TAYLOR CONE ELECTROHYDRODYNAMICS. THE MINIMUM AND MAXIMUM FLOW RATES IN ELECTROSPRAYING

The surface charge at the liquid–gas interface in cone-jet electrospraying, almost relaxed from an electrochemical point of view, is driven by the radial electric field created to supply the current to the cone tip that the microjet withdraws. The electric stress applied on the liquid surface provokes a low or high Reynolds number motions in the electrified meniscus depending on a dimensionless parameter which relates the liquid viscosity and its electrical conductivity. The analysis of the surface motion is essential to quantify the surface current convected to the cone’s tip, which is shown to be negligible compared to the one driven by bulk conduction. In the case of high Reynolds number motions, we show mathematically, and also experimentally, the emergence of an interesting self-rotation phenomenon.In addition, an analysis of the equations governing the electrohydrodynamics of the charged liquid ligament issuing from the tip of an electrified meniscus in a steady cone-jet suggests the mechanisms which set the stability limits of this steady regime. It is shown that for low and moderate liquid polarities (less than 40 times the vacuum permittivity), the minimum liquid flow rate that can be electrosprayed in a steady cone-jet is reached when the surface tension stress at the cusp from which the jet issues, which provokes a resulting pressure gradient against the flow, overcomes the electrostatic “suction” effect. To show the role of the different forces involved, we have carried the calculation of the intervening ones in the momentum equation using the digitized shape of a cone-jet close to the minimum flow rate in the case of a permittivity 6.5 times larger than the vacuum one. For larger polarities, which impose large electrical conductivities as well, the role of viscous forces, polarization forces, and charge relaxation effects is discussed. In addition, we have carried out experimental measurements of the minimum flow rate using several different liquids. These results are discussed and compared with the experimental data from different authors, as well as with other previously given scaling laws and estimations of the minimum flow rate in cone-jet electrospraying.     

Electrohydrodynamic drop-on-demand patterning in pulsed cone-jet mode at various frequencies

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.     

EXPERIMENTAL STUDY OF THE JET BREAK UP FOR EHDA OF LIQUIDS IN THE CONE-JET MODE

The scaling laws developed by Fernández de la Mora (1994, J. Fluid Mech. 260, 155–184) and by Gañán-Calvo (1994, J. Aerosol Sci. 25S, S309–S310) have been verified since then through various experiments. Chen and Pui (1997, Aerosol Sci. Technol. 27, 367–380) investigated the dependence of the current and droplet diameter with the permittivity of the liquid. Hartman et al. (1997) developed a physical model to describe the spraying of liquids in the cone-jet mode. These two approaches compare remarkably concerning the current produced by the cones. Nonetheless some differences have appeared concerning the jet break up. Namely, the scaling of the jet diameter with the flow rate and the conductivity is different in both approaches.Thus, we present here an original experimental study to investigate these differences. It was conducted using a High-Speed Spray Imaging System (HSSIS) purchased from Oxford Laser. This system consists of a digital camera (KODAK) connected to a computer which is equipped with a frame grabber. A long-distance microscopic lens is fixed to a camera. The illumination of the subject is done by an infrared laser. A control box synchronizes the camera and the laser. Pictures can be made with an illumination time down to 0.5 ms and the separation between two following pictures can be down to 15 ms. The optical system allows us to see objects down to a few micrometers but only objects bigger than 10 mm can be accurately measured. The experiments were done using five different liquids namely, ethanol, butanol, isobutanol, 2-butanone and ethylene glycol. For each liquid the conductivity and the flow rate were varied. For each situation photos were taken to determine the jet size, the break up of the jet, the droplet size and the droplet velocity. Moreover, for every situation the spray current was measured.Our results show that the jet diameter for the different liquids studied exhibits a dependence on the flow rate at a power ∼0.6. This indicates firstly, that the model developed by Hartman is correct in its calculation of the jet diameter and secondly, that the break up of the jet cannot be assimilated to the one of an unchanged jet. Nevertheless our results must be put in perspective with the fact that the jet diameter is measured at a fixed distance from the cone, while the jet length varies with the flow rate. Further, our study brought some other interesting results.First, concerning the jet break-up mechanism. It is a known fact that within the range where IμQ^1/2 the jet can break up due to varicose or kink instabilities. The results showed that for a given liquid the break up was going from varicose to kink with the flow rate going up. Second, they also showed that the number of satellites produced between two main droplets increases with the flow rate.Moreover, we have tried to define, as a function of liquid properties, a limit which separates the two types of jet break up mechanism (varicose or kink). This is of primary importance because it is known that the size distribution of the main droplets is much narrower when the jet breaks up due to varicose instability.     

