Performance optimization in electric field gradient focusing

Electric field gradient focusing (EFGF) is a technique used to simultaneously separate and concentrate biomacromolecules, such as proteins, based on the opposing forces of an electric field gradient and a hydrodynamic flow. Recently, we reported EFGF devices fabricated completely from copolymers functionalized with poly(ethylene glycol), which display excellent resistance to protein adsorption. However, the previous devices did not provide the predicted linear electric field gradient and stable current. To improve performance, Tris–HCl buffer that was previously doped in the hydrogel was replaced with a phosphate buffer containing a salt (i.e., potassium chloride, KCl) with high mobility ions. The new devices exhibited stable current, good reproducibility, and a linear electric field distribution in agreement with the shaped gradient region design due to improved ion transport in the hydrogel. The field gradient was calculated based on theory to be approximately 5.76 V/cm² for R-phycoerythrin when the applied voltage was 500 V. The effect of EFGF separation channel dimensions was also investigated; a narrower focused band was achieved in a smaller diameter channel. The relationship between the bandwidth and channel diameter is consistent with theory. Three model proteins were resolved in an EFGF channel of this design. The improved device demonstrated 14,000-fold concentration of a protein sample (from 2 ng/mL to 27 μg/mL).     

Bilinear electric field gradient focusing

Electric field gradient focusing (EFGF) uses an electric field gradient and a hydrodynamic counter flow to simultaneously separate and focus charged analytes in a channel. Previously, most EFGF devices were designed to form a linear field gradient in the channel. However, the peak capacity obtained using a linear gradient is not much better than what can be obtained using conventional CE. Dynamic improvement of peak capacity in EFGF can be achieved by using a nonlinear gradient. Numerical simulation results indicate that the peak capacity in a 4-cm long channel can be increased from 20 to 150 when changing from a linear to convex bilinear gradient. To demonstrate the increased capacity experimentally, an EFGF device with convex bilinear gradient was fabricated from poly(ethylene glycol) (PEG)-functionalized acrylic copolymers. The desired gradient profile was confirmed by measuring the focusing positions of a standard protein for different counter flow rates at constant voltage. Dynamically controlled elution of analytes was demonstrated using a monolith-filled bilinear EFGF channel. By increasing the flow rate, stacked proteins that were ordered but not resolved after focusing in the steep gradient segment were moved into the shallow gradient segment, where the analyte peak resolution increased significantly. In this way, the nonlinear field gradient was used to realize a dynamic increase in the peak capacity of the EFGF method.     

Ca ion permeation through liposome membranes with heat generation by square-wave electric field

Phospholipid liposomes that contain Ca ions in inside compartment were subjected to external alternating electric fields of square wave form at several frequencies: 10, 100 Hz, 1, 10, 100 kHz. The leakage of Ca ions from inside to bulk solution caused by the electric fields was detected by the fluorescence change due to Ca-Quin 2 (fluorescent dye) complex formation in the bulk aqueous solution. The temperature increments of the sample solution in the chamber were also measured. The amplitude of the electric field and time interval between application of the electric field were varied. The frequency dependency of both the leakage of Ca ion and the temperature increments was observed. It was observed that the efficiency of the leakage had a minimum at 1 kHz. The origin of the frequency dependency of the leakage is discussed, and it is concluded that the energy consumption on a microscopic scale in the liposome membrane zone can be the reason for membrane permeation in this experimental system.     

Structural transformations of carboxyl acids networks induced by concentration and oriented external electric field

The effects of concentration and an oriented external electric field on the transformations of hydrogenbonded structures of trimesic acid(TMA) and terephthalic acid(TPA) have been investigated at a liquidsolid interface by scanning tunneling microscopy(STM).The triangular periodic TMA framework can be transformed into a flower-like structure by changing the STM sample bias sign in situ.Networks of TMA and TPA are porous at a negative substrate bias,but typically change to relatively compact forms when the polarity of the applied bias is reversed.This change is reversible if the applied bias is reversed.The effects have potentials to locally control the capture and release of analytes in host-guest systems and the 2D morphology in multicomponent layers.     

Polyelectrolyte layer-by-layer self-assembly enhanced by electric field and their multilayer membranes for separating isopropanol–water mixtures

An electric field enhanced method is developed for fabricating layer-by-layer (LbL) self-assembly polyelectrolyte multilayer membranes. Three kinds of electric field enhanced polyelectrolyte multilayer membranes (EPEMs), poly(diallyl dimethylammonium chloride)/poly(styrenesulfonate sodium salt) (PDDA/PSS), poly(diallyldimethylammonium chloride)/poly(acrylic acid sodium salt) (PDDA/PAA) and polyethylenimine/poly(acrylic acid sodium salt) (PEI/PAA), were self-assembled on a reverse osmosis membrane (ROM). The pervaporation performances of EPEMs for separating isopropanol–water mixtures (90/10, w/w) are all superior to those of corresponding normal self-assembled polyelectrolytes membranes (PEMs), and the selectivity increases with PDDA/PSS, PDDA/PAA and PEI/PAA in order. For (PEI/PAA)₄PEI EPEM, the separation factor is 1075 and permeation flux is 4.05 kg m⁻² h⁻¹ at 70 °C. This novel method speeds up the LbL process, which makes it promising for the practical application of the LbL multilayer membrane.     

