Drying Structures of Droplets of Aluminosilicate-Based Hollow Particle Aqueous Dispersions

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript is well-structured and the experimental design is clearly presented. However, there is an opportunity to strengthen the Introduction section. In particular, the contextualization of key variables such as particle morphology and salt concentration could be improved to better position the novelty of the work within the existing literature.

1. Although the article briefly mentions in the Introduction that properties such as particle size and shape influence drying patterns (lines 38–39), this mention is tangential and lacks proper development in terms of relevant scientific background. There is no critical or even concise review of previous studies that have systematically explored the effect of particle morphology (e.g., sphericity, anisotropy, surface roughness) on droplet evaporation dynamics or structure formation. Given that this study focuses on hollow particles with buoyancy and distinct geometric properties, it is essential to contextualize it within the body of literature addressing anisotropic or non-spherical particles. A paragraph including this information in the Introduction would be a welcome addition.

2. While the effects of NaCl are discussed in detail in the Results and Discussion section, there is no dedicated paragraph in the Introduction addressing the rationale for including NaCl, nor is its use properly contextualized. Several prior studies have employed NaCl as an additive to pin the contact line, to induce aggregate formation in deposits, or to generate coffee-ring patterns. This is particularly relevant in light of publications such as:

• Carreón et al. (2021), "Texture Analysis of Dried Droplets for the Quality Control of Medicines", which reports on radial symmetry generated by crystallization patterns induced by NaCl.

• Carreón et al. (2021), "Patterns in Dried Droplets to Detect Unfolded BSA", which demonstrates the use of NaCl as an additive to detect conformational changes in proteins.

• Marin et al. (2019), "Solutal Marangoni Flow as the Cause of Ring Stains from Drying Salty Colloidal Drops", which explores Marangoni flows induced by salt concentration gradients.

A paragraph incorporating this information in the Introduction would improve the scientific framing of the work.

Author Response

Response to Reviewer 1

Dear Reviewer,

We sincerely appreciate your valuable comments and suggestions, which have significantly contributed to improving our manuscript. Below, we provide our responses to your comments and explain the corresponding revisions made to the manuscript.

 

Comments and Suggestions for Authors

The manuscript is well-structured and the experimental design is clearly presented. However, there is an opportunity to strengthen the Introduction section. In particular, the contextualization of key variables such as particle morphology and salt concentration could be improved to better position the novelty of the work within the existing literature.

Comment: Although the article briefly mentions in the Introduction that properties such as particle size and shape influence drying patterns (lines 38–39), this mention is tangential and lacks proper development in terms of relevant scientific background. There is no critical or even concise review of previous studies that have systematically explored the effect of particle morphology (e.g., sphericity, anisotropy, surface roughness) on droplet evaporation dynamics or structure formation. Given that this study focuses on hollow particles with buoyancy and distinct geometric properties, it is essential to contextualize it within the body of literature addressing anisotropic or non-spherical particles. A paragraph including this information in the Introduction would be a welcome addition.

Response: Although hollow particles exhibit unique geometric characteristics, they are essentially spherical in shape. Optical microscopy images of these particles have been presented in our previous study [31], and this point is now described in Lines 58 and 59 of the revised manuscript. Therefore, the most distinguishing feature of the particles is not their morphology, but rather their lower density compared to that of the dispersion medium (water), which has already been emphasized in the Introduction section.

[31] Kimura, H. Rapid Ascent of Hollow Particles in Water Induced by an Electric Field. Powders 2023,2, 737–748.

 

Comment: While the effects of NaCl are discussed in detail in the Results and Discussion section, there is no dedicated paragraph in the Introduction addressing the rationale for including NaCl, nor is its use properly contextualized. Several prior studies have employed NaCl as an additive to pin the contact line, to induce aggregate formation in deposits, or to generate coffee-ring patterns. This is particularly relevant in light of publications such as:

• Carreón et al. (2021), "Texture Analysis of Dried Droplets for the Quality Control of Medicines", which reports on radial symmetry generated by crystallization patterns induced by NaCl.
• Carreón et al. (2021), "Patterns in Dried Droplets to Detect Unfolded BSA", which demonstrates the use of NaCl as an additive to detect conformational changes in proteins.
• Marin et al. (2019), "Solutal Marangoni Flow as the Cause of Ring Stains from Drying Salty Colloidal Drops", which explores Marangoni flows induced by salt concentration gradients.

