Dielectrophoretic choking phenomenon of a deformable particle in a converging‐diverging microchannel

The translational motion of small particles in an electrokinetic fluid flow through a constriction can be enhanced by an increase of the applied electric potential. Beyond a critical potential, however, the negative dielectrophoresis (DEP) can overpower other forces to prevent particles that are even smaller than the constriction from passing through the constriction. This DEP choking phenomenon was studied previously for rigid particles. Here, the DEP choking phenomenon is revisited for deformable particles, which are ubiquitous in many biomedical applications. Particle deformability is measured by the particle shear modulus, and the choking conditions are reported through a parametric study that includes the channel geometry, external electric potential, and particle zeta potential. The study was carried out using a numerical model based on an arbitrary Lagrangian‐Eulerican (ALE) finite‐element method.     

Dipolophoresis of Janus nanoparticles in a microchannel

A nonlinear dipolophoretic analysis is applied to analytically explain the counterintuitive experimental results of Gangwal et al. [16, 17] that an uncharged micro/nanosize dielectric Janus particle is attracted to the wall of a microchannel when exposed to an AC‐uniform electric field in the direction parallel to the no‐slip boundaries. We employ the so‐called “weak” field assumption and consider a metallodielectric Janus colloid comprising two semispheres of distinct dielectric properties subject to an oscillating‐uniform electric field with moderate frequency (below the Maxwell–Wagner limit). The Debye scale (ratio of electric double layer thickness to particle size) is considered unrestricted. Under the low Reynolds number hypothesis, Faxén's theorem and the Green's function (Stokeslet) method of singularities, including appropriate images with respect to the no‐slip boundary, are applied under the remote‐field approximation to determine the dynamics and trajectory of a small colloid moving near a wall. When assuming maximum dielectric contrasts between hemispheres and relatively low Debye scale (compared to particle radius), a rather simple relation for the equilibrium position of the colloid (i.e. tilt angle and distance from the wall) is obtained and found to be in qualitative good agreement with the experimental observations of Gangwal et al. [16, 17] and the predictions of Kilic and Bazant [12].     

Dielectrophoretic focusing of particles in a microchannel constriction using DC‐biased AC flectric fields

This paper presents a fundamental study of particle electrokinetic focusing in a single microchannel constriction. Through both experiments and simulations, we demonstrate that such dielectrophoresis‐induced particle focusing can be implemented in a much smaller magnitude of DC‐biased AC electric fields (10 kV/m in total) as compared to pure DC electric fields (up to 100 kV/m). This is attributed to the increase in the ratio of cross‐stream particle dielectrophoretic velocity to streamwise electrokinetic velocity as only the DC field component contributes to the latter. The effects of the 1 kHz frequency AC to DC electric field ratio on particle trajectories and velocity variations through the microchannel constriction are also examined, which are found to agree with the simulation results.     

Studies of Dielectrophoretic Spectra of Levitated Tobacco Leaf Protoplast

In this paper, we have utilized the principle of dielectrophoresis to determine the dielectrophoretic spectra of single levitated tobacco leaf protoplast at various suspending media conductivities (12.5 μS/cm-520 μS/cm) over wide frequencies (10 Hz—15 MHz). From the experimental data, the membrane capacitance of the single protoplast which was 0.40±0.03μF/cm~2 has been obtained and the dipole interaction of two protoplasts in electrofusion has also been analysed. When the external electric field E_c=1.5kV/cm was applied and the frequency was higher than 20 kHz, the dipole interaction pressure ~πd would be much larger than the membrane potential pressure ~πm, and the pressure ~πd could be the main reason for membraue pores. This conclusion will provide a new basis to improve the protocol of cell electrofusion.     