Controlled generation of droplets using an electric field in a flow-focusing paper-based device

Droplet-based microfluidics is a modular platform in high-throughput single-cell and small sample analyses. However, this droplet microfluidic system was widely fabricated using soft lithography or glass capillaries, which is expensive and technically demanding for various applications, limiting use in resource-poor settings. Besides, the variation in droplet size is also restricted due to the limitations on the operating forces that the paper-based platform is able to withstand. Herein, we develop a fully integrated paper-based droplet microfluidic platform for conducting droplet generation and cell encapsulation in independent aqueous droplets dispersed in a carrier oil by incorporating electric fields. Through imposing an electric field, the droplet size would decrease with increasing the electric field and smaller droplets can be produced at high applied voltage. The droplet diameter can be adjusted by the ratio of inner and outer flow velocities as well as the applied electric field. We also demonstrated the proof of concept encapsulation application of our paper device by encapsulating yeast cells under an electric field. Using a simple wax printing method, carbon electrodes can be integrated on the paper. The integrated paper-based microfluidic platform can be fabricated easily and conducted outside of centralized laboratories. This microfluidic system shows great potential in drug and cell investigations by encapsulating cells in resource-limited environments.     

Calculation of the deuterium electric field gradients in HD and LiD using a variation-perturbation method with a Gaussian basis set

Summary We present calculations of the deuterium electric field gradients in the HD and LiD molecules obtained with a variation-perturbation method using Gaussian atomic orbitals. The differences between our theoretical values and the corresponding experimental or best calculated values are 2%. We conclude that high accuracy can be obtained with the variation-perturbation method using either Gaussian or Slater orbitals.     

Forces at the nuclei of a molecule in the presence of non-uniform electric field

The interaction Hamiltonian within the Bloch gauge for the potentials of the electric field has been used to define electric multipole moment operators. Perturbation theory has been applied to evaluate the induced electronic moments and electric field at the nuclei in the presence of spatially non-uniform electric fields of high intensity. Multipole nuclear electric shielding tensors have been defined to describe the contributions arising from a non-homogeneous electric field. These quantities are useful to rationalize linear and non-linear responses of a molecule in the presence of intense external electric perturbations.     

Investigating electric field induced molecular distortions in polypropylene using Raman spectroscopy

Polymeric electric insulators are an integral part of many electronic circuits and systems. Changes induced by an electric field can affect various mechanisms; including electrical polarisation and electromechanical properties. Changes in the dielectric material can be tracked using spectroscopic methods. This study has shown that analysing polypropylene under electric field stress using Raman spectroscopy in combination with principal component analysis allows small changes in the non-crystalline phase to be identified. We have observed that for polypropylene, vibrational motion and changes in conformation occur mostly within the tie molecules connecting the overall cluster network. Amorphous molecular chains in the spherulites were also found to orient and form into a smectic mesophase. These electromechanical changes at the micro- and macromolecular level were found to be generally reversible once the stress is removed. However, with increased aging, these changes may lead to adverse structural changes and thus, in the future, this information may be used to inform faults and defect detection within polymeric dielectric materials.     

Effect of ionic impurities on the electric field alignment of diblock copolymer thin films

In Forschung hast Du immer zu mir gesagt, Schaffe etwas!. Mein Chef, ich habe immer versucht Deinen Befehlen zu folgen. Ich widme Dir diese Veröffentlichung. Mensch, mit fünf und siebzig bist Du noch jung, schön Geburtstag!The effect of ionic impurities on the electric field alignment of lamellar microdomains in polystyrene-block-poly(methyl methacrylate) diblock copolymer thin films was studied using transmission electron microscopy (TEM) and atomic force microscopy (AFM). At lithium ion concentrations greater than ~210 ppm, the microdomain morphology in block copolymers could be aligned in the direction of an applied electric field, regardless of the strength of interfacial interactions. Complete alignment of the copolymer microdomains, even those adjacent to the polymer/substrate interface, occurred by a pathway where the applied electric field enhanced fluctuations at the interfaces of the microdomains with a wavelength comparable to L_o, the equilibrium period of the copolymer. This enhancement in the fluctuations led to a disruption of the lamellar microdomains into smaller microdomains ~L_o in size, that, in time, reconnected to form microdomains oriented in the direction of the applied field.     

