High Electric Field Gradients Research Articles

Electrokinetic particle trapping in microfluidic wells using conductive nanofiber mats.

The frequency dependence of electrokinetic particle trapping using large-area (>mm2) conductive carbon nanofiber (CNF) mat electrodes is investigated. The fibers provide nanoscale geometric features for the generation of high electric field gradients, which is necessary for particle trapping via dielectrophoresis (DEP). A device was fabricated with an array of microfluidic wells for repeated experiments; each well included a CNF mat electrode opposing an aluminum electrode. Fluorescent microspheres (1µm) were trapped at various electric field frequencies between 30kHz and 1MHz. Digital images of each well were analyzed to quantify particle trapping. DEP trapping by the CNF mats was greater at all tested frequencies than that of the control of no applied field, and the greatest trapping was observed at a frequency of 600kHz, where electrothermal flow is more significantly weakened than DEP. Theoretical analysis and measured impedance spectra indicate that this result was due to a combination of the frequency dependence of DEP and capacitive behavior of the well-based device.

Collection of serum albumin aggregate nanoparticles from human plasma by dielectrophoresis.

Dielectrophoresis (DEP) is a fast and reliable nanoparticle recovery method that utilizes nonuniform electric fields to manipulate particles based on their material composition and size, enabling recovery of biologically-derived nanoparticles from plasma for diagnostic applications. When applying DEP to undiluted human plasma, collection of endogenous albumin proteins was observed at electric field gradients much lower than predicted by theory to collect molecular proteins. To understand this collection, nanoparticle tracking analysis of bovine serum albumin (BSA) dissolved in 0.5× phosphate-buffered saline was performed and showed that albumin spontaneously formed aggregate nanoparticles with a mean diameter of 237nm. These aggregates experienceda dielectrophoretic force as a function of aggregate radius rather than the diameter of individual protein molecules which contributed to their collection. In high conductance buffer (6.8mS/cm), DEP was able to move these aggregates into regions of high electric field gradient, and in lower conductance buffer (0.68mS/cm), these aggregates could be moved into high or low gradient regions depending on the applied frequency. Disruption of BSA aggregates using a nonionic detergent significantly decreased the particle diameter, resulting in decreased dielectrophoretic collection of albumin which increased the collection consistency of particles of interest. These results provide techniques to manipulate albumin aggregates via DEP, which impacts collection of diagnostic biomarkers.

Directional Polarization of a Ferroelectric Intermediate Layer Inspires a Built-In Field in Si Anodes to Regulate Li+ Transport Behaviors in Particles and Electrolyte.

The silicon (Si) anode is prone to forming a high electric field gradient and concentration gradient on the electrode surface under high-rate conditions, which may destroy the surface structure and decrease cycling stability. In this study, a ferroelectric (BaTiO3) interlayer and field polarization treatment are introduced to set up a built-in field, which optimizes the transport mechanisms of Li+ in solid and liquid phases and thus enhances the rate performance and cycling stability of Si anodes. Also, a fast discharging and slow charging phenomenon is observed in a half-cell with a high reversible capacity of 1500.8mAhg-1 when controlling the polarization direction of the interlayer, which means a fast charging and slow discharging property in a full battery and thus is valuable for potential applications in commercial batteries. Simulation results demonstrated that the built-in field plays a key role in regulating the Li+ concentration distribution in the electrolyte and the Li+ diffusion behavior inside particles, leading to more uniform Li+ diffusion from local high-concentration sites to surrounding regions. The assembled lithium-ion battery with a BaTiO3 interlayer exhibited superior electrochemical performance and long-term cycling life (915.6mAhg-1 after 300 cycles at a high current density of 4.2Ag-1). The significance of this research lies in exploring a new approach to improve the performance of lithium-ion batteries and providing new ideas and pathways for addressing the challenges faced by Si-based anodes.

