Direct Observation of Transient Flow Kinematics of Environment-Friendly Silica-Based Alcogel at Instantaneous Gelation

Round 1

Reviewer 1 Report

In general, I find the paper interesting and well written. I only have the following comments and suggestions that must be addressed so that the paper can be accepted for publication in the journal Sustainability:

In the abstract and in several other parts of the paper the timescale for the instantaneous termination of the flow is expressed as 10⁰ ms, while in other parts of the paper (for instance, in Figure 7) it is expressed as 1 ms. I suggest that it be expressed in the latter way throughout the paper.

Line 66 on page 2 says “The state transition of liquids to objects with no mobility is classified to gelation.” In this sentence, I suggest that instead of the word "objects" the word "materials" be used.

Lines 120-121 on page 3 say “…was shorter by orders than the that of the gel time [49].” Something seems to be missing in this expression, please review it.

Line 209 on page 5 says “submicron seconds”. This time unit is unusual, please verify if it is correct.

Lines 239-240 on page 5 say “…no notable morphological alteration was not observed…”. This expression is confusing, please review it.

Lines 362-365 on page 10 say “The above result provided an unexpected view on the instantaneous gelation dealt with in the present experimental work, which told us that the arrest of the flow as the elementary step of the visible megascopic gelation occurred far more promptly by orders than in the visually perceived impression induced by our naked-eye direct observation.” This sentence is confusing, please review it.

Lines 381-385 on page 11 say “The choice of the direct observation as the main experimental method in the present study was attributed to the fact that such established methods as rheological measurements for detecting when the objects have come to the point for satisfying the criterion for the gelation were not capable of harvesting the blink-long instantaneous gelation in what we call “quick gel”.” However, the paper does not present results of rheological measurements to support this claim.

In general, the quality of the English writing of the paper is good, except for some small details that I have already pointed out in the previous section. 

Author Response

<response to reviewer 1>

Dear reviewers,

 

We are very grateful to the reviewers for the great efforts to give us the advisory comments for revising the submitted manuscript. According to the useful comments by them, we largely amended the manuscript with our best efforts to fulfill what the advisory comments are about. In some cases, we chose to address our aim and opinions in response to the reviewer’s comments in addition to the amendments in the manuscript. We sincerely hope that our efforts to improve the manuscript will smoothly lead to the help for the reviewers and potential readers to comprehend the essential aim and the results of the present study.

The modified or added parts according to your suggestions are indicated by red color. (The other colors are used for the same purpose to respond to other reviewers.)

We thank you again for having taken your time resource for our trial of publication.

 

                                                                                                  Regards,

1. Kurumada, H. Ue, and J. Sato

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In the abstract and in several other parts of the paper the timescale for the instantaneous termination of the flow is expressed as 10⁰ ms, while in other parts of the paper (for instance, in Figure 7) it is expressed as 1 ms. I suggest that it be expressed in the latter way throughout the paper.

 

=> Thank you very much for the helpful suggestion. The expression of 0-th power was replaced by much simpler “1” throughout the manuscript.

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Line 66 on page 2 says “The state transition of liquids to objects with no mobility is classified to gelation.” In this sentence, I suggest that instead of the word "objects" the word "materials" be used.

 

=> Thank you very much for the helpful suggestion. The expression of “objects” was replaced by much more common “materials” throughout the manuscript.

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Lines 120-121 on page 3 say “…was shorter by orders than the that of the gel time [49].” Something seems to be missing in this expression, please review it.

 

=> Thank you very much for the helpful suggestion.

The part “…was shorter by orders than the that of the gel time.” was replaced by “was negligible compared to the gel time.”.

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Line 209 on page 5 says “submicron seconds”. This time unit is unusual, please verify if it is correct.

 

=> Thank you for finding our mistyping. Submicron was corrected to submilli in accordance with 1000 to 5000 fps as the actual frame rate in taking the high-speed motion pictures.

