Review Reports - Hierarchy of Electrorheological Responses in Aqueous Smectite Clay Dispersions in Relation to DLVO Potential Barriers

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Comments related to the reviewed paper:

Line 11, it is not clear why the authors have selected the four clays, what is the difference between them?

Line 13, it Is not clear if the suspension of the four clays or the solids them selves.

Line 14, the abbreviation PMMA was not defined.

Line 21, why the authors have selected the conventional oil-based ER fluids as reference

Line 22, the abbreviation DLVO is not defined.

Line 24, the readers were not familiar with “smectites”?

Line 64, why the authors did not support their description by adding some references.

Line 66, references are missing

Line 68, reference is missing

Line 98, what does it mean the deionized hectorite (Ht) dispersion?

Line 102, The authors did not explain the reason between the DC and AC effect?

Line 114, The authors have stated that the present study is the first to investigate 114 the ER effect of Ht-F and Sap dispersions. However, in the abstract they have mentioned four types of clay?

Line 115, there is not a strong correlation between the title and the objective to this study.

Line 122, what is the origin of stevensite (Stv) and hectorite (Ht)?

Line 139, What deionization mean?

Line 149, the reviewer is surprised that a period of 10 years was used for deionization? Is it true?

Line 224, the reviewer did not see clearly why the PMMA was selected as reference, since it does have different chemical composition and properties than the clay suspensions?

Line 235, why the flocs were formed? Is it related to the chemical composition of PMMA? If one will use different chemical composition of the reference, did the authors expect to have the same behavior?

Lune 224, what is the reason behind the formation of clay flocs under the electric field?

Line 266, why the authors did not label the figure 2a and 2b in the text?

Line 266, what is the difference between the Ht-F and Ht in the formation of suspension?

Why the saponite has the least transparency?

Line 443, the authors have stated that the stress increase upon field application cannot be attributed to an increase in electrolyte concentration in the dispersion” Since the ratio of clay and water is fixed, how the electrolyte concertation increases in the suspension?

Line 500, the authors have stated that “Sap releases ions under applied 500

Fields” did the authors a reference to support their claim?

Ines 612 to 629, this paragraph is as a conclusion so it has to be removed in the conclusion paragraph and thus it has to be rewritten.

Author Response

Response to Reviewer 1

We sincerely thank the reviewer for the careful reading of our manuscript and for providing insightful and constructive comments. We have carefully revised the manuscript according to all suggestions. Below, we provide point-by-point responses. Line numbers refer to those in the revised manuscript.

Comments 1: Line 11, it is not clear why the authors have selected the four clays, what is the difference between them?

Response 1: We appreciate the reviewer’s comment.
The four clays (Ht-F, Stv, Ht, Sap) were chosen because they represent typical synthetic smectites with systematically varied physicochemical properties. They differ in:

primary particle diameter (≈40–110 nm),
cation exchange capacity (CEC),
origin of layer charge (octahedral vs. tetrahedral, and OH→F substitution in Ht-F), and
zeta potential, transparency, and fineness of network structure.

These differences allowed us to conduct a systematic comparison and to discuss the correspondence between ER hierarchy and DLVO potential barrier hierarchy.
A description has been added in the Introduction (Lines 143–152).

Comments 2: Line 13, it is not clear if the suspension of the four clays or the solids themselves.

Response 2: The transparency refers to that of the aqueous clay dispersions, not the dry solids. Clarification has been added to Lines 12–13.

Comments 3: Line 14, the abbreviation PMMA was not defined.

Response 3: We added the definition “poly(methyl methacrylate)” at Line 15.

Comments 4: Line 21, why the authors have selected the conventional oil-based ER fluids as reference

Response 4: To avoid confusion, the corresponding sentences on oil-based ER fluids were removed from the revised manuscript.

Comments 5: Line 22, the abbreviation DLVO is not defined.

Response 5: We added the full name “Derjaguin–Landau–Verwey–Overbeek theory” (Lines 22–23).

Comments 6: Line 24, the readers were not familiar with “smectites”?

Response 6: A concise introductory description of smectite clays has been added (Lines 51–57).

Comments 7: Line 64, why the authors did not support their description by adding some references.