Rapid Free Electron Reduction in Plasma Irradiated Microscale Water Droplets

Many reaction studies show that microdroplets can provide a new avenue for green chemistry by enabling the electrochemical activity of water molecules. The observation of enhanced chemical reaction rates in gas-phase microdroplets compared to bulk liquids, often by orders of magnitude, has sparked considerable research interest. A number of factors may be involved, including evaporation enhanced reactant concentration, partial solvation at the surface, high surface to bulk number ratio, enhanced surface rate constants, pH gradients, gas-phase reactions and mass transfer, electric field enhancement and surface charging. Plasma interactions with liquids involve the mass transfer and accommodation of reactive radicals and other plasma chemical species, often occurs in the presence of high electric field and temperature gradients, UV flux and electric currents.We have investigated the use of low temperature atmospheric pressure plasma irradiated liquid microdroplets. [1] Picolitre microdroplets are totally surrounded by plasma and their high surface area relative to volume receives chemical, photon and charge flux during flight leading to chemical reactions in the liquid at low temperature. With the inclusion of precursors, the small volume provides an excellent basis for gas phase chemical microreactors that can deliver products continuously and almost instantaneously downstream for applications such as plasma medicine and specialist chemical or nanomaterial printing.In particular, low temperature plasmas are a copious source of free electrons, up to 106 times greater than corona discharges, which can interact with the liquid surface to promote rapid reduction reactions and possibly on-water catalysis. We have observed the reduction of metal salts in flight to produce nanoparticles at rates many orders of magnitude greater than standard solution chemistry or via radiolysis. [2] We have measured the plasma gas temperature in the presence of microdroplet streams with droplet rates up to 5 x 104 s-1 and observed no significant increase in gas temperature, which is typically ~300 K, thus keeping the evaporation limited. [3] Recently, we carried out plasma simulations, coupled with downstream chemical flux measurements, to determine the evolution of gas-phase chemistry in the plasma and effluent. [4] We have also performed the first measurements of plasma charging of particles, at atmospheric pressure for diameters > 1 um. The determined average droplet charge per droplet was 2.5 x 106 electrons (400 fC). Using a number of plasma particle charging models we estimate the electron flux ranged from 5 x 1022 m-2 s-1 to 4 x 1025 m-2 s-1.In [2], at least 50% Au3+ to Au0 reduction of the droplet precursor (HAuCl4) was observed over a ~120 µs plasma flight time and the equivalent Au0 generation rate is ~1013 atoms s-1. The ratio of electron flux per droplet to metal generation rate provides a dimensionless figure of merit, e-/Au0, of ~100 - 1000. Using plasma chemistry simulations along with estimates of electron flux to the droplet surface, we then simulated the subsequent water reactions using a 1-D radial reaction – diffusion scheme. For 15 µm diameter droplets without precursor, surface H2O2 concentrations ranged from 0.1 M to 100 µM whereas surface OH· concentrations were almost constant (~1 mM), as electron flux density (m-2 s-1) increased. However the OH· concentration decayed rapidly with depth, reaching 1 nM within 1 µm. As the droplet size increased, surface concentration and penetration depth both decreased. For electron flux densities up to ~5 x 1022 m-2 s-1, the solvated electron surface concentration was ~0.1 mM and the penetration depth was ~250 nm but at higher fluxes, the penetration depth increases significantly likely due to the reduced concentration of sub-surface H2O2 and OH·, which scavenge electrons. When 1 mM HAuCl4 precursor is added, the surface electron concentration and penetration depth is significantly reduced (< 50 nm), even at very high flux densities. We also observe the loss of surface Au3+ and the increase in Au0 concentration. However, the maximum overall conversion rate is 15%, compared to the experimental observation of >50%. Enhanced conversion would likely require greater precursor transport from droplet centre to surface, either by internal fields or convection. We carried out preliminary simulations of (Au0)N (N: 2 → 20) cluster growth via simple collisions in this environment and observed almost complete conversion of Au0 to cluster (Au0)N with the concentration of (Au0)20 >> (Au0)2.[1] PD Maguire et al., Appl. Phys. Lett. 106, 224101 (2015); doi: 10.1063/1.4922034[2] PD Maguire et al., Nano Lett., 17, 1336–1343 (2017) doi: 10.1021/acs.nanolett.6b03440[3] N Hendawy et al 2020 Plasma Sources Sci. Technol. 29 085010 doi: 10.1088/1361-6595/aba2aa[4] H McQuaid et al 2022, in-press. See preprint - doi: 10.48550/arXiv.2207.04518