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Lines 239-240 on page 5 say “…no notable morphological alteration was not observed…”. This expression is confusing, please review it.

 

=> Thank you very much for pointing out the misleading expression.

“no notable morphological alteration was not observed (Figure 1G, 1H, and 1I) despite the repetition of the impingements onto both the bottoms of the shaken cylinder.” was replaced by “it stayed unchanged although it was repeatedly hit by the bottoms of the shaken cylinder (Figure 1G, 1H, and 1I).”.

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Lines 362-365 on page 10 say “The above result provided an unexpected view on the instantaneous gelation dealt with in the present experimental work, which told us that the arrest of the flow as the elementary step of the visible megascopic gelation occurred far more promptly by orders than in the visually perceived impression induced by our naked-eye direct observation.” This sentence is confusing, please review it.

 

=> Thank you very much for pointing out the misleading expression.

“The above result provided an unexpected view on the instantaneous gelation dealt with in the present experimental work, which told us that the arrest of the flow as the elementary step of the visible megascopic gelation occurred far more promptly by orders than in the visually perceived impression induced by our naked-eye direct observation.” was replaced by “Such a tiny timescale as 1 ms for the gelation to occur could have not been derived if we had merely relied on our naked-eye observation of the whole gelling system as presented in Video S1 and Video S3 which were displayed in the real time. Those who watched these motion pictures including the overall macroscopic gelling behavior were supposed to conclude that the vigorously stirred mother solution took the gel time about 0.5 s from the kinematic naked-eye visual impression. The spotted termination of the flow which lead to the ultimate gelation took only 1ms, which was much shorter than the abovementioned apparent gel time in the naked-eye impression.”.

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Lines 381-385 on page 11 say “The choice of the direct observation as the main experimental method in the present study was attributed to the fact that such established methods as rheological measurements for detecting when the objects have come to the point for satisfying the criterion for the gelation were not capable of harvesting the blink-long instantaneous gelation in what we call “quick gel”.” However, the paper does not present results of rheological measurements to support this claim.

 

=> Thank you very much for pointing out the misleading expression.

“The choice of the direct observation as the main experimental method in the present study was attributed to the fact that such established methods as rheological measurements for detecting when the objects have come to the point for satisfying the criterion for the gelation were not capable of harvesting the blink-long instantaneous gelation in what we call “quick gel”.” was replaced by “The choice of the direct observation of the quick gelation in the present study was unavoidable considering that implementing serial rheological measurements to obtain the storage and loss modulus including careful sampling and setting the sampled solution on the apparatus is impossible within only a few seconds. As seen in Video S1 or Video S2, gelation can be completed almost instantaneously, to which only direct observational methods supported by some “augmented eye” like a high-speed camera can be applied.”.

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Author Response File: Author Response.pdf

Reviewer 2 Report

The problem is timely and interesting. I recommend the publication of the manuscript after the following revisions are properly made:

 

1) The abstract should be improved.

2) For general readers, aouthors are encouraged to discuss other applications of Silica such as: [(a) - Khorasani et al. (2023), “A refined vibrational analysis of the FGM porous type beams resting on the silica aerogel substrate”, Steel and Composite Structures, 47(5), 633-644.; (b) - Garg et al. (2022), “Machine learning models for predicting the compressive strength of concrete containing nano silica”, Computers and Concrete, 30(1), 33-42.].

3) Figs. 6 and 7 should be more discussed.

4) The conclusion should be improved.

The Quality of English is good.

Author Response

<response to reviewer 2>

 

Dear reviewers,

 

We are very grateful to the reviewers for the great efforts to give us the advisory comments for revising the submitted manuscript. According to the useful comments by them, we largely amended the manuscript with our best efforts to fulfill what the advisory comments are about. In some cases, we chose to address our aim and opinions in response to the reviewer’s comments in addition to the amendments in the manuscript. We sincerely hope that our efforts to improve the manuscript will smoothly lead to the help for the reviewers and potential readers to comprehend the essential aim and the results of the present study.

