Electrokinetic Microfluidics at the Convergence Frontier: From Charge-Driven Transport to Intelligent Chemical Systems

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The paper presents a review of electrokinetic techniques used in microfluidic devices. The use of electrokinetic techniques on lab-on-a-chip systems, biotechnology, analytical chemistry etc. are on the rise. Advancement on the area have accelerated in recent times. So, the paper covers a very relevant topic which should be of significant interest to the readers of micromachines. Since it is a review paper, scope for technical comments are limited. My comments mainly focus on what discussion should be added and/or expanded upon. After some revisions, the paper should be suitable for publication.  My detailed comments are listed below:

1. In microfluidic devices, both droplets and micro/nano-samples are transported using electrokinetics. The review should briefly mention how these two cases differ in terms of challenges.

2. The review paper discusses several key electrokinetic techniques. However, it omits a few critical methods. Mainly: electrowetting and optoelectronic tweezers. These are widely used electrokinetic techniques and have been widely used in a variety of applications. Optoelectornic tweezers is a techniques based on dielectrophoresis. And electrowetting is a technique that engineers the contact angle between a droplet and a surface to induce motion. Since this is a review paper, it should cover these widely used approach. I recommend looking into the publications of Prof. Lambertus Hesselink, Prof. Shuailong Zhang, and Prof. Aaron Wheeler. At least 3-4 papers from each research group (related to optoelectronic tweezers/optically induced dielectrophoresis and electrowetting) should be cited.

3. When discussing surfactants, tween 20 was mentioned. There are other surfactants that has been used to reduce particle adhesion. For example Triton X-100 and sodium dodecyl sulfate (SDS). Please include discussion on these surfactants.

4. When discussing surfactants, only tween 20 is mentioned. Triton X-100 and sodium dodecyl sulfate (SDS) are also common surfactants. They should be mentioned in the discussion along with tween 20.

5. Traveling-wave dielectrophoresis should be discussed a bit more. In addition, a separate variant called "moving dielectrophoresis" should be mentioned. There have been published work where single particle transport was achieved using moving dielectrophoresis.

6. Electrokinetic techniques on a flowing suspension media behaves differently than in a static suspension medium. While flowing fluid case is more common, stationary suspensions media are often used for single-sample manipulation/characterization. Please briefly discuss this.

Author Response

The paper presents a review of electrokinetic techniques used in microfluidic devices. The use of electrokinetic techniques on lab-on-a-chip systems, biotechnology, analytical chemistry etc. are on the rise. Advancement on the area have accelerated in recent times. So, the paper covers a very relevant topic which should be of significant interest to the readers of micromachines. Since it is a review paper, scope for technical comments are limited. My comments mainly focus on what discussion should be added and/or expanded upon. After some revisions, the paper should be suitable for publication.  My detailed comments are listed below:

1. In microfluidic devices, both droplets and micro/nano-samples are transported using electrokinetics. The review should briefly mention how these two cases differ in terms of challenges.

Reply: Thanks to the reviewer. The authors have already briefly described the differences in challenges between these two scenarios in the Introduction. (Lines 68-75)

“Electric fields in electrokinetic microfluidic systems actuate both droplets and micro/nano-scale samples, but the underlying constraints fundamentally differ [4]. Droplet motion is dominated by multiphase electrohydrodynamics, where interfacial charge redistribution, deformation, and liquid–liquid coupling introduce instability and control complexity [2]. By contrast, transport of micro/nano-samples proceeds within a single phase and is increasingly constrained by electrical double-layer interactions, Brownian motion, Joule heating, and field-induced dispersion as dimensions shrink toward the nanoscale [10].”

2. The review paper discusses several key electrokinetic techniques. However, it omits a few critical methods. Mainly: electrowetting and optoelectronic tweezers. These are widely used electrokinetic techniques and have been widely used in a variety of applications. Optoelectornic tweezers is a techniques based on dielectrophoresis. And electrowetting is a technique that engineers the contact angle between a droplet and a surface to induce motion. Since this is a review paper, it should cover these widely used approach. I recommend looking into the publications of Prof. Lambertus Hesselink, Prof. Shuailong Zhang, and Prof. Aaron Wheeler. At least 3-4 papers from each research group (related to optoelectronic tweezers/optically induced dielectrophoresis and electrowetting) should be cited.

