Review Reports - Controlling the Polymer Ink’s Rheological Properties to Form Single and Stable Droplet

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The paper entitled “Controlling the polymer ink’s rheological properties to form single and stable droplets” by Du et al. described the formation studies of single and stable ink to produce high-quality functional films by inkjet printing. Several questions can be addressed as follows:

1. In the introduction section, was there any different ink previously reported and investigated to be employed for inkjet printing than poly3-hexylthiophene (P3HT) ink? Why was this ink selected for this work?

2. What is the viscosity of the prepared inks?

3. What is the surface tension of the prepared inks?

4. How to prepare the ink with different Weber (We) numbers?

5. Why was the concentration of 5 mg/mL ink selected for this work? Is it feasible to prepare higher or lower concentrations of PH3T ink?

6. It is suggested to apply a similar investigation at higher and lower concentrations of inks.

7. Can the contact angle data of ink droplets on the planar substrate be obtained?

8. What is the conductivity of the prepared ink from PH3T?

9. Is it feasible to apply a similar investigation with different types of inks?

Comments on the Quality of English Language

English is fine

Author Response

Response to reviewers

Dear Editor,

Thank you very much for the constructive remarks and useful suggestions on our manuscript mentioned above with ID: coatings-2991931, which has significantly raised the quality of the manuscript and has enable us to improve the manuscript. We have revised it according to the referee comments and suggestions. The modifications of the manuscript have been highlighted in the manuscript.

If there are any further questions, please let me know at your earliest convenience. Thank you for your kindness and help in advance.

Looking forward to hearing from you soon.

Sincerely yours

Yan Xue

Responds to the reviewer’s comments:

Reviewer 1:

Comments to the Author

In the introduction section, was there any different ink previously reported and investigated to be employed for inkjet printing than poly3-hexylthiophene (P3HT) ink? Why was this ink selected for this work?

Reply of the Authors:

Thank you very much for your suggestion. According to your suggestion, we have discussed several ink properties and application scenarios that can be used for inkjet printing in the revised manuscript, such as glycerin-water and glycerin-water-isopropanol mixture and linear and 6-arm star PMMA inks. P3HT is considered as a suitable material for this scenario due to its excellent properties, but its printing performance hasn’t been studied enough. And we want to use P3HT for the following device research. Therefore, the inkjet printing performance of P3HT under different working conditions is discussed in this paper. The revised content is supplemented as follows:

Page 1, Line 32:

In the early research, inks with none solute were often used in DOD inkjet printing [16-21]. Morris, Jeffrey F. et.al used the glycerin-water and glycerin-water-isopropanol mixture as the inks and the main stages of DOD drop formation were analyzed[22]. The main stages of DOD drop formation include ejection and stretching of liquid, pinch-off of liquid thread from the nozzle exit, contraction of liquid thread, breakup of liquid thread into primary drop and satellites, and recombination of primary drop and satellites. Daehwan Jang et.al have investigated the inter-relationship between ink-jet printability and physical fluid properties by monitoring droplet formation dynamics using a mixture of ethyl alcohol and ethylene glycol[23]. Berend-Jan de Gans et.al have investigated the influence of architecture on ink-jet printability of polymer solutions by comparing linear and 6-arm star PMMA inks[24].

What is the viscosity of the prepared inks?.

Reply of the Authors:

Thanks for your reminding, we have added Table 1 in the paper to clearly express the performance parameters of P3HT used, and made corresponding explanations in the paper. The revised manuscript is supplemented as follows in Page 3, Line 116:

The physical parameters of the Poly3-hexylthiophene (P3HT) ink are shown in Table 1.

Table 1. Physical parameters of the Poly3-hexylthiophene (P3HT) ink

Parameters	Value
Density, ρ	1068.40 Kg/m³
Surface tension,γ	31.0 mN/m
Viscosity, η	1.20 mPa·s
Conductivity, G	450 S/m
Z value	33.91

What is the surface tension of the prepared inks?.

