Applications of Magneto Electrochemistry and Magnetohydrodynamics in Microfluidics
Round 1
Reviewer 1 Report
This manuscript discusses about the equations governing the magnetohydrodynamics (MHD) and the applications of MHD in microfluidic systems. While the reviewer believes that MHD is an interesting research topic and this work provides good sights into the theoretical knowledge of MHD and interesting phenomena in MHD applications, there are a number of critical issues in the current manuscript, as listed below.
1. The reviewer found that there are three review papers which have summarized the governing equations of MHD, MHD pumps and actuators, chromatography, stirrers and MRI. Please cite the following paper written by the author:
Qian and Bau (2009) Magneto-hydrodynamics based microfluidics https://doi.org/10.1016/j.mechrescom.2008.06.013
and write explicitly what are the main differences between this manuscript and the author’s previous review paper (also see point 2 for further comments). In addition, it would be good if the author can have a look and cite the following review papers, and substantiate the difference between the current manuscript with these two review papers.
Gregory et al. (2016) The Magnetohydrodynamic Effect and its Associated Material Designs for Biomedical Applications: A State-of-the-Art Review. https://doi.org/10.1002/adfm.201504198
Al-Habahbeh et al. (2016) Review of magnetohydrodynamic pump applications. https://doi.org/10.1016/j.aej.2016.03.001
2. The 19-page review paper cited a total of 41 references. One paper is published in 2021, while the rest are all published in 90s and 00s. The reviewer believes that although it is important to acknowledge the scientific contributions from the old paper, this manuscript as a review paper should also highlight what the scientific advances in the recent years are. The author should cite more recent published papers.
3. The reviewer appreciates the author’s effort in highlighting the governing equation of MHD in section 2. However, the reviewer does not feel that section 2 provides any useful fundamental knowledge to other sections. The reviewer suggests the author to provide a strong link between section 2 and the sections thereafter, which will extensively improve the manuscript readability.
4. When reading Sections 3 to 9, may readers would expect that the author to summarize the current state-of-art MHD devices and various applications, point out the current limitations and provide suggestions for future improvement. However, all these points are missing in the current manuscript.
Other comments:
1. Line 92-94. “Section 8 describes the use of MEC for infinitely-long liquid chromatographs. Section 8 provides a brief discussion of MEC in magnetic resonance imaging (MRI). Section 9 concludes.” Section numbering is wrong. Section 8 describes the use of MEC for infinitely-long liquid chromatographs. Section 9 provides a brief discussion of MEC in magnetic resonance imaging (MRI). Section 10 concludes.
2. Please treat equation as part of the sentence and add comma or full stop at the end accordingly.
3. Line 105. Please consider don’t use \tilde symbol for a dimensional variable \mathbf{\tilde{u}}_k. The \tilde symbol is used by the author to denote non-dimensional variables.
4. Line 107. n_k is a new variable. Is n_k related to any of the terms in \mathbf{\tilde{u}}_k = \nu_k z_k e \mathbf{E} in line 106?
5. Line 111. The term “The mobility” is confusing to readers. Does the author mean the mobility defined by the author (as in line 107) or by other groups (as in line 111)? Please consider rewrite or reorganize the sentences in this paragraph to make the definitions clear.
6. Line 142. It is not right to refer an Equation that has not been introduced. Please reorganize Eq. 6 to somewhere before it is referred in line 142. “the RHS of the Poisson equation (6)”.
7. Line 167. “In the above r_m (kg m-3 ) is the fluid mass density; p (N m-2 ) is the pressure; r_e (C m-3 ) is the net electric charge density (C m-3 ); and m_v (N s m-2 ) is the dynamic viscosity.” The variable r_m, r_e and m_v did not appear in the text above.
8. Regarding Eq. 3 to 9. (a) I imagine the author would like to derive the simplified form of the body force from Eq. 4, and use the form in Eq. 9. However, I did not see how Eq. 5 to 8 helps. If this is not the author’s intention, please consider rewrite the section. (b) how the author recovers Ohm’s law in Eq. 8 is unclear; no definition of \sigma in Eq. 8.
9. Please explicitly define the term “The above equations” (Line 171-172).
10. Line 192. “Both the Reynolds number Re and … are typically small in microfluidic settings”. The sentence may not be true. For microfluidics, the Reynolds number varies greatly and does not guarantee that the flow is governed by Stokes flow (see DOI: 10.1039/C5LC01159K). The author may consider narrow down the scope to microfluidics in MHD applications.
