On-Chip Droplet Splitting with High Volume Ratios Using a 3D Conical Microstructure-Based Microfluidic Device.

This work reports a simple microfluidic method for splitting a mother droplet into two daughter droplets with high and precise volume ratios. To achieve this, a droplet-splitting microfluidic device embedded with a three-dimensional (3D) conical microstructure is fabricated, in which the high splitting ratios of monodisperse mother droplets are achieved. The volume ratio of the split daughter droplets can reach up to 265. In addition, we examined factors that affect the splitting ratio of the daughter droplets and found that the ratio is affected by the flow rates of the two individual outlet channels, the injection length of the conical microstructure, and the diameter of the original mother droplets. Numerical simulations of these parameters were conducted to gain a clearer understanding of the splitting behavior. The proposed droplet splitting device with a conical microstructure enables on-chip sample extraction and droplet volume control, which can be a powerful tool for various droplet-based applications in microfluidic devices such as viral infectivity assays and sequencing heterogeneous populations.

Flexible Microswimmer Manipulation in Multiple Microfluidic Systems Utilizing Thermal Buoyancy-Capillary Convection.

Flexible and accurate control of microswimmers is significant for lots of applications. Herein, we present a method for effective microswimmer manipulation in multiple microfluidic systems by thermal buoyancy-capillary convection. In the microdevice, four strips of microheaters arranged at the bottom of the microchannel are used to unevenly heat microfluids, and the convection flow forms under the influence of gravity and interfacial tension gradient. By adjusting the DC signals applied on these four heating elements, the intensity and direction of convection flow can be flexibly adjusted. Accordingly, granular samples dispersed in liquid buffer can be controllably driven to the target position by the Stokes drag. The swimming behavior of polystyrene (PS) microspheres at the solid-liquid interface of the device is first investigated. It shows that the PS microswimmers can migrate along various geometrical patterns by powering the microheaters with designed voltage combinations, and the migration velocity is positively affected by the increased voltage. Then, the butyl acrylate (BA) microswimmers are manipulated at the gas-liquid interface of the microchip. It turns out that the BA microswimmers migrate oppositely compared with PS swimmers under the same energization strategy. Additionally, the translation direction of BA swimmers can be changed over a 360° range by different voltage combinations. The multifunctionality of our approach is further demonstrated by conveniently driving the trimethylolpropane triacrylate microswimmers at the liquid-liquid interface of the microplatform along different directions and pathlines. Therefore, this technique can be promising for many cases needing granular sample control, such as cargo delivery and sensing.

Characterization of Particle Movement and High-Resolution Separation of Microalgal Cells via Induced-Charge Electroosmotic Advective Spiral Flow.

Microalgae are renewable, sustainable, and economical sources of biofuels and are capable of addressing pressing global demand for energy security. However, two challenging issues to produce high-level biofuels are to separate promising algal strains and protect biofuels from contamination of undesired bacteria, which rely on an economical and high-resolution separation technology. Separation technology based on induced-charge electroosmotic (ICEO) vortices offers excellent promise in economical microalga separation for producing biofuels because of its reconfigurable and flexible profiles and sensitive and precise selectivity. In this work, a practical ICEO vortex device is developed to facilitate high-resolution isolation of rich-lipid microalgae for the first time. We investigate electrokinetic equilibrium states of particles and particle-fluid ICEO effect in binary-particle manipulation. Nanoparticle separation is performed to demonstrate the feasibility and resolution of this device, yielding clear separation. Afterward, we leverage this technology in isolation of Chlorella vulgaris from heterogeneous microalgae with the purity exceeding 96.4%. Besides, this platform is successfully engineered for the extraction of single-cell Oocystis sp., obtaining the purity surpassing 95.2%. Moreover, with modulating parameters, we isolate desired-cell-number Oocystis sp. enabling us to investigate proliferation mode and carry out transcriptome analyses of Oocystis sp. for high-quality neutral lipids. This platform can be extended directly to economically separate other biological micro/nanosamples to address pressing issues, involving energy security, environmental monitoring, and disease diagnosis.

Aberrant functional brain network dynamics in patients with functional constipation.

