Developing an Active Microfluidic Pump and Mixer Driven by AC Field-Effect-Mediated Induced-Charge Electro-Osmosis of Metal–Dielectric Janus Micropillars: Physical Perspective and Simulation Analysis

Round 1

Reviewer 1 Report

This is a very nice piece of work. The idea is very clever, and addresses an important problem in microfluidics, which is mixing. The typically low Reynolds number that characterizes the flow in such devices prevents from utilizing turbulent flow for improving the mixing of solutes. Hence, other approaches are necessary. The use of AC in combination with an array of metal/dielectric Janus particles allows for designing flow patterns that add a convective component to the mass transport that improves mixing. The idea is similar (but not identical) to the one outlined in the papers by Velev et al. in Nature Materials, 6 (2007) pp. 235-240 and Lab-on-a-Chip, 8 (2008) pp. 117-124, which surprisingly are not cited. Adding the citations and commenting on the similarities and differences is necessary. 

One thing that need improvement is the quality of some figures. Almost all data plots look a but rough and the axis labeling font could have been a bit larger. Another advice is to reduce the overall length of the paper and make it more succinct. It is too verbose in its present form and that makes is hard to read. 

Apart from these criticisms, I support the publication of this article in Applied Sciences. 

Author Response

Please see the attachment.

Response to Reviewer 1

Reviewer 1:

Comments and Suggestions for Authors

Question 1:

This is a very nice piece of work. The idea is very clever, and addresses an important problem in microfluidics, which is mixing. The typically low Reynolds number that characterizes the flow in such devices prevents from utilizing turbulent flow for improving the mixing of solutes. Hence, other approaches are necessary. The use of AC in combination with an array of metal/dielectric Janus particles allows for designing flow patterns that add a convective component to the mass transport that improves mixing. The idea is similar (but not identical) to the one outlined in the papers by Velev et al. in Nature Materials, 6 (2007) pp. 235-240 and Lab-on-a-Chip, 8 (2008) pp. 117-124, which surprisingly are not cited. Adding the citations and commenting on the similarities and differences is necessary. 

Answer:

We thank the reviewer for the professional comments and insightful suggestions. Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches.

We have carefully read the two excellent papers recommended by the referee, which are from the group of Velev. In view of this comment, the following content has been added to the Introduction Section:

“Recently, the group of Velev [60] demonstrated how various semiconductor diodes form a new category of self-propelling particles and pumps in microfluidic systems. The energy is supplied by a global external AC electric field. A DC voltage difference is then created between the electrodes of each diode as a consequence of rectification of the global AC field. The uniform electric field between the electrodes gives rise to stable electro-osmotic flow, which could propel the floating diodes or pump the surrounding liquid. In a subsequent article from the same group [61], detailed results and discussion of the fluid pumping and flow behavior control with semiconductor diodes subjected to external AC electric field are presented. The diodes are fixed alongside the insulating channel walls and depending on their mutual orientation can work as pumps or mixers. The parameters that regulate the fluid flow are identified and their effects are studied numerically.

Although both the present work and the relevant literatures of Velev et al. made use of anisotropic solid object to actuate biased electrohydrodynamic fluid motion, the underlying physical mechanism is completely different from each other. Our currently proposed idea of Janus AC-FFET is based upon time-averaged nonlinear induced-charge electroosmosis at polarizable metal/electrolyte interfaces with symmetric breaking in flow profile along two orthogonal directions, having potential to achieve simultaneous pump and mixing of analytes in microfluidic channels, while the remotely powered microfluidic devices embedding miniature diodes are intrinsically originated by linear electroosmosis acting on native electrical double layer under a local DC voltage rectified from the globally applied AC electric field [60,61].”

Besides, some classical literatures from the group of Velev on the topic of electrokinetic micro/nanofluidics have also been quoted in the Introduction Section: “More importantly, the group led by Velev has made a series of breakthrough scientific achievements in the field of electrokinetic microfluidics during the past two decades [62-69].”

Please refer to the highlighted words in the Introduction Section.

Question 2:

One thing that need improvement is the quality of some figures. Almost all data plots look a bit rough and the axis labeling font could have been a bit larger.

Answer:

According to your professional advice, we have modified and re-output all the data curves in Fig.2-13 from the scientific drawing software, so as to enhance their quality as far as possible, chiefly in terms of two aspects. First, size of all the axis labeling font in these data plots has been unified to a fixed value of 26, which is much larger than 20-24 used in previous version. Second, the y-axis indicator has been centered with respect to the y-axis in all the data plots.

Please refer to the updated Fig.2-13 in the revised manuscript.

Question 3:

Another advice is to reduce the overall length of the paper and make it more succinct. It is too verbose in its present form and that makes it hard to read. 