Response 7: We added references on clay minerals and environmentally compatible functional materials at Lines 74–78:

Michels-Brito et al. (Langmuir, 2021)
Worasith & Goodman (Appl. Clay Sci., 2023)
Choi et al. (Materials, 2017)
Munteanu et al. (Prog. Mater. Sci., 2025)

Comments 8: Line 66, references are missing

Response 8: This sentence is directly connected to the sentence “Our group has previously demonstrated … [15–18]”.
To avoid redundancy, adding the same references again was not necessary.

Comments 9: Line 68, reference is missing

Response 9: Same reasoning as in (8). The sentence belongs to the same context as the statement citing [15–18].

Comments 10: Line 98, what does it mean the deionized hectorite (Ht) dispersion?

Response 10: We clarified that “deionized” means ion removal by mixed-bed ion-exchange resin (e.g., AG501-X8). The explanation has been added to Lines 68–71.

Comments 11: Line 102, The authors did not explain the reason between the DC and AC effect?

Response 11: The mechanisms governing particle behavior in three-dimensional aqueous dispersions under electric fields remain incompletely understood. The difference between DC- and AC-field-induced ER behavior is still an open question. Our aim was to describe the observed difference concisely.

Comments 12: Line 114, The authors have stated that the present study is the first to investigate the ER effect of Ht-F and Sap dispersions. However, in the abstract they have mentioned four types of clay?

Response 12: We revised the statement to avoid confusion:

“In this study, we investigated the ER behavior of Ht-F and Sap for the first time. Building on previous findings for Stv and Ht, we clarified the relationship between the stress responsiveness of the four smectites and their DLVO potential barriers.” (Lines 152–156)

Comments 13: Line 115, there is not a strong correlation between the title and the objective to this study.

Response 13: The revision in (12) also resolves this issue by clarifying the study objective (Lines 152–156).

Comments 14: Line 122, what is the origin of stevensite (Stv) and hectorite (Ht)?

Response 14: We emphasized that Stv and Ht were used in our previous studies and added a concise explanation of the origin of layer charge for all four clays (Lines 182–186).

Comments 15: Line 139, What deionization mean?

Response 15: Clarified as in response to comment (10).

Comments 16: Line 149, the reviewer is surprised that a period of 10 years was used for deionization? Is it true?

Response 16: The long deionization time reflects the preparation history of the stock dispersions. Although a period of 10 years is not required, we used highly deionized dispersions in this study.

Comments 17: Line 224, the reviewer did not see clearly why the PMMA was selected as reference, since it does have different chemical composition and properties than the clay suspensions?

Response 17: Revised as follows:

“The transmittance of the PMMA particle dispersion was measured for comparison because micrometer-sized particles exhibit electrically induced rapid and reversible separation (ERS) under the same field conditions.” (Lines 282–285)

Comments 18: Line 235, why the flocs were formed? Is it related to the chemical composition of PMMA? If one will use different chemical composition of the reference, did the authors expect to have the same behavior?

Response 18: The exact origin of ERS floc formation remains unclear. However, ERS occurs not only for many micrometer-sized particles (PMMA, silica, hollow particles, etc.) but also for smectites at the micrometer scale. Explanations have been added to Lines 304–311.

Comments 19: Lune 224, what is the reason behind the formation of clay flocs under the electric field?

Response 19: As above, the microscopic origin is still not fully known.

Comments 20: Line 266, why the authors did not label the figure 2a and 2b in the text?

Response 20: Corrected in the revised manuscript (Lines 317–319).

Comments 21: Line 266, what is the difference between the Ht-F and Ht in the formation of suspension? Why the saponite has the least transparency?

Response 21: Based on our previous work [11], zeta-potential differences influence aggregation states. Sap has the smallest absolute zeta potential and thus shows the lowest transparency. Added at Lines 319–321.

Comments 22: Line 443, the authors have stated that the stress increase upon field application cannot be attributed to an increase in electrolyte concentration in the dispersion” Since the ratio of clay and water is fixed, how the electrolyte concertation increases in the suspension?

Response 22: Irreversible aggregation in water is known as electrocoagulation (EC), which requires ion release from sacrificial electrodes. Our study concerns reversible aggregation and differs from EC. This distinction is already discussed in Lines 106–109.