A paragraph incorporating this information in the Introduction would improve the scientific framing of the work.

Response: NaCl is the most basic and simple monovalent salt, and it allows for the adjustment of ionic strength without introducing specific ion effects. For this reason, it is one of the most commonly used electrolytes in studies on drying patterns, and we have newly cited the work by Carreón et al. (2021) [33] to reflect this point. In our manuscript, discussions based on DLVO theory assume a 1:1 electrolyte system, and the use of NaCl as a representative 1:1 electrolyte is therefore well justified. Among the references you kindly suggested, the study by Marin et al. (2019) is already cited in our manuscript as reference [10].

[33] Carreón, Y.J.P.; Díaz-Hernández, O.; Escalera Santos, G.J.; Cipriano-Urbano, I.; Solorio-Ordaz, F.J.; González-Gutiérrez, J.; Zenit, R. Texture Analysis of Dried Droplets for the Quality Control of Medicines. Sensors 2021, 21, 4048.

 

Once again, we appreciate the constructive feedback and believe that the revisions have improved the manuscript. We look forward to any further comments you may have.

Best regards,
Hiroshi Kimura
On behalf of all authors

Reviewer 2 Report

Comments and Suggestions for Authors

In this manuscript, the drying patterns of sessile droplets of aqueous dispersions of aluminosilicate-based hollow particles were systematically investigated for the first time. Since hollow particles are less dense than the liquid and float in water, the resulting drying patterns were markedly different from the conventional patterns observed for particles denser than the solvent. The findings of this study provide fundamental insights into the drying structures of various types of colloidal droplets. Some issues should be addressed to improve this work before publication.

1. In section introduction, the languageis recommended to be more succinct. It should clearly present the knowledge gap and the novelty of the paper. Some references could be cited to furtherly improve the section introduction ([Surfaces and Interfaces, 2024, 46: 104036], [Applied Surface Science, 2019, 473: 614-621], [Journal of colloid and interface science, 2018, 529: 234-242]).
2. What is the shape of the hollow particles? The particle shapemay influence their movement within droplets and drying patterns, and it is suggested to add the explanation.
3. The manuscript selects a volume fraction range of hollow particles from 0.001 to 0.1 and a salt concentration range of 0 to 0.1 mol/L. Is the selection of these parameters supported by theoretical considerations or preliminary experiments? For instance, if the salt concentration were higher (e.g., 0.5 mol/L), might new drying pattern variations emerge?
4. In section 3, the authors compare gravity and diffusionwhen identifying buoyancy as the primary cause of drying pattern formation. Why are capillary forces and Marangoni effects not considered in this analysis?
5. What is the significance of the experiment where the central floating cluster was directly stimulated with a needle to demonstrate the formation of a flexible and elastic layer? How is it related to the research objectives of this study?
6. Some problems about academic standards exist in the manuscript, such as (not limited): The sharpness of Figure 4(b-i) could be enhanced for better interpretation. The font size of text in Figuresshould be reduced to match the font size of the main text.

Author Response

Response to Reviewer 2

Dear Reviewer,

We sincerely appreciate your valuable comments and suggestions, which have significantly contributed to improving our manuscript. Below, we provide our responses to your comments and explain the corresponding revisions made to the manuscript.

 

Comments and Suggestions for Authors

In this manuscript, the drying patterns of sessile droplets of aqueous dispersions of aluminosilicate-based hollow particles were systematically investigated for the first time. Since hollow particles are less dense than the liquid and float in water, the resulting drying patterns were markedly different from the conventional patterns observed for particles denser than the solvent. The findings of this study provide fundamental insights into the drying structures of various types of colloidal droplets. Some issues should be addressed to improve this work before publication.

Comment: In section introduction, the languageis recommended to be more succinct. It should clearly present the knowledge gap and the novelty of the paper. Some references could be cited to furtherly improve the section introduction ([Surfaces and Interfaces, 2024, 46: 104036], [Applied Surface Science, 2019, 473: 614-621], [Journal of colloid and interface science, 2018, 529: 234-242]).