Reply: We thank the reviewer for this important and constructive suggestion. Electrowetting and optoelectronic tweezers indeed represent widely adopted electrokinetic paradigms that complement classical EOF-, EP-, and DEP-based approaches.

In response, a new dedicated section has been added to the revised manuscript (now Section 7, Emerging Electrokinetic: Electrowetting and Optoelectronic Tweezers), which systematically introduces the physical principles, operational characteristics, and representative applications of electrowetting-on-dielectric and optoelectronic tweezers. The original Conclusions section has been renumbered as Section 8 accordingly.

This new section places electrowetting in the context of interfacial energy modulation for droplet actuation, and optoelectronic tweezers as an optically addressable extension of dielectrophoresis, thereby completing the conceptual landscape of electrokinetic microfluidics discussed in this review. Representative works from Prof. Aaron Wheeler, Prof. Lambertus Hesselink, and Prof. Shuailong Zhang have been incorporated to reflect the breadth and impact of these techniques.

3. When discussing surfactants, tween 20 was mentioned. There are other surfactants that has been used to reduce particle adhesion. For example Triton X-100 and sodium dodecyl sulfate (SDS). Please include discussion on these surfactants.

Reply: Thanks to the reviewer. The authors have briefly described the properties of two surfactants, Triton X-100 and sodium dodecyl sulfate (SDS), in electrokinetic microfluidics in Section 4.2. (Lines 380-388)

“For example, the commonly used surfactant Tween 20 is often used to reduce particle adhesion, but studies have indicated that it significantly reduces electroosmotic migration in PDMS microchannels. Increasing the Tween 20 concentration continues to reduce mobility, suggesting it should be used with care in applications where flow rate is a critical parameter [43]. Triton X-100, a nonionic surfactant, disrupts hydrophobic interactions at solid–liquid interfaces, thereby limiting particle attachment and reducing surface fouling within microfluidic channels [95]. Sodium dodecyl sulfate (SDS) inhibits particle attachment by imparting strong electrostatic repulsion, but its pronounced protein-disruptive behavior makes it unsuitable for assays requiring preserved biomolecular structure [96].”

4. When discussing surfactants, only tween 20 is mentioned. Triton X-100 and sodium dodecyl sulfate (SDS) are also common surfactants. They should be mentioned in the discussion along with tween 20.

Reply: Thanks to the reviewer. The authors have briefly described the properties of two surfactants, Triton X-100 and sodium dodecyl sulfate (SDS), in electrokinetic microfluidics in Section 4.2. (Lines 380-388)

5. Traveling-wave dielectrophoresis should be discussed a bit more. In addition, a separate variant called "moving dielectrophoresis" should be mentioned. There have been published work where single particle transport was achieved using moving dielectrophoresis.

Reply: Thanks to the reviewer. The authors have added a detailed explanation of traveling wave dielectrophoresis to the text and introduced the effect of moving dielectrophoresis on single-particle transport phenomena. (Lines 424-428)

“In addition, traveling-wave DEP regulates single-particle transport via a frequency-dependent interaction between Coulomb and dielectrophoretic forces. This balance determines directional motion, establishes a size-sensitive transport threshold, and offers a mechanistic foundation for the predictable, selective manipulation of individual particles in electrokinetic micro- and mesoscale systems [115].”

6. Electrokinetic techniques on a flowing suspension media behaves differently than in a static suspension medium. While flowing fluid case is more common, stationary suspensions media are often used for single-sample manipulation/characterization. Please briefly discuss this.

Reply: Thanks to the reviewer. The authors have added a detailed explanation of the use of static suspension media for single-sample manipulation/characteristic analysis in the main text. (Lines 83-86)

“It additionally distinguishes electrokinetic behavior in static suspensions—dominated by inherent electrical double-layer polarization and particle–particle interactions—from that in flowing systems, where convection, shear, and field–flow coupling fundamentally modify the resulting forces and transport pathways [15].”

 

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This manuscript presents a timely and comprehensive review of electrokinetic microfluidics, covering a wide range of charge-driven transport phenomena, electroosmotic flow modulation, electrohydrodynamic instabilities, droplet manipulation, and enrichment strategies, with relevance to emerging intelligent chemical systems. The topic fits well within the scope of Micromachines, and the authors have surveyed a substantial body of recent literature. However, in its current form, the manuscript remains largely descriptive and lacks the level of conceptual synthesis and critical analysis expected of a high-quality review article. The discussion primarily summarizes individual studies sequentially, without sufficiently articulating a unifying physical framework, critically evaluating limitations and trade-offs, or clearly distinguishing between proof-of-concept demonstrations and approaches closer to practical implementation. Substantial revision is therefore required to strengthen the intellectual contribution of the review by improving conceptual integration, critical depth, and clarity regarding the maturity and applicability of the reviewed electrokinetic techniques.