The modified or added parts according to your suggestions are indicated by green color. (The other colors are used for the same purpose to respond to other reviewers.)

We thank you again for having taken your time resource for our trial of publication.

 

                                                                                                  Regards,

1. Kurumada, H. Ue, and J. Sato

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

1) The abstract should be improved.

 

=> Thank you very much for the useful advice. The abstract was fully modified from the original one as given in the revised manuscript.

Abstract: This study was intended to exploit the possibility of using the quick gelation of alcogel which is induced by adding catalytic imidazole into silicate-oligomer-based solution. For this purpose, the experimental viability of direct observation of the gelation behavior was actually examined. The silicate oligomer derived from tetraethyl orthosilicate to be hydrolyzed under acidic condition (pH ~ 5) was used as the quickly gelling mother solution. The capability of the oligomer solution to form a non-flowable matter in only a few seconds triggered by the addition of the catalytic solution of imidazole is promising, for example, for stabilizing sandy ground surface due to its simplicity. From the practical viewpoint, how long the gelation could take ( = gel time ) is a crucial parameter when a choice of an appropriate gelling chemical species needs to be made. Thus, this study focused its interest on as a simple experimental method as possible for evaluating the gel time of the gelling systems which actually underwent instantaneous gelation. Silicate oligomer solution was an appropriate material both in its quick gelling behavior and environmental friendliness. For such quick gelation, rheological approaches are not applicable to detecting the boundary in the mechanical properties which delineates the regime of “gel”. In this study, instead, direct observation was employed to capture the short interval during which the gelation was completed. The silicate-oligomer-based gelling solution was observed to lose its flowability within only 0.2 s as seen to come off the bottom of the shaken cylinder at 5 Hz. For more quantitative estimation, the same gelling solution was observed by high-speed motion picture. The high-speed motion picture could clearly capture the instantaneous gelation as a sudden arrest of the flow. The submilli-second direct observation of the gelation behavior revealed that the timescale of the instantaneous termination of the flow was as quick as 1 ms in order of magnitude. Such instantaneous gelation in the sub-milli-second-order timescale could not be forecasted from the observable megascopic gelation, which appeared to last for 10² ms to 10³ ms in our naked-eye observation. The noteworthy gap between the timescale of the naked-eye-observed gelation and that of the true gel time at a localized spot determined by the high-speed motion picture should be noted to avoid excess agitation, which can result in total collapse into gel fragments of the just solidifying or already solidified gel under strong deformational influence by mechanical agitation, for example.

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2) For general readers, aouthors are encouraged to discuss other applications of Silica such as: [(a) - Khorasani et al. (2023), “A refined vibrational analysis of the FGM porous type beams resting on the silica aerogel substrate”, Steel and Composite Structures, 47(5), 633-644.; (b) - Garg et al. (2022), “Machine learning models for predicting the compressive strength of concrete containing nano silica”, Computers and Concrete, 30(1), 33-42.].

 

=> Thank you very much for the sincere advice.

The below description was added in the introduction with the reference number 3 and 4.

“In general, incorporating silica into construction materials can be regarded as a useful and effective prescription for enhancing the strength and resilience of those materials [3, 4].”

< in the references >

3. Khorasani, M.; Lampani, L.; Tounsi, A. A refined vibrational analysis of the FGM porous type beams resting on the silica aerogel substrate. Steel Compos. Struct. 2023, 47(5), 633-644. doi: https://doi.org/10.12989/scs.2023.47.5.633
4. Garg, A.; Aggarwa; P.; Aggarwal, Y.; Belarbi, M. O.; Chalak, H. D.; Tounsi, A.; Gulia, R. Machine learning models for predicting the compressive strength of concrete containing nano silica. Comput. Concr. 2022, 30(1), 33-42. doi: https://doi.org/10.12989/cac.2022.30.1.033

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Figs. 6 and 7 should be more discussed.