Response: The ring-like pattern, which most frequently appears in the drying of sessile droplets, is formed as a result of capillary flow transporting particles toward the droplet edge. Since this flow occurs primarily near the bottom of the droplet, conventional sedimenting particles are readily entrained in it. In contrast, hollow particles tend to float in water and may therefore escape the influence of capillary flow. This explanation has been newly added to Lines 51–55 of the revised manuscript. Among the suggested references concerning the influence of dispersion medium composition on drying patterns, we have newly incorporated the study by Al-Milaji et al. (2018) [12] into Line 38.

 

[12] Al-Milaji, K.N.; Radhakrishnan, V.; Kamerkar, P.; Zhao, H. pH-Modulated Self-Assembly of Colloidal Nanoparticles in a Dual-Droplet Inkjet Printing Process. J. Colloid Interface Sci. 2018, 529, 234–242.

 

Comment: What is the shape of the hollow particles? The particle shapemay influence their movement within droplets and drying patterns, and it is suggested to add the explanation.

Response: The hollow particles are spherical in shape [31]. This information has been added to Lines 58 and 59 of the revised manuscript. Optical microscopy images of the hollow particles were presented in our previous study [31].

[31] Kimura, H. Rapid Ascent of Hollow Particles in Water Induced by an Electric Field. Powders 2023,2, 737–748.

 

Comment: The manuscript selects a volume fraction range of hollow particles from 0.001 to 0.1 and a salt concentration range of 0 to 0.1 mol/L. Is the selection of these parameters supported by theoretical considerations or preliminary experiments? For instance, if the salt concentration were higher (e.g., 0.5 mol/L), might new drying pattern variations emerge?

Response: The selected range of particle volume fractions is considered to be typical for studies on the drying patterns of sessile droplets. As clearly shown in Figure 5, at a volume fraction of 0.1, the particle concentration is sufficiently high, and hollow particles tend to spread throughout the entire area of the dried droplet. Regarding the salt concentration, from the perspective of electrostatic interactions between particles, 0.1 mol/L is sufficiently high. At concentrations higher than this, the proportion of NaCl crystals increases drastically, and the entire pattern becomes dominated by crystal deposition, which falls outside the scope of this study. For volume fractions lower than 0.001, it became difficult to visually identify coffee-eye or coffee-ring structures; therefore, we set 0.001 as the lower limit of the volume fraction range.

 

Comment: In section 3, the authors compare gravity and diffusionwhen identifying buoyancy as the primary cause of drying pattern formation. Why are capillary forces and Marangoni effects not considered in this analysis?

Response: We sincerely appreciate this important comment. In our study, we focused on the comparison between gravity (buoyancy) and diffusion. This is because the hollow particles used in this work do not sediment under gravity like conventional particles; instead, they exhibit unique behavior by floating in water due to their lower density (approximately 0.77 g/cm³). This buoyancy-driven ascent significantly affects the vertical distribution of the particles and serves as a primary factor in determining the final drying pattern.

Capillary flow is a well-known mechanism that transports particles toward the contact line and causes the so-called “coffee-ring effect.” However, in the present system, the particles move upward due to buoyancy, counteracting such radial flow. As a result, although a ring-shaped deposition is present, it is not prominent, indicating suppression of the coffee-ring formation. Therefore, capillary flow is not considered the dominant transport mechanism in our system.

Marangoni flow is driven by surface tension gradients at the liquid–air interface of the droplet and typically involves a surface flow component and an induced bulk circulation. Under deionized conditions, this flow is relatively weak, but it may be enhanced with increasing [NaCl]. However, in the drying patterns observed within the range of 10⁻⁴ to 10⁻² mol/L NaCl, no significant changes attributable to Marangoni flow were detected. Thus, under the present experimental conditions, the effect of Marangoni flow is considered to be limited.

These considerations have been briefly incorporated into the revised manuscript in Lines 164–177.

 

Comment: What is the significance of the experiment where the central floating cluster was directly stimulated with a needle to demonstrate the formation of a flexible and elastic layer? How is it related to the research objectives of this study?

Response: The purpose of this experiment was to demonstrate that the particle assembly floating at the center of the droplet during drying is not merely a loosely aggregated cluster, but a flexible and elastic network structure. When the central floating cluster was gently stimulated with a needle, reversible deformation was observed, indicating that the particles were loosely bound in a soft, interconnected structure. This observation suggests that the floating cluster was formed not by interfacial adsorption but rather through direct physical contact between hollow particles.