The aberrant static functional connectivity of brain network has been widely investigated in patients with functional constipation (FCon). However, the dynamics of brain functional connectivity in FCon patients remained unknown. This study aimed to detect the brain dynamics of functional connectivity states and network topological organizations of FCon patients and investigate the correlations of the aberrant brain dynamics with symptom severity. Eighty-three FCon patients and 80 healthy subjects (HS) were included in data analysis. The spatial group independent component analysis, sliding-window approach, k-means clustering, and graph-theoretic analysis were applied to investigate the dynamic temporal properties and coupling patterns of functional connectivity states, as well as the time-variation of network topological organizations in FCon patients. Four reoccurring functional connectivity states were identified in k-means clustering analysis. Compared to HS, FCon patients manifested the lower occurrence rate and mean dwell time in the state with a complex connection between default mode network and cognitive control network, as well as the aberrant anterior insula-cortical coupling patterns in this state, which were significantly correlated with the symptom severity. The graph-theoretic analysis demonstrated that FCon patients had higher sample entropy at the nodal efficiency of anterior insula than HS. The current findings provided dynamic perspectives for understanding the brain connectome of FCon and laid the foundation for the potential treatment of FCon based on brain connectomics.

Microbes vs. chemistry in the origin of the anaerobic gut lumen.

The succession from aerobic and facultative anaerobic bacteria to obligate anaerobes in the infant gut along with the differences between the compositions of the mucosally adherent vs. luminal microbiota suggests that the gut microbes consume oxygen, which diffuses into the lumen from the intestinal tissue, maintaining the lumen in a deeply anaerobic state. Remarkably, measurements of luminal oxygen levels show nearly identical pO2 (partial pressure of oxygen) profiles in conventional and germ-free mice, pointing to the existence of oxygen consumption mechanisms other than microbial respiration. In vitro experiments confirmed that the luminal contents of germ-free mice are able to chemically consume oxygen (e.g., via lipid oxidation reactions), although at rates significantly lower than those observed in the case of conventionally housed mice. For conventional mice, we also show that the taxonomic composition of the gut microbiota adherent to the gut mucosa and in the lumen throughout the length of the gut correlates with oxygen levels. At the same time, an increase in the biomass of the gut microbiota provides an explanation for the reduction of luminal oxygen in the distal vs. proximal gut. These results demonstrate how oxygen from the mammalian host is used by the gut microbiota, while both the microbes and the oxidative chemical reactions regulate luminal oxygen levels, shaping the composition of the microbial community throughout different regions of the gut.

Flexible Particle Focusing and Switching in Continuous Flow via Controllable Thermal Buoyancy Convection.

We present a novel approach that utilizes thermal buoyancy convection to achieve flexible particle focusing and switching in continuous flow of a microfluidic system. In this platform, three strip microheaters, A, B, and C, are symmetrically distributed at the bottom of microchannel, and they are isolated from the particle suspension by a thin glass slide. Continual transverse convection flow forms when the microheaters are energized by dc signals. The flow patterns are readily tuned by changing the energization strategies of the microheater array, leading to the modulation of the position of flow stagnation region. Accordingly, microparticles dispersed in fluids are rapidly focused to the flow stagnation region by the Stokes drag and thus form a continuous particle beam. The particle beam can also be switched to different lateral positions by adjusting the control voltages. This particle manipulation method is first demonstrated by respectively energizing these three microheaters and subsequently switching silica particles into different outlets. The lateral position of the particle beam then is flexibly controlled by simultaneously energizing microheaters A and B (or B and C) and adjusting the voltage applied on microheater A (or C). Furthermore, the versatility of this approach is proved by focusing and switching of microsized droplets, that is, oil-in-water and water-in-oil-in-water emulsion droplets. Finally, we use poly(ethylene glycol) diacrylate microgels, excellent reactant carriers, as an experimental sample and flexibly manipulate them in this microdevice, demonstrating this strategy's applicability for the cargo delivery. Therefore, this technique can be attractive for many particle preprocessing applications.

Continuous Particle Trapping, Switching, and Sorting Utilizing a Combination of Dielectrophoresis and Alternating Current Electrothermal Flow.

We propose a simplified multifunctional traffic control approach that effectively combines dielectrophoresis (DEP) and alternating current electrothermal (ACET) flow to realize continuous particle trapping, switching, and sorting. In the designed microsystem, the combined DEP and ACET effects, which are symmetrically generated above a bipolar electrode surface, contribute to focus the incoming colloidal particles into a thin beam. Once the bipolar electrode is energized with an electric gate signal completely in phase with the driving alternating current (AC) signal, the spatial symmetry of the electric field can be artificially reordered by adjusting the gate voltage through field-effect traffic control. This results in a reshapable field stagnant region for precise switching of particles into the region of interest. Moreover, the integrated particle switching prior to the scaled particle trapping experiment is successfully conducted to demonstrate the feasibility of the combined strategy. Furthermore, a mixture of two types of particle sorting (i.e., density, size) with quick response performance is achieved by increasing the driving voltage with a maximum gate voltage offset, thus, extending the versatility of the designed device. Finally, droplet switching and filtration of the satellite droplets from the parent droplets is performed to successfully permit control of the droplet traffic. The proposed traffic control approach provides a promising technique for flexible manipulation of particulate samples and can be conveniently integrated with other micro/nanofluidic components into a complete functional on-chip platform owing to its simple geometric structure, easy operation, and multifunctionality.