THE SURFACE CHARGE IN ELECTROSPRAYING: ITS NATURE AND ITS UNIVERSAL SCALING LAWS

The electrospraying of liquids in steady cone-jet mode follows a well-defined EHD mechanism described and quantified in this work using a hybrid experimental–numerical technique: a collection of emitted microjet shapes corresponding to several liquids and different flow rates have been digitized and introduced in a quasi-one-dimensional analytical model. A universal value of the surface charge on the liquid microjet and the resulting charged droplets, independent of their size and of the liquid permittivity, has been found. The surface charge is shown to be always in equilibrium, being the liquid bulk quasi-neutral. From these findings, we finally present a consistent general scaling of all EHD variables involved which is experimentally verified. In this scaling, the electric current I and the characteristic microjet radius R_o are both proportional to the square root of the emitted flow rate, Q^1/2, and independent of the liquid permittivity ε_i.     

Single event electrospraying of water

Single event electrospraying (SEE) is a method for on-demand deposition of picoliter volumes of liquids. The steady electrospraying of water in the cone–jet mode is difficult under normal operating conditions. This paper shows that SEE can be used for water to deposit very small volumes (1 pL) and spray water in a stable pulsed cone–jet mode. The stability regime for SEE of deionized water with regard to pulse time and frequency of applied voltage was determined and was explained by the analysis of the characteristic times involved in SEE. With our unique set-up it is possible to use a pulsed cone–jet mode for a stable ejection on-demand. The pulse time for stable SEE of water is limited due to the time of formation of a space charge in between the nozzle and the counter electrode. Moreover, the maximum pulse frequency is limited to two times the natural oscillation frequency of the liquid. Additionally, the volume of liquid deposited per SEE can be calculated using a theoretical consideration of the electrohydrodynamic displacement of water inside the capillary.     

Increase of electrospray throughput using multiplexed microfabricated sources for the scalable generation of monodisperse droplets

The development, fabrication, and testing of a compact multiplexed system of electrosprays are presented with the dual goal of increasing by orders of magnitude the liquid flow rate to be dispersed and of retaining the quasi-monodispersity of the generated droplets. The system was microfabricated as an array of nozzles etched in silicon, with a density of 250 sources/cm². Although the operation of a single electrospray is rather forgiving with respect to the electrode geometry, successful performance of the multiplexed system is critically dependent on a careful selection of the electrode configuration, which in the present work entails an extractor electrode mounted at a distance from the spray sources that is comparable to the distance between sources (on the order of 0.5 mm). The electrode has the dual function of limiting electric field cross-talk between neighboring sources and minimizing space charge feedback from the spray cloud. Measurements of current and droplet size as a function of flow rate and of droplet size distribution using ethanol demonstrated that the system may be optimized to produce uniform droplets simultaneously from all parallelized electrosprays, each one operating as an isolated spray in the quasi-monodisperse cone-jet mode. Ease of operation and uniformity in size from spray to spray require strategies to increase the pressure drop in the liquid flow path and/or to uniformize the electric field at the spray sources.     

Electret state in Bi4Ti3O12 ceramics

The effect of thermal treatment atmosphere (argon, air or oxygen) on the electret behavior of bismuth titanate (Bi₄Ti₃O₁₂) ceramics was investigated. Polycrystalline ferroelectric ceramics were prepared by reactive liquid phase sintering. Sintered samples were polarized applying an electric field, E_p = 1 MV m^?1 at 100 °C for 60 min. Effective surface density, σ_eff of free charges was determined by compensation voltage measurements. Thermally treated samples in oxygen atmosphere exhibit a significant decrease of σ_eff while those treated in argon exhibit an increase of σ_eff accompanied by the improvement in stability of electret charge. In all cases homocharge with a time independent sign was obtained.     

ONE-DIMENSIONAL SIMULATION OF THE BREAKUP OF CAPILLARY JETS OF CONDUCTING LIQUIDS. APPLICATION TO E.H.D. SPRAYING

Nonlinear breakup of charged liquid jets is numerically analyzed in this work in the limit of a very small electrical Strouhal number T_e/T_b≪1 (i.e. negligible charge relaxation effects, applicable to highly conducting liquids), where T_e is the electric relaxation time of charges, and T_b is the breakup time in a Lagrangian framework following the liquid jet at its average axial velocity. The influence of the electrical Bond’s number and viscosity on (i) the capillary Rayleigh’s most probable breakup length, (ii) the breakup time, (iii) the volume of the satellite, and (iv) the charge of both main drop and satellite, are analyzed. The model is related to the microjet break-up phenomena in the electrospraying of liquids in steady cone-jet mode, and its range of applicability to those particular problems discussed. Previous experimental results [Mutoh et al., 1979, Convergence and disintegration of liquid jets induced by an electrostatic field. J. Appl. Phys. 50, 3174–3179; Clopeau and Prunet-Foch, 1989, Electrostatic spraying of liquids in cone-jet mode. J. Electrostatics 22, 135–159.] support our numerical finding that the influence of the electrical Bond’s number on Rayleigh’s length is small within the usual parametrical limits of stability of a steady Taylor cone-jet at atmospheric pressure.     