Comments 23: Line 500, the authors have stated that “Sap releases ions under applied fields” did the authors a reference to support their claim?

Response 23: Our discussion (Lines 516–555) synthesizes experimental observations and prior literature relevant to ion release from smectites. We have added citations appropriately.

Comments 24: Lines 612 to 629, this paragraph is as a conclusion so it has to be removed in the conclusion paragraph and thus it has to be rewritten.

Response 24: As suggested, we substantially revised both the end-of-Results summary (Lines 664–697) and the Conclusions section (Lines 705–730) to clearly separate their roles.

We sincerely appreciate Reviewer 1’s detailed and constructive comments, which have significantly improved the clarity and quality of our manuscript. All concerns have been addressed in the revised version.

Reviewer 2 Report

Comments and Suggestions for Authors

1. The core of this article lies in distinguishing the ER behavior of four types of clay, but lacks key physicochemical parameters. DLVO analysis requires accurate input parameters. To this end, the authors are requested to supplement the key parameters of four types of clay (Ht-F, Stv, Ht, Sap) in the "Materials and Methods" section, including specific surface area, cation exchange capacity (CEC), surface charge density, and specific distribution positions of charges in the crystal layer (such as octahedra or tetrahedra), which are crucial for explaining the DLVO potential energy calculation results.
2. It should be noted that rheological measurement results (especially yield stress and viscosity) are highly dependent on testing conditions. The reviewer believes that the author needs to appropriately explain the experimental details of ER testing in the relevant section of research methods, especially: a) the pre shear process before measurement (shear rate and time); b) The equilibrium time before applying an electric field; c) The method for determining yield stress (such as shear rate scanning or stress scanning).
3. In this manuscript, the authors compared the differences between micrometer sized PMMA (ERS) and nanometer sized clay (ER), but lacked quantitative particle size data support. Therefore, the reviewers believe that the authors need to provide quantitative data on the particle size distribution of PMMA and four clay dispersions in the results or materials section to confirm that the significant difference in particle size is the main factor leading to the differentiation between ERS and ER phenomena. Meanwhile, the authors need to appropriately emphasize the essential differences between ERS (settlement/separation) and traditional ER (field induced network/yield stress).
4. The double layer around clay particles is not only important in materials science, but also crucial in oil and gas development.
5. The reviewer found that the title of the manuscript emphasizes DLVO, but the discussion section may not fully and clearly relate the DLVO potential energy curves of the four clays to their final ER performance level order. Please add a section in the discussion section to summarize and clarify how the specific differences in the DLVO potential barriers of the four clays (Ht-F, Stv, Ht, Sap) (such as the depth of the secondary potential well/the height of the primary potential barrier) gradually lead to the observed differences in ER response intensity levels.
6. The conclusion section of this article lacks sufficient strength and prospects for future research. The reviewer suggests that the authors highlight the order of ER response levels and the explanation of the DLVO model in the conclusion section. At the same time, briefly propose future research directions, such as how to further adjust the DLVO potential barrier of these clays through surface modification to optimize ER performance.

Author Response

Response to Reviewer 2

We sincerely thank the reviewer for the positive evaluation of our work and for the thoughtful comments provided. We have carefully addressed each point and revised the manuscript accordingly. Our detailed responses are given below. Line numbers refer to those in the revised manuscript.

Comments 1: The core of this article lies in distinguishing the ER behavior of four types of clay, but lacks key physicochemical parameters. DLVO analysis requires accurate input parameters. To this end, the authors are requested to supplement the key parameters of four types of clay (Ht-F, Stv, Ht, Sap) in the "Materials and Methods" section, including specific surface area, cation exchange capacity (CEC), surface charge density, and specific distribution positions of charges in the crystal layer (such as octahedra or tetrahedra), which are crucial for explaining the DLVO potential energy calculation results.
Response 1: Thank you for this important comment.
We fully agree that SSA, CEC, surface charge density, and the origin of layer charge are crucial physicochemical features of smectites. Although these parameters are not directly used as inputs in the DLVO potential calculations performed in this study, they indeed influence the ζ-potential, aggregation behavior, and ion-release tendencies of each clay. These properties, in turn, determine the relative heights of the DLVO potential barriers. We have added all available information—SSA, CEC, and the octahedral/tetrahedral origin of layer charge—to the Materials and Methods section (Lines 173–179 and 182–186). This revision clarifies the physicochemical background that underpins the hierarchy of DLVO barriers.

