coating, but its water-repellent properties stay unaltered.

Remarks:

1. It is necessary to quantitatively characterize the areas of different colors of a spot in Figure 2 (right image). Specify in the text, which area of this spot is shown in Figures 3 and 4.

Response:

We strongly appreciate the useful comments provided by reviewer 2. According to his/her recommendation, we have now quantitatively characterized the intensity of laser radiation based on the different colors of the spot in Figure 2. Also, we have specified in the text which area of the spot is shown in Figures 3 and 4.

Change in Manuscript:

Novel information concerning the intensity of the Gaussian beam and the specific area of the spot shown in Figures 3-4 is now available on Page 3, Lines 95-96 in the revised manuscript.

2. The phrase on the line 96 ‘(qualitatively defined)’ looks as unfounded one. Please, indicate the method of quantification.

Response:

Well, the number of soot agglomerates is defined by a “naked eye” inspection of the SEM images. This clarification is now available on Page 3, Line 101 in the revised manuscript.

3. Table 1. Please, specify an accuracy of measuring the percentage of C and O elements.

Response:

Thanks for this kind suggestion. The measurement uncertainty of the EDX instrument is ±0.1 at.%.  

Change in Manuscript:

The required information is available in the caption of Table 1.