Compound-Droplet-Pairs-Filled Hydrogel Microfiber for Electric-Field-Induced Selective Release.

The separate co-encapsulation and selective controlled release of multiple encapsulants in a predetermined sequence has potentially important applications for drug delivery and tissue engineering. However, the selective controlled release of distinct contents upon one triggering event for most existing microcarriers still remains challenging. Here, novel microfluidic fabrication of compound-droplet-pairs-filled hydrogel microfibers (C-Fibers) is presented for two-step selective controlled release under AC electric field. The parallel arranged compound droplets enable the separate co-encapsulation of distinct contents in a single microfiber, and the release sequence is guaranteed by the discrepancy of the shell thickness or core conductivity of the encapsulated droplets. This is demonstrated by using a high-frequency electric field to trigger the first burst release of droplets with higher conductivity or thinner shell, followed by the second release of the other droplets under low-frequency electric field. The reported C-Fibers provide novel multidelivery system for a wide range of applications that require controlled release of multiple ingredients in a prescribed sequence.

Microparticle separation using asymmetrical induced-charge electro-osmotic vortices on an arc-edge-based floating electrode.

Five arc-shaped gaps were designed on the bipolar electrode to actuate alternately opposite-direction asymmetrical induced-charge electro-osmosis (AICEO) vortices, and we developed a microfluidic device using such asymmetrical vortices to realize particle separation. When the buoyancy force dominates in the vertical direction, particles stay at the channel bottom, experiencing a left deflection under the vortices in the convex arc areas. In contrast, when the levitation force induced by AICEO vortices overcomes the buoyancy force, particles are elevated to a high level and captured by right vortices, undergoing a right deflection under the vortices in the concave arc areas. Moreover, when particles pass through the concave or convex arc areas every time, their right or left deflections are enlarged gradually and the separation becomes more complete. Remarkably, as the light/small particles at low voltage, heavy/large particles can be elevated to a new high level and undergo right deflection by increasing the voltage. We first explicitly proved the separation principle and analyzed numerically its capability in density- and size-based separation. Depending on the study of the voltage-dependent AICEO characterization of 4 µm silica and 4 µm PMMA particles, we experimentally verified the feasibility of our device in density-based separation. According to the investigation of sensitivity to particle size, we separated multi-sized yeast cells to confirm the capability of our device in size-based separation. Finally, we extracted yeast cells from impeding particles, obtaining 96% purity. Additionally, we designed a 500 µm distance between the focusing and separation region to circumvent the problems caused by electric-field interaction. Our AICEO-based separation method holds potential to serve as a useful tool in transesterification of microalgal lipids to biodiesel and solar cell processing because of its outstanding advantages, such as gentle conditions, contact-free separation, high-sensitivity and high-efficiency separation capability.

High-Throughput Separation, Trapping, and Manipulation of Single Cells and Particles by Combined Dielectrophoresis at a Bipolar Electrode Array.

Microfluidic systems have been developed widely in scaled-down processes of laboratory techniques, but they are usually limited in achieving stand-alone functionalities. It is highly desirable to exploit an integrated microfluidic device with multiple capabilities such as cell separation, single-cell trapping, and cell manipulation. Herein, we reported a microfluidic platform integrated with actuation electrodes, for separating cells and microbeads, and bipolar electrodes, for trapping, rotating, and propelling single cells and microbeads. The separation of cells and microbeads can be first achieved by deflective dielectrophoresis (DEP) barriers. Trapping experiments with yeast cells and polystyrene (PS) microbeads suspended in aqueous solutions with different conductivities were then conducted, showing that both cells and particles can be trapped at the center of wireless electrodes by negative DEP force. Upon application of a rotating electric field, yeast cells exhibit translational movement along the electrode edges, and self-rotation is seen at an array of bipolar electrodes when electrorotational torque and traveling wave DEP force are applied on the cells. The current approach allows us to switch the propulsion and rotation direction of cells by varying the frequency of the applied electric field. Beyond the achievements of single-cell manipulation, this system permits effective control of several particles or cells simultaneously. The integration of parallel sorting and single trapping stages within a microfluidic chip enables the prospect of high-throughput cell separation, single trapping, and large-scale cell locomotion and rotation in a noninvasive and disposable format, showing great potential in single-cell analysis, targeted drug delivery, and surgery.