Comments 2: It should be noted that rheological measurement results (especially yield stress and viscosity) are highly dependent on testing conditions. The reviewer believes that the author needs to appropriately explain the experimental details of ER testing in the relevant section of research methods, especially: a) the pre shear process before measurement (shear rate and time); b) The equilibrium time before applying an electric field; c) The method for determining yield stress (such as shear rate scanning or stress scanning).

Response 2: We appreciate the reviewer’s thoughtful comment.
We agree that rheological results, especially yield stress and viscosity, depend strongly on the testing protocol. In our ER experiments, we followed the same rheological procedures used in our previous studies. To improve clarity, we have added the following details to Materials and Methods (Lines 277–279):

(a) Pre-shear treatment:
A pre-shear of 100 s⁻¹ for 60 s was applied to all samples to reset the initial structure, consistent with our earlier work (e.g., [15]).

(b) Resting time before field application:
After pre-shear, samples were allowed to rest for 60 s to stabilize the structure prior to field application.

(c) Yield-stress determination:
As the present study does not report yield-stress values, items related to yield-stress evaluation were not applicable. We included (a) and (b), which directly affect the ER measurements presented.

Comments 3: In this manuscript, the authors compared the differences between micrometer sized PMMA (ERS) and nanometer sized clay (ER), but lacked quantitative particle size data support. Therefore, the reviewers believe that the authors need to provide quantitative data on the particle size distribution of PMMA and four clay dispersions in the results or materials section to confirm that the significant difference in particle size is the main factor leading to the differentiation between ERS and ER phenomena. Meanwhile, the authors need to appropriately emphasize the essential differences between ERS (settlement/separation) and traditional ER (field induced network/yield stress).

Response 3: Thank you for pointing this out.
We agree that particle-size information is essential to establish the distinction between ER (nanometer-scale smectites) and ERS (micrometer-scale particles).

Quantitative size data have been added or emphasized at:

Lines 103–106
Lines 282–285
Lines 304–311

Comments 4: The double layer around clay particles is not only important in materials science, but also crucial in oil and gas development.

Response 4: We appreciate this valuable perspective.
The importance of EDL-controlled swelling, ion exchange, and dispersion stability in oil/gas development is now highlighted in the Introduction, supported by the added reference:

Lyu et al., J. Nat. Gas Sci. Eng. (2015)

This discussion is added at Lines 84–88, strengthening the cross-disciplinary significance of clay EDL behavior.

Comments 5: The reviewer found that the title of the manuscript emphasizes DLVO, but the discussion section may not fully and clearly relate the DLVO potential energy curves of the four clays to their final ER performance level order. Please add a section in the discussion section to summarize and clarify how the specific differences in the DLVO potential barriers of the four clays (Ht-F, Stv, Ht, Sap) (such as the depth of the secondary potential well/the height of the primary potential barrier) gradually lead to the observed differences in ER response intensity levels.

Response 5: We fully agree with the reviewer.
Although the Conclusions already mentioned this correspondence, a more explicit discussion within the Discussion section was necessary.

We have added a dedicated explanatory paragraph (Lines 680–697), which systematically connects:

barrier height
secondary minimum depth
interparticle attraction under electric fields
formation of field-induced flocs
resulting ER responsiveness

This new section clarifies why the observed experimental hierarchy:

Stv < Ht-F < Ht < Sap

corresponds directly to the DLVO barrier hierarchy.

Comments 6: The conclusion section of this article lacks sufficient strength and prospects for future research. The reviewer suggests that the authors highlight the order of ER response levels and the explanation of the DLVO model in the conclusion section. At the same time, briefly propose future research directions, such as how to further adjust the DLVO potential barrier of these clays through surface modification to optimize ER performance.