Apart from these criticisms, I support the publication of this article in Applied Sciences. 

Answer:
We appreciate your professional and insightful comment.

Considering the logical coherence of the entire scientific content in current study, all the contents constitute the complete architecture of the present manuscript, any omissions may be detrimental to the presentation of relevant details. Even so, considering your advice, we have tried our best to make the paper more concise from two aspects. First, some insignificant analyzes of the microscopic physical description of electrothermal convection and basic nonlinear feature of induced-charge electrokinetic phenomenon are deleted directly from the Introduction Section. Second, the original Section 2.3 Numerical simulation methodology and Section 3.1 Model Validation against standard benchmarks were both moved to the Supplementary Information (SI).

  Please refer to the highlighted words in the Introduction Section, as well as Section A. and Section B. in the SI.

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript titled " Developing active microfluidic pump and mixer driven by AC field-effect-mediated induced-charge electroosmosis of metal-dielectric Janus micropillars: Physical perspective and simulation analysis" is a well planned study with comprehensive details on each aspect of concern.

Authors have carefully addressed the current issues and also concluded the shortcomings of the present study and possibilities of deviations from the presented results at varied experimental conditions.

The paper can be accepted with just a handful of minor corrections.

1. In line 160, how do authors claim that their physical model of ICEO is accurate than the standard model?

2. In line 475, "modal" is typed instead of "model". This is just a minor spelling error.

Use of language is professional and advanced. Just a few minor adjustments would be needed in terms of spellings in some places. This might be due to the mistakes happened during typing. 

Author Response

Please see the attachment.

Response to Reviewer 2

Reviewer 2:

Comments and Suggestions for Authors

Question 1:

The manuscript titled " Developing active microfluidic pump and mixer driven by AC field-effect-mediated induced-charge electroosmosis of metal-dielectric Janus micropillars: Physical perspective and simulation analysis" is a well planned study with comprehensive details on each aspect of concern.

Authors have carefully addressed the current issues and also concluded the shortcomings of the present study and possibilities of deviations from the presented results at varied experimental conditions.

The paper can be accepted with just a handful of minor corrections.

1. In line 160, how do authors claim that their physical model of ICEO is accurate than the standard model?

Answer:

We thank the reviewer for the professional comments and insightful suggestions. Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches.

We sincerely appreciate your positive and constructive comments on our manuscript, which are of great importance to help us enhance its quality.

The standard model of ICEO was proposed by Squires and Bazant in a pioneering JFM paper [Journal of Fluid Mechanics 2004, 509, 217-252]. In the mathematical derivation of the standard model, the polarizable solid surface in contact with the electrolyte solution is assumed to be a perfect metal conductor of an equal body potential when subjected to a background DC or low-frequency AC electric field. So, it may be difficult for the standard model to describe the phenomenon of ICEK adjacent to non-metallic solid objects.

To investigate Janus AC-FFET in detail, under the approximation of thin IDL and small IZP, starting from leaky dielectric theory, we derived a rigorous macroscopic mathematical model, which is able to capture inhomogeneous electrochemical polarization and time-averaged ICEO slipping at the pillar/electrolyte interfaces, on both the metal and dielectric-phase surface of the Janus post. Considering a high impedance of dielectric phase, capacitive charging of IDL on an insulating surface is usually at least two orders of magnitude weaker than that on conducting surface, so it can be safely disregarded in most situations. However, at the transition region between metal and dielectric phase, there is appreciable ICEO slipping velocity, which is identical to the strong ICEO jet observed around sharp dielectric corners. That is, the standard model of ICEK may malfunction when dielectric solid phase having sharp boundaries is involved in microfluidic channels. In this sense, by resolving the phenomenon of ICEO streaming on solid surfaces with any electrical polarizability, our physical description on ICEO is more accurate than the standard model obtained by linear RC circuit theory on perfect conductors. It is worth to note that, the standard model is generally correct when mere metal conductors are utilized to actuate ICEO convection under the Debye-Hückel limit.

Please refer to the highlighted words in Page 4 of the revised manuscript.

Question 2:

2. In line 475, "modal" is typed instead of "model". This is just a minor spelling error.

Answer:

Thanks for your warm correction.

Since this section has been moved to the Supplementary Information (SI), we have corrected this spelling error in the SI.

Question 3:

3. Comments on the Quality of English Language

Use of language is professional and advanced. Just a few minor adjustments would be needed in terms of spellings in some places. This might be due to the mistakes happened during typing. 

Answer:

We appreciate your positive comments on our English writing of current manuscript.

According to your advice, we have carefully gone through the manuscript, and corrected spelling errors as far as we can.

Author Response File: Author Response.pdf