 

=> Thank you very much for the sincere advice. The below two descriptions were added to the discussive statements as to Figure 6 and Figure 7, respectively.

 

Figure 6

The slanted pattern in the left-hand side of Figure 6 represents the continuation of the unidirectional flow until it was instantaneously arrested due to the gelation. The present interest was focused on the transition which appeared as the disappearance of the slant in Figure 6. In order to evaluate the timescale in which the slant abruptly vanished, the appropriate part for the purpose of finding the arrest of the flow was selected as depicted with a drawn rectangle in Figure 6.

 

Figure 7

As seen Figure 6 and Figure 7, the slant did not reappear once it turned plateau. This irreversibility obviously corresponded to the irreversible transition from the macroscopically mobile state to immobile state. This result showed that the gelation occurred as a straightforward and entirely irreversible transition toward the completely immobile state and intrinsically differed from a diminishing semi-periodic repetition between the mobile and immobile state in the midst of the course toward the ultimate gelation. The visual impression of oscillatory behavior which turned up in watching the megascopic gelation captured in motion pictures like Video S1 or Video S3 was shown due to not the solidification behavior in an oscillatory manner between liquid and solid but the macroscopic rotational agitation.

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The conclusion should be improved.

 

=> Thank you very much for the useful advice. The conclusions were largely modified from the original section as given in the revised manuscript. It should be noted that the amended part according to the suggestion by another reviewer is included.

 

4. Conclusions

  The most straightforward method for evaluating the gel time of a quickly gelling system was proposed. The direct observation and the following image analyses of the high-speed motion picture taken at the frame rate of 5000 per second was used. In the pre-sent study, the gelation of silica-based alcogel derived from silicate oligomer through its quick polycondensation reaction was recorded in the high-speed motion picture. The choice of the direct observation of the quick gelation in the present study was unavoidable considering that implementing serial rheological measurements to obtain the storage and loss modulus including careful sampling and setting the sampled solution on the apparatus is impossible within only a few seconds. As seen in Video S1 or Video S2, gelation can be completed almost instantaneously, to which only direct observational methods supported by some “augmented eye” like a high-speed camera can be applied.

  The 5000-fps high-speed motion picture could capture the exact moment of the quick gelation. The image analyses of the serial translation of the flowing gelling liquid showed that the arrest of the flow occurred in the timescale of 1 ms in order of magnitude, which was much shorter than that elucidated from the naked-eye observation. In observing the flow behavior using the high-speed camera at thousands of frames per second, the vigorous blade agitation which incessantly entrained the outside air from the undulating upper surface of the gelling liquid was of use since the entrained air immediately fragmented to submillimeter-sized bubbles which worked as appropriate visualization tracers of the turbulent flow of the agitated gelling liquid. The time-series snapshots of the gelling liquid in the vicinity of the instant of the gelation showed the invisibly quick gelation which occurred in only 1 ms.

Quickly gelling materials like silica based alcogel investigated in this study need to be swiftly handled considering that the irreversible elimination of the flowability can be brought about during the gelation as promptly as in only a few milliseconds once the gelation is triggered. Such quick loss of the flowability can be an appeal for its use in stabilizing weak sandy ground surfaces and similar fragile natural or artificial structures.

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Author Response File: Author Response.pdf

Reviewer 3 Report

In this work, the authors have proposed a simple, direct-visual experimental method for evaluating gel time of gelling system (like silica-based alcogel in this case). The authors claim to fill the gaps, not possible using the conventional rheological approaches for such quick-gelation phases. The research is interesting, however there are several concerns/questions that must be addressed and highlighted in the manuscript before it is published online and made public.

 

After careful reading, I would like to offer my comments as follows:

 

1.     The research presented by the authors is very niche and at this point it’s difficult to make a case for it to be suitable to published in MDPI sustainability. However, I would like to give the authors a chance to make an appeal. Additionally, they should try to tailor their Introduction section to ensure it falls within the focus and scope of sustainability journal.