This finding directly supports one of the central objectives of the study—namely, to clarify what kind of structures are formed by hollow particles that float (rather than sediment) during the drying process. The formation of a flexible and elastic layer at the droplet surface highlights the distinct self-organization dynamics of hollow particles, which differ from those of conventional particle systems. Moreover, this result indicates that buoyancy not only transports particles vertically but also contributes to particle–particle interactions and network formation near the liquid–air interface during evaporation.

This discussion has been added to the revised manuscript in Lines 326–335.

 

Comment: Some problems about academic standards exist in the manuscript, such as (not limited): The sharpness of Figure 4(b-i) could be enhanced for better interpretation. The font size of text in Figuresshould be reduced to match the font size of the main text.

Response: Regarding the font size in Figure 4 and other figures, we believe that the text used in our figures falls within a generally acceptable range. Since the final size of figures is determined during the production process, it is difficult to make precise adjustments to the font size in advance. As for the resolution, drying patterns of sessile droplets are susceptible to interference from lighting conditions. Therefore, we minimized the light intensity as much as possible and used indirect illumination rather than direct lighting to avoid affecting the pattern formation. Due to these constraints and the limitations of our current equipment, obtaining high-clarity images is technically challenging. However, we believe that this limitation does not significantly affect the interpretation of our experimental results.

 

Once again, we appreciate the constructive feedback and believe that the revisions have improved the manuscript. We look forward to any further comments you may have.

Best regards,
Hiroshi Kimura
On behalf of all authors

Reviewer 3 Report

Comments and Suggestions for Authors

The article focuses on investigating the unique features of drying droplets containing hollow nanoparticles with a density lower than that of water, which is an interesting and novel topic. Overall, the manuscript is well-structured and informative. It has the potential to attract interest from the journal’s readership and could be considered for publication after eliminating the comments, some of which are of a fundamental nature.

 

Major Remarks: 

1. Figure 1: The features described by the authors are barely visible in the figure. Readers should be able to clearly see the aspects highlighted by the authors. It is necessary to consider ways to better emphasize these features. For example, image processing could be applied to present pixel brightness profiles or to show selected droplet regions at significantly higher magnification.

 

Minor Remarks: 

1. Factual inaccuracies:

- In the sentence (line 30) "This capillary flow transports colloidal particles, particularly those larger than the micrometer scale," the phrase "larger than the micrometer scale" is imprecise. Size comparisons are usually made with specific units, such as "larger than one micrometer" or "micrometer-sized particles and above." The term "micrometer scale" refers to a range, not a precise size. 

 

2. Terminology consistency: It is important to maintain uniform terminology and avoid ambiguity. For example, in lines 13–14 of the Abstract, the term "bump-shaped structure" is used to describe particle accumulation at the center, whereas the main text uses "coffee-eye" to describe a similar pattern. If these terms refer to the same phenomenon, consistent usage is recommended for clarity.

 

3. Clarity of exposition: For instance, in lines 14–15 of the Abstract, it is stated that particle transport toward the droplet periphery, typically observed during drying, is "rarely seen." However, the main text mentions the formation of a very thin peripheral ring. This may be perceived as contradictory. It should be clarified that while the ring is present, it is very narrow and less pronounced.

 

4. Sentence complexity: Some sentences are overly long and complex. For example, line 34 on page 1 contains the sentence: "The balance between capillary flow and Marangoni flow causes the drying pattern to vary under different conditions, including the composition of the dispersion medium..." The extensive list of conditions makes the sentence cumbersome. Breaking it into shorter, clearer sentences would enhance readability.

 

5. Article usage: The manuscript should be checked for correct use of articles. For example, in line 9 of the Abstract, "macroscopic pattern formation" would be better phrased as "the macroscopic pattern formation."

 

6. Formula explanations: In line 105 on page 3, explanations of variables should be placed immediately after the formula, separated by commas, e.g., "where d is the particle diameter, ..."

 

7. Figure captions: In line 139 on page 4, captions for Figure 1 and other figures should include descriptions of all highlighted features, such as the meaning of dashed or solid lines, arrows, etc. Time and scale bars should be indicated on the figures. It is also important to specify the illumination method used for the images.

