Search

Electrohydrodynamic (EHD) inkjet printing is a type of potential non-contact micro/nanoscale manufacturing technology. The printed droplet dimension plays an important role in EHD inkjet printing due to its significant influence on printing quality and the resolution of patterns. The complexity of the mechanism and the limited process optimization techniques present a challenge in obtaining the desired printing resolution, eventually becoming an expensive and time consuming endeavor. Recent developments in computational fluid dynamics (CFD) bring an effective alternative to alleviate the aforementioned challenges. In this study, a CFD model is proposed to investigate the mechanism of the cone-jet printing process in EHD inkjet printing. The complete cone-jet printing process is successfully simulated with four phases: Taylor cone formation, cone-jet generation, jet break and droplet expansion. A further analysis predicts the jetting diameter and printed droplet diameter with different operating parameters and substrates. The simulation has a satisfactory agreement with experiments in predicting the printing behavior and printing quality (jetting diameter, printed droplet diameter). Such advancement in modeling can be utilized to guide the quick determination of operation parameters for the desired printing resolution when given a new ink.

Electrospray thrusters with small size, high precision specific impulse, low power consumption are the most recommended propulsive equipments for micro-nano satellites. Molecular dynamics simulation is able to be used to predict the fundamental physics of electrospray and evaluate the propulsive performance. However, the accuracy of molecular dynamics simulation results critically depends on the accuracy of the interaction potential models used. Therefore, the present paper discusses the effect of different interaction potential models on the molecular dynamics simulations of electrospray thrusters with ionic liquid 1-ethyl-3-methyl-imidazolium tetrafluoroborate. The reduced charge all-atoms model, full charge all-atoms model, effective-force coarse-grained model and Merlet coarse-grained model reported in the literature are used for comparison, and comparison is also made to experimental data to elucidate which interaction potential model might be the most realistic. The process of Taylor cone formation, electrospray currents, energy characteristics and velocity distribution are analyzed under varying operating conditions. Our calculations indicate that the reduced charge all-atoms model is the most accurate model for molecular dynamics simulation of electrospray. In contrast, the coarse-grained models fail to reveal the energy characteristics. The full charge all-atoms model is not able to characterize the velocity characteristics. The full charge all-atoms model and coarse-grained models underestimate and overestimate the propulsive performance respectively. It is also found that the energy characteristics and velocity distribution of particles within the Taylor cone are almost unchanged with varying operating conditions.

The electrospray thruster relies on a large number of emission sites that simultaneously emit ions or charged droplets to generate thrust. The operation uniformity of the emission sites is of great significance to ensure the stable work of the thruster and improve its thrust level. However, there was little in-depth study of it before, especially of its influence on the thruster's characteristics. To this end, four porous electrospray thrusters with different degrees of non-uniform operation of emission sites were adopted, and their emission site distribution characteristics, volt-ampere characteristics, and current fluctuation characteristics were investigated. Emission site distribution of all thrusters showed a certain degree of unevenness. On their emission surfaces, there were some dark zones where few emission sites located on, highlight zones where emission sites were too concentrated, and intermediate zones where emission sites repeatedly appeared and disappeared. The appearance of a dark zone could cause a decrease in the growth rate of emitted current with voltage, and electric sparks that happened in the highlight zones could directly interrupt the operation of thrusters and suppress the increase of thrusters' thrust level. A periodic fluctuation, which should be caused by the flow rate difference between emission sites, emerged on the plume current of almost all thrusters. Various experimental results indicated that part of the liquid in high-flow-rate areas spread to low-flow-rate areas while the current fluctuated periodically. It could effectively reduce negative effects caused by the non-uniform operation of emission sites. The flow resistance of one of the four thrusters was greatly reduced. As a result, the operation uniformity of emission sites was greatly improved, but the overflow problem in high-flow-rate areas was exacerbated.

A robust, nanofibrous membrane with dual-layer composite structure and remarkable features has been developed in this work. The new membrane is based on the affordable styrene-acrylonitrile (SAN) polymer and a commercial hydrophilic nonwoven for the support layer. This can help to step forward the industrialization of the direct contact membrane distillation (DCMD) process for treating the hot dyeing effluent in the textile industry. The modified electrospinning with a gas-jet surrounding the Taylor cone was used for fabricating the membrane samples. Using the gas-assisted electrospinning, a nanofibrous nonwoven structure embedded with micro-cups and micro-spheres can be achieved in a one-step, gas-assisted electrospinning. The hydrophilic substrate was used for enhancing the permeate flux of the resultant composite membrane. The fabricated membrane showed remarkable features in comparison with a commercial polytetrafluoroethylene (PTFE) membrane, including high surface hydrophobicity (≥148°), higher porosity (≥81%), and smaller tortuosity factor (1.71). The unique properties of the membrane with the best performance provided a promising permeate flux (28.31 kg.m−2.h−1) and contaminant rejection (RCOD: 98.15%) compared to the commercial membrane (flux: 18.50 kg.m−2.h−1 and RCOD: 97.10%), without considerable pore wetting. However, the new membrane possessed lower liquid entry pressure (LEP) owing to its higher porosity, larger maximum pore size (0.76 μm), and thinner structure (150 μm). Results indicated that the decline in the permeate flux was mainly caused by the cake layer formation on the surface of the membrane rather than the partial pore wetting during the long-term operation for 48 h. Therefore, the newly developed nanofibrous SAN membrane with hydrophobic/hydrophilic composite structure is a promising, robust candidate for DCMD application in the textile wastewater treatment.

To solve the problems of low production, needle blockage, and Taylor cone stability, a self-made free surface electrospinning (FSE), namely spherical section FSE (SSFSE), is developed to obtain high-throughput production of nanofibers, which can use solution reservoirs with different radii. The surface morphology of polyacrylonitrile (PAN) nanofibers fabricated using SSFSE is investigated by scanning electron microscopy (SEM). According to the SEM data, response surface methodology (RSM) is used to model and optimize the SSFSE process for evaluating the effect of spinning parameters on the nanofiber diameter, and the established RSM model is applied to determine the optimal spinning parameters. The results reveal that the solution reservoir radius and the applied voltage both have great effects on nanofiber diameter, and nanofibers with the highest quality and yield are achieved using a solution reservoir with a radius of 25 mm at 40 kV. In addition, the electric field distributions produce in the SSFSE processes with solution reservoirs of different radii are simulated by Maxwell 3D to study the preparation mechanism of the SSFSE device. The electric field simulation results are in keeping with the experimental data and indicate that these five SSFSE devices have the potential to increase the yield, and the device using a solution reservoir with a radius of 25 mm provides the highest quality and yield of nanofibers.