11. Line 134, 186 and 274. Is the potential difference defined as \delta V or DV?
12. Page 11, Figure 3. It would be better if the author can fit in more descriptive text in the figure. Eg. What does the grid, green and red arrow represents?
13. Page 12, Figure 4. Please label where is (A) and (B) in the figure.
14. Page 15, Line 529. “MRI has revolutionized medical diagnostics. enabling….” The “.” After diagnostics should be “,”
Author Response
See attached file
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments for author File: Comments.pdf
Author Response
See attached file
Author Response File: Author Response.pdf
Reviewer 3 Report
“Here, I focus” please be impersonal. And also be focus at 3th person.
Please details “topics of growing current interest” which are the current growing interest ?
How this review differentiate compare with what is in the literature and what the readers can take on – this is not clear from your introduction.
“Section 9 concludes.” what concludes ? some formulation are weak in this manuscript
The figure should be consistent in terms of font size for example figure 2 has much bigger font size compare to Figure1.
Your conclusion looks now rather summary and not a proper conclusion of this work – please revise it
Some quantitative details are required in conclusion
Some future perspective may helps to improve the quality of this work
“This review has focused on applications of magneto electrochemistry” this should not be used in the last paragraph.
Some recent references are required.
Author Response
See attached file
Author Response File: Author Response.pdf
Reviewer 4 Report
This paper presents a review of different applications of electromagnetic fields in microfluidics when the working fluid is an electrolyte. It is an update of a previous review that the author published some years ago. The topic is interesting since the use of electromagnetic fields in the microfluidic area is extensive and innovations have appeared in recent years. Particularly, it is relevant for biophysical and medical applications since most biofluids are electrolytes.
However, I have some concerns about the presentation. In some parts, I consider that the manuscript is confusing or could be misleading. Although the review is pertinent, different applications are unevenly addressed, in fact for some applications only a few references are provided. In some parts the phenomenology that occurs with electrolytes and liquid metals is not clearly differentiated. In my opinion, the author should consider the following issues before the paper can be considered for publication:
-The author seems to use the terms Magneto Electro Chemistry (MEC) and Magnetohydrodynamics (MHD) as if they were synonyms. It appears to me that this is not the case. Most of the examples presented in the review are related with the use of Lorentz forces to pump, control, or stir the fluid, originating flows that can be considered as MHD (without chemical reactions). Although it is mentioned that the injection of an electric current in the electrolyte through electrodes may give rise to electrochemical reactions that alter the properties of the electrolyte and produce unwanted products, in the discussed examples this kind of phenomena are not considered. That is, electrolyte is considered as an ohmic conductor with a uniform conductivity. In many cases this is a very good approximation that has been tested successfully in many investigations. It would be probably more adequate to avoid the term MEC when dealing with MHD flows in absence of electrochemical reactions.
-The section about the fundamental equations that govern the flow of electrolytes subjected to electromagnetic interactions pretends to be general but in some parts is confusing. For instance, why is the Lorentz force said to be an apparent body force? What is it that makes it "apparent"?
Is the electric field in Eq. (4) an externally imposed field or does it appear due to charge redistribution?
The fundamental MHD equations, once the MHD approximation is used, include the Navier-Stokes equation with the Lorentz body force, the mass conservation (or incompressibility condition), the Faraday’s law, Ampère’s law, Gauss’s law for magnetism and Ohm’s law. Gauss law for the electric field (Eq. (6)) is usually not considered since it is not required to close the system of equations. It is necessary only if the charge distribution is of interest. Is that the case for electrolytes? It should be stated why Ampère’s law is not considered while Gauss’s law for magnetism should be stated for completeness.
-It should be stated explicitly which is the complete system of equations that describe the flow under determined conditions. In most of the examples presented in the paper where the Lorentz force is produced by the interaction of an injected current with an imposed magnetic field, the flow is described by the Navier-Stokes equation (or the Stokes equations) and the incompressibility condition since the electric current density and the magnetic field are known. In fact, the current is given in terms of the gradient of the imposed electric potential as stated by Eq. (12), the Ha^2 term being negligible. As stated by the author, induced currents are negligible when the working fluid is an electrolyte.
-The appearance of electric force in Eq. (10) is not consistent with MHD approximation. It can be shown that it is negligible comparing with the magnetic force. Why is it included? If there is a particular consideration, it should be mentioned explicitly. Moreover, the charge density should be expressed in terms of the other electric parameters.
-To mention induction pumping as a possible application is misleading since this is not an option with electrolytes. In any case, it can be stated that this method works with high conductivity fluids such as liquid metals. It is also mentioned in line 356: “.. and by induction pumping with travelling waves”.