Flexible Continuous Particle Beam Switching via External-Field-Reconfigurable Asymmetric Induced-Charge Electroosmosis.

Continuous sample switching is an essential process for developing an integrated platform incorporating multiple functionality with applications typically ranging from chemical to biological assays. Herein we propose a unique method of external-field-reconfigurable symmetry breaking in induced-charge electroosmosis above a simple planar bipolar electrode for continuous particle beam switching. In the proposed system, the spatial symmetry of a nonlinear electroosmotic vortex flow can be artificially reordered to achieve an asymmetric electrically floating-electrode polarization by regulating the configurations of the external ac signals, thus contributing to flexible particle beam switching. This switching system comprises an upstream flow-focusing region where particles are prefocused into a beam on the bipolar electrode by transversal electroconvective mass transfer, and a deflecting region in which the resulting particle beam is deflected to generate a steerable lateral displacement to enter the desired region via the action of an asymmetric polarization-induced reshapable electroosmotic flow stagnation line in a controllable background field gradient. A lateral particle displacement on the order of hundreds of micrometers can be achieved in a deterministic manner by varying the voltage, frequency, and inlet flow rate, thereby enabling multichannel particle switching. Furthermore, the versatility of the switching mechanism is extended by successfully accomplishing fluorescent nanoparticle beam switching, yeast cell switching, five-outlet particle switching, and simultaneous switching of two particle types. The proposed switching approach provides a promising technique for flexible electrokinetic sample preconcentration prior to any subsequent analysis and can be conveniently integrated with other micro/nanofluidic components into a complete functional on-chip platform owing to its simple electrode structure.

Flexible particle flow-focusing in microchannel driven by droplet-directed induced-charge electroosmosis.

We report herein a novel microfluidic particle concentrator that utilizes constriction microchannels to enhance the flow-focusing performance of induced-charge electroosmosis (ICEO), where viscous hemi-spherical oil droplets are embedded within the mainchannel to form deformable converging-diverging constriction structures. The constriction region between symmetric oil droplets partially coated on the electrode strips can improve the focusing performance by inducing a granular wake flow area at the diverging channel, which makes almost all of the scattered sample particles trapped within a narrow stream on the floating electrode. Another asymmetric droplet pair arranged near the outlets can further direct the trajectory of focused particle stream to one specified outlet port depending on the symmetry breaking in the shape of opposing phase interfaces. By fully exploiting rectification properties of induced-charge electrokinetic phenomena at immiscible water/oil interfaces of tunable geometry, the expected function of continuous and switchable flow-focusing is demonstrated by preconcentrating both inorganic silica particles and biological yeast cells. Physical mechanisms responsible for particle focusing and locus deflection in the droplet-assisted concentrentor are analyzed in detail, and simulation results are in good accordance with experimental observations. Our work provides new routes to construct flexible electrokinetic framework for preprocessing on-chip biological samples before performing subsequent analysis.

Sequential Coalescence Enabled Two-Step Microreactions in Triple-Core Double-Emulsion Droplets Triggered by an Electric Field.

Advances in microfluidic emulsification have enabled the generation of exquisite multiple-core droplets, which are promising structures to accommodate microreactions. An essential requirement for conducting reactions is the sequential coalescence of the multiple cores encapsulated within these droplets, therefore, mixing the reagents together in a controlled sequence. Here, a microfluidic approach is reported for the conduction of two-step microreactions by electrically fusing three cores inside double-emulsion droplets. Using a microcapillary glass device, monodisperse water-in-oil-in-water droplets are fabricated with three compartmented reagents encapsulated inside. An AC electric field is then applied through a polydimethylsiloxane chip to trigger the sequential mixing of the reagents, where the precise sequence is guaranteed by the discrepancy of the volume or conductivity of the inner cores. A two-step reaction in each droplet is ensured by two times of core coalescence, which totally takes 20-40 s depending on varying conditions. The optimal parameters of the AC signal for the sequential fusion of the inner droplets are identified. Moreover, the capability of this technique is demonstrated by conducting an enzyme-catalyzed reaction used for glucose detection with the double-emulsion droplets. This technique should benefit a wide range of applications that require multistep reactions in micrometer scale.