Paper Spray (PS) is an ambient ionisation technique that is conceptually simple, economical, direct and versatile. In its simplest form, the procedure for operation relies on the application of an electric potential to a triangular-shaped paper substrate with the addition of an extraction/spray solvent to generate charge droplets, without requiring any pneumatic assistance. Despite its promise and rapidly growing popularity, there are some practical challenges associated with PS implementation with scope to enhance its performance further. One such challenge relates to only a small fraction of the Taylor cone expansion being sampled at the MS inlet. In this work we propose a new arrangement for PS, which retains the inherent advantages of this popular technique without introducing additional variables, by using an ambient ion focusing lens that is held at the same potential as the paper substrate. A thorough investigation, consisting of visual spray inspection, electric field simulations and analytical evaluation, including analysis of paracetamol from saliva, shows that ambient ion focusing can provide up to a 50% improvement in spray stability, 60% increase in signal intensity and a 30% improvement in detection limits for routine paper spray mass spectrometry analysis.

In the case of nanofluids, the nanoparticle concentration has a significant effect on individual physical properties such as density, surface tension, and viscosity. However, there have been few studies on the comprehensive, interactive effects of nanoparticle concentration and electrification. This study demonstrates the synergetic effects of nanoparticle concentration and electrification on the breakup performance of nanofluid fuel (Al/n-decane). The secondary breakup behaviours associated with the interactive actions of gravity, surface tension, viscous forces, and electric-field force were investigated. When the Al-nanoparticle concentration of the n-decane increased, there was a linear increase in the density, and a trinomial increase in the viscosity; however, there was a piecewise influence on the surface tension. The addition of the Al nanoparticles simultaneously influenced the electrification of the nanofluid fuel, and synergistically contributed to the deformation of the Taylor cones and the secondary breakup of the jets. The effective surface tension was calculated and analysed; the results demonstrated that the Weber number decreased when the Al-nanoparticle concentration was elevated. This resulted in enlarged Taylor cone angles and minimized the average diameter of the breakup droplets. More importantly, at a critical Al-nanoparticle concentration (7.9 mg/ml), the effective surface tension was minimized; consequently, the average diameter of the breakup droplets was minimized (72 μm).

Electrospinning technology is taken as the most versatile process to generate continuous nanofibers. However, effects should be taken to achieve better understanding and precisely control the actual mechanics in the formation of nanofibers through better system design. In this article, the diameter and the deposition area of the as-prepared nanofibers can be controlled by added helix tube which can provide external electric-magnetic field. The spinning process can be controlled employing a magnetic flux (B) analytical model which is based on the magnetic field calculation model of the solution and the electrostatic charge repulsion in the jet. This method includes the thermo-mechanical properties based on as-prepared electrospun nanofiber films and the capability of tribology-mechanical and mathematical physics. A novel investigation of the whole output increase at per unit spinning area is presented in this article to describe the whipping instability which is an important feature of the Taylor Cone phenomenon. The present design and process for electrospinning are based on a solution differential calculus, which a single stainless steel spinning nozzle can generate multi-squirt flow. With the purpose of providing useful jet initial behaviour design for the further technology applications, we attempt to set up a differential calculus method with evenly distributed circumferential electric field, yet giving more design guidance to produce controllable nanofibers through magnetic-field-assisted electrospinning technology.

An electrospinning process is to produce nanofibers from a polymer solution by a high electrostatic force, which is used to eject jets from a Taylor cone formed on a nozzle. When a moving jet approaches to a receptor within few nanometers during the electrospinning process, a Casimir-like attraction is formed, which attracts the jet’s end to the surface of the receptor. Thousands of thousands nanofibers will be attached on the receptor’s surface or attached on each other, as a result a nanofiber membrane is formed. The geometric potential was implemented to explain the nanoscale adhesion, which is greatly affected by the attachment oscillation. Mathematical models are established to describe the attachment oscillation under the Lennard-Jones potential and the geometric potential, respectively, and their amplitude-frequency relationship is elucidated analytically. The present theory gives an alternative explanation of the gecko effect and molecule scale forces. Keywords: Lennard-Jones potential, Van der Waals force, Casimir force, Geometrical potential, Molecule oscillation, Smart adhesion, Parthenocissus tricuspidata, Gecko effect, Macromolecular electrospinning

In this work, we review the physics of the liquid ejection by the application of electric fields, paying special attention to the steady cone-jet mode of electrospray. We aim to provide a comprehensive view on the role of electrohydrodynamics effects, and how the full electrokinetic equations can be reduced or simplified into the Taylor-Melcher leaky dielectric model. We provide an extensive review of the steady Taylor cone-jet mode, considering both its predicting scaling laws and limits of operation. Coaxial Taylor cone-jets are also briefly reviewed. Finally, we outline a revision of AC electrohydrodynamics and electrospraying.

Electrospray is a relatively mature research field, with sustained and growing activity. It has attracted researchers from diverse application fields. Electrospray is perhaps best known as a tool which has enabled the mass spectrometry analysis of large biomolecules (proteins and DNA). It has also been proposed as a colloidal thruster for spacecraft propulsion. And also, it has been used as a general methodology for converting precursor droplets to microspheres and other particle morphologies and films, with controlled nanostructure. In parallel to this body of applied research, other efforts have been aimed at improving the understanding of the underlying physics of electrosprays. While many reviews exist on applications of this system, wherein the physics of the process is described to a certain extent, the present review focuses on the fundamentals. We begin by describing the different modes developing in liquid menisci to which an electrical potential is applied. Due to its applications, the mode of greatest interest is the cone-jet mode in which a steady microscopic jet is emitted and breaks up periodically into uniformly sized droplets. The physics of this mode is then explored in detail, first by reviewing the remarkable effort in the field aimed at identifying so called scaling laws. These are simple scaling relationships connecting the variables of the problem which can be applicable to many operating conditions. We then describe the dynamics of generation of the jet which emerges from the electrified conical meniscus, as well as the physics associated with the spray plume development. We conclude with the extension of these concepts to multi-fluid configurations, such as coaxial and parallel flows.