-In section 6 the power conversion and hydrogen production is mentioned. However, MHD power generation with an electrolyte as working fluid is not a reliable option. In MHD generators power output goes as sigma U^2 B^2, and due to the small conductivity (sigma) of electrolytes, small velocity and magnetic field the electric power is extremely low. In any case, it may be of interest as hydrogen producer but not as electric generator.
-In the second paragraph of the conclusion section, it is mentioned that
“When an electrical current (e.g., provided by a power supply) is transmitted in a conductor, such as an electrolyte, in the presence of an applied magnetic field, the electrical current interacts with the applied magnetic field to produce the Lorentz body force.” Immediately it says: “An electrical current can also be induced in a conductive medium with a time-varying magnetic field.” This is misleading since electric current can be induced with a time-varying magnetic field only if the conducting media has a high conductivity, as occurs with a liquid metal. This is not possible with an electrolyte.
-Although it is mentioned that MHD facilitates control of surface morphology, no examples or references are provided.
Minor points:
-Several misprints appear after Eq. (10): rho_m, rho_e, mu_v, DV.
-Instead of the symbol “Delta”, it is written D along the manuscript.
-It is said “Unless J and B are perpendicular to each other...” it should say “parallel to each other...”
-In Line 216 it is said “local electric flux...” This could be misleading, it is clearer to call it an induced current density.
-It would be convenient to define what “operating under galvanistic conditions” or “potentiometric conditions” means.
Author Response
I thank the reviewer for his time and constructive suggestions. Below, I reproduce the reviewer’s comments and my responses.
Reviewer: -The author seems to use the terms Magneto Electro Chemistry (MEC) and Magnetohydrodynamics (MHD) as if they were synonyms. It appears to me that this is not the case. Most of the examples presented in the review are related with the use of Lorentz forces to pump, control, or stir the fluid, originating flows that can be considered as MHD (without chemical reactions). Although it is mentioned that the injection of an electric current in the electrolyte through electrodes may give rise to electrochemical reactions that alter the properties of the electrolyte and produce unwanted products, in the discussed examples this kind of phenomena are not considered. That is, electrolyte is considered as an ohmic conductor with a uniform conductivity. In many cases this is a very good approximation that has been tested successfully in many investigations. It would be probably more adequate to avoid the term MEC when dealing with MHD flows in absence of electrochemical reactions.
Response: I agree that magneto electro chemistry and MHD are not synonyms. I attempted to clarify as much in the revised title and the revised version of my manuscript.
Reviewer: -The section about the fundamental equations that govern the flow of electrolytes subjected to electromagnetic interactions pretends to be general but in some parts is confusing. For instance, why is the Lorentz force said to be an apparent body force? What is it that makes it "apparent"?
Response: In an electrolyte solution, the Lorentz force acts on the migrating ions - not on the solvent. The ions drag the solvent. Thus, the Lorentz force is an apparent body force in the sense of mean field theory.
Reviewer: Is the electric field in Eq. (4) an externally imposed field or does it appear due to charge redistribution?
Response: E in equation (4) is the electric field from any source; applied electric field included.
Reviewer: The fundamental MHD equations, once the MHD approximation is used, include the Navier-Stokes equation with the Lorentz body force, the mass conservation (or incompressibility condition), the Faraday’s law, Ampère’s law, Gauss’s law for magnetism and Ohm’s law. Gauss law for the electric field (Eq. (6)) is usually not considered since it is not required to close the system of equations. It is necessary only if the charge distribution is of interest. Is that the case for electrolytes? It should be stated why Ampère’s law is not considered while Gauss’s law for magnetism should be stated for completeness.
Response: Equation (6) is essential for electrolytes. In the bulk of the electrolyte, the net charge is very small but not nil. The RHS of equation (6) comprises the ratio of two small quantities: the charge and the permittivity. The electric field is not divergence free and the potential cannot, in general, be determined from the Laplace equation (a common mistake in the literature). Laplace equation is valid only in the presence of supporting electrolyte. The electric field is determined by equating the RHS of the Poisson equation (6) to zero. Poisson equation is also needed to resolve the electric field in electric double layers that contain net charge. Such electric fields affect the ion distribution. I have further clarified this in the revised manuscript and specified explicitly the equation to determine the potential field (equation 7). As a result, the numbering of the equations was altered.
As I indicated earlier in my manuscript, processes in the electrolyte have an insignificant effect on the magnetic field. The determination of the magnetic field is decoupled from the electrolyte problem. I added in the revised manuscript that when permanent magnets are used, the magnetic field is determined from Gauss law of Magnetism and when electromagnets are used from Ampere’s law. I did not want to reproduce Ampere’s law in the manuscript out of fear that its presence would lead to confusion as it is driven by electric fields that may differ from the ones present in the electrolyte.