Effect of electric field during gel-layer controlled ultrafiltration of synthetic and fruit juice

Electric field enhanced ultrafiltration of pectin–sucrose mixture (synthetic juice) and mosambi (Citrus sinensis (L.) Osbeck) fruit juice using 50,000 (MWCO) polyerthersulfon membrane is studied in a cross-flow cell. Pectin, completely rejected by the membrane, forms a gel type layer over the membrane surface. Under the application of an external dc electric field across the membrane, gel-layer formation is restricted leading to an enhancement of permeate flux. During ultrafiltration of synthetic juice, application of dc electric field (800 V/m) increases the permeate flux to almost threefold compared to that with zero electric field. A theoretical model based on integral method assuming suitable concentration profile in the boundary layer is developed. The proposed model is used to predict the permeate flux in gel-layer governed electric field enhanced ultrafiltration. Predictions of the model are successfully compared with the experimental results under a wide range of operating conditions. Experiments with fruit juice also demonstrated significant increase in flux with the application of a suitable electric field.     

Localized electric field induced transition and miniaturization of two‐phase flow patterns inside microchannels

Strategic application of external electrostatic field on a pressure‐driven two‐phase flow inside a microchannel can transform the stratified or slug flow patterns into droplets. The localized electrohydrodynamic stress at the interface of the immiscible liquids can engender a liquid‐dielectrophoretic deformation, which disrupts the balance of the viscous, capillary, and inertial forces of a pressure‐driven flow to engender such flow morphologies. Interestingly, the size, shape, and frequency of the droplets can be tuned by varying the field intensity, location of the electric field, surface properties of the channel or fluids, viscosity ratio of the fluids, and the flow ratio of the phases. Higher field intensity with lower interfacial tension is found to facilitate the oil droplet formation with a higher throughput inside the hydrophilic microchannels. The method is successful in breaking down the regular pressure‐driven flow patterns even when the fluid inlets are exchanged in the microchannel. The simulations identify the conditions to develop interesting flow morphologies, such as (i) an array of miniaturized spherical or hemispherical or elongated oil drops in continuous water phase, (ii) “oil‐in‐water” microemulsion with varying size and shape of oil droplets. The results reported can be of significance in improving the efficiency of multiphase microreactors where the flow patterns composed of droplets are preferred because of the availability of higher interfacial area for reactions or heat and mass exchange.     

A microfluidic viscometer: Translation of oscillatory motion of a water microdroplet in oil under electric field

The electric field induced motion of a charged water droplet suspended in a low-dielectric oil medium is exploited to evaluate the rheological properties of the suspending medium. The time-periodic electrophoretic motion of the droplet between the electrodes decorated in a polymeric micro-well is translated into a proof-of-concept microfluidic prototype, which can measure viscosities of the unknown fluid samples. The variations in the instantaneous velocities of the migrating droplet have been measured inside silicone oil of known physical properties at different electric field intensities. Subsequently, a balance between the electric field to the viscous force has been employed to evaluate the experimental charge density on the droplet surface. Thereafter, a comprehensive scaling law has been devised to find a correlation between the charge on the droplet to the dielectric permittivity of the surrounding medium, size of the water droplet, and the applied electric field intensity. Following this, the scaling law and force balance have been employed together to evaluate the unknown viscosity of an array of suspending mediums by simply analyzing the electrophoretic motion of water droplet. The model proposed is also found to be consistent when a solid amberlite microparticle has been employed as a probe instead of the water droplet. In such a scenario, minor changes in the exponents of the scaling law are found to be necessary to reproduce the results obtained using the water droplet. The method paves the way for the making of an economical and portable microfluidic rheometer with further finetuning and translational developments.     

Deviation from Ohm's law in electric field assisted capillary liquid chromatography

Earlier studies of electric field assisted LC (EF-LC) have shown that the effect on charged analytes of the application of an electric field over a capillary LC column is relatively small. Charged analytes can only be affected by the electric field while present in the mobile phase, which makes the effective time for influence of the electric field t(0) independent of retention time. Because the charged analytes only can be affected for a short time the electric field strength ought to be high in order to increase the impact of the electric field on the separation. We have, however, found that only a relatively low electric field strength can be used in EF-LC when pressure is used as main driving force. The useful field strength was limited by a dramatic increase in the current. This increase in current was found to origin from an increased concentration of buffer ions that have an electrophoretic mobility towards the pumped flow.     