Ultrathin fibers have been widely used in thermal insulation, catalysts, drug delivery, tissue scaffolds, wound dressing, energy storage, energy harvesting, environment engineering, and textile industries. Electrospinning is a facile, universal, and cost-efficient technology for fabricating ultrathin nanofiber materials. On the other hand, porous structure and functional modification can be generated through interfacial assembly method. Accordingly, interfacial electrospinning assembly is emerged as an excellent strategy to prepare multifunctional ultrafine nanofibers. However, the traditional electrospinning using a single Taylor cone is facing a major challenge–low production efficiency, which can no longer meet the actual demand. Therefore, diverse multi-Taylor cone sited electrospinning have been developed, such as multi-needle, various needleless electrospinning and so on. In this review, different electrospinning methods of preparing ultrathin fibers are summarized, especially from the point of view of single Taylor cone site to multi-Taylor cone sites. The precursor materials species, electrospinning apparatus, nanofiber structure and scale-up equipment are critically discussed. It is foreseeable that multi-Taylor cone sited electrospinning can overcome the drawbacks of one Taylor cone sited electrospinning and make the large-scale industrial production possible.

We present a new approach for micro- and nanoparticle production by in-situ charge reduction of electrospray droplets, which prevents their Coulombic instabilities and allow the efficient transport (extraction) of the particles. A unipolar ion source based on corona discharge generates a controllable ion flux of opposite polarity to the electrospray. The ions are introduced axially into the spray, while the Taylor cone is screened from the ions by an extractor ring electrode. Efficient and steady droplet discharge and extraction through an orthogonal aerosol-extraction tube was attained when the inlet of the tube was near the spray emission and the ring electrode, resulting in dramatic changes in droplets’ trajectories. The best extraction conditions (highest filter collections) were associated with the best discharging (lowest residual electrical charge) and the most globular particles. The size distributions on the particles collected on the filters were monomodal and homogeneous, with small relative standard deviations (as small as 10.6%). The use of corona ions significantly expands the range of polymer concentrations over which globular particles with monomodal size distribution can be made by electrospray.

A cone-jet regime in electrohydrodynamic atomization (EHDA) is widely applied into printing, electro-spinning, spray-coating, and biological mass spectrometry. Regulating different instabilities of jet is crucial for optimizing its operating parameters. In present work, an atypical, hemispherical capillary with inner diameter of 0.40 mm and cap size of 6.20 mm was employed as atomization nozzle, through which a steady cone-jet regime was confirmed versus suitable feeding liquid flow rate and electric potential. A permanent and highly charged jet emits from a Taylor cone apex and further disintegrates into fine highly charged drops in the form of varicose or whipping instabilities. The results indicate that a steady cone-jet is observed in the specific ranges of the feeding liquid flow rate, electric potential and conductivity. The typical breakup instabilities including varicose and whipping are clearly observed with an increase in liquid flow rate for a fixed electric potential for most of liquids, and the transition from varicose to whipping instability easily occurs. For a steady cone-jet, the jet breakup instabilities usually depends on the competition between interfacial forces on the electrified jet. The transition process involving a low-viscosity jet with a large flow rate is observed, and the general parameter G = Γ 2 δ μ 0.52 is calculated to account for surface charge, surface tension, and viscosity effects, where Γ is the electric force to surface tension ratio and δμ is the inertia to viscous force ratio. Finally, the varicose, transition regime, and whipping are classified with first and second critical threshold values G = 40 and G = 102, respectively. Meanwhile, as the electrified jet transforms from varicose to whipping instabilities, the jet breakup length shows a trend of increasing first and then decreasing.

Electrospraying is a highly attractive method for generation of small particles and also for measurement applications of particles in suspensions. In the cone jet mode, the particle generation rate increases with decreasing flow rate. For applications in the field of aerosol technology flow rates of less than 10µl/h are attractive as particle sizes of less than 200nm can be obtained in this flow rate range. At the same time generation frequency of drops reaches the values required for size measurements by Scanning Mobility Particle Sizers (SMPS). A special challenge in operation of electrosprays at small flow rates is the change of liquid composition due to mass transfer processes between the Taylor cone and the surrounding gas phase. Typically used organic solvents such as alcohols, acetone or acetonitrile evaporate fast (or even evaporate completely) and mixtures containing such volatile species will change their composition significantly due to evaporation from the Taylor cone. Based on key experiments we show a) that the evaporating solvent can be reduced by as much as a factor of 20 by controlling the gas phase saturation. This allows spraying volatile solvents such as e.g. methanol at flow rates down to less than 1µl/h and makes the attractive sub-10µl/h flow rate range accessible to quite volatile solvents as well. b) that changes in the liquid composition by not considering the mass transfer (evaporation) problem lead to significant differences in liquid properties between the feed liquid and the Taylor cone. For example, an increase of surface tension by a factor of 2 is observed for water and t-butanol feed liquid. c) that employing a saturated sheath gas flow allows for very effective manipulation of the solvent composition in the Taylor cone. Based on water as feed liquid we show that a sufficient amount of organic solvent is easily transferred to the Taylor cone liquid from the gas phase to allow spraying the water without electron scavenging gases. Measurement results of the Taylor cone liquid composition are in good agreement with predictions by the presented new model describing the mass transfer between the Taylor cone and the surrounding gas phase. The efficiency of the mass transfer, the predictability of the process and the opportunity of comparably quick changes of the Taylor cone liquid by changing the gas phase saturation state result in a powerful tool for the entire field of electrospraying at low rates.

This work investigates the effect of gravity during melt electrospinning writing (MEW) and explores the feasibility of developing MEW devices with lateral and upside down print head configurations. Average fibre diameters of printed constructs using different printing directions and their variability reveal particular differences. These give an insight into the effect of gravity, which was observed to affect the Taylor cone, yet interestingly not the fibre jet. Additionally, stable processing conditions were studied and showed that it is possible to reproducibly print from all directions using MEW.