1. While the manuscript is divided into sections according to different electrokinetic mechanisms, the connections among these mechanisms are not sufficiently articulated. The authors should introduce a clearer overarching framework that links electroosmotic flow, electrophoresis, electrohydrodynamic instability, and enrichment phenomena through common physical principles such as charge distribution, electric field gradients, and fluid–ion coupling. This framework should be explicitly stated in the Introduction and revisited in the Conclusions.
2. The review predominantly summarizes prior work without adequately evaluating its limitations. For a major revision, the authors should critically assess key challenges such as Joule heating, electrode degradation, sensitivity to fluid properties, scalability, and robustness in biologically relevant samples. A balanced discussion of both strengths and weaknesses is essential for a review article.
3. Many application examples are presented, but the manuscript does not clearly distinguish between proof-of-concept demonstrations and approaches that are closer to practical implementation. The authors should clarify the technology readiness level of major electrokinetic strategies, particularly for point-of-care and integrated lab-on-a-chip systems.
4. Several closely related topics are treated in separate sections with limited cross-reference, which disrupts the logical flow. For example, electroosmotic flow modulation and electrokinetic instabilities are physically linked but discussed largely in isolation. Stronger transitional text and explicit cross-linking are required.
5. The Conclusions section should move beyond summarizing the reviewed content and provide a more forward-looking and critical outlook. Key open questions, fundamental limitations, and promising future directions should be explicitly identified. Without this, the manuscript does not fully meet the expectations of a comprehensive review.

The manuscript has the potential to become a valuable review for the microfluidics and electrokinetics community, but substantial revisions are necessary to enhance its conceptual integration, critical depth, and practical relevance.

 

Author Response

This manuscript presents a timely and comprehensive review of electrokinetic microfluidics, covering a wide range of charge-driven transport phenomena, electroosmotic flow modulation, electrohydrodynamic instabilities, droplet manipulation, and enrichment strategies, with relevance to emerging intelligent chemical systems. The topic fits well within the scope of Micromachines, and the authors have surveyed a substantial body of recent literature. However, in its current form, the manuscript remains largely descriptive and lacks the level of conceptual synthesis and critical analysis expected of a high-quality review article. The discussion primarily summarizes individual studies sequentially, without sufficiently articulating a unifying physical framework, critically evaluating limitations and trade-offs, or clearly distinguishing between proof-of-concept demonstrations and approaches closer to practical implementation. Substantial revision is therefore required to strengthen the intellectual contribution of the review by improving conceptual integration, critical depth, and clarity regarding the maturity and applicability of the reviewed electrokinetic techniques.

1. While the manuscript is divided into sections according to different electrokinetic mechanisms, the connections among these mechanisms are not sufficiently articulated. The authors should introduce a clearer overarching framework that links electroosmotic flow, electrophoresis, electrohydrodynamic instability, and enrichment phenomena through common physical principles such as charge distribution, electric field gradients, and fluid–ion coupling. This framework should be explicitly stated in the Introduction and revisited in the Conclusion.

Reply: We thank the reviewer for highlighting the importance of a clearer unifying framework. In response, the revised manuscript now articulates an explicit physical perspective that links electroosmotic flow, electrophoresis, electrohydrodynamic instabilities, and electrokinetic enrichment within a single conceptual structure.

The Introduction has been revised to clarify that these phenomena originate from the same interplay between interfacial charge organization, applied electric fields, and fluid–ion interactions under confinement. Rather than treating EOF, EP, instability-driven transport, and enrichment as separate techniques, their distinctions are now described in terms of how electric-field gradients, EDL polarization, and ion transport respond to geometry, boundary conditions, and field nonuniformity. (Lines 68-75, 117-120)

In the Conclusions, this viewpoint is taken up again by framing linear electrokinetic transport, nonlinear electrohydrodynamic responses, and field-driven enrichment as related behaviors within a single response continuum. Taken together, the revisions sharpen the conceptual thread of the manuscript and make explicit how different electrokinetic strategies can be coherently interpreted under a shared physical basis. (Lines 883-891, 911-913)

2. The review predominantly summarizes prior work without adequately evaluating its limitations. For a major revision, the authors should critically assess key challenges such as Joule heating, electrode degradation, sensitivity to fluid properties, scalability, and robustness in biologically relevant samples. A balanced discussion of both strengths and weaknesses is essential for a review article.