Flexoelectric vibration control of parabolic shells

The presented work focuses on the spatially distributed actuation behavior of flexoelectric actuators on a parabolic cylindrical shell panel with simply supported boundary conditions. A flexoelectric actuator with significant electric field gradients could generate actuation forces and control moments to drive or control system oscillations. For an engineering structure with initial excitations, flexoelectric actuators could be employed to achieve vibration control with designed electric inputs. The dynamic equations of the parabolic cylindrical panel laminated with an arbitrary flexoelectric actuator are defined first. The transverse displacement excited by an external mechanical loading and flexoelectric actuators is calculated. Actuation effects and vibration control efficiency contributed by various design parameters including flexoelectric patch thickness and atomic force microscopy (AFM) probe radius are studied. As the purpose of this study is to accelerate the vibration attenuation, the open-loop control usually causes insufficient control or over-actuation effects. Using displacement feedback, velocity feedback, and acceleration feedback, the effectiveness of these three closed-loop control methods is analyzed and evaluated by the modal response. Analyses suggest that the velocity feedback could effectively attenuate oscillations with the lowest voltage input than in the other two cases. Specifically, the system damping ratio can be adjusted with velocity feedback. Lower voltage input could benefit system design and release stress concentrations caused by high electric field gradients. This research provides a basis for vibration control based on flexoelectric parabolic shell structures and provides an optimal choice for the design of flexoelectric drive closed-loop control for parabolic shell structures.

Perturbed Angular Correlation Technique at ISOLDE/CERN Applied for Studies of Hydrogenated Titanium Dioxide (TiO2): Observation of Cd-H Pairs

Profound understanding of the local electronic and defect structure in semiconductors always plays a vital role in the further developing of applications of such materials. In the present work an investigation of the electronic structure in hydrogenated TiO2 (rutile) thin films is conducted by virtue of Time-Differential γ-γ Perturbed Angular Correlation spectroscopy (TDPAC or PAC) with 111mCd/Cd isotope, produced and implanted at ISOLDE/CERN. The measurements were performed at 581 K as a function of the temperature of the samples during hydrogenation. Despite the fact, that rutile single crystals usually show the presence of two local environments, when are studies with Cd/In isotopes, the current pristine thin films sample had a single electric field gradient. Upon various degrees of hydrogenation, Cd probe atoms showed underwent alterations, resulting in up to 3 different local surroundings, generally with high electric field gradients. Broad EFG distributions are likely due to randomly distributed point defects in the neighbourhood of Cd acceptors. Observed results suggest that hydrogenations performed at RT and 423 K are not able to promote unique defect configurations, while in the range of 473-573 K the formation of such configurations is observed. Therefore, one may assume that the formation of Cd-defect complexes (Cd-H pairs) is temperature enhanced. At higher levels of hydrogenation (663 K), the samples become partly amorphous that further hinders any atomistic studies with strong damped PAC spectra. Cd-H complexes seem to be stable up to annealing up to 581 K annealing. The obtained results give a deep insight into complex hydrogen defects, their interactions and bond formations with Cd acceptor.

Coalescence and counterflow of droplets on needle electrode with negative corona discharge

The coalescence of droplets on the discharge electrode surface in high humidity environments has rarely been studied, which may affect discharge characteristics. Meanwhile, directional transport of droplets is of great significance for many applications ranging from fluidic processing to thermal management. Here, corona discharge in a needle-plate electrode is adopted to explore the coalescence rule of droplets attached on the discharge electrode surface in a high-humidity environment, and to realize the counterflow of droplets. The experimental results show that the number of coalesced droplets on the needle electrode surface reaches the maximum under −7.5 kV at relative humidity ∼94% and ambient temperature ∼20 °C. When the applied voltage increases from −6 kV to −11 kV, the droplet moves up 2.76 mm in 5 s. The size of the attached droplet depends on the balance of coalescence and evaporation. The coalescence is mainly attributed to the dielectrophoretic force caused by the high electric field gradient. The evaporation is related to the ionic wind generated by the corona discharge. With regard to the counterflow phenomenon of the droplet, we speculate that the high concentration gradient of positive ions near the needle electrode provides a driving force for the negatively charged droplets. Meanwhile, the electrons and negative ions below the needle tip offer a repulsive force to the droplet. The shape and moving direction of the droplet attached to the needle surface can be manipulated by changing the voltage applied to the needle electrode, which shows the potential application value in realizing the self-cleaning of electrodes, liquid lenses and so on.