 

2.     The authors have provided an engaging introduction to the research topic. However, amidst the content, the novelty of the present work is unclear and is lost. What distinguishes the current work from rest of the published studies?

 

To help make the distinction clear and bring out the work’s significance, it would be helpful to highlight the novelty using the proposed layout: (a) key highlights of the previous studies, (b) crucial points lacking in those studies, (c) importance of the missing pieces from a big-picture perspective, (d) how does the current study help to address those to advance the understanding, and (e) how does the current findings help the field – future scope and applications of the observations. This will help the reader to appreciate the current study and clarify the impact of the work.

 

 3.     In the Introduction section, in lines 107 and 108, the authors have made a reference to 46+ articles, which is highly unusual. I would encourage the authors to severely cut down the list and make reference only to a few articles that are relevant and important.

 

 4.     As per my understanding, the entire purpose of this article is to use silica-based alcogel for stabilizing sandy layer (unstable ground surface). However, the authors do not mention how they use the learnings from their direct quick gelling observation to apply the alcogel for stabilization purposes. There is a need for demonstration which is critically missing. It is important not to confuse the reader as to what’s the main point of this work and how it applies to real-life problems.

 

 5.     With regards to the gelation experiments, have the authors considered the effects of the following conditions and can they expound more? 

A)   Effect of temperature – soil temperatures could have an effect if the gelation is delayed or instantaneous.

B)   Effect of pH on gelation

Minor editing of English language required

Author Response

<response to reviewer 3>

 

Dear reviewers,

 

We are very grateful to the reviewers for the great efforts to give us the advisory comments for revising the submitted manuscript. According to the useful comments by them, we largely amended the manuscript with our best efforts to fulfill what the advisory comments are about. In some cases, we chose to address our aim and opinions in response to the reviewer’s comments in addition to the amendments in the manuscript. We sincerely hope that our efforts to improve the manuscript will smoothly lead to the help for the reviewers and potential readers to comprehend the essential aim and the results of the present study.

The modified or added parts according to your suggestions are indicated by blue color. (The other colors are used for the same purpose to respond to other reviewers.)

We thank you again for having taken your time resource for our trial of publication.

 

                                                                                                  Regards,

1. Kurumada, H. Ue, and J. Sato

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

1. The research presented by the authors is very niche and at this point it’s difficult to make a case for it to be suitable to published in MDPI sustainability. However, I would like to give the authors a chance to make an appeal. Additionally, they should try to tailor their Introduction section to ensure it falls within the focus and scope of sustainability journal.

 

2. The authors have provided an engaging introduction to the research topic. However, amidst the content, the novelty of the present work is unclear and is lost. What distinguishes the current work from rest of the published studies?

To help make the distinction clear and bring out the work’s significance, it would be helpful to highlight the novelty using the proposed layout: (a) key highlights of the previous studies, (b) crucial points lacking in those studies, (c) importance of the missing pieces from a big-picture perspective, (d) how does the current study help to address those to advance the understanding, and (e) how does the current findings help the field – future scope and applications of the observations. This will help the reader to appreciate the current study and clarify the impact of the work.

 

4. As per my understanding, the entire purpose of this article is to use silica-based alcogel for stabilizing sandy layer (unstable ground surface). However, the authors do not mention how they use the learnings from their direct quick gelling observation to apply the alcogel for stabilization purposes. There is a need for demonstration which is critically missing. It is important not to confuse the reader as to what’s the main point of this work and how it applies to real-life problems.

 

=> We are extremely grateful for the very essential points which the reviewer showed us through the careful thinking of a sort of a complication lying in the question of “in which stage of study we should present the research results to the public”. What the reviewer has pointed out concerning the scope of the Sustainability journal is quite central and we were all profoundly hit by the highly authentic insight provided by the reviewer. At the same time, we have faced a complex problem of how much content could be neatly addressed just in a research report of 10 pages or so. Now we are on the corner of how we could make the focus of the study both in a comprehensible way and in a scientifically meaningful way, at the same time.