Reply of the Authors:

Thank for your reminding, we have added the surface tension data the paper in Table 1 to clearly express the performance parameters of P3HT used in Page 3, line 118.

How to prepare the ink with different Weber (We) numbers?.

Reply of the Authors:

Thank for your asking, Weber numbers are used to evaluate droplet morphology and stability. In this article, through regulating the ink droplet velocity, the P3HT inks with different Weber number features. The revised content is supplemented as follows in Page 3, Line 119:

As previously stated, in drop-on-demand (DOD) inkjet printing, the production of uniform and stable ink droplets is crucial for fabricating high-quality functional films. The morphology of these droplets is chiefly influenced by ink viscosity, surface tension, and ejection velocity, with the Weber number (We), a dimensionless quantity repre-senting the relative importance of inertial forces to surface tension, commonly em-ployed to comprehend the droplet shape and stability. This study, by regulating the ink ejection velocity, examines the droplet ejection characteristics of P3HT ink under var-ious Weber number conditions as depicted in Table 2, thereby enhancing the under-standing and controlling of its application in DOD inkjet printing processes.

Table 2. The value of We for the Poly3-hexylthiophene (P3HT) at different dropping velocity

Velocity (m/s)	Re	We
1.1	50.8	2.2
1.8	81.1	5.7
2.3	103.1	9.3
2.6	115.8	11.7

Why was the concentration of 5 mg/mL ink selected for this work? Is it feasible to prepare higher or lower concentrations of PH3T ink?

Reply of the Authors:

Thanks for your advice, the concentration of the ink directly affects the Z value, and the change of the Z value will influence the inkjet printing dynamic process of the ink (as shown in Figure 7). In the previous part of this paper, We mainly discussed the influence of different We number on droplet morphology when the ink concentration is 5mg/ml. Of course, this method is feasible to prepare higher concentrations of PH3T ink. In the later part (FIG. 7), we also discussed the influence of different Z values on the droplet morphology. But due to our unclear expression, the influence of concentration on Z value was not clearly stated. According to your suggestion, We added Z values corresponding to different ink concentrations in order to better understand the droplet formation process. In subsequent studies, we will focus on the dynamic properties of droplets at smaller concentrations, i.e. smaller Z values. The revised content is supplemented as follows in Page 4, line 133:

Table 3. Z value of different concentrations of Poly3-hexylthiophene (P3HT)

Concentration (mg/mL)	Z
5	33.91
10	24.31
15	18.30
20	12.75

It is suggested to apply a similar investigation at higher and lower concentrations of inks.

Reply of the Authors:

Thank you very much for your suggestion. We are very sorry for your confusing because of our unclear expression. We have added Table 3 to illustrate the Z values corresponding to different concentrations, thus corresponding to the droplet behavior of different Z values in FIG. 6. The modified content is as follows in Page 4, Line 133

The concentration of the ink directly affects the Z value, and the change of the Z value will influence the inkjet printing dynamic process of the ink. Inks with different Z value was also investigated in Table 3.

Table 3. Z value of different concentrations of Poly3-hexylthiophene (P3HT)

Concentration (mg/mL)	Z
5	33.91
10	24.31
15	18.30
20	12.75

Can the contact angle data of ink droplets on the planar substrate be obtained?

Reply of the Authors:

Thank you very much for your advice. The printing effect of inkjet printing is affected by many factors, such as the type of ink substrate material, ink concentration, ink drop speed, etc. Of course, the contact Angle of the ink drop on the flat substrate has a very large impact on the printing effect of inkjet printing. This paper focuses on the dynamic process of droplets of P3HT inks with different Weber numbers from a single nozzle to contact with the substrate. In our follow-up work, we will focus on the development of ink droplets on the substrate, including the determination of contact Angle.

What is the conductivity of the prepared ink from PH3T?