Janus particles with a functional gold surface for control of surface plasmon resonance

We report in this study the presence of Janus particles, which are candidates for use with electronic color papers. We used negatively charged polystyrene particles (370 nm) as the core particles, and gold was then sputtered onto their packed monolayer under several conditions. The sputtered particles were next redispersed into the aqueous medium by gentle sonication. Gold nanoparticles localized on one side of the cores could also serve as seeds for subsequent shell growth by electroless gold plating. Through these treatments, a series of well-dispersed Janus particles were obtained with gold nanostructures of different size and shape only on one side. Their dispersions showed different colors originating from the surface plasmon resonance absorption of gold nanoparticles localized on the hemisphere. The particles obtained by this approach have potential applications such as in sensors and electronic color paper.     

Experimental study of dielectrophoresis and liquid dielectrophoresis mechanisms for particle capture in a droplet

This work presents a microfluidic system that can transport, concentrate, and capture particles in a controllable droplet. Dielectrophoresis (DEP), a phenomenon in which a force is exerted on a dielectric particle when it is subjected to a non-uniform electric field, is used to manipulate particles. Liquid dielectrophoresis (LDEP), a phenomenon in which a liquid moves toward regions of high electric field strength under a non-uniform electric field, is used to manipulate the fluid. In this study, a mechanism of droplet creation presented in a previous work that uses DEP and LDEP is improved. A driving electrode with a DEP gap is used to prevent beads from getting stuck at the interface between air and liquid, which is actuated with an AC signal of 200 V(pp) at a frequency of 100 kHz. DEP theory is used to calculate the DEP force in the liquid, and LDEP theory is used to analyze the influence of the DEP gap. The increment of the actuation voltage due to the electrode with a DEP gap is calculated. A set of microwell electrodes is used to capture a bead using DEP force, which is actuated with an AC signal of 20 V(pp) at a frequency of 5 MHz. A simulation is carried out to investigate the dimensions of the DEP gap and microwell electrodes. Experiments are performed to demonstrate the creation of a 100-nL droplet and the capture of individual 10-μm polystyrene latex beads in the droplet.     

Pickering乳液模板法制备Janus粒子

本文以SiO₂粒子稳定的水包油(O/W)型Pickering乳液作为模板, 在乳液连续相进行SI-ATRP, 将聚合物刷接枝到SiO₂粒子外半表面, 破乳得到半修饰的Janus粒子.     

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.     

Continuous dielectrophoretic separation of particles in a spiral microchannel

Particle separation is a fundamental operation in the areas of biology and physical chemistry. A variety of force fields have been used to separate particles in microfluidic devices, among which electric field may be the most popular one due to its general applicability and adaptability. So far, however, electrophoresis‐based separations have been limited primarily to batchwise processes. Dielectrophoresis (DEP)‐based separations require in‐channel micro‐electrodes or micro‐insulators to produce electric field gradients. This article introduces a novel particle separation technique in DC electrokinetic flow through a planar double‐spiral microchannel. The continuous separation arises from the cross‐stream dielectrophoretic motion of particles induced by the non‐uniform electric field inherent to curved channels. Specifically, particles are focused by DEP to one sidewall of the first spiral, and then dielectrophoretically deflected toward the other sidewall of the second spiral at a particle‐dependent rate, leading to focused particle streams along different flow paths. This DEP‐based particle separation technique is demonstrated in an asymmetric double‐spiral microchannel by continuously separating a mixture of 5/10 μm particles and 3/5 μm particles.     

A new droplet breakup phenomenon in electrokinetic flow through a microchannel constriction

A completely new droplet breakup phenomenon is reported for droplets passing through a constriction in an electrokinetic flow. The breakup occurs during the droplet shape recovery process past the constriction throat by the interplay of the dielectrophoretic stress release and the interface energy for droplets with smaller permittivity than that of the ambient fluid. There are conditions for constriction ratios and droplet size that the droplet breakup occurs. The numerical predictions provided here require experimental verification, and then can give rise to a novel microfluidic device design with novel droplet manipulations.     

Selective Janus Particle Assembly at Tipping Points of Thermally‐Switched Wetting

Thermal wetting can simply, selectively and reversibly join patchy particles into clusters (2D and 3D) and also colloidal crystals over the narrow temperature range of 1–2 °C. This is demonstrated with Janus particles (gold half‐coated silica spheres) immersed in a binary mixture of water/2,6‐lutidine, such that the relative strength of gold–gold bonding through hydrophobic interaction and silica–silica bonding through the wetting‐induced attraction is reversibly switched according to temperature.     

