Effects of Non-Thermal Plasma on the Transition from Nano-Crystalline to Amorphous Structure in Water and Subsequent Effects on Viscosity

Round 1

Reviewer 1 Report (Previous Reviewer 2)

The authors have addressed my comments and concerns in the revised version. I recommend accepting the paper for publications.

Reviewer 2 Report (Previous Reviewer 1)

The revised manuscript is improved and it is now suitable for acceptance.

Reviewer 3 Report (Previous Reviewer 3)

The authors improved their manuscript and incorporate my suggestion or argued professionally well founded against them. 

Only minor English checks.

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

In this theoretical paper, a model for calculating PAW viscosity is presented, the paper is suitable for the mentioned special issue. The work continues on a series of publications by the group from Drexel University, in which the interesting properties of plasma gliding arc treated water with a relatively low pH value were previously demonstrated. Unfortunately, the phenomenon of lower temperature viscosity was demonstrated only for two pH values 2.95 and 3.04. PAW is characterized only by pH value, which somewhat limits the scope of applications in PAW research. What are the characteristics of PAW with such low pH values with regard to the concentrations of peroxides, nitrites and nitrates and the possible presence of metal ions and stability in the light of applications.

In the list of references ref 11 needs title.

Author Response

Dear Reviewer, thank you for reviewing and letting us know your valuable comments and areas of improvement in our manuscript. We have made changes to all the areas you have asked us to, please find our replies and information about the changes attached.  

PAW produced with pH in the range of 2.76 to 3.04 has nitrate concentration in the range of 100 to 250 mg/L and peroxide in the range of 25 to 150 mg/L. Since gliding arc discharge was used to produce PAW, electrode corrosion and thereby metal in the water is not expected. The viscosity of PAW has been tested during period of two during which the viscosity values measured were consistent with each other. Please find the detailed PAW characteristics in manuscript in lines 239 and 288.

Reviewer 2 Report

Review on Effects of non-thermal plasma on the nano-crystalline to amorphous structural transition in water and subsequent effects on viscosity

I have completed my review of manuscript Plasma-2574108, entitled, “Effects of non-thermal plasma on the nano-crystalline to amorphous structural transition in water and subsequent effects on viscosity.”

This study investigates the distinct physical properties of plasma-activated water (PAW) compared to distilled water through a developed model. Integrating the two-state model of water with the Debye-Huckel theory, the model explains PAW's unique characteristics, including lower viscosity at specific temperatures. The model elucidates that PAW's effects surpass colligative behavior due to non-colligative contributions. Notably, the model's predictions closely match experimental data. The proportion of nano-crystalline structures in PAW plays a pivotal role in determining its properties beyond viscosity. This comprehensive understanding can extend to other characteristics like surface tension, contact angle, and conductivity, fostering applications in eco-friendly surfactants, disinfectants, and more. The model showcases the potential for predictive insight into diverse PAW properties, with promising practical implications.

The subject and findings of this article are interesting and useful. A major revision is required before publication. Consider my comment during revisions.

Comments for authors

Comment 1: As the field of plasma is broad, the background information provided by the authors may be insufficient. It would be informative for readers if the authors included information regarding nonthermal plasma and its applications in the background section. The following recent article could be a useful resource to incorporate into the background information. [Review on the Biomedical and Environmental Applications of Nonthermal Plasma. Catalysts 2023, 13.]

Comment 2: Figure 1 is not so efficient and informative, consider revising it to present it in a better way.

Comment 3: Authors literature review is poor. Most of the cited data is outdated and lacking. I recommend increasing the literature review on the subject of the study.

Comment 4: Could you provide further insight into the mechanism by which non-thermal plasma treatment leads to the observed changes in the physical properties of water, such as viscosity, surface tension, and contact angle? How do these changes relate to the structural alterations in the water clusters and molecules? This needs to be discussed with extensive details in the manuscript.