Reply: Thanks to the reviewer. Practical constraints of electrokinetic microfluidics are now discussed explicitly. Issues such as Joule heating, electrode degradation, sensitivity to fluid properties, scalability, and operational robustness in biologically relevant matrices are now analyzed in terms of their physical origins and practical consequences. Where possible, representative mitigation strategies reported in the literature are discussed alongside their remaining constraints.

       The Conclusion and Future Outlook section brings these considerations together by placing demonstrated capabilities alongside unresolved constraints. As a result, the review no longer reads as a catalogue of techniques but as a design-aware assessment of electrokinetic microfluidics, reflecting both practical feasibility and current boundaries relevant to fundamental studies and system-level development.  (Lines 820-831)

3. Many application examples are presented, but the manuscript does not clearly distinguish between proof-of-concept demonstrations and approaches that are closer to practical implementation. The authors should clarify the technology readiness level of major electrokinetic strategies, particularly for point-of-care and integrated lab-on-a-chip systems.

Reply: Thanks to the reviewer. The revised manuscript now explicitly differentiates proof-of-concept studies from approaches closer to practical implementation by clarifying their approximate technology readiness levels (TRLs). Droplet electrodynamics, including the transport and breakup of Janus droplets, is defined as mechanism-driven proof-of-concept work (TRLs 2–4), elucidating interfacial electrohydraulic dynamics but remaining sensitive to chemical properties and electric field homogeneity [37,49–53]. In contrast, electrokinetic injection and microcrystal electrophoresis exhibit higher readiness (TRLs 4–7) and have examples of integrated separation and portable analyzers suitable for wafer lab deployments [63,64]. Electrokinetic preconcentration strategies, particularly ICP and stacking on paper-based or hybrid wafers, are considered most relevant to point-of-care testing (POC) (TRLs 5–9) and have been validated using whole blood, serum, and field-compatible readout formats [179,186,188,190,191,212–214]. (Lines 892-903)

4. Several closely related topics are treated in separate sections with limited cross-reference, which disrupts the logical flow. For example, electroosmotic flow modulation and electrokinetic instabilities are physically linked but discussed largely in isolation. Stronger transitional text and explicit cross-linking are required.

Reply: Thank you to the reviewer. We agree that several closely related electrodynamic topics need closer connection. In the revised manuscript, we have added clear transitional text to clarify the physical continuity between electroosmotic flow (Section 4) and electrodynamic instabilities and mixing (Section 5). A transitional sentence, “Building on EOF modulation strategies discussed in Section 4…” has been added at the beginning of Section 5. Closer cross-linking among related electrokinetic topics has been incorporated by clarifying the physical continuity between electroosmotic flow and electrokinetic instabilities. Surface-charge–mediated modulation of EOF is now explicitly discussed as an initiating condition that, under stronger electric fields or conductivity gradients, transitions into instability-driven vortical flow and chaotic mixing. The discussion traces a continuous transition from linear electrokinetic transport to nonlinear instability and field-driven enrichment, clarifying the conceptual linkage between Sections 4 and 5 and tightening the overall physical narrative.

5. The Conclusions section should move beyond summarizing the reviewed content and provide a more forward-looking and critical outlook. Key open questions, fundamental limitations, and promising future directions should be explicitly identified. Without this, the manuscript does not fully meet the expectations of a comprehensive review.

Reply: Thanks to the reviewer. In the Conclusions, this viewpoint is taken up again by framing linear electrokinetic transport, nonlinear electrohydrodynamic responses, and field-driven enrichment as related behaviors within a single response continuum. Taken together, the revisions sharpen the conceptual thread of the manuscript and make explicit how different electrokinetic strategies can be coherently interpreted under a shared physical basis. (Lines 883-891, 911-913)

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The paper has been revised satisfactorily. It can be considered for publication now. A minor optional comment: When discussing moving dielectrophoresis and single particle transport, the authors may choose to refer to the following papers:
https://doi.org/10.1038/s41378-024-00750-0

https://doi.org/10.1021/acs.langmuir.2c02235

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have revised the manuscript to make it publishable.