Polyolefin‐Based Janus Separator for Rechargeable Sodium Batteries

Rechargeable sodium batteries are a promising technology for low‐cost energy storage. However, the undesirable drawbacks originating from the use of glass fiber membrane separators have long been overlooked. A versatile grafting–filtering strategy was developed to controllably tune commercial polyolefin separators for sodium batteries. The as‐developed Janus separators contain a single–ion‐conducting polymer‐grafted side and a functional low‐dimensional material coated side. When employed in room‐temperature sodium–sulfur batteries, the poly(1‐[3‐(methacryloyloxy)propylsulfonyl]‐1‐(trifluoromethanesulfonyl)imide sodium)‐grafted side effectively enhances the electrolyte wettability, and inhibits polysulfide diffusion and sodium dendrite growth. Moreover, a titanium‐deficient nitrogen‐containing MXene‐coated side electrocatalytically improved the polysulfide conversion kinetics. The as‐developed batteries demonstrate high capacity and extended cycling life with lean electrolyte loading.     

A Self‐Digitization Dielectrophoretic (SD‐DEP) Chip for High‐Efficiency Single‐Cell Capture,On‐Demand Compartmentalization,and Downstream Nucleic Acid Analysis

The design and fabrication of a self‐digitization dielectrophoretic (SD‐DEP) chip with simple components for single‐cell manipulation and downstream nucleic acid analysis is presented. The device employed the traditional DEP and insulator DEP to create the local electric field that is tailored to approximately the size of single cells, enabling highly efficient single‐cell capture. The multistep procedures of cell manipulation, compartmentalization, lysis, and analysis were performed in the integrated microdevice, consuming minimal reagents, minimizing contamination, decreasing lysate dilution, and increasing assay sensitivity. The platform developed here could be a promising and powerful tool in single‐cell research for precise medicine.     

Tailored Synthesis of Octopus‐type Janus Nanoparticles for Synergistic Actively‐Targeted and Chemo‐Photothermal Therapy

A facile, reproducible, and scalable method was explored to construct uniform Au@poly(acrylic acid) (PAA) Janus nanoparticles (JNPs). The as‐prepared JNPs were used as templates to preferentially grow a mesoporous silica (mSiO₂) shell and Au branches separately modified with methoxy‐poly(ethylene glycol)‐thiol (PEG) to improve their stability, and lactobionic acid (LA) for tumor‐specific targeting. The obtained octopus‐type PEG‐Au‐PAA/mSiO₂‐LA Janus NPs (PEG‐OJNP‐LA) possess pH and NIR dual‐responsive release properties. Moreover, DOX‐loaded PEG‐OJNP‐LA, upon 808 nm NIR light irradiation, exhibit obviously higher toxicity at the cellular and animal levels compared with chemotherapy or photothermal therapy alone, indicating the PEG‐OJNP‐LA could be utilized as a multifunctional nanoplatform for in vitro and in vivo actively‐targeted and chemo‐photothermal cancer therapy.     

Electrogenerated Chemiluminescence Imaging of Electrocatalysis at a Single Au‐Pt Janus Nanoparticle

Noble metal nanoparticles are promising catalysts in electrochemical reactions, while understanding the relationship between the structure and reactivity of the particles is important to achieve higher efficiency of electrocatalysis, and promote the development of single‐molecule electrochemistry. Electrogenerated chemiluminescence (ECL) was employed to image the catalytic oxidation of luminophore at single Au, Pt, and Au‐Pt Janus nanoparticles. Compared to the monometal nanoparticles, the Janus particle structure exhibited enhanced ECL intensity and stability, indicating better catalytic efficiency. On the basis of the experimental results and digital simulation, it was concluded that a concentration difference arose at the asymmetric bimetallic interface according to different heterogeneous electron‐transfer rate constants at Au and Pt. The fluid slip around the Janus particle enhanced local redox reactions and protected the particle surface from passivation.     