 

We revised the introduction with our best efforts as far as we could at present within the range of what we could mention on this ongoing study. As the reviewer pointed out, what we could fully present in this report is the result of having exploited the possibility of 1/5000 sec level high-speed motion picture in capturing the very instant of gelation which occurred entirely instantaneously in our naked-eye observation. The identification of the timescale for the very quick gelation in silica-based alcogel is a fundamental and pertinent parameter in actual practices of stabilization of a sandy layer since such operations always accompany mechanical agitation or so. The irreversibility of gelation behavior which can be applied to the stabilization is a crucial property. Therefore, the authors tried to reinforce the statement to make it clear that determination methods of the gel time for various gelling systems are of mandatory use. At the same time, more actual investigations including the incorporation of the gelling media in unstable sandy or muddy states need to be further carried out being followed by more comprehensive reports in engineering-science study fields. This is exactly what the reviewer sharply saw and must be covered in the following research reports, which the authors have seriously recognized, too.

 

︾

1. Introduction

Civil engineering frequently encounters situations in which excessively flowable objects need to be stabilized in construction or for mitigating disasters. Sandy ground surface is one of the examples with such excessively and riskily flowable properties. Particularly, a sandy layer whose interparticle gap is filled with water is highly likely to undergo a sudden liquefaction or avalanche in case of even subtle oscillation. Since the lubricative effect of the interparticle water causes the excessive flowability, filling the interparticle gap with immobile matrices can be a viable approach to effective and lasting suppression of the risky flowability of sandy ground surface layers. Gelation induced in such excessively flowable granular locations can be a technological candidate to mitigate the likelihood of the occurrence of troubles due to their mechanical weakness. More specifically, the possibility of using silica-based gel for the above purpose is to be thoroughly examined to provide fundamentals in applying that to various actual situations. Since silica is the dominant chemical moiety which constitutes sand, it is superior in its surface chemical affinity and compatibility. Hence, the use of silica-based immobilization medium is practically promising both in that it can function well in stabilizing sandy grounds and that the environmental alteration due to artificially introduced chemical component in the sandy layer can be minimized. Furthermore, the extreme chemical stability of silica as an inorganic solid species is an advantage considering that silica is not involved in chemical reactions under common moderate conditions. Actually, Remzova et al. succeeded in enhancing the mechanical strength of sandy stones by mixing silicate oligomer followed by its polycondensation [1]. Zarzuela et al. proposed a method to enhance the performance of porous portlandite-based solid as a cementation material by filling the pores inside which silica oligomer polycondensed to a firm solid filler [2]. The irreversibility in the solidification of silicate oligomer to silica-based solids or non-flowable matrices can be regarded as an advantage in applying these materials to stabilizing sandy ground surface layers. In general, incorporating silica into construction materials can be regarded as a useful and effective prescription for enhancing the strength and resilience of those materials [3, 4].

In the present study, the main subject was put on how to deal with the relatively quick gelation of silica-based materials. More specifically, the timescale for the gelation in those chemical systems should be elucidated since no operations which can take more time than the gelation itself are not supposed to help us to finish the fabrication or construction works with the quickly gelling materials. Generally, the state transition of liquids to materials with no mobility is classified to gelation. In considering bringing the stabilization due to the gelation into practical use, how much time the gelation requires to be completed is an experimentally obtained parameter of pragmatic importance. Obviously, the required timescale for completion of gelation (gel time) which is long enough to guarantee the utilitarian stability crucially depends on the chemical species of the gelling objects. The authors have been particularly interested in experimental evaluation of the gel time in quickly gelling systems which start to have the outside appearance of non-flowable gels within only 10 seconds or so. Such quick gelation can be detected or perceived through direct observation of the kinematic behavior. For the purposes of suppressing the excess flowability of undesirably unstable objects like sandy layers, we have had a prospect of applying the gelation of silica-based alcogel since its gelation can be triggered and controlled through the addition of proper catalytic agents. As an indispensable fundamental parameter, we need to estimate the gel time of the silica-based gelling materials. To fulfill this purpose, developing a viable and reliable experimental method to identify the gel time is absolutely necessary. Therefore, to establish reliable measurement methods for evaluating the gel time of silica-based alcogel as the stabilizing matrix has been of fundamental use. The authors implemented a preliminary observational experiment by means of high-speed motion picture to show the possibility of direct capturing of the instant of gelation in silica-based alcogel [5].