A novel approach, continuous needleless electrospinning from the tips of magnetization-induced self-assembling PVA/ferrofluid cone array, was proposed to prepare magnetic nanofibers. A PVA/ferrofluid was synthesized, the needleless electrospinning process was observed, and the morphology and magnetic properties of magnetic nanofibers were investigated. The results showed that the PVA/ferrofluid could remain stable and homogeneous for 21 days under the magnetic field gradient (2.2 mT/mm). “Taylor cone” array of PVA/ferrofluid was self-assembled under both the magnetic and electric fields. As the electric voltage reached 25 kV, the jets were emitted from the “Taylor cone” array, resulting in needleless electrospinning of magnetic nanofibers. Magnetic nanofibers were homogeneous and continuous with an average diameter of 73.6 nm. Magnetic nanofibers showed a good magnetic response property and relatively high saturated magnetization (1.71 emu/g), which is expected to be applied in the biomedical field.

A simplified Eulerian model for the description of steady state electric charge dispersion in vacuum, including space-charge effects, is introduced. The model is based on a potential velocity field for the charged particles, which is valid if all the particles are emitted with the same kinetic plus potential energy. Despite the high simplicity of the present model, the mathematical system still encompasses several types of singularities, which are analyzed here in full detail, thus enabling the numerical solution of the system. The model is applied to the description of electrospray emission in vacuum, in the limit of high electric conductivity liquids, assuming that all particles are emitted with zero initial velocity. Our main result in this regard is a relation existing between the cone semiangle α and the electrospray beam angular width. In particular, we show that the cone semiangle can only take values between a minimum angle ca. 21.89 deg, and a maximum value given by Taylor's angle (ca. 49.29 deg), with lower values of α corresponding to wider beams, while higher values of α correspond to narrower beams.

With regard to improving the blending performance of biodiesel with hydrogen, it is important to generate smaller hydrogen bubbles in biodiesel at low energy consumption. In this context, a strong non-uniform direct current (DC) electric field has been created between a needle-ring electrode configuration, which is used to study the effect of EHD on hydrogen bubble behaviors in biodiesel considering the various gas flow rates and applied voltages. The special geometry of gas cones similar to the Taylor cone in liquid droplets under an electric field is demonstrated. The hydrogen bubble features—which are characterized by the bubble heights, frequencies, velocities, bubble sizes, and shapes—show that the bubble geometry and dynamic characteristics in the electric field are performed in manners different from those reported in the literature. It is found that the hydrodynamic behavior of the bubbles is also modified by the EHD effect. The distributions of bubble size in different cases follow the Gaussian distribution. Small bubbles are more likely to be associated with high separation velocities. Further, the diversity of bubble shapes at the separation is demonstrated, and two regimes of impact mechanism based on the surface tension and electric forces are revealed.

Hypromellose (HPMC)-based hydrophilic composites (HCs) used for rapid dissolution of ferulic acid (FA) were investigated. Electrospun and casting HCs were prepared from a solution containing HPMC, FA, and polyethylene glycol. Ethanol was used as sheath fluid during coaxial processes, and the effects of its flow rates on the Taylor cone and straight fluid jet were investigated. The morphology, component state, hydrophilicity, and drug dissolution rate of the HCs were characterized. Results demonstrated that all HCs were amorphous materials, and their components were compatible. However, the dissolution rate of electrospun HCs was 10 times faster than that of casting HCs. The smaller the diameters of electrospun HCs were, the better their performances were. The mechanism of electrospun HCs was suggested. By utilizing modified coaxial electrospinning and combinations of drug carriers, new types of HPMC-based HCs can provide an alternative approach for the effective delivery of poorly water-soluble drugs.

A novel fiber-spray/mass spectrometry method using a piece of a porous hollow fiber is described. An adapter, fabricated by a 3D-printer, was designed to connect the fiber and a standard ESI (electrospray ionization) needle, in which a Taylor cone was formed and the resulting ions were detected by a mass spectrometer. Since a porous hollow fiber is usually used for microextraction, the so-called hollow fiber liquid-phase microextraction (HF-LPME) method, a new methodology that involves coupling HF-LPME and fiber-spray, was developed for the first time. We report herein on the design and testing of a microextraction kit. The kit consists of a centrifuge tube (reservoir for the sample solution) and a piece of a porous hollow fiber (for sample extraction). The kit was placed between the mass inlet and the ESI needle. Using the kit, analytes can be extracted from a very dilute solution and then evaporate and escape from the fiber surface. When they make contact with the ESI plume, which arises from the ESI needle tip, the molecules are ionized and then detected by a mass spectrometer. Using the setup, it was possible to improve the limit of detection after microextraction by ∼360-fold when 3,4-MDMA (3,4-methylenedioxymethamphetamine) was analyzed, and the limit of detection achieved was 2 ng/mL.

The influence of the conductivity of a water droplet formed at a capillary electrode on the negative corona discharge and production of nano-sized droplets was investigated. Conductivity of a water droplet was adjusted from 1 μS/cm of deionized water to 48 mS/cm of nitric acid water solution. The size distribution of nanometer sized water particles yielded at a disruption of a Taylor cone was measured. The higher conductivity of a droplet, the larger corona pulses appeared and the more number of charged droplets was generated.

The purpose of this study was to investigate how the size distribution of taurine particles produced by electrostatic atomization in a liquid–liquid two-phase system without using a nozzle was affected by the process parameters. A Taylor cone was observed to be formed at the liquid–liquid interface during the process, with many droplets ejected from the tip of the cone when ring and needle electrodes were used to apply the electric field perpendicularly to the interface. The droplet size was controlled by varying the angle of the Taylor cone between 60° and 75° through adjustment of the process parameters such as the inter-electrode distance, electrical conductivity of the water phase, and ring diameter. The effect of the ring electrode diameter on the electric field and the interface was examined by solving the Poisson equation. The established variability of the mean taurine particle diameter by adjusting the inter-electrode distance and conductivity of the solution phase can be used to achieve continuous production by avoiding clogging and electrical corrosion of the system.

In the present experimental study, the effects of electrical conductivity on electrospraying modes and its produced droplets are investigated. A metallic nozzle with 0.6 mm ID as high voltage electrode were employed in a point to ring electrode configuration in this work. Experiments were carried out in still room temperature. Four different aqueous KCl solutions were sprayed in various high voltages and flow rates. Results confirmed that spraying modes are more dependent on electrical conductivity rather than flow kinetics. Applied electrical field required to form a cone shape on the nozzle tip decreases with increasing conductivity. Moreover, results revealed that electrical conductivity of dispersed solution plays a decisive role on elongation of the cones in ES procedure. Smaller size of produced droplets was obtained by increasing conductivity of dispersed liquid while velocity of filaments emanated from liquid jet decreases at the same time. In this study, we have presented insight to the role of physicochemical properties on the pulsating modes of spraying.

