PONE-D-19-34425

Formation of a hard surface layer during drying of a heated porous media

PLOS ONE

Dear Dr. Kumar,

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Jie Zheng, Ph.D

Academic Editor

PLOS ONE

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Partly

Reviewer #2: Partly

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2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: N/A

Reviewer #2: Yes

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3. Have the authors made all data underlying the findings in their manuscript fully available?

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Reviewer #1: Yes

Reviewer #2: No

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Reviewer #1: No

Reviewer #2: Yes

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5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: This paper reports the crust formation in heated porous media during water evaporation and concludes that the remaining water, as liquid bridge, is responsible for the formation and strength of the crusts. The liquid bridge shown in microscopic pictures are impressive. However, the organization of the manuscript should be carefully revised so I suggest that major revision is needed before this manuscript is considered publishable on PLOS One. Line numbers are not seen in this manuscript. Comments are listed below:

Abstract

1. The author should use past tense when describing the results of this study.

2. “Amount of the trapped water was ~1.5% (this is ~10 times higher than in the samples with caking), which was confirmed qualitatively using SEM images”.

Please rewrite this sentence.

Introduction

Page 1: How does “addition of a small amount of water” change soil strength, increase or reduce?

Page 2: Description of Stage 2 and 3 is missing.

Page 2: “textual layered”. I think a noun is missing here.

Page 3: “soil” was not seen in Materials and Methods, did the author mean “sand”?

Materials and Methods

1. The source (manufacturer and country of manufacturing) and grade/purity of each material should be described in this section.

2. FTIR, TGA and SEM procedures should be described here rather than in the later Results and Discussion section.

3. What were the containers used and what are the purposes?

4. What was the (volume) ratio of isooctane: FOTS?

Results and Discussion

1. Page 5: What was the porosity of the glass beads? Did hydrophobic and hydrophilic glass beads differ in porosity?

2. Page 6: The “bouncing stainless steel ball method” can be moved to Supporting Information.

3. Page 7: Regarding Figure 4, suggest adding a control picture (without liquid bridges) to compare.

4. Some data like FTIR and TGA in Supporting Information are even more important than those pictures in Figure 3, 5 and 6. There are no restrictions on number of figures so the author should consider including all important figures in the main text.

5. Page 11: please check the unit in equation (3).

6. Page 11: Table 1, how did the author get “no. of layers”?

7. Hydrophilic glass beads vs. hydrophobic glass beads: the author should note which glass beads were used in each experiment.

Conclusions

The conclusions were too long (655 words). Please shorten this section and only summarize the most important findings/conclusions here.

Reviewer #2: In this manuscript, the authors discuss the surface hardening process in a medium consisting of sub-millimeter spheres. Through control experiments with varied particle size, particle surface wettability, liquid medium and characterization with FTIR, TGA, SEM, etc, they determine that the capillary bridge plays a major role in the hardening phenomenon. In general, the manuscript is well written in academic format, the data is relatively solid and can support their conclusions, thus it can be published in Plos One if authors can address following issues after minor revisions:

1. As this paper conclude that the liquid bridge, or the capillary bridge, is the dominant contributor to the surface hardening, then the surface water wettability should be a major factor in the hardening result. Authors are using ‘hydrophobic’ spheres for most of their experiments. Could the authors provide some characterizations, such as contact angle, of the surface wettability before and after the hydrophobic treatment? The hydrophobic surface should have a water contact angle higher than 90 degrees. Maybe after the treatment, even though the contact angle increase, it is still less than 90 degrees and cannot be considered a ‘hydrophobic’ surface. Also, In Figure S11, the authors discuss a scheme for hydrophilic surface but not a hydrophobic surface.

2. This reviewer assumes that comparing to strength, the hardness may be a more important parameter describing the hardening process. In page 6, section-time of hardening-bouncing stainless steel ball method, the authors use the dropping ball methods to describe the hardening process, do authors have enlarged surface image after the ball ‘indentation’? Have authors tried any standard hardness measurement to characterize or these indentation images to calculate the hardness?

3. On page 7, first sentence, ‘these clumps could not be held between the fingers as they fell apart’ is not a very scientific expression, please rephrase.

4. Figure 3, please provide high-resolution images and clearly label the images, such as marking important features in the image, add scale bar, etc. Right now it's very unreadable.

5. Figure 4b, please add scale bar, Figure 4c, d, please enlarge the scale bar

6. Figure 5b, is the center brown region the crusted thin upper layer? If so, please mark carefully.

7. Figure 6, again, please label clearly on all subfigures, and please provide a scale bar for d and e.

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Reviewer #1: No

Reviewer #2: No

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Formation of a hard surface layer during drying of a heated porous media

PONE-D-19-34425R1

Dear Dr. Kumar,

We are pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it complies with all outstanding technical requirements.

Within one week, you will receive an e-mail containing information on the amendments required prior to publication. When all required modifications have been addressed, you will receive a formal acceptance letter and your manuscript will proceed to our production department and be scheduled for publication.

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With kind regards,

Jie Zheng, Ph.D

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

Reviewers' comments: