One-directional flow of ionic solutions along fine electrodes under an alternating current electric field

Electric fields are widely used for controlling liquids in various research fields. To control a liquid, an alternating current (AC) electric field can offer unique advantages over a direct current (DC) electric field, such as fast and programmable flows and reduced side effects, namely the generation of gas bubbles. Here, we demonstrate one-directional flow along carbon nanotube nanowires under an AC electric field, with no additional equipment or frequency matching. This phenomenon has the following characteristics: First, the flow rates of the transported liquid were changed by altering the frequency showing Gaussian behaviour. Second, a particular frequency generated maximum liquid flow. Third, flow rates with an AC electric field (approximately nanolitre per minute) were much faster than those of a DC electric field (approximately picolitre per minute). Fourth, the flow rates could be controlled by changing the applied voltage, frequency, ion concentration of the solution and offset voltage. Our finding of microfluidic control using an AC electric field could provide a new method for controlling liquids in various research fields.


Do you have any ethical concerns with this paper? No
Have you any concerns about statistical analyses in this paper? I do not feel qualified to assess the statistics

Recommendation?
Major revision is needed (please make suggestions in comments)

Comments to the Author(s)
The authors explore the transport of aqueous electrolytes under imposed high frequency (MHz) electric fields in a geometry that consists of a planar electrode upon which the electrolyte drop sits and a vertically held carbon nanotube coated tungsten wire in contact with the drop. Under certain conditions, they observe that the electrolyte migrate up the tungsten wire.
They make the assertion that the this is a previously unreported behavior and that it cannot be explained by existing theories, namely, dielectrophoresis, induced charge electroosmosis, travelling wave, AC, and rectified AC electroosmosis. While this may indeed be a novel phenomenon, it is my opinion that they have not done an adequate job of exploring the details of the other theories that they present, nor have they included all electrokinetic phenomena. Furthermore, they present their experimental observations, but provide no theoretical explanation for the observed behavior. I would recommend publication only after major revisions to the manuscript. My specific comments are below: 1. In the introduction, dielectrophoresis, induced charge electroosmosis, travelling wave, AC, and rectified AC electroosmosis are all listed as possibly related and the authors attempt to explain why each theory is inadequate. The treatment is qualitative and sparse. I suggest the authors include a more detailed treatment of each of these theories, presenting the relevant equations that explain fluid flow under each theory and then explain why the presented experiment does not fit with theory.
2. On line 26, the statement "the mechanism of 'ICEO' is based on the movement of small particles, however, the discovered phenomenon happens without any particles" is false. ICEO can occur around any polarizable surface. The authors cite: Squires, T. M.; Bazant, M. Z. Induced-Charge Electro-Osmosis. Journal of Fluid Mechanics 2004, 509, 217-252. I encourage the authors to re-read section 5 that describe ICEO flow around electrically floating electrodes in microfluidic devices.
3. There are several other electrokinetic phenomena that the authors neglect to consider as possible explanations of the observed effect: Electrowetting: The presented setup is similar to those found in the electrowetting literature. The following review may be helpful in assessing: Mugele, Frieder, and Jean-Christophe Baret. "Electrowetting: From Basics to Applications." Journal of Physics: Condensed Matter 17, no. 28 (2005) 4. The authors should perform a flow visualization experiment, such as micro-PIV, to look at flow patterns in the system during the experiment. This will help to determine a possible mechanism.
5. The authors provide no competing theory to explain their observations. If it isn't a well established theory, then what is the physical mechanism that is driving the flow?
6. The authors should provide more detail on the experimental setup. Specifically, details on the electronics utilized for the experiment. For instance, what is the model number of the power supply used? How much current was flowing during the experiment? What is the conductivity of the electrolytes?
With these significant revisions, I feel that the manuscript would be more impactful to the community.

03-Aug-2018
Dear Professor Shin, The editors assigned to your paper ("One-directional flow of ionic solutions along fine electrodes under an alternating current electric field") have now received comments from reviewers. We would like you to revise your paper in accordance with the referee and Associate Editor suggestions which can be found below (not including confidential reports to the Editor). Please note this decision does not guarantee eventual acceptance.
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We suggest the following format: AB carried out the molecular lab work, participated in data analysis, carried out sequence alignments, participated in the design of the study and drafted the manuscript; CD carried out the statistical analyses; EF collected field data; GH conceived of the study, designed the study, coordinated the study and helped draft the manuscript. All authors gave final approval for publication.
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Comments to Author:
Reviewers' Comments to Author: Reviewer: 1 Comments to the Author(s) Overall the paper is interesting -it reports on some unusual experimental results that are not easily explained by any theory I am aware of. In the intro, the authors review several possibilities and conclude none of them explain. I have a few comments in no particular order.
1) The authors claim there are no reactions because there are no bubbles. Did they try to assess reactions in a more quantitative way, or does the small droplet size make this difficult -i.e. measure pH?
2) I didn't really understand the brackets and the stars in all the figures? I found it distracting from the otherwise pretty simple plots.
3) I didn't see a comment in the paper about how many repeats were done at each measurement. Are the error bars the scatter at that condition? Is it the n=7? If so, be more explicit -i.e. say each experiment was repeated 7 times. If it was repeated, was it with a new CNT or the same CNT but different flow measurement? Just a sentence or two to explain how repeatable your results are.
4) The table at the end could be better formatted -or maybe all this data would be organized as supplementary material. In any case it was hard to figure out what you were showing there. 5) I dont understand why at 40 MHZ you have 200 nL/min but can't measure lower than that. A little more explanation would be helpful here. I don't doubt the difficulty due to the scale here. 6) Caption for figure 7 plots frequency on one axis and then reports a frequency in the caption. That seems confusing.
7) The comment in the caption about values compared by a t-test seems unusual. I don't really know what you mean by this comment. Are you trying to claim the data are statistically significantly different? Why not just show the data with the error bars?
Reviewer: 2 Comments to the Author(s) The authors explore the transport of aqueous electrolytes under imposed high frequency (MHz) electric fields in a geometry that consists of a planar electrode upon which the electrolyte drop sits and a vertically held carbon nanotube coated tungsten wire in contact with the drop. Under certain conditions, they observe that the electrolyte migrate up the tungsten wire.
They make the assertion that the this is a previously unreported behavior and that it cannot be explained by existing theories, namely, dielectrophoresis, induced charge electroosmosis, travelling wave, AC, and rectified AC electroosmosis. While this may indeed be a novel phenomenon, it is my opinion that they have not done an adequate job of exploring the details of the other theories that they present, nor have they included all electrokinetic phenomena. Furthermore, they present their experimental observations, but provide no theoretical explanation for the observed behavior. I would recommend publication only after major revisions to the manuscript. My specific comments are below: 1. In the introduction, dielectrophoresis, induced charge electroosmosis, travelling wave, AC, and rectified AC electroosmosis are all listed as possibly related and the authors attempt to explain why each theory is inadequate. The treatment is qualitative and sparse. I suggest the authors include a more detailed treatment of each of these theories, presenting the relevant equations that explain fluid flow under each theory and then explain why the presented experiment does not fit with theory.
2. On line 26, the statement "the mechanism of 'ICEO' is based on the movement of small particles, however, the discovered phenomenon happens without any particles" is false. 4. The authors should perform a flow visualization experiment, such as micro-PIV, to look at flow patterns in the system during the experiment. This will help to determine a possible mechanism.
5. The authors provide no competing theory to explain their observations. If it isn't a well established theory, then what is the physical mechanism that is driving the flow? 6. The authors should provide more detail on the experimental setup. Specifically, details on the electronics utilized for the experiment. For instance, what is the model number of the power supply used? How much current was flowing during the experiment? What is the conductivity of the electrolytes?
With these significant revisions, I feel that the manuscript would be more impactful to the community.

Author's Response to Decision Letter for (RSOS-180657.R0)
Thank you for your kind handling of our manuscript. In this manuscript, we responded to the reviewer comments and all changes in our manuscript were highlighted.

Do you have any ethical concerns with this paper? No
Have you any concerns about statistical analyses in this paper? I do not feel qualified to assess the statistics

Comments to the Author(s)
The authors have included additional details of electrokinetic theories in the text as I recommended in the initial review; however, some of the analysis given, in particular, on electrowetting is difficult to follow. I believe more analysis needs to be performed beyond the qualitative explanation given by the authors. In specific: Will the authors please provide more explanation for this statement: "However, electrowetting differs from the phenomenon we describe in that an insulating layer is placed between the mother droplet and the plane electrode; this renders monodirectional flow impossible" To my knowledge, electrowetting usually performed on a dielectric layer to limit Faradaic reactions. Electrowetting can and does occur on bare metal electrodes a well. Regardless, I do not see how the presences of an insulator mean that monodirectional flow is impossible. Please explain.
Also, the field is very high near the tip of the nanotube. The authors state that the CNT has a diameter of ~1 nm, and the effect appears at >5Vpp. This means that peak electric fields near the CNT tip are on the order of 10^9 V/m. This is much higher than the dielectric breakdown strength of water which is ~70*10^6 V/m. Is it possible that this effect isdue to electrical breakdown?