Two model solutions, one containing organic salt and the other none, of poly(N-isopropylacrylamide) (PNIPAM) in DMF solvent with the same concentration of 17 wt% were electrospun to investigate the extension rate profile in the straight jet as well as the subsequent jet bending (whipping) behavior. Both model solutions have identical rheological properties, while the solution with added salt possesses electrical conductivity 16 fold higher than the other. At a given flow rate of Q, light scattering experiments were carried out to derive the jet diameters of the straight segment dj at different jet positions z so that the z-dependence of jet velocity (vj, = 4Q/πdj2) was feasibly calculated, from which the profile of extension rate ( e ˙ ) was obtained by dvj/dz. Using high speed video imaging, the onset of jet bending was observed, and the motion of the first bend was traced, from which the extension rate of the bending jet could be also estimated. Compared to the neat solution, the solution with salt exhibits a bigger Taylor cone and a shorter straight segment with a larger jet-end diameter to proceed the subsequent jet bending process. Moreover, the magnitudes of e ˙ in the straight jet and in the bending jet are all dramatically enhanced with solution conductivity, yielding PNIPAM fibers with a smaller diameter on the grounded collector. Our results show that it is the extension rate developed in the spin line plays an important role in determining the final fiber diameter. The onset of bending instability is attributed to the local buckling in the end section of the straight jet, which is unambiguously observed by video imaging with a high frame rate of 104 fps. The formation of jet buckling is due to the compressive forces associated with the air drag to resist the electric stretching force induced by the setup electric field. The dynamics of bending jet was elucidated in this work.

Molecular modeling has revealed that the presence of charged macromolecules (macroions) in liquid droplets dramatically changes the pathways of droplet fission. These mechanisms are not captured by the traditional theories such as ion-evaporation and charge-residue models. We review the general mechanisms by which macroions emerge from droplets and the factors that determine the droplet fission. These mechanisms include counter-intuitive “star” droplet formations and extrusion of linear macroions from droplets. These findings may play a direct role in determining macromolecule charge states in electrospray mass spectrometry experiments.

A family of medicated Janus fibers that provides highly tunable biphasic drug release was fabricated using a side-by-side electrospinning process employing a Teflon-coated parallel spinneret. The coated spinneret facilitated the formation of a Janus Taylor cone and in turn high quality integrated Janus structures, which could not be reliably obtained without the Teflon coating. The fibers prepared had one side consisting of polyvinylpyrrolidone (PVP) K60 and ketoprofen, and the other of ethyl cellulose (EC) and ketoprofen. To modulate and tune drug release, PVP K10 was doped into the EC side in some cases. The fibers were linear and had flat morphologies with an indent in the center. They provide biphasic drug release, with the PVP K60 side dissolving very rapidly to deliver a loading dose of the active ingredient, and the EC side resulting in sustained release of the remaining ketoprofen. The addition of PVP K10 to the EC side was able to accelerate the second stage of release; variation in the dopant amount permitted the release rate and extent this phase to be precisely tuned. These results offer the potential to rationally design systems with highly controllable drug release profiles, which can complement natural biological rhythms and deliver maximum therapeutic effects.

Taylor cone can be easily observed in the electrospinning process, and its height can be used to control the fiber diameter. This paper gives a theoretical analysis of the relationship between the cone height and fiber diameter.

Water electrospray in the cone jet mode was performed by a polymer micro-atomizer. A high aspect ratio dielectric polymer micro-nozzle and an in-plane extractor were proposed to concentrate the electric field at the tip of the micro-nozzle, which prevent electrical discharge caused by the high surface tension of water. The effect of the in-plane extractor was confirmed by electric field simulation and electrospray experiment. A micro-pillar array was also proposed to prevent surface wetting of the polymer micro-atomizer during the electrospray. Water electrospray of the polymer micro-atomizer was operated in a flow rate of 0.15–0.30 ml/h with a voltage of 1.5–3.0 kV, −2.0 kV, and −4.0 kV at water, a guide ring, and a collector, respectively. The key features of the cone jet mode such as Taylor cone, jet, and water micro-droplets were visualized by using nanosecond pulsed laser illumination.

Numerical models that are capable to quantitatively elaborate the formation of novel two-dimensional (2D) nanonets during electrospinning/netting would have broad technological implications for areas ranging from sensing devices and individual protection to industrial filtration and separation; however, creating such models has proved extremely challenging. Here we report novel numerical models for clarifying the origin, evolution, and regulation of the nanonets via combining the ejection modes of Taylor cone apex and the phase separation of charged droplets. Our models propose two critical condition formulas for the generation of jets and droplets, and a LCST phase diagram combined with concentration sweep paths for the nanonets or non-porous films. The resulting model predications exhibit excellent agreement with the experimental results obtained via varying the solution properties and process parameters. The successful model derivation for the nanonets formation may provide new insights into the design and development of 2D multifunctional nanomaterials for various applications.

A three-dimensional optical tomography technique was developed to reconstruct three-dimensional objects using a set of two-dimensional shadowgraphic images and normal gray images. From three high-speed cameras, which were positioned at an offset angle of 45° between each other, number, size, and location of electrohydrodynamic jets with respect to the nozzle position were analyzed using shadowgraphic tomography employing multiplicative algebraic reconstruction technique (MART). Additionally, a flow field inside a cone-shaped liquid (Taylor cone) induced under an electric field was observed using a simultaneous multiplicative algebraic reconstruction technique (SMART), a tomographic method for reconstructing light intensities of particles, combined with three-dimensional cross-correlation. Various velocity fields of circulating flows inside the cone-shaped liquid caused by various physico-chemical properties of liquid were also investigated.

This technical note documents the use of ballpoint pen tips as robust electrospray emitters. The embedded microsphere helps stabilize Taylor cones by effectively reducing the height of the liquid meniscus and significantly decreasing the viscous damping time of the Taylor cone. The microsphere also provides a mechanism for the Taylor cone to continuously adapt its base diameter to match the voltage applied. As a result, the ballpoint pen electrospray emitter noticeably expands the operation envelope in the flow rate-voltage space.

