A Simple and Efficient Strategy for Preparation of Flexible Strain Sensors Based on Marangoni Effect

Round 1

Reviewer 1 Report

 Article is fine for publication.

Author Response

Thanks a lot for your affirmation and we will revise our manuscript according to the comments of other reviewers.

Reviewer 2 Report

This manuscript from Bai et all proposes a method to graphene/carbon nanotubes composite films assembled by using the Marangoni effect as strain sensors. While the general concept is seducing, the article suffers from several major issues that require extensive corrections 

Remarks on the form of the manuscript:

-  the sample names are incredibly tedious and make the discussion difficult to read. Please simplify the names and provide a table with name/sample correspondance

- Define GF (present in figures 2, 3, 4

- Expand the figure descriptions (legends) as the figures used are both large and not self-explanatory

- Correct the units of fig 2g

- Provide SEM images with the same magnification in fig2

- Replace unspecific vocabulary (eg "ohm behavior" line 228, "fast speed" line 35, "attached" line 151 ..) by precise scientific vocabulary

Remarks on the content of the manuscript: 

- The experimental section is blurry and does not allow for reproduction of the experiments. More precisely, please indicate: 

           - the yield of graphene synthesis

            - The temperatures and times for all experiments

           - the dimensions of the copper wires and its resistivity

           - the conditions of resistance measurements (T, humidity)

           - describe how the sensitivity and working range were determined (provide equations if necessary)

- Could author discuss on the expected interaction between GN and CNT ? 

 - According to the descriptions and units given, there is a confusion between resistance, resistivity and even conductivity throughout the manuscript. Please choose one descriptor and homogenize the article. 

- The raw materials are poorly characterized. Please provide TEM analysis demonstrating the graphene nature of the raw GN, as well as different magnification images of  GN and CNT to appreciate their size distribution/homogeneity

- The films characterization needs to be completed: 

        - Provide SEM or AFM analysis of the films at different magnification to enable a proper appreciation of the film's quality and homogeneity. 

          - Ellipsometric measurements are mentioned but not shown. Please provide the measurements + fit data of all films and add a table with the thickness of all films 

- Questions related to the resistance change of the films: 

       - What is the resistance (and its change vs stretching) of the support ? This control is missing. 

        - There is an obvious (and expected) influence of the film thickness on its conduction properties (both static and under stretch). The authors should discuss their resistance measurement data in the context of the film thickness. 

         - If not performed, the influence of the temperature and humidity on the film's resistance should be determined. Indeed, wearable sensors are supposed to be used in environment with changing humidity/temperature (eg human skin)

          - The mechanism described on line 175-179 appears to be speculative. Authors should support it with concrete evidences and/or references     

Questions related to the films stability and cyclability: 

        - post-mortem SEM or AFM measurements are needed after cycles of  low stretchting as well after moderate/high stretching to detect any change in their morphology/quality

        - Could author comments on how does the thickness of the film evolve with stretching and is this change reversible ?

 - In the introduction, electrodeposition should be mentioned among the method for sensor assembly (eg: small 11 (36), 4638-4642 // Materials Today Vol 62,  2023, Pages 129-150 )

A very large number of typos and english mistakes are disseminated throughout the manuscript.

A few non-exhaustive examples: 

- "propertion of GN and grephene" line 143

- conducive, legend of figure 2

- "after the frozen" figure 1

- all subsection titles are either difficult to understand or contain typos

Note that most of these mistakes do not seem related to the author's proficiency in english, but rather to a poor reading/correction of the draft. 

Author Response

Thanks for the meticulous review and valuable feedback from the reviewer. We will reply to comments point-by-point.

Q1. The sample names are incredibly tedious and make the discussion difficult to read. Please simplify the names and provide a table with name/sample correspondence.
A1: Because a few factors such as the CNTs proportion, the number of conductive film layers, substrate, and assembly method that affect sensor performance were discussed in this work, and many samples were involved, there was indeed some confusion in sample naming. We have provided a principle for naming sensors (lines 147-158), for example, simplifying the 'paste two + paste once' sensor to [2+1] sensor in revised manuscript.

Q2. Define GF (present in figures 2, 3, 4)
A2: We have added the definition of GF in line 75-78 in revised manuscript.

Q3. Expand the figure descriptions (legends) as the figures used are both large and not self-explanatory
A4: We have expanded the figure descriptions to enhace the accessibility and readability.

Q4. Correct the units of fig 2g
A4: We have corrected the units of sheet resistance as "kΩ sq-1" (Fugre 3 in revised manuscript).