Response 6: We greatly appreciate this suggestion.
The Conclusion has been substantially rewritten (Lines 705–730) to:

highlight the central finding:ER hierarchy ←→ DLVO barrier hierarchy
emphasize the predictive value of physicochemical properties
discuss future work:
- frequency-dependent ER behavior
- extension to smectites with broader layer-charge variation
- quantitative modeling integrating DLVO and electric-field structuring
- applications to biocompatible ER materials and responsive rheological systems

This revision provides a clearer and more forward-looking conclusion.

We thank Reviewer 2 again for these insightful comments. They greatly improved the clarity, completeness, and scientific depth of the manuscript. All points have now been fully addressed in the revised version.

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript "Hierarchy of Electrorheological Responses in Aqueous Smectite Clay Dispersions in Relation to DLVO Potential Barriers" is interesting. It deals with a very specific topic. I think it deserves to be accepted, but the authors need to answer a few insufficiently explained things.

Explain in detail in the Introduction how the results of the rheological analysis of clay suspension in water during and after exposure to an electric field can be used.
Why compare the results obtained for clay suspensions in water with the results obtained for PMMA? (eg lines 145-147) This is not clear even from the text written in the Abstract and need to be explained.
To define exactly why measurements of particle size and zeta potential of clay were made on two different devices when it is well known that Zetasizer can determine the both. In addition to this, it is even more unclear why the same measurements for PMMA were performed at completely different devices than those used to examine clays.
Precisely state the reason why specific values for strain and angular frequency were chosen, which were kept constant in oscillatory measurements.

Author Response

Response to Reviewer 3

Comments 1: Explain in detail in the Introduction how the results of the rheological analysis of clay suspension in water during and after exposure to an electric field can be used.

Response 1: Thank you for this valuable comment.
In the original manuscript, the Introduction did not sufficiently explain how rheological analysis during and after electric-field application can be utilized for studying clay dispersions. In response, we have added a new explanatory passage in the revised version (Lines 88–97).

Comments 2: Why compare the results obtained for clay suspensions in water with the results obtained for PMMA? (eg lines 145-147) This is not clear even from the text written in the Abstract and need to be explained.

Response 2: We appreciate this important point.
The comparison with PMMA was introduced because micrometer-sized particles in water exhibit the electrically induced rapid and reversible separation (ERS) effect—one of the key mechanistic contrasts with nanoscale smectites.

We have revised multiple sections to make this rationale explicit:

Lines 282–285
Lines 304–311
Lines 669–679

These revisions clarify that:

ER (clays): nanometer-sized sheets → network formation without sedimentation
ERS (PMMA etc.): micrometer-sized spheres → accelerated sedimentation/flotation due to floc growth

Despite the contrasting macroscopic behaviors, both phenomena share a universal mechanism of electric-field-induced floc formation, which we emphasize in the revised text.

Comments 3: To define exactly why measurements of particle size and zeta potential of clay were made on two different devices when it is well known that Zetasizer can determine the both. In addition to this, it is even more unclear why the same measurements for PMMA were performed at completely different devices than those used to examine clays.

Response 3: Thank you for raising this question.
The clay dispersions used in this study were prepared and measured over different periods within our broader research program. At those times, different instruments were available in our laboratory for clay and PMMA measurements. This historical reason is now stated explicitly in the revised manuscript (Lines 179–182 and 200–202).

Despite the difference in instrumentation, the measurement principles (dynamic light scattering for size and electrophoretic light scattering for ζ-potential) are equivalent, and all measurements were performed using appropriate calibration and repeated trials.

Comments 4: Precisely state the reason why specific values for strain and angular frequency were chosen, which were kept constant in oscillatory measurements.

Response 4: We appreciate this valuable comment.
We have now clarified the rationale at Lines 268–272:

A strain amplitude ofgamma = 0.1 was chosen because strain-sweep experiments confirmed that this value lies well within the linear viscoelastic region (LVE) for all four clay dispersions.
The angular frequencyomega = 0.63 rad/s was selected because it falls within the previously established range (0.13–6.3 rad/s) used in our earlier ER studies and provides the most stable torque response and measurement reproducibility.

This explanation has been added to the revised manuscript.

We are grateful to Reviewer 3 for recognizing the significance of our study and for the constructive suggestions, which have helped improve both clarity and scientific rigor. All points have been fully addressed in the revised version.