Chemical Enhancement vs Molecule–Substrate Geometry in Plasmon-Enhanced Spectroscopy

Light interaction with metal nanostructures exposes exciting phenomena such as strong amplification and localization of electromagnetic fields. In surface-enhanced Raman spectroscopy (SERS), the strong signal amplification is attributed to two fundamental mechanisms, electromagnetic and chemical enhancement (EM and CM, respectively). While the EM mechanism is accepted as the main responsible for signal amplification, a long-standing controversy on the CM mechanism’s role still prevails. The CM contribution can be evidenced when compared to the nonenhanced (or bulk) Raman signal as a change in intensity ratios, peak shifts, or appearance of new Raman modes. However, it is also possible to induce similar spectral variations by changing the relative orientation between the electric field and molecule or when a high electric field gradient is achieved. Therefore, in this work, we show specific spectral changes in SERS affected by the molecular orientation, while changes in other modes can be attributed to chemical enhancement. On the basis of our experimental and quantum chemical results for cobalt phthalocyanine, we identify low-frequency Raman modes (LFMs) sensitive to charge-transfer compared to high-frequency modes (HFMs) that are rather sensitive to geometrical effects and temperature changes. These results provide new evidence on the role of molecule excitation/polarization that comes now as a more general and dominant effect than the chemical enhancement mechanism so far attributed to charge-transfer processes. These findings make it possible to engineer multifunctional Raman molecular probes with selective sensitivity to the local environment (HFMs) and charge-transfer processes (LFMs).

Growth mechanism for nanotips in high electric fields

In this work we show using atomistic simulations that the biased diffusion in high electric field gradients creates a mechanism whereby nanotips may start growing from small surface asperities. It has long been known that atoms on a metallic surface have biased diffusion if electric fields are applied and that microscopic tips may be sharpened using fields, but the exact mechanisms have not been well understood. Our Kinetic Monte Carlo simulation model uses a recently developed theory for how the migration barriers are affected by the presence of an electric field. All parameters of the model are physically motivated and no fitting parameters are used. The model has been validated by reproducing characteristic faceting patterns of tungsten surfaces that have in previous experiments been observed to only appear in the presence of strong electric fields. The growth effect is found to be enhanced by increasing fields and temperatures.

High-throughput dielectrophoretic filtration of sub-micron and micro particles in macroscopic porous materials

State-of-the-art dielectrophoretic (DEP) separation techniques provide unique properties to separate particles from a liquid or particles with different properties such as material or morphology from each other. Such separators do not operate at throughput that is sufficient for a vast fraction of separation tasks. This limitation exists because high electric field gradients are required to drive the separation which are generated by electrode microstructures that limit the maximum channel size. Here, we investigate DEP filtration, a technique that uses open porous microstructures instead of microfluidic devices to easily increase the filter cross section and, therefore, also the processable throughput by several orders of magnitude. Previously, we used simple microfluidic porous structures to derive design rules predicting the influence of key parameters on DEP filtration in real complex porous filters. Here, we study in depth DEP filtration in microporous ceramics and underpin the previously postulated dependencies by a broad parameter study (Lorenz et al., 2019). We will further verify our previous claim that the main separation mechanism is indeed positive DEP trapping by showing that we can switch from positive to negative DEP trapping when we increase the electric conductivity of the suspension. Two clearly separated trapping mechanisms (positive and negative DEP trapping) at different conductivities can be observed, and the transition between them matches theoretical predictions. This lays the foundation for selective particle trapping, and the results are a major step towards DEP filtration at high throughput to solve existing separation problems such as scrap recovery or cell separation in liquid biopsy.Graphical abstract