Due to the relative ease for fabricating fibers with a core–shell structure, emulsion electrospinning has been investigated intensively for making nanofibrous delivery vehicles for local and sustained release of biomolecules such as growth factors. In this investigation, water-in-oil (w/o) emulsions consisting of deionized (DI) water or phosphate buffer saline (PBS), poly(lactic-co-glycolic acid) (PLGA) solution and a surfactant were electrospun into fibers for studying the evolution of core–shell structures in emulsion electrospun fibers. Optical microscopy, high speed imaging, fluorescence microscopy and transmission electron microscopy were employed to examine the morphology of the water phase in fibers at different locations of the jet (or fiber) trajectory during emulsion electrospinning. The results indicated that the water phase in emulsion jets underwent multi-level stretching and breakup. When the jet (or fiber) diameter was large (i.e., for the jet near the Taylor cone), the water phase within the jet (or fiber) exhibited a discrete morphology whereas in ultrafine fibers collected beyond certain distance from the Taylor cone, a mostly continuous water phase core was formed. The Rayleigh/capillary instability determined the breakup point of the deformed water phase. When the water phase in emulsions for electrospinning was changed from DI water to PBS, it changed from a continuous state to a discontinuous state in electrospun nanofibers.

The combination of electrospinning and forcespinning (otherwise called electrostatic-centrifugal spinning) was first proposed in our laboratory. A viscoelastic jet has a stretching force in the axial ( a x → ) and tangential ( a y → ) directions under electrostatic and centrifugal forces. The jet radius (R0) will be dramatically decreased, and the jet velocity (υ0) will be substantially increased due to a particular stretching force. The radius of curvature, the jet length, and the Taylor cone of a viscoelastic jet will be completely different when the jet velocity (υ0) increases. A strong stretching force and a fast extension speed will form after optimizing the radius of curvature (by Reynolds (Re) and Weber (We) numbers), jet length (by Peclet (Pe) and Epsilon (e) numbers), and Taylor cone (by dimensionless stress (Π1) and Ohnesorge (Oh) numbers). Through dimensionless number and group analysis, higher Re(PC = 9.38 × 10−2, PLA = 1.52 × 10−1 and PAN = 1.53 × 10−2) and We(PC = 13.8, PLA = 15 and PAN = 13.3) number of all three systems can collect the bead-free and uniform nanofibers. Similarly, higher П1(PC = 1.83 × 10−5, PLA = 2.86 × 10−5 and PAN = 5.13 × 10−9) and Oh(PC = 39.60, PLA = 29.22 and PAN = 238.94) numbers refer to viscosity and electrostatic force dominate the jet behavior and obtained the nanofiber with high modulus, hardness, crystallinity and good molecular orientation. Hence, the uniaxially aligned polycarbonate, polylactic acid and polyacrylonitrile nanofibers with superior physical properties (modulus ⩾ 2.77 GPa, 3.3 GPa and 1.46 GPa, hardness ⩾ 0.32, 0.26 and 0.22 and crystallinity ⩾ 3%, 37% and 21%) can be successfully manufactured in this process.

We report the fabrication and characterization of thin films from the water soluble polymer sodium poly[2-(3-thienyl)-ethyloxy-4-butylsulfonate] (PTEBS) by electrospray deposition (ESD). Contiguous thin films were created by adjusting the parameters of the electrospray apparatus and solution properties to maintain a steady Taylor cone for uniform nanoparticle aerosolization and controlling the particle water content to enable coalescence with previously deposited particles. The majority of deposited particles had diameters less than 52 nm. A thin film of 64.7 nm with a root mean square surface roughness of 20.2 nm was achieved after 40 min of ESD.

Corona discharge from a fine water droplet always involves deformation of the droplet shape or Taylor-cone formation, emission of fine water jets or disruption of droplet. Therefore, corona discharge from a water droplet always manifests complicated aspects. In addition, disruption of Taylor cone simultaneously affects not only discharge current but also motion of water droplet. To confirm corona discharge phenomena from a water droplet protruded from a tip of a metal capillary tube with a diameter of 1 mm, negative corona discharge was investigated by using a water droplet located at a tip of grounded rod electrode facing a ring electrode with positive dc voltage superimposed by ac one. Since the droplet has inherent resonant vibrating frequency defined by the size or volume, the volume of water droplet was adjusted at 20 nL where the corresponding resonant frequency was 500 Hz. The period of the event of successive corona discharge is exactly consistent with resonant frequency defined by the size of the water droplet. As a result, corona pulse trains with a definite duration appeared intermittently corresponding to its resonant vibration. When dc voltage superimposed by ac voltage with resonant frequency of 500 Hz was applied to the water droplet, corona pulse trains appeared at the period corresponding to the frequency. The maximum value of corona current reasonably increased with the applied voltage. Even when the frequency of ac field superimposed on dc field was varied from the resonant frequency, corona pulse trains occur corresponding to not only the superimposed field frequency but also resonant frequency.

Electrified interfaces have been used to produce fine particles. Here, we investigate the behavior of planar electrified interface in the presence of vertical electric field. Corn oil and four KCl aqueous solutions with different concentrations are employed. During the experiments, for the first time, four distinct behaviors of electrified interfaces were disclosed and named as: Hump figure of interface, Taylor cone appearance and its rotary motion on the interface, intermittent jetting and thorough ascending of the interface. Taylor cones’ mean droplet diameter and their angles, discharging velocity at the tip of the cone, the frequency of Taylor cones appearance, and their merging to each other are studied in this paper. Additionally, the circumstances of interface levitation affected by electric field are some of the major results of this study to gain some insight into the electrified interfaces issue.

The paper investigated describes the patterning of a series of drops generated by pulsed cone-jet electrohydrodynamic atomization. The authors concluded that when the voltage frequency is higher than the drop generation frequency, the drops formed and the spacing between one another are no longer uniform. The authors did not calculate the frequency where this transition occurs. A formula has been proposed that can be used to calculate this fulcrum frequency. It is noted that this relation only applies when the capillary is itself conductive. The proposed relation relies primarily on a formula proposed by Suvorov and Zubarev to calculate the Taylor cone formation time.