Reviewer 4 Report

Comments and Suggestions for Authors

This paper reported the hierarchy of ER responses in smectite clay dispersions. The results demonstrate that the ER hierarchy of aqueous smectites can be rationalized by DLVO interactions and provide design guidelines for environmentally compatible ER fluids. The result is interesting and I recommend it to publish after suitable revision.

Please provide the rheological of pure water and ionic water under AC electric field for comparison.
Whether does the ER effect depend on the gap between the inner and outer cylinders?
Whether does an electrolysis effect affect the ER effect?
It is better to observe the structures of clay flocs induced under the fields with an optical microscopy in order to support it.

Author Response

Response to Reviewer 4

We sincerely thank the reviewer for the positive evaluation of our manuscript and for the constructive comments. All points have been carefully considered, and revisions have been made accordingly. Detailed responses are provided below, with line numbers referring to the revised manuscript.

Comments 1: Please provide the rheological of pure water and ionic water under AC electric field for comparison.

Response 1: Thank you for raising this point.
Although we did not perform independent rheological measurements of pure water or ionic water under AC fields in this study, previous work has already established that water does not exhibit detectable ER responses, even under DC electric fields of similar magnitude.

In particular, Kimura et al., Rheol. Acta (2013) demonstrated that ultrapure water shows no measurable viscosity change under a DC field of 6.0 V/mm. If water exhibits no ER response under DC fields—where a directional field component is maintained—it is physically reasonable that water would also show no ER response under AC fields, where the time-averaged field direction is zero.

We have added an explanation to this effect in the revised manuscript (Lines 343–350).
This clarifies that the rheological changes observed in our study arise from the clay particles, not from the solvent.

Comments 2: Whether does the ER effect depend on the gap between the inner and outer cylinders?

Response 2: We appreciate this thoughtful question.
While we did not systematically vary the gap in this study, ER behavior in such systems is primarily governed by the electric-field strength (V/mm) rather than the absolute gap width. As long as the same field strength is established between the electrodes, the qualitative ER response does not depend strongly on the gap size.

However, increasing the gap requires a proportionally higher applied voltage to maintain the same field strength. At higher voltages, electrolysis and undesired secondary reactions may occur more easily in aqueous systems, which can suppress or distort ER behavior. This potential limitation is characteristic of water-based ER fluids.

We added this explanation in Lines 439–444.

Comments 3: Whether does an electrolysis effect affect the ER effect?

Response 3: Electrolysis, when it occurs, disrupts or eliminates the ER effect. This is consistent with our response in (2). In our experiments, electrolysis was indeed observed in certain conditions—most notably for Sap dispersions under DC fields—and this led to irreversible gel-like film formation on the positive electrode surface.

Comments 4: It is better to observe the structures of clay flocs induced under the fields with an optical microscopy in order to support it.

Response 4: We appreciate this important suggestion.
We have repeatedly attempted optical microscopic observations under electric fields. However, due to the nanometric size of smectite clay sheets and the high transparency of the dispersions—even under field-induced aggregation—it is not currently feasible to directly visualize the flocs with conventional optical microscopy.

Furthermore, standard electrode geometries provide only a thin 2D observation window, whereas ER flocculation occurs within a three-dimensional volume, making direct observation inherently challenging.

Nevertheless:

The existence of field-induced flocculation is clearly supported byreversible stress and modulus changes
The behavior is fully consistent with DLVO barrier analysis
The macroscopic ER signatures confirm the presence of field-induced structural evolution

We also mention a future plan: using micron-sized PMMA particles as tracer probes to indirectly visualize the spatial heterogeneity induced by clay flocs.

Because inserting the full methodological discussion would disrupt the narrative flow, we have not included these details in the manuscript at this stage.

We sincerely thank Reviewer 4 for recommending our work for publication after revision. Their comments have improved the clarity of the manuscript and strengthened several key interpretations. All revisions have now been incorporated.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have replied to my comments

Reviewer 2 Report

Comments and Suggestions for Authors

accepted

Reviewer 3 Report

Comments and Suggestions for Authors

The authors made an effort to improve the manuscript according to the suggestions, so I believe that the manuscript should be accepted and published in its current form.