Intuitively, direct observation of the instant of the prompt gelation by high-speed visualization tends to be considered as a simple experimental task. However, unexpected difficulties are found because the vanishing flowability is invisible itself and we can never perceive it only by observing the gelation in a static manner. In other words, visual capturing of the vanishing flowability needs to be made possible while the object in flow is being “frozen” during its kinematic motion. For example, capturing the process of gelation by the tube inversion is impossible, although it is often employed in order to obtain tangible evidence of the absence of macroscopic flowability due to its simplicity [6, 7]. Although the tube inversion is a quite versatile method in checking whether the observed object ought to be classified to gel or not, it cannot be applied to investigating the transient gelation process. Direct observation with naked eyes is obviously the most straightforward method for taking a view of the vanishing flowability and was used even in recent studies [8-11]. Furthermore, an auxiliary procedure of dispersing tiny particles in the gelling solution for enhancing the observability was revealed to work effectively in helping the observers to trace the in-situ gelation process [12-14]. This method is particularly helpful in observing the gelling objects whose transient behavior toward the completion of the gelation occurs relatively quickly. Krause et al. succeeded in tracing the fading flowability in a system where the sol-gel transition was induced and recorded the result of the particle image velocimetry (PIV) [14].

As will be stated in the following sections, the gelation occurred much more instantaneously than the above case. Thus, a different experimental approach to capturing the moment of the gelation was required.

In previous laboratory-scale studies, rheological methods have been conventionally and popularly employed for the determination of the gel time. Among them, the criterion concerning the storage and loss modulus which was proposed by Winter and Chambon [15] has been predominantly adopted as the most reliably established experimental scale [16-62]. This criterion has a noteworthy content for the justification which tells us the view that the dominance of the storage modules to the loss modules corresponds to the marked appearance of the elastic properties versus the behavior as the viscous liquid. This physically intelligible picture can support the criterion by Winter and Chambon and enable us to regard it as a sound experimental evidence of actual gelation. A technically serious complication is found in the fact that a rheological measurement for checking the satisfaction of the abovementioned criterion for gelation requires the timescale at least as long as 10¹ s in order of magnitude. In the previous studies, there were examples in which the completion of gelation was demonstrated based on the results of the rheological measurements [19, 36, 51, 60]. For example, Huang et al. determined the gel time as a few thousands of seconds for polypropylene melt mixed with carbon black particles. Obviously, in that study, the applicability of the rheological measurements was guaranteed because the required timescale for the individual rheological measurement was negligible compared to the gel time [51].

On the other hand, a set of established measurements are not applicable to what undergoes quick gelation just in a blink. For such quick gels, the only realistic method to capture the moment of the gelation was anticipated to be straightforward high-speed motion picture. Therefore, the objectives of the present study were to take as a clear view of the instantaneous gelation of silica-based alcogel as possible in a high-speed motion picture at thousands of the frame rate per second and to create a viable method for the evaluation of such a very short gel time [63].

