Micro/Nano-Chip Electrokinetics

Micro/nanofluidic chips have found increasing applications in the analysis of chemical and biological samples over the past two decades. Electrokinetics has become the method of choice in these micro/nano-chips for transporting, manipulating and sensing ions, (bio)molecules, fluids and (bio)particles, etc., due to the high maneuverability, scalability, sensitivity, and integrability. The involved phenomena, which cover electroosmosis, electrophoresis, dielectrophoresis, electrohydrodynamics, electrothermal flow, diffusioosmosis, diffusiophoresis, streaming potential, current, etc., arise from either the inherent or the induced surface charge on the solid-liquid interface under DC and/or AC electric fields. To review the state-of-the-art of micro/nanochip electrokinetics, we welcome, in this Special Issue of Micromachines, all original research or review articles on the fundamentals and applications of the variety of electrokinetic phenomena in both microfluidic and nanofluidic devices

Editorial for the Special Issue on Micro/Nano-Chip Electrokinetics, Volume II

There has been a rapidly increasing interest in the use of micro/nanofluidics to develop various point-of-care technologies for global health [1,2]. Electrokinetics is often the method of choice in these micro/nano-chips for an accurate transport and manipulation of fluids and samples [3,4]. This special issue in Micromachines is the continuation of our successful first volume on Micro/Nano-Chip Electrokinetics [5]. It consists of 22 contributions, which cover multiple aspects of electrokinetics related phenomena for various chemical and biological applications. We divide these papers into three primary categories and summarize them briefly below

Particle focusing and separation in xanthan gum solutions

There has been increasing interest in the use of magnetic fluids to manipulate diamagnetic particles in microfluidic devices. Focusing particles (both biological and synthetic) into a single tight stream is usually a necessary upstream operation in numerous microfluidic applications. Current methods for diamagnetic-particle focusing in magnetic fluids require either a pair of repulsive magnets or a diamagnetic sheath flow, which can work efficiently for very small particles by simply increasing the flow-rate ratio between the sheath fluid and particle suspension. This approach, however, becomes difficult to implement if particles need to be focused in both the horizontal and vertical directions for so-called three dimensional focusing. Therefore, a variety of sheathless particle-focusing approaches have been developed in microfluidic devices, which are classified as either passive or active depending on the source of the involving force. We demonstrate herein a passive, tunable, sheathless focusing of diamagnetic particles in a microchannel ferrofluid flow with a single set of overhead permanent magnets. Particles are focused into a single stream near the bottom wall of a straight rectangular microchannel, where a magnetic-field minimum is formed because of the magnetization of the ferrofluid. This focusing can be readily switched off and on by removing and replacing the permanent magnets. More importantly, the particle-focusing position can be tuned by shifting the magnets with respect to the microchannel. We perform a systematic experimental study of the parametric effects of the fluid-particle-channel system on diamagnetic particle focusing in terms of a defined particle-focusing effectiveness

Editorial for the Special Issue on Micro/Nano-Chip Electrokinetics

Micro/nanofluidics-based lab-on-a-chip devices have found extensive applications in the analysis of chemical and biological samples over the past two decades. Electrokinetics is the method of choice in these micro/nano-chips for transporting, manipulating and sensing various analyte species (e.g., ions, molecules, fluids and particles, etc.) [1,2]. This Special Issue in Micromachines is aimed to provide the recent development in the field of Micro/Nano-Chip Electrokinetics. It consists of 15 papers, which cover both fundamentals and applications, original research and review

Particles Focusing in Ferrofluids with Magnet

There has been increasing interest in the use of magnetic fluids to manipulate diamagnetic particles in microfluidic devices. Focusing particles (both biological and synthetic) into a single tight stream is usually a necessary upstream operation in numerous microfluidic applications. Current methods for diamagnetic-particle focusing in magnetic fluids require either a pair of repulsive magnets or a diamagnetic sheath flow, which can work efficiently for very small particles by simply increasing the flow-rate ratio between the sheath fluid and particle suspension. This approach, however, becomes difficult to implement if particles need to be focused in both the horizontal and vertical directions for so-called three dimensional focusing. Therefore, a variety of sheathless particle-focusing approaches have been developed in microfluidic devices, which are classified as either passive or active depending on the source of the involving force. We demonstrate herein a passive, tunable, sheathless focusing of diamagnetic particles in a microchannel ferrofluid flow with a single set of overhead permanent magnets. Particles are focused into a single stream near the bottom wall of a straight rectangular microchannel, where a magnetic-field minimum is formed because of the magnetization of the ferrofluid. This focusing can be readily switched off and on by removing and replacing the permanent magnets. More importantly, the particle-focusing position can be tuned by shifting the magnets with respect to the microchannel. We perform a systematic experimental study of the parametric effects of the fluid-particle-channel system on diamagnetic particle focusing in terms of a defined particle-focusing effectiveness

Analytical Guidelines for Designing Curvature-Induced Dielectrophoretic Particle Manipulation Systems.

Curvature-induced dielectrophoresis (C-iDEP) is an established method of applying electrical energy gradients across curved microchannels to obtain a label-free manipulation of particles and cells. This method offers several advantages over the other DEP-based methods, such as increased chip area utilisation, simple fabrication, reduced susceptibility to Joule heating and reduced risk of electrolysis in the active region. Although C-iDEP systems have been extensively demonstrated to achieve focusing and separation of particles, a detailed mathematical analysis of the particle dynamics has not been reported yet. This work computationally confirms a fully analytical dimensionless study of the electric field-induced particle motion inside a circular arc microchannel, the simplest design of a C-iDEP system. Specifically, the analysis reveals that the design of a circular arc microchannel geometry for manipulating particles using an applied voltage is fully determined by three dimensionless parameters. Simple equations are established and numerically confirmed to predict the mutual relationships of the parameters for a comprehensive range of their practically relevant values, while ensuring design for safety. This work aims to serve as a starting point for microfluidics engineers and researchers to have a simple calculator-based guideline to develop C-iDEP particle manipulation systems specific to their applications

Viscoelastic effects on electrokinetic particle focusing in a constricted microchannel

Focusing suspended particles in a fluid into a single file is often necessary prior to continuous-flow detection, analysis, and separation. Electrokinetic particle focusing has been demonstrated in constricted microchannels by the use of the constriction-induced dielectrophoresis. However, previous studies on this subject have been limited to Newtonian fluids only. We report in this paper an experimental investigation of the viscoelastic effects on electrokinetic particle focusing in non-Newtonian polyethylene oxide solutions through a constricted microchannel. The width of the focused particle stream is found NOT to decrease with the increase in DC electric field, which is different from that in Newtonian fluids. Moreover, particle aggregations are observed at relatively high electric fields to first form inside the constriction. They can then either move forward and exit the constriction in an explosive mode or roll back to the constriction entrance for further accumulations. These unexpected phenomena are distinct from the findings in our earlier paper [Lu et al., Biomicrofluidics 8, 021802 (2014)], where particles are observed to oscillate inside the constriction and not to pass through until a chain of sufficient length is formed. They are speculated to be a consequence of the fluid viscoelasticity effects. (c) 2015 AIP Publishing LLC

IMECE2008-66729 THERMODYNAMIC ANALYSIS OF ELECTROKINETIC TRANSPORT IN MICRO/NANOFLUIDICS

ABSTRACT Presented herein is an Onsager reciprocal relations-based thermodynamic analysis of the electrokinetic transport of fluids and ions in micro/nanofluidics. This analytical approach provides a straightforward understanding of electrokinetic energy conversion, streaming potential and streaming current measurements, and electrokinetic flow control in micro/nanoscale channels or networks