Reviewer: It should be stated explicitly which is the complete system of equations that describe the flow under determined conditions. In most of the examples presented in the paper where the Lorentz force is produced by the interaction of an injected current with an imposed magnetic field, the flow is described by the Navier-Stokes equation (or the Stokes equations) and the incompressibility condition since the electric current density and the magnetic field are known. In fact, the current is given in terms of the gradient of the imposed electric potential as stated by Eq. (12), the Ha^2 term being negligible. As stated by the author, induced currents are negligible when the working fluid is an electrolyte.
Response: In general, we determine the concentration and flow fields in the electrolyte by solving the nonlinear, coupled equations 5, 7, 10, and 11 with the appropriate boundary conditions. The electric current is defined in equation 3. I worked out an example in the paper when excess electrolyte is present. The excess electrolyte does not participate in electrode reactions and controls the electrolyte conductivity, thereby decoupling the electric field from the concentration fields. This is a special case that is amenable to an analytical solution. In most cases, the resolution of the concentration fields would require a numerical solution as indicated in the references.
Reviewer: -The appearance of electric force in Eq. (10) is not consistent with MHD approximation. It can be shown that it is negligible comparing with the magnetic force. Why is it included? If there is a particular consideration, it should be mentioned explicitly. Moreover, the charge density should be expressed in terms of the other electric parameters.
Response: Although the bulk of the electrolyte is nearly electrically neutral, this is not the case next to interfaces, solid surfaces and electrodes. The electric double layer is charged and the electrostatic forces may be significant. The expression for the electric charge density is given now in line 159.
Reviewer:-To mention induction pumping as a possible application is misleading since this is not an option with electrolytes. In any case, it can be stated that this method works with high conductivity fluids such as liquid metals. It is also mentioned in line 356: “.. and by induction pumping with travelling waves”.
Response: Done. Reference 22 on travelling waves was removed.
Reviewer: -In section 6 the power conversion and hydrogen production is mentioned. However, MHD power generation with an electrolyte as working fluid is not a reliable option. In MHD generators power output goes as sigma U^2 B^2, and due to the small conductivity (sigma) of electrolytes, small velocity and magnetic field the electric power is extremely low. In any case, it may be of interest as hydrogen producer but not as electric generator.
Response: The reviewer is correct. MHD power generation efficiency with electrolytes is very small. Hartmann numbers as high as 50 are, however, possible in electrolytes as I have indicated in the revised manuscript
Reviewer: -In the second paragraph of the conclusion section, it is mentioned that
“When an electrical current (e.g., provided by a power supply) is transmitted in a conductor, such as an electrolyte, in the presence of an applied magnetic field, the electrical current interacts with the applied magnetic field to produce the Lorentz body force.” Immediately it says: “An electrical current can also be induced in a conductive medium with a time-varying magnetic field.” This is misleading since electric current can be induced with a time-varying magnetic field only if the conducting media has a high conductivity, as occurs with a liquid metal. This is not possible with an electrolyte.
Response: Significant inductive current is possible in high conductivity electrolytes in the presence of large magnetic field. For example, Andreev et al report Hartmann numbers as high as 50 in electrolyte solution.
Reviewer: -Although it is mentioned that MHD facilitates control of surface morphology, no examples or references are provided.
Response: Indeed, one of the main applications of MHD is in metallurgy in controlling, among other things, deposit density and morphology. I added a few references.
Minor points:
Reviewer: -Several misprints appear after Eq. (10): rho_m, rho_e, mu_v, DV.
Response: I wonder whether this is an issue of software compatibility. The various symbols appear correctly in my hands.
Reviewer: -Instead of the symbol “Delta”, it is written D along the manuscript.
Response: Again, this may be a result of software incompatibility.
Reviewer: -It is said “Unless J and B are perpendicular to each other...” it should say “parallel to each other...”
Response: The revised version of the manuscript was modified.
Reviewer: -In Line 216 it is said “local electric flux...” This could be misleading, it is clearer to call it an induced current density.
Reviewer: -It would be convenient to define what “operating under galvanistic conditions” or “potentiometric conditions” means.
Response: Done
Reviewer 5 Report
Reviewer Report
Magnetochemistry (ISSN 2312-7481)
Title: Applications of Magneto Electro Chemistry (MEC) in Microfluidics
Recommendation: Major Revision
Reviewer's comments:
Paper is of current interest and falls in the scope of the journal, however, there are the following suggestions authors should address and then I welcome for publication:
Comment 1: Title of the paper should be modified.