AC electrokinetic immobilization of organic dye molecules

The application of inhomogeneous AC electric fields for molecular immobilization is a very fast and simple method that does not require any adaptions to the molecule’s functional groups or charges. Here, the method is applied to a completely new category of molecules: small organic fluorescence dyes, whose dimensions amount to only 1 nm or even less. The presented setup and the electric field parameters used allow immobilization of dye molecules on the whole electrode surface as opposed to pure dielectrophoretic applications, where molecules are attracted only to regions of high electric field gradients, i.e., to the electrode tips and edges. In addition to dielectrophoresis and AC electrokinetic flow, molecular scale interactions and electrophoresis at short time scales are discussed as further mechanisms leading to migration and immobilization of the molecules.Graphical

Passivated-electrode insulator-based dielectrophoretic separation of heterogeneous cell mixtures.

Rapid and accurate purification of various heterogeneous mixtures is a critical step for a multitude of molecular, chemical, and biological applications. Dielectrophoresis has shown to be a promising technique for particle separation due to its exploitation of the intrinsic electrical properties, simple fabrication, and low cost. Here, we present a geometrically novel dielectrophoretic channel design which utilizes an array of localized electric fields to separate a variety of unique particle mixtures into distinct populations. This label-free device incorporates multiple winding rows with several nonuniform structures on to sidewalls to produce high electric field gradients, enabling high locally generated dielectrophoretic forces. A balance between dielectrophoretic forces and Stokes' drag is used to effectively isolate each particle population. Mixtures of polystyrene beads (500nm and 2μm), breast cancer cells spiked in whole blood, and for the first time, neuron and satellite glial cells were used to study the separation capabilities of the design. We found that our device was able to rapidly separate unique particle populations with over 90% separation yields for each investigated mixture. The unique architecture of the device uses passivated-electrode insulator-based dielectrophoresis in an innovative microfluidic device to separate a variety of heterogeneous mixture without particle saturation in the channel.

Molybdenum Oxides Coatings for High Demanding Accelerator Components

Large electric gradients are required for a variety of new applications, notably including the extreme high brightness electron sources for X-ray free electron lasers (FELs), radio-frequency (RF) photo-injectors, industrial and medical accelerators, and linear accelerators for particle physics colliders. In the framework of the INFN-LNF, SLAC (USA), KEK (Japan), UCLA (Los Angeles) collaboration, the Frascati National Laboratories (LNF) are involved in the modelling, development, and testing of RF structures devoted to particles acceleration by high gradient electric fields of particles through metal devices. In order to improve the maximum sustainable gradients in normal-conducting RF-accelerating structures, both the RF breakdown and dark current should be minimized. To this purpose, studying new materials as well as manufacturing techniques are mandatory to identify better solutions to such extremely requested applications. In this contribution, we discuss the possibility of using a dedicated coating on a solid copper sample (and other metals) with a relatively thick film to improve and optimize breakdown performances and to minimize the dark current. We present here the first characterization of MoO3 films deposited on copper by pulsed-laser deposition (PLD).

Low-Voltage Flow-Through Electroporation Membrane and Method.

Electroporation uses high electric field gradients to create pores within the membrane of living cells in order to deliver a substance, for example a gene, into the cytoplasm. To achieve such gradients, current electroporation devices deliver voltage pulses in the kV range to the cell medium. We describe here a new device based on gold-microtube membranes that can accomplish electroporation with voltage pulses that are orders of magnitude smaller, 4V. The percentages of electroporated bacteria were found to be more than an order of magnitude higher than obtained with a commercial electroporator.