 

8. Terminology clarification: In line 152 on page 4, the term "coffee-eye structure" is introduced without explanation, which may confuse readers if it has not been defined earlier. If it is a synonym or part of the "coffee-ring" effect, this should be clarified.

 

9. Word choice: In line 154 on page 4, the phrase *"a ring-shaped pattern instantly appeared"* uses *"instantly,"* which may be too strong for scientific description. Consider using *"rapidly"* or *"quickly."*

 

10. Particle size distribution: In section 3.2, the authors report particle size distribution obtained via optical microscopy and note the difficulty in assessing particles smaller than one micron with this method. It would be beneficial to include electron microscopy images to better evaluate the size distribution.

 

11. Péclet number interpretation: In line 205 on page 5, the Péclet number of approximately 0.55 for 1.4 μm particles indicates comparable gravity and diffusion effects, which is correct. However, the text states that buoyancy dominates for most particles, which contradicts the fact that smaller particles (less than 1.4 μm) have significantly lower Péclet numbers and are diffusion-dominated.

 

12. DLVO theory and Hamaker constant: In line 594 on page 13, the Hamaker constant A_H = 3.2 × 10^-20 J is derived from interaction potential analysis, which is a typical value for silicate particles and confirms correctness. Please verify the proper formatting of formula (6).

 

14. Conclusion: Consider adding a brief mention of potential applications or future research directions to make the conclusion more comprehensive.

 

14. Discussion: The discussion should elaborate more on possible mechanisms for the aggregation of microparticles into clusters. For instance, capillary forces at the liquid surface and the presence of nano- and microbubbles, which are always present in liquids, can induce particle clustering [DOI: 10.1088/1612-2011/11/11/116001]. Additionally, the importance of interactions among particles of different sizes and their dynamics under non-equilibrium drying conditions should be discussed (see e.g., [DOI: 10.1016/j.colsurfa.2024.135965]).

 

15. It is recommended to include a discussion on drying behaviors of biological fluids, as these systems often exhibit complex mass accumulation both at the droplet edges and at the center. Such patterns arise due to the interplay of multiple factors including particle size and density distribution, interactions, evaporation dynamics, and the presence of biomolecules, which can significantly influence the drying morphology. Incorporating this perspective would broaden the scope of the study, highlight the relevance of the findings to biologically relevant systems, and provide valuable context for understanding similar pattern formation mechanisms in complex fluids.

 

16. Drying-induced structures: When describing structures formed during drying of droplets containing nano- and microparticles, mention should be made of the possible formation of pronounced cracks [DOI: 10.1021/acs.langmuir.8b01721] and network structures arising from non-equilibrium dynamic self-organization processes [DOI: 10.1038/nature02087].

Comments on the Quality of English Language

The article is generally well written. Individual comments on the text are provided to the authors above.

Author Response

Response to Reviewer 3

Dear Reviewer,

We sincerely appreciate your valuable comments and suggestions, which have significantly contributed to improving our manuscript. Below, we provide our responses to your comments and explain the corresponding revisions made to the manuscript.

 

Comments and Suggestions for Authors

The article focuses on investigating the unique features of drying droplets containing hollow nanoparticles with a density lower than that of water, which is an interesting and novel topic. Overall, the manuscript is well-structured and informative. It has the potential to attract interest from the journal’s readership and could be considered for publication after eliminating the comments, some of which are of a fundamental nature.

 

Major Remarks: 

Comment: Figure 1: The features described by the authors are barely visible in the figure. Readers should be able to clearly see the aspects highlighted by the authors. It is necessary to consider ways to better emphasize these features. For example, image processing could be applied to present pixel brightness profiles or to show selected droplet regions at significantly higher magnification.

Response: Thank you very much for your valuable comment. In response, we have added four magnified images highlighting key regions in Figure 1. Corresponding modifications have also been made to the figure caption (Lines 141–148 of the revised manuscript).

Minor Remarks:

Comment: Factual inaccuracies:
- In the sentence (line 30) "This capillary flow transports colloidal particles, particularly those larger than the micrometer scale," the phrase "larger than the micrometer scale" is imprecise. Size comparisons are usually made with specific units, such as "larger than one micrometer" or "micrometer-sized particles and above." The term "micrometer scale" refers to a range, not a precise size.
Response: Thank you for your insightful comment. We have revised the relevant part to read “micrometer-sized particles and above” (Lines 30–32 of the revised manuscript).