Comment 5: In your model combining the two-state model of water with the Debye-Huckel theory, how did you account for the potential interactions between plasma-generated species and water molecules? Could these interactions affect the linearity assumption made for the Debye-Huckel term? This needs to be clarified during your explanation in the manuscript.

Comment 6: In this model combining the two-state model of water with the Debye-Huckel theory, how did the authors account for the potential interactions between plasma-generated species and water molecules? Could these interactions affect the linearity assumption made for the Debye-Huckel term?

Comment 7: Can you elaborate on the conditions or circumstances under which the effects of plasma-generated species on water properties might deviate from linearity, potentially introducing more complex relationships than those captured by the current model?

Comment 8: Can you explain how this developed model addresses potential variations in plasma conditions or treatment methods? Are there specific limitations or scenarios where the model might need to be adapted to different types of non-thermal plasma sources?

Comment 9: The paper contains errors and typos that make it difficult to understand and distort its intended meaning. I encourage authors to reread carefully and fix any grammatical errors.

Comment 10: Check the manuscript to avoid repeating the full form of an abbreviation after its short form has been introduced. For example, 'plasma activated water’ and ‘PAW.'

The paper contains errors and typos that make it difficult to understand and distort its intended meaning. I encourage authors to reread carefully and fix any grammatical errors.

Author Response

Dear Reviewer, thank you for reviewing and letting us know your valuable comments and areas of improvement in our manuscript. These constructive comments from you have helped us present our work to its best form yet. We have made changes to all the areas you have asked us to, please find our replies and information about the changes attached.  

Comment 1: As the field of plasma is broad, the background information provided by the authors may be insufficient. It would be informative for readers if the authors included information regarding nonthermal plasma and its applications in the background section. The following recent article could be a useful resource to incorporate into the background information. [Review on the Biomedical and Environmental Applications of Nonthermal Plasma. Catalysts 2023, 13.]

The introduction have been enriched with information about plasma, types of plasma, and applications of non-thermal plasma. Please see pages 1-2.

Comment 2: Figure 1 is not so efficient and informative, consider revising it to present it in a better way.

Figure 1 has been changed to better express the ideas of crystalline and amorphous structure. Please see page 3.

Comment 3: Authors literature review is poor. Most of the cited data is outdated and lacking. I recommend increasing the literature review on the subject of the study.

The literature review have been undertaken to better address the subject of study.

Comment 4: Could you provide further insight into the mechanism by which non-thermal plasma treatment leads to the observed changes in the physical properties of water, such as viscosity, surface tension, and contact angle? How do these changes relate to the structural alterations in the water clusters and molecules? This needs to be discussed with extensive details in the manuscript.

The mechanism of physical property change have been discussed in page 2, line 66 to 85.

Comment 5: In your model combining the two-state model of water with the Debye-Huckel theory, how did you account for the potential interactions between plasma-generated species and water molecules? Could these interactions affect the linearity assumption made for the Debye-Huckel term? This needs to be clarified during your explanation in the manuscript.

Comment 6: In this model combining the two-state model of water with the Debye-Huckel theory, how did the authors account for the potential interactions between plasma-generated species and water molecules? Could these interactions affect the linearity assumption made for the Debye-Huckel term?

Comment 7: Can you elaborate on the conditions or circumstances under which the effects of plasma-generated species on water properties might deviate from linearity, potentially introducing more complex relationships than those captured by the current model?

Viscosity of PAW exhibits a dual nature – lower viscosity relative to water at lower temperatures and higher viscosity relative to water at higher temperatures. The mechanism of this change was explained in the previous publication: “Physical Properties of Plasma-Activated Water” by Shaji et al.

Due to their highly polar nature, water molecules have the tendency to create hydrogen bonds between one another even before their freezing point. As a result, liquid water can exist in two possible structures: the amorphous state, where water molecules move freely and are not bonded to one another, and the nano-crystalline or structured state, where molecules begin to form bonds between one another.