We propose an algorithm based on dissipative particle dynamics (DPD) for simulations of conducting fluids in the presence of an electric field. In this model, the electrostatic equations are solved in each DPD time step to determine the charge density at the fluid surfaces. These surface charges are distributed on a thin layer of fluid particles near the interface, and the corresponding interfacial electric forces are added to other DPD forces. The algorithm is applied to the electrospinning process at the Taylor cone formation stage. It is shown that, when the applied voltage is sufficiently high, the algorithm captures the formation of a Taylor cone with analytical apex angle 98.6°. Our results demonstrate the potential of the presented DPD algorithm for simulating two-phase problems in the presence of an electric field with non-periodic boundary conditions.

Electrospun polymeric fibers are attractive candidates in the development of scaffolds for the tissue engineering and for providing new systems for delivery of bioactive molecules. Co-axial fibers have emerged as an efficient tool to protect the core material from the adverse conditions of electrospinning process, to spin difficult-to-process fluids and to generate fibers with much more control of the delivery of encapsulated bioactive molecules. Currently, there is very little reported work on the optimization of the processing parameters of electrospinning, especially core–shell electrospinning. This study extends the understanding of the role of solution viscosity as a vital material parameter for electrospinning of fibers. The spinning solutions were characterized for viscosity and optical imaging of the compound Taylor cone for spinnability, and the fibers were imaged by Scanning Electron Microscopy (SEM). Our experimental results, using PLGA as the model polymer, confirm that the solution concentration be above the entanglement concentration (C e ) to obtain uniform beadless monolithic fibers; for core–shell fibers, the shell solution must fulfill the above criterion for spinnability and, further, the ratio of the viscosities of core and shell solutions (η core /η shell ) has to be greater than a threshold value to get a stable compound Taylor cone and therefore to obtain uniform beadless core–shell fibers. Addition of surfactant led to reduction of the threshold η core /η shell (from 0.55 to 0.18) for the PVA–PLGA system.

In this paper, we propose an electrospraying microfluidic chip which utilizes the microfluidic cross-junction and strong electric field to generate uniform emulsions and fiber. Our strategy was based on flow-focusing for the formation of a series emulsions by changing concentration of span 80 and applying the electric field to control the size of emulsion. In the results, as the electric filed increased, the size of the emulsion decreased. In additional, the fiber is generated at high voltage of 5000 V. When applying the electric field, the Taylor cone is form and generates the smaller emulsions and fibers, the minimum PLGA emulsion size is about 4 μm and fiber is about 7 μm width.

Electrospinning is a method to produce submicron polymer fibers for a wide range of applications and it is desirable to understand what determines the fiber diameter distribution in this process. A well instrumented and machine vision based system had been developed to provide the measurement capability for this study. This paper investigated several important process variation factors as well as the operating conditions which resulted in minimal Taylor cone and jet fluctuations. It was found that the dynamic variations of Taylor cone volume, electric current, and charge density correlated to the resulting fiber diameter distributions.

Using chloroform/dimethylformamide (CF/DMF) co-solvent, electrospinning of poly[( R )-3-hydroxybutyrate] (PHB) solutions was carried out at ambient temperature. The effects of the applied voltage ( V ), flow-rate ( Q ), and solution viscoelastic properties on the Taylor cone, electrified jet, and fiber morphology were investigated. In addition, the electric field developed by the needle-plate electrode configuration was calculated using a finite element analysis to reveal the tip-to-collector ( H ) effect. Among the processing parameters ( V , Q and H ), it was found that Q played a key role in determining the jet diameter ( d j ) and electrospun fiber diameter ( d f ), and scaling laws existed between them, i.e., d j – Q 0.61 and d f – Q 0.33 . The diameter reduction ratios of D o / d j ( D o is the needle diameter) and d j / d f were measured as 50–120 and 5–10, respectively; it suggested that major jet stretching took place in the straight electrified jet region, and further chain orientation could be gained by the subsequent process of jet whipping. By changing PHB concentrations from 5 to 15 wt%, the solution viscosity ( η o ) was increased from 100 to 4900 cP, whereas the surface tension and solution conductivity remained unchanged; it provided a good model solution to exclusively reveal the η o effect on the electrospinning process. Our results showed that the η o -dependence of d j and d f also followed simple scaling laws: d j – η o 0.06 , and d f – η o 0.39 , with a prefactor depending on the processing variables, mainly the flow-rate. Regardless of the PHB concentrations used, the obtained PHB fibers showed a similar crystallinity fraction of ca. 0.63 and possession of major α-crystals together with a small amount of β-crystals with zigzag chain conformation.

The patterning of a series of drops was investigated by the electrohydrodynamic printing method in the drop-on-demand fashion. A positive pulse voltage was applied to the capillary nozzle periodically to eject a pulsed liquid jet. The ejected jet was directed to the moving substrate, to which DC bias voltage was applied. High-speed imaging revealed that a Taylor cone was established at the nozzle tip during the ejection of the liquid jet, and that the jet directly struck the substrate to form a drop without the jet break-up. The frequency of drop generation can be controlled precisely, because the frequency of the pulsed voltage was almost same as the pulsating frequency of the liquid in pulsed cone-jet mode. The deposited patterns showed a series of uniformly sized drops with a regular spacing. At the pulse voltage frequency of 25 Hz, the diameter of the drops was approximately 95 μm. Using this drop-on-demand method, it is feasible to produce a variety of patterns of dots and continuous/discontinuous lines.

A novel method of characterization of viscoelastic longitudinal stresses in electrospun jets of semi-dilute and concentrated polymer solutions and melts is introduced. The measured longitudinal stresses at the beginning of the thin jet region in the jets of a 6 wt% aqueous solution of polyethylene oxide (Mw = 400 kDa) were of the order of 100 kPa, which is two orders of magnitude larger than in any other free viscoelastic jets issued from nozzles and orifices. This is attributed to elongation-driven stretching of polymeric liquids in the transition zone, between the preceding modified Taylor cone zone and the beginning of the thin jet region, where the stretching rates are of the order of 100–1000 s−1. The Rouse relaxation times found were in the range of 3–8 ms, and the moduli of elasticity were of the order of 100 Pa. A novel explanation of the reasons for the formation of the straight sections in the electrospun jets is proposed. The straight sections are stabilized by the high initial longitudinal stresses in the jet generated due to strong electrically driven stretching in the transition zone. The further electrically driven stretching in the jet (after the transition zone) is relatively weak, and viscoelastic Rouse relaxation prevails. The relaxation distance of the longitudinal stresses along the jet increases with the applied voltage (which generates higher initial stresses in the transition zone) and thus the length of straight section of the jet should increase as the applied voltage increases. The results also point at an opportunity to develop an elongational rheometer for concentrated polymeric systems with stretching rates of the order of 100–1000 s−1. The proposed rheometer employs excitation of electrically driven jets by single lateral pulses, and observation of the pulse propagation and widening along the jet. This reveals the level of the longitudinal stresses along the jet and allows evaluation of the viscoelastic Rouse relaxation time, modulus of elasticity and the elongational viscosity in the jet.