Reply of the Authors:

Thanks for your reminding, we have added the conductivity data in Table 1 in the revised manuscript to clearly express the performance parameters of P3HT in Page 3, Line 118.

Is it feasible to apply a similar investigation with different types of inks?

Reply of the Authors:

Thanks for your question. According to our understanding, for the different types of inks with certain working conditions (Weber number, Z value, etc.), their dynamic process changes may show different mechanisms, but their research methods should be similar. In this paper, P3HT is selected as the research object due to its excellence properties. In the later research, we will explore different ink types suitable for DOD, to further understanding the formation of a single and stable ink droplet.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This is and interesting experimental work devoted to the rheology of inks used in ink-jet printing. However, I noticed a few weak points that require clarification. I point them out below.

It is commonly assumed that the dynamics of droplet formation is described by the dimensionless Weber number (We), whereas the fluid flow is described by the dimensionless Reynolds number (Re). Also important is the Z number, which is the ratio of the Reynolds number to the square root of the Weber number. To determine the above dimensionless numbers, the values of the droplet velocity, characteristic geometric parameter, fluid density, fluid viscosity and surface tension are needed.

However, based on the results included in the manuscript, we only can get an idea about the vlue of drop velocity. The values of the remaining parameters are missing and therefore there is no certainty that the values of the Weber number or Z number presented in the paper were determined correctly. As for ink viscosity and surface tension, only how and with what instruments they were measured is given, but there are no results (there is also a question whether the tested ink is a Newtonian fluid, characterized by constant value of viscosity, or a non-Newtonian fluid, i.e. a fluid with variable viscosity). There is also no information about the characteristic geometric parameter and ink density.

Therefore, in order to decide on the possibility of printing the reviewed manuscript in Coatings, it should be supplemented with the above-mentioned parameters and characteristics.

Comments on the Quality of English Language

Generally, the Authors incorrectly write the following fragments: "... cell guidance[1-8]" (line 26), "... diodes[9-13]" (line 27) and many others - these expressions should be placed with a space. .. cell guidance [1-8]", "... diodes [9-13]".

Author Response

Response to reviewers

Dear Editor,

If there are any further questions, please let me know at your earliest convenience. Thank you for your kindness and help in advance.

Looking forward to hearing from you soon.

Sincerely yours

Yan Xue

Reviewer 2:

Comments to the Author

This is and interesting experimental work devoted to the rheology of inks used in ink-jet printing. However, I noticed a few weak points that require clarification. I point them out below.

It is commonly assumed that the dynamics of droplet formation is described by the dimensionless Weber number (We), whereas the fluid flow is described by the dimensionless Reynolds number (Re). Also important is the Z number, which is the ratio of the Reynolds number to the square root of the Weber number. To determine the above dimensionless numbers, the values of the droplet velocity, characteristic geometric parameter, fluid density, fluid viscosity and surface tension are needed.

Therefore, in order to decide on the possibility of printing the reviewed manuscript in Coatings, it should be supplemented with the above-mentioned parameters and characteristics.

Reply of the Authors:

Thank you very much for your suggestion. The tested ink is a non-Newtonian fluid. And the above-mentioned parameters and characteristics have been supplemented in the revised manuscript as follows:

Page 3, Line 116 (the values of the droplet velocity, fluid density, fluid viscosity and surface tension):

The physical parameters of the Poly3-hexylthiophene (P3HT) ink are shown in Table 1.

Table 1. Physical parameters of the Poly3-hexylthiophene (P3HT) ink

Parameters	Value
Density, ρ	1068.40 Kg/m³
Surface tension,γ	31.0 mN/m
Viscosity, η	1.20 mPa·s
Conductivity, G	450 S/m
Z value	33.91

Page 4, Line 128 (the values of the droplet velocity):

Table 2. The value of We for the Poly3-hexylthiophene (P3HT) at different dropping velocity

Velocity (m/s)	Re	We
1.1	50.8	2.2
1.8	81.1	5.7
2.3	103.1	9.3
2.6	115.8	11.7

Page 2, Line 90 (the characteristic geometric parameter):

The ink-jet printing experiments were conducted using the AD-P-8000 Printing System with high resolution built-in CCD shown in Figure 1, provided by Microdrop Technologies GmbH. Its nozzle diameter is 50 μm.