Rapid and Efficient Separation of Oil from Oil‐in‐Water Emulsions Using a Janus Cotton Fabric

A novel bi‐functional Janus cotton fabric is used to separate oil from oil‐in‐water emulsions. This fabric is superhydrophobic on one surface and polyamine‐bearing on the other. When used as a filter, the polyamine‐bearing side causes the micrometer‐sized oil droplets to coalesce. The coalesced oil then fills fabric pores on the superhydrophobic side and selectively permeates it. Oil separation using this method is rapid and the separated oil is pure. Furthermore, the content of the model oil hexadecane (HD) in water after a separation can be reduced to less than 0.03±0.03 vol %. These features demonstrate the practical potential of this technology.     

Salt‐assisted liquid–liquid extraction in microchannel

In this study, for the first time, salt‐assisted liquid–liquid extraction was performed in a microchannel system. The proposed design is based on the increase of contact surface area between target analytes and extracting phase during the sample and extracting phase transfer in microchannel. In this method, first sample solution, extracting solvent, and salt were mixed by stirrer and simultaneously delivered into a microchannel using a syringe pump. In order to optimize the influential parameters on the extraction efficiency of the proposed method, zidovudine and tenofovir disoproxil fumarate were selected as model analytes. The main parameters such as extracting solvent and its volume, salt amount, pH of sample solution, and microchannel shape, length, and its inner diameter were investigated and optimized. Under the optimized conditions, the proposed method was linear in the range of 0.1–30 µg/mL and R² coefficients were equal to 0.9922 and 0.9947 for zidovudine and tenofovir disoproxil fumarate, respectively. Extraction efficiency of the proposed method was compared with conventional salt‐assisted liquid–liquid extraction. The results show that the proposed design has higher extraction efficiency than conventional salt‐assisted liquid–liquid extraction. Finally, the proposed method was successfully applied for the determination of zidovudine and tenofovir disoproxil fumarate in plasma samples.     

Aerosol damage to a microchannel plate analysed by X‐ray photoelectron spectromicroscopy

Localised corrosion on a microchannel plate has been studied using X‐ray photoelectron spectromicroscopy. Curve fitting to the spectra at each pixel in the image data set reveals the presence of sodium and carbonate species at the corrosion sites, which optical microscopy suggests originated as droplets. It is proposed that aerosol contamination of the alkali enriched microchannel plate during removal of particulate material using a stream of compressed gas was responsible for the initiation of corrosion. Copyright © 2006 John Wiley & Sons, Ltd.     

Magneto‐Plasmonic Janus Vesicles for Magnetic Field‐Enhanced Photoacoustic and Magnetic Resonance Imaging of Tumors

Magneto‐plasmonic Janus vesicles (JVs) integrated with gold nanoparticles (AuNPs) and magnetic NPs (MNPs) were prepared asymmetrically in the membrane for in vivo cancer imaging. The hybrid JVs were produced by coassembling a mixture of hydrophobic MNPs, free amphiphilic block copolymers (BCPs), and AuNPs tethered with amphiphilic BCPs. Depending on the size and content of NPs, the JVs acquired spherical or hemispherical shapes. Among them, hemispherical JVs containing 50 nm AuNPs and 15 nm MNPs showed a strong absorption in the near‐infrared (NIR) window and enhanced the transverse relaxation (T₂) contrast effect, as a result of the ordering and dense packing of AuNPs and MNPs in the membrane. The magneto‐plasmonic JVs were used as drug delivery vehicles, from which the release of a payload can be triggered by NIR light and the release rate can be modulated by a magnetic field. Moreover, the JVs were applied as imaging agents for in vivo bimodal photoacoustic (PA) and magnetic resonance (MR) imaging of tumors by intravenous injection. With an external magnetic field, the accumulation of the JVs in tumors was significantly increased, leading to a signal enhancement of approximately 2–3 times in the PA and MR imaging, compared with control groups without a magnetic field.