An analysis is presented of positive and negative vacuum electrospray currents from the ionic liquid, [Emim][Im], wetted on a sharp tungsten needle. The source is operated in a cone–jet configuration established with a low extraction voltage of approximately ±1 kV. Emission current and mass flow are measured as a function of the emitter angle with respect to the instrument axis. Mass spectra of field evaporated ions and mass-resolved energy distributions of emitted charges are recorded as a function of emitter angle. The measurements at both source polarities indicate that the cone–jet emits a mixture of ions and charged droplets. While the charged droplets are emitted with a narrow angular distribution centered on axis, the angular distributions of ions are significantly broader than those of the droplets and centered off axis. Pure ion emission is observed at emission angles larger than ∼15°. The main mass spectral peaks are from X ± {[Emim][Im]} n ions (X = Emim or Im for positive and negative polarities, respectively) with n = 0, 1, and 2. Quantum chemical calculations of the n = 0 and 1 ions and the isolated ion pairs are presented, yielding thermochemical information on the observed ionic species. The present results demonstrate that the cone–jet emits droplets from the tip of the jet, and provide direct evidence that ions are produced in an Iribarne–Thomson field-evaporation mechanism at the transition region between the Taylor cone and jet. Electrochemical modification of the cone–jet emitter properties are observed when maintaining the emitter at a positive polarity for a prolonged (minutes to hours) period. New ions emerge consisting of clusters containing a neutralized Im − . The associated current suppression can be mitigated through polarity alternation at 1 Hz. Measurements are also presented with the tungsten needle heated to 373 K, thereby reducing the liquid viscosity and increasing the flow rate. The average mass of the droplets is observed to increase with temperature while the mass distributions of the ions are not found to change noticeably.

The electrospinning process uses electrical force to produce nanofibers. A charged droplet acquires a conical shape known as the Taylor cone and then becomes unstable. A charged jet emerges from the vertex and develops a spiral path due to the electrically driven bending instability, which makes it possible, in a small space, for the jet to elongate by a large amount and produce nanofibers. Evaporation and the associated solidification were identified as important factors that affect the diameter of electrospun nanofibers. In this study, the evaporation rate and solidification of the charged jet were controlled by varying the relative humidity during electrospinning of poly(ethylene oxide) from aqueous solution. As the relative humidity increased, the solidification process became slower, allowing elongation of the charged jet to continue longer and thereby to form thinner fibers. As the relative humidity increased from 5.1% to 48.7%, the diameter of the solidified fiber decreased from 253 nm to 144 nm. As the relative humidity increased above 50%, beads formed on the thinner fibers, indicating that the capillary instability occurred before the jet solidified. The vapor concentration of solvent is an effective electrospinning process control parameter of fiber diameter that also produces a systematic change in the development of beads on the fibers.

A capillary with an external electrode for cone-jet mode of electrospray has been developed to spot a droplet accurately on a substrate surface. The external electrode is made by gold deposition around tip of a glass capillary. The electrospray was made by applying a positive pulsed dc voltage to the solution in the capillary. Using a positive bias voltage to the external electrode, the meniscus of the solution at the tip deformed to be more sharp, and center of the meniscus was prolonged. This deformation stabilized the trajectory of the jet from the Taylor cone at the tip. From the experimental result, accuracy of positioning of the droplet having 0.3 pL volume was improved with the standard deviation of 1.1 μm, from that of 2.5 μm for conventional capillary without the external electrode.

Electrosprays are devices in which nanometer sized droplets and/or solvated ions are electrically extracted from a liquid surface and accelerated to high velocities. They are usually constructed from conductive capillaries with one of the ends tapered down to a sharp tip with the purpose of enhancing the local electric field that produces the instability that develops into the Taylor cone structure from which emission occurs. In an alternative configuration, the conductive needles are replaced by small holes through a dielectric, nonwetting flat block. An electrostatic model shows that if the emitter material has low dielectric constant then the local electric field near the emission site is enhanced in a very similar way as with sharp metallic needles. Furthermore, the combination of the nonwetting property of the material and the sharp corner formed in the hole–surface interface effectively anchors the Taylor cone to the edge of the hole, thus simplifying the process of manufacturing. The possible microfabrication of this configuration makes it especially attractive for producing arrays of large numbers of individual emitters. Such arrays may find use as space propulsion thrusters and in the analytical industry to improve the characteristics of mass-spectrometric analyses.

An experimental investigation on the electrified co-axial jets of two immiscible liquids issuing from a structured Taylor cone (Science 295 (5560) (2002) 1695) has been carried out. The structure of these almost conical electrified menisci consists of an outer meniscus surrounding an inner one. The liquid threads which issue from the vertex of each one of the menisci give rise to a two-concentric layered jet whose eventual breakup results in an aerosol of relatively monodisperse compound droplets with the outer liquid encapsulating the inner one. The effect of the flow rates of both liquids on the current transported by these coaxial jets and on the size of the compound droplets has been investigated. Several couples of liquids have been used to explore the influence on the spraying process of the properties of the liquids: i.e. the electrical conductivity K , dielectric constant β , interfacial tension of the liquid couple γ , viscosity μ , etc. We have found that the measurements of the current emitted through the coaxial jet when they are made dimensionless fit satisfactorily the current scaling law of regular electrosprays. Data of the mean diameter of the compound droplets have been obtained using a non-intrusive laser system. As expected the breakup process and therefore the droplet size are strongly dependent on the liquid viscosities and on the ratio of the liquid flow rates.