Comment 2: Improve the quality of the paper by fixing some typos errors.
Comment 3: The Abstract should contain problem studied, methods used, and important results and should be modified with fruitful results.
Comment 4: There are too many symbols, I suggest adding a nomenclature.
Comment 5: Figures have a very poor caption. Captions must describe the figure clearly and text-independently.
Comment 6: Results and discussion part is week. Improve it by giving physical meaning of each profile.
Comment 7: The lack of physical argumentation is a concern that should be rectified in the revised version.
Comment 8: Every equation must be named and discussed to well present Mathematical modeling. Model simplifications must be discussed, and all variables must be defined.
Comment 9: The originality of the paper needs to be stated clearly. It is of importance to have sufficient results to justify the novelty of a high-quality journal paper.
Comment 10: Finally, applications of the current investigation have to be highlighted.
Author Response
Comment 1: Title of the paper should be modified.
Response: I modified the paper’s title
Comment 2: Improve the quality of the paper by fixing some typos errors.
Response: Typos have been fixed
Comment 3: The Abstract should contain problem studied, methods used, and important results and should be modified with fruitful results.
Response: This is an excellent suggestion for a research paper. This current paper is, however, a review of numerous works and applications. Hence, the suggested format is not applicable to this work.
Comment 4: There are too many symbols, I suggest adding a nomenclature.
Response: It seems uncommon to have a nomenclature in the Journal of Magnetochemistry. Perhaps this something that should be left to the editorial office.
Comment 5: Figures have a very poor caption. Captions must describe the figure clearly and text-independently.
Response: I fully agree with the reviewer that captions should be clear and descriptive. In my judgement, they are. It would have been helpful if the reviewer had pointed out specific deficiencies.
Comment 6: Results and discussion part is week. Improve it by giving physical meaning of each profile.
Comment 7: The lack of physical argumentation is a concern that should be rectified in the revised version.
Response: Specific examples would be helpful.
Comment 8: Every equation must be named and discussed to well present Mathematical modeling. Model simplifications must be discussed, and all variables must be defined.
Response: The equations are numbered as is common in scientific publications and, to my knowledge, all variables are defined. I would have appreciated it f the reviewer had pointed out anything that I have missed.
Comment 9: The originality of the paper needs to be stated clearly. It is of importance to have sufficient results to justify the novelty of a high-quality journal paper.
Response: This is a review paper – not a research paper.
Comment 10: Finally, applications of the current investigation have to be highlighted.
Response: I believe that I have done so.
Round 2
Reviewer 1 Report
Although the author has addressed most of the comments, the critical issues on how this paper is significantly different from past review papers is not clear.
Author Response
Reviewer: “Although the author has addressed most of the comments, the critical issues on how this paper is significantly different from past review papers is not clear.”
Author: I am pleased that the reviewer is familiar with my earlier work. By its very nature, as a review, this paper reproduces prior work. This paper includes, however, significant new material that was not included in my earlier review such as (A) demonstration of the presence of velocity maximum in section 2; (B) clearer presentation of MHD networks in section 4; (C) new section 5; (D) new section 6; (E) New section 9; and (F) new section 10. I did not include a statement to this effect in the paper since I think that it would not be relevant to most readers who are likely to be outside the mechanics community and unfamiliar with my earlier review.
Reviewer 3 Report
-
Author Response
This reviewer has not provided any comments.
Reviewer 4 Report
My previous comments and concerns were addressed by the author so I consider that the present version of the paper can be considered for publication. I have, however, two minor comments that I think that the author should consider:
- The author should avoid the notation Re_m for the Reynolds number (see eq. 12) since the magnetic Reynolds number is usually denoted in this way.
- As I commented in my previous review, in section 6, it should be explicitly stated that MHD generators based on electrolytes produce an extremely low electrical power. The use of liquid metal is evidently more appropriate for these devices.
Author Response
I thank the reviewer for his comments. Below, I reproduce the reviewer's comments and my response.
Reviewer: The author should avoid the notation Re_m for the Reynolds number (see eq. 12) since the magnetic Reynolds number is usually denoted in this way.
Author: I modified my notation.
Reviewer: - As I commented in my previous review, in section 6, it should be explicitly stated that MHD generators based on electrolytes produce an extremely low electrical power. The use of liquid metal is evidently more appropriate for these devices.
Author: I added a comment to the address the reviewer's concern
Reviewer 5 Report
The authors made all required changes
Author Response
I have addressed all this reviewer's comments in my earlier re-submission. This reviewer had no comments.