 

Comment: Terminology consistency: It is important to maintain uniform terminology and avoid ambiguity. For example, in lines 13–14 of the Abstract, the term "bump-shaped structure" is used to describe particle accumulation at the center, whereas the main text uses "coffee-eye" to describe a similar pattern. If these terms refer to the same phenomenon, consistent usage is recommended for clarity.

Response: We have clearly indicated in both the Abstract and the main text that the term “bump-shaped structure” refers to what is commonly known as a “coffee-eye,” and we have unified the terminology by primarily using “coffee-eye” throughout the manuscript (Lines 12–16, 379–382).

 

Comment: Clarity of exposition: For instance, in lines 14–15 of the Abstract, it is stated that particle transport toward the droplet periphery, typically observed during drying, is "rarely seen." However, the main text mentions the formation of a very thin peripheral ring. This may be perceived as contradictory. It should be clarified that while the ring is present, it is very narrow and less pronounced.

Response: We have revised the description to clarify that “while a ring pattern is present, it is extremely narrow and barely noticeable” (Lines 15–16 and 169–170).

 

Comment: Sentence complexity: Some sentences are overly long and complex. For example, line 34 on page 1 contains the sentence: "The balance between capillary flow and Marangoni flow causes the drying pattern to vary under different conditions, including the composition of the dispersion medium..." The extensive list of conditions makes the sentence cumbersome. Breaking it into shorter, clearer sentences would enhance readability.

Response: We have revised the sentence by dividing it into two parts to improve clarity (Lines 36–40):

Comment: Article usage: The manuscript should be checked for correct use of articles. For example, in line 9 of the Abstract, "macroscopic pattern formation" would be better phrased as "the macroscopic pattern formation."

Response: We have revised the text as suggested (Line 10 of the revised manuscript).

 

Comment: Formula explanations: In line 105 on page 3, explanations of variables should be placed immediately after the formula, separated by commas, e.g., "where d is the particle diameter, ..."

Response: We have revised the text as suggested (Line 117 of the revised manuscript).

 

Comment: Figure captions: In line 139 on page 4, captions for Figure 1 and other figures should include descriptions of all highlighted features, such as the meaning of dashed or solid lines, arrows, etc. Time and scale bars should be indicated on the figures. It is also important to specify the illumination method used for the images.

Response: We have corrected the deficiencies in the figure captions (Lines 141–148, 183–185, 363, and 364). For images taken during the drying process, all time information has been clearly indicated in the captions. Scale bars have also been described for all figures. In addition, the illumination method has been briefly noted in the text (Lines 103–105).

 

Comment: Terminology clarification: In line 152 on page 4, the term "coffee-eye structure" is introduced without explanation, which may confuse readers if it has not been defined earlier. If it is a synonym or part of the "coffee-ring" effect, this should be clarified.

Response: Thank you for pointing this out. The term “coffee-eye” in this context should have been written as “FC,” and we have corrected it accordingly (Line 158 of the revised manuscript).

 

Comment: Word choice: In line 154 on page 4, the phrase *"a ring-shaped pattern instantly appeared"* uses *"instantly,"* which may be too strong for scientific description. Consider using *"rapidly"* or *"quickly."*

Response: As you correctly pointed out, we have revised “instantly” to “rapidly” (Line 160 of the revised manuscript).

 

Comment: Particle size distribution: In section 3.2, the authors report particle size distribution obtained via optical microscopy and note the difficulty in assessing particles smaller than one micron with this method. It would be beneficial to include electron microscopy images to better evaluate the size distribution.

Response: The evaluation of particle size distribution was conducted using not only optical microscopy observations but also the results of dynamic light scattering (DLS) measurements. The combined data allowed us to clarify the particle size distribution, as shown in Figure 3.

 

Comment: Péclet number interpretation: In line 205 on page 5, the Péclet number of approximately 0.55 for 1.4 μm particles indicates comparable gravity and diffusion effects, which is correct. However, the text states that buoyancy dominates for most particles, which contradicts the fact that smaller particles (less than 1.4 μm) have significantly lower Péclet numbers and are diffusion-dominated.