Promising graphene/carbon nanotube foam@π-conjugated polymer self-supporting composite cathodes for high-performance rechargeable lithium batteries

Three-dimensional (3D) graphene foam materials are highly favored due to large accessible surface and excellent conductive network, which can be commendably applied as self-supporting electrodes for advanced rechargeable lithium batteries (RLBs). Here, promising graphene nanosheets/acid-treated multi-walled carbon nanotubes (GNS/aMWCNT)-supported 1,5-diaminoanthraquinone (DAA) organic foams [oGCTF(DAA)] are prepared by organic solvent displacement method followed by solvothermal reaction. And then electrochemical polymerization is carried out to obtain 3D porous GNS/aMWCNT organic foam-supported poly(1,5-diaminoanthraquinone) (oGCTF@PDAA) nanocomposites, which achieves the ordered growth of homogeneous PDAA nanoparticles on GNS/aMWCNT surface due to the role of oGCTF(DAA). Such structure largely improves PDAA utilization, facilitates charge transportation and suppresses the dissolution of PDAA. As a result, the oGCTF@PDAA cathode for RLBs delivers a high discharge capacity of 289 mAh g⁻¹ at 30 mA g⁻¹ and still retains 122 mAh g⁻¹ at extreme 10 A g⁻¹ for rapid charging/discharging. Moreover, superior cycling stability is achieved with only 14.8% capacity loss after 2000 cycles even at a high current density of 1 A g⁻¹.     

Layer-by-Layer assembly and redox properties of undoped HPHT diamond particles

The surface properties of undoped diamond particles are investigated by a combination of zeta potential measurements in solution and electrochemical studies in thin layer assemblies. High-Pressure High-Temperature (HPHT) 500 nm diamond particles exhibit positive and negative zeta potentials depending on pH. The estimated point of zero zeta potential (pzzp) was 6.6, while mobility measurements provided an average charge per particle of ?(843 ± 31)e at high pH. The charge indicates that approximately 50 ppm of surface atoms involves ionisable impurities. The positive charge measured at low pH is of similar magnitude and could be related to nitrogen impurities. The surface charge in basic solutions allows the electrostatic adsorption of diamond particles on poly(diallyldimethylammonium chloride) (PDADMAC) modified In-doped SnO₂ electrodes (ITO). The particle number density shows a strong dependence on pH, with a maximum value of (1.7 ± 0.3) × 10⁸ cm^?2. Electrochemical studies carried out in the absence of redox species in solution revealed signals associated with sp² type surface states. Analysis of electrochemical responses concluded that 1 × 10⁴ redox centres per particle are involved in a single electron transfer process. We demonstrate that this simple yet versatile approach is rather sensitive to the extent of sp² hybridisation at the surface of diamond powders.     

Microstructure and corrosion properties of Ni-TiN nanocoatings prepared by jet pulse electrodeposition

Ni–TiN nanocoatings were successfully prefabricated by jet pulse electrodeposition. The effect of jet rate on cross-sectional composition, microstructure, microhardness, and corrosion properties of nanocoatings was examined by X-ray photoelectron spectroscopy, high-resolution transmission electron microscope, atomic force microscopy, microhardness tester and electrochemical workstation. Results illustrated that Ni–TiN nanocoatings deposited at jet rate of 3 m/s exhibited high concentration of Ni and Ti with average concentrations of Ni and Ti of 54.5 at% and 19.8 at%, respectively. Average diameters of Ni grains and TiN nanoparticles in Ni–TiN nanocoatings prepared at 3 m/s were 47.8 nm and 30.5 nm, respectively. Nanocoatings deposited at 1 m/s, 3 m/s and 5 m/s showed surface root-mean-square roughness value of 95.431, 30.091 and 58.454 nm, respectively, and presented maximum microhardness of 789.5, 876.2, and 849.9 HV, respectively. Ni–TiN nanocoating obtained at 3 m/s demonstrated minimum I_corr and E_corr values of 1.02 × 10⁻³ mA/cm² and − 0.551 V, respectively, signifying to offer the best corrosion resistance.     

Phase transitions in single crystal tubes formed from C60 molecules under high pressure

Pure single crystal tubes formed from C₆₀ molecules, with a face-centered cubic (fcc) structure were fabricated by a liquid–liquid interfacial precipitation method using C₆₀ powder. A bulk transition from fcc to a simple cubic structure and a surface transition from (1 × 1) to (2 × 2) have been observed around 246 K (bulk transition temperature T_B) and 214 K (surface transition temperature T_S), respectively, during the measurement of the temperature dependence of electrical resistance. The initiation of the two transitions under pressure was investigated using a piston cylinder high pressure apparatus and it was found that both T_B and T_S increase with increasing pressure. And the C₆₀ molecules at the surface of the tube exhibit the same behavior of that in the bulk at a pressure of about 2.1 GPa.     