11-Dec-2018
Dear Professor Shin: On behalf of the Editors, I am pleased to inform you that your Manuscript RSOS-180657.R1 entitled "One-directional flow of ionic solutions along fine electrodes under an alternating current electric field" has been accepted for publication in Royal Society Open Science subject to minor revision in accordance with the referee suggestions. Please find the referees' comments at the end of this email.
The reviewers and Subject Editor have recommended publication, but also suggest some minor revisions to your manuscript. Therefore, I invite you to respond to the comments and revise your manuscript.
• Ethics statement If your study uses humans or animals please include details of the ethical approval received, including the name of the committee that granted approval. For human studies please also detail whether informed consent was obtained. For field studies on animals please include details of all permissions, licences and/or approvals granted to carry out the fieldwork.
• Data accessibility It is a condition of publication that all supporting data are made available either as supplementary information or preferably in a suitable permanent repository. The data accessibility section should state where the article's supporting data can be accessed. This section should also include details, where possible of where to access other relevant research materials such as statistical tools, protocols, software etc can be accessed. If the data has been deposited in an external repository this section should list the database, accession number and link to the DOI for all data from the article that has been made publicly available. Data sets that have been deposited in an external repository and have a DOI should also be appropriately cited in the manuscript and included in the reference list.
If you wish to submit your supporting data or code to Dryad (http://datadryad.org/), or modify your current submission to dryad, please use the following link: http://datadryad.org/submit?journalID=RSOS&manu=RSOS-180657.R1 • Competing interests Please declare any financial or non-financial competing interests, or state that you have no competing interests.
• Authors' contributions All submissions, other than those with a single author, must include an Authors' Contributions section which individually lists the specific contribution of each author. The list of Authors should meet all of the following criteria; 1) substantial contributions to conception and design, or acquisition of data, or analysis and interpretation of data; 2) drafting the article or revising it critically for important intellectual content; and 3) final approval of the version to be published.
All contributors who do not meet all of these criteria should be included in the acknowledgements.
We suggest the following format: AB carried out the molecular lab work, participated in data analysis, carried out sequence alignments, participated in the design of the study and drafted the manuscript; CD carried out the statistical analyses; EF collected field data; GH conceived of the study, designed the study, coordinated the study and helped draft the manuscript. All authors gave final approval for publication.
• Acknowledgements Please acknowledge anyone who contributed to the study but did not meet the authorship criteria.
• Funding statement Please list the source of funding for each author.
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Kind regards, Andrew Dunn
Royal Society Open Science Editorial Office Royal Society Open Science openscience@royalsociety.org on behalf of Dr Oliver Jensen (Associate Editor) and R. Kerry Rowe (Subject Editor) openscience@royalsociety.org Associate Editor Comments to Author (Dr Oliver Jensen): Associate Editor: 1 Comments to the Author: Please make a further revision of your paper to address the specific points raised by the reviewer. In particular, please remove the assertion (sentence 2 of the Conclusions) that "The discovered phenomenon ... cannot be explained using previous theories." This comment is not fully supported by your analysis.
In addition to dielectric breakdown as a candidate mechanism of the effect, please also comment on the possibility of acoustic (steady) streaming as a possible mechanism.
Reviewer comments to Author: Reviewer: 2 Comments to the Author(s) The authors have included additional details of electrokinetic theories in the text as I recommended in the initial review; however, some of the analysis given, in particular, on electrowetting is difficult to follow. I believe more analysis needs to be performed beyond the qualitative explanation given by the authors. In specific: Will the authors please provide more explanation for this statement: "However, electrowetting differs from the phenomenon we describe in that an insulating layer is placed between the mother droplet and the plane electrode; this renders monodirectional flow impossible" To my knowledge, electrowetting usually performed on a dielectric layer to limit Faradaic reactions. Electrowetting can and does occur on bare metal electrodes a well. Regardless, I do not see how the presences of an insulator mean that monodirectional flow is impossible. Please explain.
Also, the field is very high near the tip of the nanotube. The authors state that the CNT has a diameter of ~1 nm, and the effect appears at >5Vpp. This means that peak electric fields near the CNT tip are on the order of 10^9 V/m. This is much higher than the dielectric breakdown strength of water which is ~70*10^6 V/m. Is it possible that this effect isdue to electrical breakdown?
Author's Response to Decision Letter for (RSOS-180657.R1) See Appendix A.