Large-Scale Single Particle and Cell Trapping based on Rotating Electric Field Induced-Charge Electroosmosis.

We propose a simple, inexpensive microfluidic chip for large-scale trapping of single particles and cells based on induced-charge electroosmosis in a rotating electric field (ROT-ICEO). A central floating electrode array, was placed in the center of the gap between four driving electrodes with a quadrature configuration and used to immobilize single particles or cells. Cells were trapped on the electrode array by the interaction between ROT-ICEO flow and buoyancy flow. We experimentally optimized the efficiency of trapping single particles by investigating important parameters like particle or cell density and electric potential. Experimental and numerical results showed good agreement. The operation of the chip was verified by trapping single polystyrene (PS) microspheres with diameters of 5 and 20 µm and single yeast cells. The highest single particle occupancy of 73% was obtained using a floating electrode array with a diameter of 20 µm with an amplitude voltage of 5 V and frequency of 10 kHz for PS microbeads with a 5-µm diameter and density of 800 particles/µL. The ROT-ICEO flow could hold cells against fluid flows with a rate of less than 0.45 µL/min. This novel, simple, robust method to trap single cells has enormous potential in genetic and metabolic engineering.

NIR light-triggered bursting of double-emulsion drops (DEDs) for microdroplet generation.

Microdroplets have significant applications in microbiology, pharmaceuticals, cosmetics, and synthetic materials. Herein, we present for the first time, a near-infrared (NIR)-light-triggered double-emulsion drop (DED) bursting method for generating a large number of micro-droplets with a size of several microns. Under the irradiation of NIR light, the inner water phase of the DED containing a trace amount of Prussian blue (PB) rapidly heats up and evaporates rapidly to generate microbubbles due to the photothermal property of PB. By controlling the light intensity, the DED could be inflated by the constant coalescence of microbubbles, which then burst immediately and tear the middle oil phase to form a large number of microdroplets. The performance of the microdroplets generated by NIR-light-triggered DED bursting was investigated by varying the oil shell thickness (HO), oil phase viscosity (ηO) and oil type. HO and ηO were the key factors affecting the generation of microdroplets. DEDs with lower HO and ηO generated lower polydispersity and a large number of microdroplets via NIR-triggered DED bursting. The proportion of microdroplets of sizes below 10 µm reached up to 95%. Furthermore, camellia oil, as the middle oil phase of the DEDs, generated lower polydispersity and a large number of microdroplets measuring several microns. The as-developed bursting method has great potential to generate micro-droplets for micro-/nano- and biotechnology applications.

One-Step Microfluidic Fabrication of Bioinspired Microfibers with a Spindle-Knot Structure for Fog Harvest.

Many biomimetic microfibers have been designed from spider silk to collect water efficiently from humid air as a result of its periodic spindle-knot structure, which enhances the direct movement and convergence of captured fog droplets. Here, a hydrodynamic flow-focusing microfluidic device with a theta-shaped tube is designed for the one-step fabrication of bioinspired microfibers with a spindle-knot structure for fog harvest. The morphology of the structured microfibers, including height, width, and spacing of spindle knots, can be adjusted readily by regulating the flow rate of specific phases. The production rate of these structured microfibers can reach 1100 cm/min. Moreover, the capture, transportation, and collection performance of fog droplets on various microfibers are investigated in a fog collection platform. It is demonstrated that our one-step microfluidic device presents a ready method for the fabrication of structured microfibers with spindle knots, which provide a significant facilitation on efficient fog capture and water collection.

High-Speed Generation of Microbubbles with Constant Cumulative Production in a Glass Capillary Microfluidic Bubble Generator.

This work reports a simple bubble generator for the high-speed generation of microbubbles with constant cumulative production. To achieve this, a gas-liquid co-flowing microfluidic device with a tiny capillary orifice as small as 5 µm is fabricated to produce monodisperse microbubbles. The diameter of the microbubbles can be adjusted precisely by tuning the input gas pressure and flow rate of the continuous liquid phase. The co-flowing structure ensures the uniformity of the generated microbubbles, and the surfactant in the liquid phase prevents coalescence of the collected microbubbles. The diameter coefficient of variation (CV) of the generated microbubbles can reach a minimum of 1.3%. Additionally, the relationship between microbubble diameter and the gas channel orifice is studied using the low Capillary number (Ca) and Weber number (We) of the liquid phase. Moreover, by maintaining a consistent gas input pressure, the CV of the cumulative microbubble volume can reach 3.6% regardless of the flow rate of the liquid phase. This method not only facilitates the generation of microbubbles with morphologic stability under variable flow conditions, but also ensures that the cumulative microbubble production over a certain period of time remains constant, which is important for the volume-dominated application of chromatographic analysis and the component analysis of natural gas.