Q5. Provide SEM images with the same magnification in fig2
A5: We have replaced SEM images of CNTs and GN with the same magnification in Figure 2 (Figure 3 in revised manuscript).

Q6. Replace unspecific vocabulary (eg "ohm behavior" line 228, "fast speed" line 35, "attached" line 151 ..) by precise scientific vocabulary
A6: We have replaced unspecific vocabulary by precise scientific vocabulary and expression. "Ohm behavior" has been deleted together with the current-voltage diagram for the neat composition of manuscript. "fast speed" has been replaced by "fast film-formation rate"  (line 39) and "attached" has been replaced by "distributed" (line 218).

Q7. The experimental section is blurry and does not allow for reproduction of the experiments. More precisely, please indicate: (1) the yield of graphene synthesis (2) the temperatures and times for all experiments (3) the dimensions of the copper wires and its resistivity (4) the conditions of resistance measurements (T, humidity) (5) describe how the sensitivity and working range were determined (provide equations if necessary)
A7: We have refined the experimental and characterization conditions, and we have expanded the introduction to sensitivity and working range (lines 75-80).

Q8. Could author discuss on the expected interaction between GN and CNT ?
A8: We have polished the discussion on the interaction between GN and CNT in lines 241-246:  GN is with a lamellar structure, which means it is easy to slip during stretching and re-sults in damage to the conductive network. When the composite film was stretched to large strains, CNTs worked as "bridges", which was helpful to reduce the agglomeration between GN nanosheets, increase the number of conductive paths and enhance the overall electrical conductivity to a large extent.

Q9. According to the descriptions and units given, there is a confusion between resistance, resistivity and even conductivity throughout the manuscript. Please choose one descriptor and homogenize the article.
A9: Resistance (kΩ) was used to evaluate the conductivity of fabricated sensors, while sheet resistance (kΩ sq-1) is employed to evaluate the conductivity of the conductive films (CNTs/GN films). We have tried our best to homogenize the article.

Q10. The raw materials are poorly characterized. Please provide TEM analysis demonstrating the graphene nature of the raw GN, as well as different magnification images of GN and CNT to appreciate their size distribution/homogeneity
A10: In the revised manuscript, GN and CNT were characterized by TEM, AFM, SEM and Raman spectrometer. Considering the length of the paper, we have included part of the results in the Supporting Information file.

Q11. The films characterization needs to be completed: (1) Provide SEM or AFM analysis of the films at different magnification to enable a proper appreciation of the film's quality and homogeneity. (2) Ellipsometric measurements are mentioned but not shown. Please provide the measurements + fit data of all films and add a table with the thickness of all films.
A11： (1) SEM images of the typical films at different magnification are supplied in Figure 3, 4 in manuscript and Figure S2, S3 in SI file. (2) We used an ellipsometer to measure the thickness of two typical films and supplied certain data support. However, we must send the samples to a testing center in another city, which is very time-consuming for us, so we will conduct the additional testing later.

Q12. What is the resistance (and its change vs stretching) of the support ? This control is missing
A12: In fact, the substrate of the sensor is insulated, and its resistance is much higher than that of the conductive film. Moreover, during the packaging of the sensor, electrodes specifically connected to the conductive film were already made using conductive silver paste and copper wires (lines 158-159), so we did not test the resistance of the substrate.

Q13. There is an obvious (and expected) influence of the film thickness on its conduction properties (both static and under stretch). The authors should discuss their resistance measurement data in the context of the film thickness.
A13: It is a very important issue that we have also taken into account, however, we did not accurately measure the thickness of the film, but indirectly represented the thickness of the film using the number of conductive layers. For example, the thickness of the conductive films of [2+2] sensor should be thicker than that of [2+1] and [1+1] sensors.

Q14. If not performed, the influence of the temperature and humidity on the film's resistance should be determined. Indeed, wearable sensors are supposed to be used in environment with changing humidity/temperature (eg human skin)
A14: It is a very valuable suggestion, and we will conduct relevant research in subsequent experimental.

Q15. The mechanism described on line 175-179 appears to be speculative. Authors should support it with concrete evidences and/or references
A15: We have cited relative references ([43, 44] in line 241-242). 

A16. post-mortem SEM or AFM measurements are needed after cycles of low stretching as well after moderate/high stretching to detect any change in their morphology/quality
A16: It is indeed necessary to detect the change in morphology/quality after cycles of low stretching as well after moderate/high stretching. We added SEM images of 50% CNTs film based on 3M4910 after several cycles of 30% stretching in the manuscript, in which we can see fold network on the substrate (Figure S4 in SI). The SEM image is helpful to explain the change of ΔR/R0. While SEM images of the film after high stretching is unavailable because we didn't set aside plenty of samples for latter test.