Conduction stability of high-density polyethylene/carbon black composites due to electric field action

Conduction stability of high-density polyethylene/carbon black (HDPE/CB) composites with a CB volume fraction slightly above the percolation threshold is studied in relation to electric field action at various ambient temperatures below the melting point of HDPE. It is found that resistance of the composites shows considerable changes after the electric field is switched off. Influence of irradiation crosslinking of HDPE on the conduction stability is also discussed.     

Dynamics of electric field induced deformations in flexoelectric nematic layers

The dynamics of deformations induced by DC electrical fields in homeotropically aligned layers containing a flexoelectric nematic material with negative dielectric anisotropy has been studied numerically. The rise time constants, characterising the development of deformations after switching on the external voltage, and the decay time constants, describing the decay of deformation after switching off the voltage, were calculated as a function of the parameters essential for the behaviour of the layer. In particular, the influence of flexoelectricity was studied. It was found that the stronger the flexoelectric properties of the nematic, the lower is its viscosity, the higher is the bias voltage, the weaker is the surface anchoring, the thinner is the layer and the higher is the ion concentration, the more rapid was the onset of deformation. Similarly, the lower the viscosity, the thinner is the layer, the stronger the anchoring and the larger the ion content, the more rapid was the decay of deformation. Neither the voltage previously applied nor the flexoelectric properties were found to affect the decay time.     

Strong deformations induced by a DC electric field in homeotropic flexo-electric nematic layers

Elastic deformations of homeotropic nematic liquid crystal layers subjected to a DC electric field were studied numerically in order to determine their development under increasing voltage. Both signs of the dielectric anisotropy, Δ?, and of the sum of flexo-electric coefficients, e ₁₁ + e ₃₃, were considered, as were also low, moderate and high ion concentrations. The electrical properties of the layer are described in terms of a weak electrolyte model. Quasi-blocking electrode contacts and a finite surface anchoring strength were adopted. Director orientation, the electric potential and the ion concentrations were each calculated as a function of the coordinate normal to the layer. The director distributions turned out to depend not only on the sign of Δ? but also on the sign of e ₁₁ + e ₃₃, due to the difference in the mobility of anions and cations. The importance of ion content was also confirmed.     

有机磁合成化学研究进展

通过磁场对聚合反应、酯化反应、光还原反应和不对称合成等的影响，论述了有机磁化学的理论研究和应用进展。磁场在一定程度上影响有机反应的反应速率、产率、反应途径和产物构成。同时初步探讨了磁场影响化学反应速率的机理，并展望了磁化学的发展前景。     

Influence of varying electroosmotic flow on the effective diffusion in electric field gradient separations

Recently the use electric field gradient focusing (EFGF) to enhance focusing of proteins has been proposed and explored to provide significant improvement in separation resolution. The objective of EFGF is to focus proteins of specific electrophoretic mobilities at distinct stationary locations in a column or channel. This can be accomplished in a capillary by allowing the electric potential to vary in the streamwise direction. Because the electric field is varying, so also is the electrokinetic force exerted on the proteins and the electroosmotic velocity of the buffer solution. Due to the varying electric field, the Taylor diffusion characteristics will also vary along the column, causing a degradation of peak widths of some proteins, dependent on their equilibrium positions and local velocity distributions. The focus of this paper is an analysis that allows characterization of the local Taylor diffusion and resulting protein band peak width as a function of the local magnitude of the EOF relative to the average fluid velocity for both cylindrical and rectangular channels. In general the analysis shows that as the ratio of the local electroosmotic velocity to the average velocity deviates from unity, the effective diffusion increases significantly. The effectiveness of EFGF devices over a range of protein diffusivities, capillary diameters, flow velocities, and electric field gradient is discussed.     

Effect of electric field on the partitioning behavior of solutes in entropic interaction chromatography

In this study, a novel column design with a round cross‐section was proposed to be suitable for a transverse electric field (EF). Additionally, two beads for entropic interaction chromatography (EIC) were prepared by grafting glycidyl methacrylate onto Toyopearl HW‐65F (T65F) beads. Solute partitioning was then investigated to elucidate the role of graft polymerization with and without an EF. In a T65F column, solute partitioning was attributed to the distinct pore structure in the beads and was governed by pore flow. Under EF, partition coefficients (K_p) for solutes decreased with increasing EF strength. In the two EIC columns, a decrease of K_p was also observed without an EF while the fractionation windows were extended. It was more pronounced in the EIC column with a high grafting density (T65F‐H). This was explained by the decrease in the effective pore size of solutes caused by the steric hindrance of polymer chains. Under an EF, the solutes showed different partitioning behaviours in the T65F‐H column. With increasing EF strength, K_p for vitamin B₁₂ and myoglobin was decreased. In contrast, K_p for large solutes increased as a result of concentration polarization on the bead surface. Both behaviors were related to the modulation of graft polymerization to residual charge on the matrix and the pore size of the solutes.