Continuous microfluidic fabrication of anisotropic microparticles for enhanced wastewater purification.

Anisotropic microparticles containing functional nanomaterials have attracted growing interest due to their enhanced performance in diverse applications ranging from catalysts to environmental remediation. However, the preparation of anisotropic microparticles with highly controlled morphologies and dimensions usually suffers from a limited material choice. Here, we develop a facile strategy to continuously prepare anisotropic microparticles with their shapes changing from spherical to pear-like, maraca-like and rod-like for enhanced water decontamination. Anisotropic microparticles are produced by deforming oil-droplet templates within microfibers and then locking their shapes via thermo/photo-polymerization. The sizes and geometries of the oil-droplet templates are precisely controlled by varying the fluid flow conditions. In addition, porous spherical and rod-like microparticles are functionalized for photocatalytic degradation of organic contaminants by incorporating functional TiO2 and Fe3O4 nanoparticles. Compared to spherical microparticles with equal volume, functionalized rod-like microparticles exhibit better performance in removal of contaminants due to their larger specific surface area, which facilitates the contact between the loaded catalysts and organic pollutants. Moreover, the magnetic rod-like microparticles can be easily recovered and reused without deterioration of catalytic performance. The proposed strategy in this study is useful for producing anisotropic microparticles with well-tailored shapes via different polymerization methods and extending their potential applications.

Fabrication of syntactic foam fillers via manipulation of on-chip quasi concentric nanoparticle-shelled droplet templates.

Syntactic foams with fly ash cenospheres or commercial microballoons as fillers have been widely used in various applications ranging from aerospace to marine fields and the automotive industry. However, these two extensively adopted fillers possess multiple shortcomings, such as variations in the composition, material degeneration and distinct structural heterogeneity, which will inevitably hamper accurate prediction of the structure-property relationship and the corresponding design of the syntactic foams, reducing material utilization. Here, we present a microfluidic-based approach integrated with a subsequent heat treatment process to engineer syntactic foam fillers with a predefined composition, specified dimensional scope and reduced structural heterogeneity. These fillers are fully guaranteed by the synergy of the flexible and controllable generation of droplet templates with hydrodynamic regulation and rational selection of the nanoparticle dynamic response with respect to the heating temperature. In addition, two distinct surface morphologies have been observed with a temperature demarcation point of 1473 K, further endowing the fillers with multiplicity and optionality, simultaneously laying the foundation to regulate the properties of the syntactic foams through the diversity of the filler selection. Then, we fabricated a syntactic foam specimen by mold casting, and the integrity of the fillers inside was verified using an elaborate buoyancy comparison experiment, exhibiting its potential value in lightweight related applications. As the fillers derived from our approach show significant advantages over conventional ones, they will provide considerable benefits for the regulation and improvement of syntactic foam fillers in many practical applications.

Induced charge electro-osmotic particle separation.

Vortex-based separation is a promising method in particle-particle separation and has only been demonstrated theoretically some years ago. To date, a continuous-flow separation device based on vortices has not been conceived because many known vortices were either unstable or controlling them lacked precision. Electro-convection from induced charge electro-osmosis (ICEO) has advantages, such as adjustable flow profiles, long-range actuation, and long-lived vortices, and offers an alternative means of particle separation. We found though a different ICEO focusing behaviour of particles whereby particles were trapped and concentrated in two vortex cores. Encouraged by these features of ICEO vortices, we proposed a direct method for particle separation in continuous flow. In various experiments, we first characterized the ICEO-induced focusing performances of various kinds of particle samples in a straight channel embedded with an individual central bipolar electrode, presenting a justifiable explanation. Second, the combined dependences of ICEO particle separation on the sample size and mass density were investigated. Third, an application to cell purification was performed in which we obtained a purity surpassing 98%. Finally, we investigated the ICEO characteristics of nanoparticles, exploiting our method in isolating nanoscale objects by separating 500 nm and 5 µm polystyrene beads, gaining clear separation. Certain features of this method, such as having ease of operation, simple structure, and continuous flow, and being prefocusing free and physical property-based, indicate its good potential in tackling environmental monitoring, cell sorting, chemical analysis, isolation of uniform-sized graphene and transesterification of micro-algal lipids to biodiesel.