Q17. Could author comments on how does the thickness of the film evolve with stretching and is this change reversible ?
A17: When the senor is stretched gradually, the thickness of the film decrease in limited range because of the Poisson effect. The change in thickness is reversible under certain stains, which can be proved by cyclic stretching-releasing tests.

Q18. In the introduction, electrodeposition should be mentioned among the method for sensor assembly (eg: small 11 (36), 4638-4642 // Materials Today Vol 62, 2023, Pages 129-150)
A18: We have replenished the method of electrodeposition in the introduction and the relative paper has been cited in our manuscript. ([37] in line 66)

Q19. A very large number of typos and english mistakes are disseminated throughout the manuscript. A few non-exhaustive examples: (1) "propertion of GN and grephene" line 143 (2) conducive, legend of figure 2 (3) "after the frozen" figure 1 (4) all subsection titles are either difficult to understand or contain typos. Note that most of these mistakes do not seem related to the author's proficiency in English, but rather to a poor reading/correction of the draft.
A19: Thanks a lot for professional comments, which benefit us a lot. We have carefully proofread and polished our manuscript.

Reviewer 3 Report

The authors report on the fabrication and characterization of GN-CNT films by the Marangoni effect and its application as a strain sensor.

I find their work suitable for publication after minor revision. Following a few comments:

-The authors should describe further the morphology and size distribution of the GN they prepared, citing the relevant literature.

-The GN CNT ratio is not clear, do the percentages refer to the volume or mass of the dispersion or of the dry films?

Author Response

Q1. The authors should describe further the morphology and size distribution of the GN they prepared, citing the relevant literature.
A1: We have described further about the morphology and size distribution of GN (Section 3.1 added) , and relevant literature has been cited (Reference [43]).

Q2. The GN CNT ratio is not clear, do the percentages refer to the volume or mass of the dispersion or of the dry films?
A2: The percentages refer to the CNTs mass proportions in carbonous materials (CNTs and GN) in the dispersion. We clarified in section 3.2 that the CNTs ratios are the "mass proportions" (lines 212-214).

Reviewer 4 Report

The manuscript titled "A Simple and Efficient Strategy for Preparation of Flexible Strain Sensors Based on Marangoni Effect" presents the authors' work on the preparation of ultrathin conductive films using graphene (GN) and carbon nanotubes (CNTs) through the Marangoni effect. These films were then transferred twice onto flexible substrates and assembled face to face to create strain sensors. The study primarily focuses on the properties of these conductive films and sensors. The researchers investigated the impact of various factors such as the proportion of GN and CNTs, the volume of the carbon/ethanol dispersion, and the number of conductive films on the performance of the sensors. The results showed that the ratio of GN and CNTs influenced the working range and sensitivity of the sensors, while the volume of carbon/ethanol dispersion affected the film density and light transmittance. The number of conductive films transferred also had an effect on sensor conductivity. Overall, the study successfully demonstrated the fabrication process of flexible strain sensors using the Marangoni effect and provided insights into the structural and electrical properties of the conductive films.

- Results and discussion: While some results are mentioned and a few SEM images are shown, a more detailed discussion of the results is necessary. It would be beneficial to provide a deeper analysis and interpretation of the obtained data in the context of the study objectives. Expanding the discussion of results to provide a deeper analysis and interpretation would help readers understand the significance and implications of the findings more clearly.

- Terminology and technical details: Some technical terms and concepts may require further explanation or clarification, particularly for readers who are not familiar with the specific field or subject matter. Providing additional explanations where necessary would enhance the paper's accessibility.

- Language and grammar: There are instances of grammatical errors, awkward phrasing, and incomplete sentences. Careful proofreading and editing would significantly improve the overall quality and coherence of the writing.

- Expand the conclusion: Summarize the key findings and their implications more comprehensively in the conclusion section.

- Terminology and technical details: Some technical terms and concepts may require further explanation or clarification, particularly for readers who are not familiar with the specific field or subject matter. Providing additional explanations where necessary would enhance the paper's accessibility.

- Language and grammar: There are instances of grammatical errors, awkward phrasing, and incomplete sentences. Careful proofreading and editing would significantly improve the overall quality and coherence of the writing.

Author Response

Many thanks for the valuable feedback provided by the reviewer. We have carefully revised the manuscript, especially in language polishing, to enhance the paper's accessibility.