              The authors were also interested in the gap between how long the quick gel took to lose the observable macroscopic flowability and how instantly the local gelation ( = termination of the flow) occurred as determined from the analysis of the time-series high-speed motion pictures. Grasping the abovementioned gap between the timescales of the megascopic “apparent” gelation and spotted “true” gelation were expected to provide, for example, how quickly the agitation to facilitate the quick gelation should be switched off for avoiding the detrimental effect due to the vigorous agitation in trying to stabilize a sandy layer with silica-based alcogel. Therefore, the quite fundamental approach for evaluating the timescale required for effective stabilization of sandy ground is expected to help us to choose a proper method for reinforcing sand or soil. How quickly the stabilization remedy should be carried out in actual and practical situations can be linked to our knowledge of the gel time in various quickly gelling systems. Thus, this research report presents our straightforward trials to capture the instant of the gelation in a simply derived silica-based materials from silicate oligomer. The gelation is characterized by its extreme rapidity when catalyzed with basic additives. Visual presentations of the rapidity and the experimental techniques for determining the quantitative gel time applied to such a quickly gelling system were introduced with the obtained gel time, which showed non-negligible discrepancy from our anticipation based on our common naked-eye observation.

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3. In the Introduction section, in lines 107 and 108, the authors have made a reference to 46+ articles, which is highly unusual. I would encourage the authors to severely cut down the list and make reference only to a few articles that are relevant and important.

 

=> We are very grateful for the reviewer’s sincere advice as to the pretty large number of the cited studies. The authors have had a serious discussion on this issue and still now in a sort of hesitant consideration on whether we should make a brave omission of the Winter-Chambon rheology.

              The primary problem for us is the present point of the study is how we could manage to work out an alternative experimental method in facing the difficulty caused by the blink-quick gelation. At the same time, the respect to the established method for determining the state of the gelling material should be presented considering the great contribution of the rheological approaches. Indeed, the direct observation obviously differs from the rheological method, which might have lead to the idea that the number of the studies picked up in the present report should be minimized. Nevertheless, identifying the criterion for whether a study based on the rheological methods should be regarded as sufficiently related or not to the present study which adopted a completely distinct principle from that of the rheological studies is an extremely difficult question to answer confidently.

              There is another problem in selecting the cited studies because of the research viewpoint of the present study. Since quick gelation tends to be difficult to trace, it has not been a popular object of physicochemical studies. Therefore, finding previous studies which dealt with quick gelation is not quite easy as it can be seen from the small number of the cited articles. In particular, the kinetic trace of physicochemical change which could reach the end in a few seconds is quite unlikely to be selected as the object of kinematic trace in experiments.

              As a result of that, the policy for citing the previous studies here could not help being to show that the case where the researchers could rely on the rheological measurements were quite popular in comparison to that where quicker and more direct approaches need to be employed.

              The authors sincerely hope that the citing policy that we could only show that the dependence on the rheological experiments was very common will be understood by those looking over this study.

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5. With regards to the gelation experiments, have the authors considered the effects of the following conditions and can they expound more?
6. A) Effect of temperature – soil temperatures could have an effect if the gelation is delayed or instantaneous.
7. B) Effect of pH on gelation

 

=> As in the above comment, we recognize that there are details to be investigated further. Although we are not ready for making the thorough statements on these issues, the following supplementary information can be provided:

 

1. A) The same gelation could be observed irrespective of the air temperature and soil temperature unless it is entirely beyond the common likely values of 20 deg. C. ± 20 K. The soil temperature was forced to be very close to the solution temperature when the surface of the constituent particles was made wet after scattering the mother solution containing silicate oligomer. Therefore, the temperature condition employed in the present study could be considered valid during outdoors experiments or practices.

 

10. B) The value of pH is dominated by the preparation conditions. The mother solution is always prepared under pH~5 in coexistence of some typical usable acids and the gelation is triggered by imidazole which functions as a pH stabilizer at 9 to 10. Thus, the pH condition is considered sufficiently stabilized by the preparation condition itself. Common soil does not influence the pH condition enough to hinder or prevent the anticipated chemical reactions which start from the hydrolysis and end with the polycondensation reaction of the silicate oligomer.

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Round 2

Reviewer 3 Report

I thank the authors for their sincere effort in addressing the concerns raised by the reviewers.

Minor editing of English language required