Modeling Microgravity Using Clinorotation in a Microfluidic Environment: A Numerical Approach

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript reports numerical modeling of clinorotation in a microfluidic channel as a way to simulate microgravity-like conditions. By systematically varying channel radius, particle diameter, and rotation speed, the authors illustrate how particle trajectories transition from gravitational settling to centripetal-dominated orbits, identifying narrow parameter ranges in which gravitational forces are effectively averaged out. The work is well-structured and offers insights into the design of microgravity experiments. While the manuscript is thorough, futher refinements are requried to strengthen the clarity. Below are point-by-point revision suggestions.

1. Although the paper models clinorotation numerically, it would help to briefly discuss any existing experimental setups or published studies that have tested 1-axis clinostats in a microfluidic environment. Any numerical model must be validated first against experimental measurements.
2. The manuscript briefly describes the meshes (Table 1) and notes the total element count. However, it is necessary to show mesh convergence results for at least one representative case. For example, the authors might consider comparing velocity fields or particle trajectories for progressively finer meshes.
3. Please indicate if any refinements in boundary layers proved necessary for accurate drag force calculations.
4. The velocity field is established before particle release. Provide more detail on how long the “spin-up” phase lasts (i.e., from t=0 until the fluid is considered stable) for each rotation speed.
5. If the flow does not fully stabilize by 5 seconds for certain larger channels or slower rotations, explain how partial stabilization might affect subsequent particle trajectories.
6. In some cases, rotation speeds exceed 100–150 rpm for sustaining “microgravity” with larger particles in small channels. Is this a practial condition for experimental test in a lab setting?
7. The study applies a rotating frame (ALE approach) and a simplified laminar flow assumption.If the rotation speeds are high, will the flow transition to turbulent regimes near the channel periphery? Provide quantitative explanation.
8. Please explain if any secondary flow (e.g., Coriolis effects) might become important at higher rpm or for larger geometries.
9. The current approach tracks particles individually under Stokes drag and gravity. In experiments with multiple particles (especially at higher concentrations), particle–particle interactions becomes an important consideration, which can dramatically change the particle dynamics. Please discuss how the model might extend to moderate or high particle concentrations, or whether it is strictly valid for dilute suspensions.
10. Please justify the choice of 1% downward displacement as a cutoff for maintaining microgravity condition. Why not using other threshold (2%, 5%)?
11. There are certain typo and formatting issues. For example, in section “b. Effect of the Channel Dimensions and Particle size”, all subsections start with the same index “i”. Please also proofread for minor grammatical or typographical inconsistencies.

Author Response

responses attached

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Microgravity is an important field in scientific research. The simulations which described by the authors are useful, the challenge is the connection with real physical systems. I think, at this point the authors could be add some suggestions. The paper is well organized and can be published in present form.

Author Response

Reviewer 2

Microgravity is an important field in scientific research. The simulations which described by the authors are useful, the challenge is the connection with real physical systems. I think, at this point the authors could be add some suggestions. The paper is well organized and can be published in present form.

The authors thank the reviewer for the positive comments and recognition of this work.

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript of the article is dedicated to the numerical modeling of microgravity under terrestrial conditions. The authors propose an original methodology that allows for the calculation of optimal clinorotation parameters, enabling the simulation of microgravity conditions on Earth. The article is relevant, original, and addresses a specific gap in the field of study.

The manuscript is well-written in scientific language, clear, well-structured, and pertinent. The references to the sources used are appropriate and current. The manuscript includes a scientifically justified concept, a detailed experimental plan with a description of the methods and materials sufficient for the experimental validation of the scientific conclusions.

The conclusions are well-structured, the research methods are adequate, and the data are reliable. The methodological foundations and practical results of this study can be widely replicated. The references are relevant and sufficient. The tables and figures do not have any remarks. The article is suitable for publication in its current form.

Author Response

Reviewer 3

The manuscript of the article is dedicated to the numerical modeling of microgravity under terrestrial conditions. The authors propose an original methodology that allows for the calculation of optimal clinorotation parameters, enabling the simulation of microgravity conditions on Earth. The article is relevant, original, and addresses a specific gap in the field of study.

The manuscript is well-written in scientific language, clear, well-structured, and pertinent. The references to the sources used are appropriate and current. The manuscript includes a scientifically justified concept, a detailed experimental plan with a description of the methods and materials sufficient for the experimental validation of the scientific conclusions.

The conclusions are well-structured, the research methods are adequate, and the data are reliable. The methodological foundations and practical results of this study can be widely replicated. The references are relevant and sufficient. The tables and figures do not have any remarks. The article is suitable for publication in its current form.

The authors thank the reviewer for the positive comments regarding the work.

 

Reviewer 4 Report

Comments and Suggestions for Authors

1. The annotation and explanation of some graphics are not clear enough. For example, in Figure 6, Figure 7, Figure 9, etc., although the particle trajectory and velocity distribution are shown, the specific meaning of color bar and how it relates to microgravity conditions are not clearly explained.
2. In Figure 13 and Figure 14, for the particle trajectory analysis with a radius of 10 mm, it is not clear why 54 seconds is chosen as the analysis time instead of 5 seconds.
3. When determining the minimum rotation speed, only 1% displacement tolerance is used as the standard, but the scientific basis of this standard is not explained. In this paper, only numerical simulation is carried out, and experimental verification is lacking.
4. In the discussion part, the limitations of the results are not deeply analyzed, and how to overcome these limitations is not fully discussed.

Author Response

responses attached

Author Response File: Author Response.pdf

Reviewer 5 Report

Comments and Suggestions for Authors

This study presents a numerical investigation of particle behavior in a clinostat-driven microfluidic channel, aiming to simulate microgravity conditions. A computational model was developed in COMSOL Multiphysics to analyze the impact of channel size, particle diameter, and rotational speed on particle trajectories and establish sets of parameters for assuring microgravity conditions. The work conducted in this paper can promote the understanding of a microgravity simulation method, which contains some worthwhile information for the researchers. However, some issues need to be addressed before the paper can be published. The comments are as follows:

1. The study relies entirely on numerical simulations without experimental validation. While the results are promising, the lack of empirical data to corroborate the simulations weakens the conclusions. The authors should discuss potential experimental setups or cite existing studies that could validate their findings.
2. The manuscript does not mention whether a grid independence test was conducted to ensure that the simulation results are independent of the mesh resolution. A proper grid independence test is critical for validating the numerical model, as insufficient mesh refinement may lead to inaccurate solutions, while excessive refinement increases computational cost without significant improvements.
3. However, the justification for selecting specific parameter ranges (e.g., channel radii, particle sizes, and rotational speeds) is not thoroughly explained. The authors should clarify how these ranges were chosen and whether they are based on prior experimental data or theoretical considerations. Additionally, the criteria for determining "stable microgravity-like conditions" (e.g., the 1% tolerance for downward displacement) should be more explicitly defined and justified.
4. The authors have chosen to employ a two-dimensional (2D) computational model to simulate particle dynamics in a rotating microfluidic channel. While this approach offers computational efficiency, the rationale for not using a three-dimensional (3D) model is not sufficiently justified in the manuscript.

Author Response

responses attached

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have done an excellent job revising the manuscript, and it is now suitable for publication. I have no further comments or required changes.

One minor point of caution for the authors: the perfectly flat line in the mesh independence study is anomalous. While your justification regarding the simple geometry is noted, be aware that this result is unconventional and may raise questions from others in the numerical analysis community regarding the study's setup.