Hyperfine interactions and diffusion of Cd in TiO2 (rutile)

In the current work, we present an investigation of the electronic and defect structure in (TiO2) rutile monocrystals by virtue of time differential perturbed angular γ-γ correlation spectroscopy. Studies were performed using 111mCd, implanted at ISOLDE/CERN complemented with diffusion studies and density functional theory calculations. Hyperfine field parameters have been probed as a function of temperature between 298 K and 873 K. The results demonstrate that 111mCd/Cd implanted rutile has two local environments. The first environment is characterized with parameters attributed to Cd localized at the cationic site which goes relatively along with a specific case where a charged supercell Cd:Ti(2e−) is in the scope. The origin of the second fraction could be rising from the subsurface regions where according to a tracer diffusion study the major part of implant is bounded featuring different diffusion mechanisms. Performed ab initio calculations suggest that the disruptive surface environment could contain apical or equatorial vacancies near the probe, inducing high electric field gradients for the second fraction. Current results seem to differ from those obtained before with different methods of probing (Ag/Cd and In/Cd).

An Investigation of the Role of Rapid Chlorine Hydrolysis on the Performance of Electrochemically-Mediated Desalination

A recent United Nations report estimates that more than two billion people presently lack access to clean drinking water and more than four billion people lack access to proper sanitation. Furthermore, steadily rising populations, a changing climate, and growth in the agriculture, energy, and manufacturing sectors jeopardize future water security in many parts of the world. Accordingly, energy-efficient separations of salts and trace contaminates from water represent one of the most significant (and difficult) challenges facing humankind. To address this problem, we have previously reported the development of a promising approach to the separation of salts from seawater which we refer to as electrochemically-mediated desalination (EMD). EMD is an electrokinetic technique which utilizes the electric field gradient formed local to an anode during chloride oxidation to separate charged species from pure water. Despite experimental and computational study of EMD, a deep understanding of phenomena fundamental to the technique has yet to be attained. To this end, we performed time-resolved high resolution electric field gradient measurements and direct conductivity measurements during electrochemical chloride oxidation within a simple system composed of microelectrodes embedded in a straight microchannel. We were surprised by the findings of these measurements. Specifically, a large electric field gradient was measured upstream of the anode during chloride oxidation, however, a local increase in solution conductivity was measured at the same axial position along the channel length during chloride oxidation. To address these seemingly contradictory findings, the experimental system was simulated by finite element modeling. Simulation results showed that electrochemically generated chlorine undergoes rapid hydrolysis and produces substantial quantities of HOCl, H+, and Cl- near the anode. This finding corroborates the experimentally observed increase in solution conductivity near the anode during chloride oxidation. Consequently, we hypothesize that the observed electric field gradient is not indicative of local solution conductivity, but rather is related to a surface interaction between a product of chloride oxidation and the Pt microband electrode used to probe the electric field gradient. Ongoing research seeks to confirm this claim. Ultimately, we conclude that complex aqueous chlorine chemistry significantly limits the utility of chlorine generation for the formation of a local electric field gradient and seawater desalination.

Field emission from laser-processed niobium (110) single crystals

Niobium is a technologically important material, which is typically used for superconducting radio-frequency applications. Superconductive cavities made of niobium require contamination-free and smooth surfaces to ensure the best performance of a particle accelerator. Interior surfaces of niobium cavities are usually obtained by centrifugal barrel polishing, buffered chemical polishing, and electropolishing or a combination of these methods. However, a standard inspection of the inner cavity surface after the treatment still shows the presence of sharp features limiting cavity performance. Laser polishing is a potential alternative or can be used as an additional final step for a more efficient surface treatment toward higher electric field gradients. In the present study, a chemically polished (110)-oriented single crystal niobium surface was processed with a focused pulsed laser with a pulse duration of 3.5 ns and a repetition rate of 10 Hz. The laser fluence and the number of pulses were varied in the range from 0.68 up to $4.27\text{ }\text{ }\mathrm{J}/{\mathrm{cm}}^{2}$ and from 20 up to 200, respectively. The magnitude of laser-induced surface structures and boiling traces was systematically studied by means of scanning electron microscopy, atomic force microscopy, and optical profilometry. Finally, the local field emission behavior was investigated and correlated with the observed surface modifications. Typical current-field characteristics and a field enhancement statistic of laser-processed areas are presented in the study. The surface processing with a rather low laser fluence of $0.68\text{ }\text{ }\mathrm{J}/{\mathrm{cm}}^{2}$ yielded high onset electric fields of $650--940\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ with field enhancement factors below 10. The processing with a higher laser fluence and/or a higher number of pulses resulted in boiledlike structures emitting at $70--190\text{ }\text{ }\mathrm{MV}/\mathrm{m}$.