Response: The relationship between particle size and volume fraction is as follows. According to the DLS measurement results, particles with diameters smaller than 1.3 µm account for approximately 20% of the total volume, and those smaller than 1.0 µm account for less than 10%. Therefore, buoyancy is the dominant factor for the larger particle population, which comprises approximately 80% of the total volume. We have added and revised the text accordingly (Lines 232–236 of the revised manuscript).

 

Comment: DLVO theory and Hamaker constant: In line 594 on page 13, the Hamaker constant A_H = 3.2 × 10^-20 J is derived from interaction potential analysis, which is a typical value for silicate particles and confirms correctness. Please verify the proper formatting of formula (6).

Response: Thank you for confirming the validity of the Hamaker constant. We have used the equation editor to correctly format and present the equation (Line 626 of the revised manuscript).

 

Comment: Conclusion: Consider adding a brief mention of potential applications or future research directions to make the conclusion more comprehensive.

Response: We have provided a brief mention of potential future research directions at the end of the Conclusion section (Lines 706–709 of the revised manuscript).

 

Comment: Discussion: The discussion should elaborate more on possible mechanisms for the aggregation of microparticles into clusters. For instance, capillary forces at the liquid surface and the presence of nano- and microbubbles, which are always present in liquids, can induce particle clustering [DOI: 10.1088/1612-2011/11/11/116001]. Additionally, the importance of interactions among particles of different sizes and their dynamics under non-equilibrium drying conditions should be discussed (see e.g., [DOI: 10.1016/j.colsurfa.2024.135965]).

Response: We have focused on electrostatic interactions between particles as a key mechanism for the formation of microparticle clusters. As suggested by the discussion based on DLVO theory and the results shown in Figure 5, at NaCl concentrations of approximately 0.1 mol/L or higher, the formation of three-dimensional networks driven by London dispersion forces is likely. On the other hand, in the low-salt concentration range, Figure 2 shows that the floating clusters (FC) observed during drying exhibit elastic properties. When the number density of particles becomes high, the particles themselves may behave as electrolytes, effectively increasing the local ionic strength within dense regions. As a result, elastic structures similar to the three-dimensional networks observed at high salt concentrations may have been formed. This discussion has been added to the revised manuscript (Lines 326–335).

 

Comment: It is recommended to include a discussion on drying behaviors of biological fluids, as these systems often exhibit complex mass accumulation both at the droplet edges and at the center. Such patterns arise due to the interplay of multiple factors including particle size and density distribution, interactions, evaporation dynamics, and the presence of biomolecules, which can significantly influence the drying morphology. Incorporating this perspective would broaden the scope of the study, highlight the relevance of the findings to biologically relevant systems, and provide valuable context for understanding similar pattern formation mechanisms in complex fluids.

Response: Our research is primarily focused on fundamental colloid science, and we do not have sufficient expertise in complex systems such as biological or ecological environments. Therefore, we have decided to refrain from including speculative or superficial discussions on such topics.

 

Comment: Drying-induced structures: When describing structures formed during drying of droplets containing nano- and microparticles, mention should be made of the possible formation of pronounced cracks [DOI: 10.1021/acs.langmuir.8b01721] and network structures arising from non-equilibrium dynamic self-organization processes [DOI: 10.1038/nature02087].

Response: No cracks were observed in this study, which we believe indicates the flexibility of the films formed by hollow particles. This point has been added to the revised manuscript (Lines 351 and 352).

 

Once again, we appreciate the constructive feedback and believe that the revisions have improved the manuscript. We look forward to any further comments you may have.

Best regards,
Hiroshi Kimura
On behalf of all authors

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

I have carefully reviewed the revised version of the manuscript. The authors have significantly improved the Introduction and have addressed my previous suggestions appropriately. I have no further comments, and I consider the manuscript suitable for publication in its present form.

Reviewer 2 Report

Comments and Suggestions for Authors

The authors has made the revisions as requested and the manuscript can be published.

Reviewer 3 Report

Comments and Suggestions for Authors

The authors have carried out a certain amount of work, made the necessary corrections and additions, which significantly improved the quality of the article. In its current form, the article presents some interest to the journal's readers and can be recommended for publication.