Generally, the Authors incorrectly write the following fragments: "... cell guidance[1-8]" (line 26), "... diodes[9-13]" (line 27) and many others - these expressions should be placed with a space. .. cell guidance [1-8]", "... diodes [9-13]".

Reply of the Authors:

Thank you very much for your suggestion. We have revised the reference in the article updated with the new references and added a space in the expressions.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

In the paper "Controlling the Polymer Ink’s Rheological Properties to form single and stable droplet", the authors propose a DOD-specific experimental study. The novelty element is brought by carefully investigating the ink droplet formation behavior of Poly3-hexylthiophene (P3HT) at different Weber numbers (We) and evaluating the impact of Z value on the formation of single ink droplets. The paper follows a methodologically correct approach with results demonstrable by other authors due to sufficient details for reproducing the experiments. On the other hand, some shortcomings or insufficient approaches can be pointed out whose correction can add value to this work.

1. In the first paragraph of the introduction, 32 out of 35 references are reviewed. I propose to address the content of these references more consistently throughout the paper. Some of them can be used in a Discussion section independent of the Results section where a comparative study with the literature would be a plus.

2. Also, some figures can be spliced out for a more homogeneous presentation of the results and pagination.

3. Each figure should have a caption that makes clear reference to the content of each sub-figure ... (a) ... (b) ... not only in Figure 3.

4. A picture or diagram of the experimental setup adds to the Materials and Methods section, which is excessively brief anyway. All equipment used should be described including the high speed CCD.

5. Since 32 citations were used in the first 7 lines of text I think the paper in this approach should eventually reach an even higher number of citations. The number of citations is not high but I invite you to pay more attention to this aspect including self citations.

Author Response

Response to reviewers

Dear Editor,

If there are any further questions, please let me know at your earliest convenience. Thank you for your kindness and help in advance.

Looking forward to hearing from you soon.

Sincerely yours

Yan Xue

Reviewer 3:

Comments to the Author

In the first paragraph of the introduction, 32 out of 35 references are reviewed. I propose to address the content of these references more consistently throughout the paper. Some of them can be used in a Discussion section independent of the Results section where a comparative study with the literature would be a plus.

Reply of the Authors:

Thank you very much for your advice. We have revised the introduction and corrected the references. The details are as shown in the revised manuscript.

Also, some figures can be spliced out for a more homogeneous presentation of the results and pagination.

Reply of the Authors:

Thanks a lot for your suggestion. We have reviewed all the diagrams and reformatted Figure 2 and 3.
3. Each figure should have a caption that makes clear reference to the content of each sub-figure ... (a) ... (b) ... not only in Figure 3.

Reply of the Authors:

Thanks a lot for your advice. We have reviewed the captions of sub-figure in Figure 2, 3 and 5.

A picture or diagram of the experimental setup adds to the Materials and Methods section, which is excessively brief anyway. All equipment used should be described including the high speed CCD.

Reply of the Authors:

Thank you very much for your suggestion. We are very sorry for your confusing because of our unclear expression. According to your suggestion, we have added the schematic diagram of experimental setup and made corresponding explanations in the paper in Page 2, Line 90. All equipment description including the high speed CCD have been added and sample preparation was introduced in detail in this section.

The revised content is as follows:

Page 2, Line 90:

The ink-jet printing experiments were conducted using the AD-P-8000 Printing System with high resolution built-in CCD, provided by Microdrop Technologies GmbH. Its nozzle diameter is 50 μm. Its viscosity and surface tension range are 0.4 – 100 mPa•s and 28 – 72 mN/m. And its max printing frequency is 2000 Hz.