PAW at low temperatures has more amorphous structure relative to water. Amorphous structure is characterised by lower viscosity and lower surface tension relative to crystalline structure and therefore PAW at lower temperatures has lower viscosity than water. As temperature increases, the effect of changes in structural states is reduced, and the plasma species in PAW will lead to it having a higher viscosity than water. This is same as the presence of foreign particles increasing viscosity in water.

The increase in viscosity due to plasma species could lead to non-linear changes in viscosity and will be difficult to model due to the kinetics of interaction between active species and water molecules. This model can act as a qualitative indicator of PAW viscosity behaviour during the non-linear plasma species driven changes at higher temperatures. 

Comment 8: Can you explain how this developed model addresses potential variations in plasma conditions or treatment methods? Are there specific limitations or scenarios where the model might need to be adapted to different types of non-thermal plasma sources?

To some extent this model can be adapted to different types of non-thermal plasma sources, as the active species formed across different discharges are proportional but vary in concentration depending upon the energy supplied. As Debye-Huckel term accounts for concentration of ions, this viscosity model can be applied in a qualitative manner to water activated by different discharges and not in a quantitative manner. 

Comment 9: The paper contains errors and typos that make it difficult to understand and distort its intended meaning. I encourage authors to reread carefully and fix any grammatical errors.

The manuscript has been reviewed carefully and grammatical errors have been addressed.

Comment 10: Check the manuscript to avoid repeating the full form of an abbreviation after its short form has been introduced. For example, 'plasma activated water’ and ‘PAW.'

Care has been taken to make sure abbreviations are not repeated.

Reviewer 3 Report

Review: Effects of non-thermal plasma on the nano-crystalline to amorphous structural transition in water and subsequent effects on viscosity

The authores postulate a model, which describes different physical behaviours of distilled water and PAW. For their model, the authors combine statements about a spectrum of water structures with a crystalline structure at one end and a very disordered structure at the other end and the Debeye-Hückel-theory to explain the solvation of ions in liquids. For instance, the author aim to explain the unusual drop in viscosity in PAW for temperatures around 15 °C.

The model is based on a number of assumptions and limitations that are at least worth discussing. For example, many effects are described in the literature, which are described by the respective authors on the basis of water structure models. Also in the presence of ions or other charged particles such as proteins. In short, I lack a bit of an overview of the literature. Many criticisms, such as the limitation of the Debeye-Hückel term to a simple statement of pH, are explained and discussed by the authors in their summary. Despite the limitations of a basic model, the authors' work convinces me and I find it worth publishing the manuscript in "plasma".

Comments for author File: Comments.pdf

The manuscript is well written and only needs minor English corrections.

Author Response

Dear Reviewer, thank you for taking the time to review this work of ours and letting us know your valuable comments. 

Round 2

Reviewer 2 Report

The authors have made revisions to the manuscript. Some corrections are still required.
1. The introduction requires improvement, as the authors should consider conducting a comprehensive literature review related to their specific field of study.
2. It is advisable to investigate and provide a detailed analysis of the potential benefits and drawbacks associated with using PAW with different viscosities as compared to DI water.
3. Avoid repeating the full form when the abbreviation is already introduced. e.g. Plasma-Activated Water and PAW

Author Response

Thank you for your valuable comments. We have tried to address these comments in this recent submission. Kindly see our responses below:

1. The introduction requires improvement, as the authors should consider conducting a comprehensive literature review related to their specific field of study.

The introduction has been modified to include PAW applications in line 50 and review on previous PAW physical properties investigated from lines 66 to 69.

2. It is advisable to investigate and provide a detailed analysis of the potential benefits and drawbacks associated with using PAW with different viscosities as compared to DI water.

Details on potential PAW applications owing to its physical property and limitations are added from line 91 to 130.

3. Avoid repeating the full form when the abbreviation is already introduced. e.g. Plasma-Activated Water and PAW

Care have been placed to include PAW full form only once in the abstract and in introduction, after that the abbreviation “PAW” have been used in the manuscript.