Direct Numerical Simulation of Particles in Spatially Varying Electric Fields †

A numerical scheme is developed to simulate the motion of dielectric particles in the uniform and nonuniform electric fields of microfluidic devices. The motion of particles is simulated using a distributed Lagrange multiplier method (DLM) and the electric force acting on the particles is calculated by integrating the Maxwell stress tensor (MST) over the particle surfaces. One of the key features of the DLM method used is that the fluid-particle system is treated implicitly by using a combined weak formulation, where the forces and moments between the particles and fluid cancel, as they are internal to the combined system. The MST is obtained from the electric potential, which, in turn, is obtained by solving the electrostatic problem. In our numerical scheme, the domain is discretized using a finite element scheme and the Marchuk-Yanenko operator-splitting technique is used to decouple the difficulties associated with the incompressibility constraint, the nonlinear convection term, the rigid-body motion constraint and the electric force term. The numerical code is used to study the motion of particles in a dielectrophoretic cage which can be used to trap and hold particles at its center. If the particles moves away from the center of the cage, a resorting force acts on them towards the center. The MST results show that the ratio of the particle-particle interaction and dielectrophoretic forces decreases with increasing particle size. Therefore, larger particles move primarily under the action of the dielectrophoretic (DEP) force, especially in the high electric field gradient regions. Consequently, when the spacing between the electrodes is comparable to the particle size, instead of collecting on the same electrode by forming chains, they collect at different electrodes.

A dielectrophoretic study of the carbon nanotube chaining process and its dependence on the local electric fields

The chaining process of a system of interacting carbon nanotubes (CNTs) under an alternating current electric field is investigated at two regions of different electric field characteristics. For the region of uniform electric field (far from the electrodes), a two-dimensional multiparticle approach based on the dielectrophoretic (DEP) theory and classical mechanics is proposed to investigate the CNT rotational and translation motion. For this scenario, CNT rotation and alignment along the electric field direction occurs first, followed by the translation and chaining processes which were found to be highly dependent on the CNT-to-CNT initial configuration. On the other hand, the presence of high electric field gradients governs the CNT chaining at regions near the electrodes. DEP forces caused by such gradients were computed by finite element analysis and compared to the magnitude of the CNT-to-CNT interacting forces at zones of uniform electric fields. A critical distance of CNT-to-CNT separation was estimated, which determines if a CNT is attracted towards the electrode or if it is attracted by other CNTs away from the electrodes. Experimental evidence of CNTs dynamic motion under electric fields is presented to support the predicted trends.

Dielectrophoresis-Based Protein Enrichment for a Highly Sensitive Immunoassay Using Ag/SiO2 Nanorod Arrays.

A nanoscale insulator-based dielectrophoresis (iDEP) technique is developed for rapid enrichment of proteins and highly sensitive immunoassays. Dense arrays of nanorods (NDs) by oblique angle deposition create a super high electric field gradient of 2.6 × 1024 V2 m-3 and the concomitant strong dielectrophoresis force successfully traps small proteins at a bias as low as 5 V. 1800-fold enrichment of bovine serum albumin protein at a remarkable rate of up to 180-fold s-1 is achieved using oxide coated Ag nanorod arrays with pre-patterned sawtooth electrodes. Based on this system, an ultrasensitive immunoassay of mouse immunoglobulin G is demonstrated with a reduction in the limit of detection from 5.8 ng mL-1 (37.6 pM) down to 275.3 fg mL-1 (1.8 f M), compared with identical assays performed on glass plates. This methodology is also applied to detect a cancer biomarker prostate-specific antigen spiked in human serum with a detection limit of 2.6 ng mL-1 . This high sensitivity results from rapid biomarker enrichment and metal enhanced fluorescence through the integration of nanostructures. The concentrated proteins also accelerate binding kinetics and enable signal saturation within 1 min. Given the easy fabrication process, this nanoscale iDEP system provides a highly sensitive detection platform for point-of-care diagnostics.