Page 3, Line 98:

Figure 1. Schematic diagram of the experimental setup

Since 32 citations were used in the first 7 lines of text I think the paper in this approach should eventually reach an even higher number of citations. The number of citations is not high but I invite you to pay more attention to this aspect including self citations.

Reply of the Authors:

Thank you very much for your suggestion. We have revised the introduction section according to your suggestions and the reference in the article have been updated with the new references.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The revised manuscript is now fine and can be considered for publication.

Author Response

Response to reviewers

Dear Editor,

If there are any further questions, please let me know at your earliest convenience. Thank you for your kindness and help in advance.

Looking forward to hearing from you soon.

Sincerely yours

Yan Xue

Responds to the reviewer’s comments:

Reviewer 2:

Comments to the Author

The revised version of this manuscript takes into account most of my comments and therefore is more interesting and contains more useful information than the original version.

However, the analyzed medium is a non-Newtonian fluid. Therefore, I am asking for information on how its viscosity changes when the drop velocity changes from 1.1 to 2.8 m/s and how this change affects the values of the We, Re and Z numbers.

I suggest supplementing the article with this information.

Reply of the Authors:

Thank you very much for your suggestion. For non-Newtonian fluid, its viscosity curve does not conform to Newton's law of viscosity (viscosity keeps constant with the shear rate change). There is a plateau in the curve of non-Newtonian fluid’s viscosity changing with shear rate, during which the viscosity changes a little. Our research focus on the behavior of forming single and stable ink droplet of P3HT. So, the droplet velocity changing from 1.1 to 2.8 m/s was chosen to study the dynamic process with relatively small shear rate, unable to cause a large change in viscosity. In our later work，we will focus on the effect of high shear rate on ink formation.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The revised version of this manuscript takes into account most of my comments and therefore is more interesting and contains more useful information than the original version.

However, the analyzed medium is a non-Newtonian fluid. Therefore, I am asking for information on how its viscosity changes when the drop velocity changes from 1.1 to 2.8 m/s and how this change affects the values of the We, Re and Z numbers.

I suggest supplementing the article with this information.

Author Response

Dear Editor,

If there are any further questions, please let me know at your earliest convenience. Thank you for your kindness and help in advance.

Looking forward to hearing from you soon.

Sincerely yours

Yan Xue

Responds to the reviewer’s comments:

Reviewer 2:

Comments to the Author

The revised version of this manuscript takes into account most of my comments and therefore is more interesting and contains more useful information than the original version.

However, the analyzed medium is a non-Newtonian fluid. Therefore, I am asking for information on how its viscosity changes when the drop velocity changes from 1.1 to 2.8 m/s and how this change affects the values of the We, Re and Z numbers.

I suggest supplementing the article with this information.

Reply of the Authors:

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The authors of the revised version of the paper “Controlling the Polymer Ink’s Rheological Properties to form single and stable droplet” have substantially completed this version with better graphic, tablet and explanatory content in line with the recommendations in the original version. The work in this version is very well developed and structured in accordance with general practice. In its present form, I consider that the paper meets the conditions for publication.

Author Response

Response to reviewers

Dear Editor,

If there are any further questions, please let me know at your earliest convenience. Thank you for your kindness and help in advance.

Looking forward to hearing from you soon.

Sincerely yours

Yan Xue

Responds to the reviewer’s comments:

Reviewer 2:

Comments to the Author

The revised version of this manuscript takes into account most of my comments and therefore is more interesting and contains more useful information than the original version.

However, the analyzed medium is a non-Newtonian fluid. Therefore, I am asking for information on how its viscosity changes when the drop velocity changes from 1.1 to 2.8 m/s and how this change affects the values of the We, Re and Z numbers.

I suggest supplementing the article with this information.

Reply of the Authors:

Author Response File: Author Response.pdf