Q1. Results and discussion: While some results are mentioned and a few SEM images are shown, a more detailed discussion of the results is necessary. It would be beneficial to provide a deeper analysis and interpretation of the obtained data in the context of the study objectives. Expanding the discussion of results to provide a deeper analysis and interpretation would help readers understand the significance and implications of the findings more clearly.
A1: We have made comprehensive revisions to the entire manuscript, and added some new data and analysis of experimental results, hoping to provide readers with better reference value.

Q2. Terminology and technical details: Some technical terms and concepts may require further explanation or clarification, particularly for readers who are not familiar with the specific field or subject matter. Providing additional explanations where necessary would enhance the paper's accessibility.
A2: We have supplied further explanation and clarification for technical terms and concepts, including gauge factor (GF), working range, response time, stability, etc.

Q3. Language and grammar: There are instances of grammatical errors, awkward phrasing, and incomplete sentences. Careful proofreading and editing would significantly improve the overall quality and coherence of the writing.
A3: We have thoroughly and carefully polished the manuscript, hoping to meet the publication requirements and provide better readability. 

Q4. Expand the conclusion: Summarize the key findings and their implications more comprehensively in the conclusion section.
A4: We have carefully rewritten the conclusion section (lines 391-411), more data and key findings are included.

Round 2

Reviewer 2 Report

The authors have brought some improvements to their paper but failed to adress most experimental flaws (eg Q11, Q13, Q14, Q17). Under these conditions I cannot recommed  publication of this work.

Author Response

Q11. The films characterization needs to be completed: (1) Provide SEM or AFM analysis of the films at different magnification to enable a proper appreciation of the film's quality and homogeneity. (2) Ellipsometric measurements are mentioned but not shown. Please provide the measurements + fit data of all films and add a table with the thickness of all films.
A11: (1) SEM images of all kinds of films at different magnifications have been supplied in Figure 3, 4, 6 in manuscript and Figure S2, S3, S4 in SI file. (2) We used an ellipsometer to measure the thickness of two typical films in order to prove that the films prepared by our method are ultrathin enough. We have listed the data and described the measuring method detailedly in the manuscript.

Q13. There is an obvious (and expected) influence of the film thickness on its conduction properties (both static and under stretch). The authors should discuss their resistance measurement data in the context of the film thickness.
A13: Thanks a lot for your valuable suggestion. It is a pity that the resistance measurement data in the context of the film thickness was not discussed in our manuscript mainly for the following two reasons. (1) The thickness of ultrathin conductive films based on flexible substrates is difficult to measure accurately, especially under stretch. We are advised to use rigid substrates for thickness measurement, in which case the films can’t be stretched. (2) Obviously, the more layers of the conductive film, the greater the total thickness of the conductive film. The thickness, transmittance, and resistance of conductive films are related. The relationship between the number of conductive layers and resistance (DOI:10.1016/j.nanoen.2019.02.036) has been reported in literatures as well as the relationship between the thickness and light transmittance of conductive films (DOI:10.1002/adfm.201504717). We did not quantitatively discuss the relationship between the thickness and resistance of conductive films, because it is difficult to measure the thickness accurately, but we qualitatively analyzed the relationship between the number of conductive film layers and transmittance (Figure 5a) and resistance (Figure 5b) . The relationship between film thickness and resistance was discussed indirectly, and the sensitivity and working range of different sensors were characterized, which was consistent with our expectations.

Q14. If not performed, the influence of the temperature and humidity on the film's resistance should be determined. Indeed, wearable sensors are supposed to be used in environment with changing humidity/temperature (eg human skin)
A14: According to the literature (DOI:10.1007/s42765-023-00270-y), temperature and humidity have certain effect on sensing performance of the sensors, which is negligible compared with mechanical strains. Our sensors are still some distance away from practical applications in human skin, therefore, we didn’t design systematic study of temperature and humidity on sensing performance. Your advice is much enlightening to us, and we will conduct relevant research in subsequent experimental.

Q17. Could author comments on how does the thickness of the film evolve with stretching and is this change reversible?
A17: When the sensor is stretched gradually, the thickness of the film decreases in limited range because of the Poisson effect. When the sensor recovers to initial length, the thickness of the film recovers to initial state, as well. The change in thickness is reversible under certain strains, which can be proved by cyclic stretching-releasing tests.

Round 3

Reviewer 2 Report

The authors have brought complementary corrections to their manuscript while providing justifications for the issues that they did not address. 

I thus recommend publication of this article. 

Best regards