Determination of surface area of carbon-black by simple cyclic-voltammetry measurements in aqueous H2SO4

Determination of the surface area of commercial carbon-black (CB) by cyclic-voltammetry (CV) measurements of the electrochemical double-layer charge (Q) in aqueous sulfuric acid was investigated. Various factors that affect the Q value associated with CB, including: the presence of redox-reversible function-groups, the binders used for the formation of thin-film CB electrodes, the scan rates of the CV measurement, H₂SO₄ electrolyte concentration and the volume of air contained in the pores of the CB samples were examined. Conditions for measuring Q without interference from these factors were investigated and the data derived, without interference, was found to correlate well with the surface area of the CB. The results show that the total Brunauer–Emmett–Teller (BET) surface area shows good correlation (R² = 0.993) with the Q value corresponding to a full charge/discharge of the CB (Q₀), obtained by extrapolation at a zero scan rate; additionally, the CB micropore surface area (diameter < 2 nm) shows good linear correlation with the Q deficiency (Q₀–Q), measured at 5 mV s^?1 (R² = 0.998).     

Nucleation and growth surface conductivity of H-terminated diamond films prepared by DC arc jet CVD

High-quality polycrystalline diamond film has been extremely attractive to many researchers, since the maximum transition frequency (f_T) and the maximum frequency of oscillation (f_max) of polycrystalline diamond electronic devices are comparable to those of single crystalline diamond devices. Besides large deposition area, DC arc jet CVD diamond films with high deposition rate and high quality are one choice for electronic device industrialization. Four inch free-standing diamond films were obtained by DC arc jet CVD using gas recycling mode with deposition rate of 14 μm/h. After treatment in hydrogen plasma under the same conditions for both the nucleation and growth sides, the conductivity difference between them was analyzed and clarified by characterizing the grain size, surface profile, crystalline quality and impurity content. The roughness of growth surface with the grain size about 400 nm increased from 0.869 nm to 8.406 nm after hydrogen plasma etching. As for the nucleation surface, the grain size was about 100 nm and the roughness increased from 0.31 nm to 3.739 nm. The XPS results showed that H-termination had been formed and energy band bent upwards. The nucleation and growth surfaces displayed the same magnitude of square resistance (R_s). The mobility and the sheet carrier concentration of the nucleation surface were 0.898 cm/V s and 10¹³/cm² order of magnitude, respectively; while for growth surface, they were 20.2 cm/V s and 9.97 × 10¹¹/cm², respectively. The small grain size and much non-diamond carbon at grain boundary resulted in lower carrier mobility on the nucleation surface. The high concentration of impurity nitrogen may explain the low sheet carrier concentration on the growth surface. The maximum drain current density and the maximum transconductance (g_m) for MESFET with gate length L_G of 2 μm on H-terminated diamond growth surface was 22.5 mA/mm and 4 mS/mm, respectively. The device performance can be further improved by using diamond films with larger grains and optimizing device fabrication techniques.     

Dielectric properties and tunability of Ba(ZrxTi1−x)O3 ceramics under high DC electric field

Ba(Zr_xTi_1−x)O₃ (BZT) (x = 0.25, 0. 3, 0.35, 0.4) ceramics were prepared by the traditional ceramic processing and their structural, surface morphological, dielectric properties, tunable properties as well as the mechanism of their nonlinear dielectric constant under DC electric field were systemically examined. The Zr ions substitution of Ti ions has a strong effect on the dielectric properties and the grain sizes. The results show Ba(Zr_xTi_1−x)O₃ (x = 0.25, 0.3, 0.35) ceramics to be promising candidates for the DC electric field tunable materials for microwave electronics application, because they exhibit high tunability (27.6%, 26.3%, 19.4%, respectively) as the strength of electric field is up to 2 kV/mm, low dielectric loss (0.001–0.002, 0.001–0.002, 0.004–0.005, respectively) at 10 kHz at room temperature and low temperature coefficient of capacitance.     

Carbamazepine removal from water by dielectric barrier discharge: Comparison of ex situ and in situ discharge on water

Dielectric barrier discharges (DBD) were used for the degradation of carbamazepine (CBZ) in aqueous solution. The electric discharge was generated either ex situ or in situ directly on the water surface. To maintain the same ozone concentration of 40 ppm in both instances, the power injected was 0.7 W in the ex situ discharge and 12 W in the in situ discharge. The results showed 100% CBZ removal after 3 min of treatment with the ex situ discharge, while the in situ discharge only removed 81% of the CBZ after 60 min. According to measurements of UV absorbance at 285 nm and 254 nm, and of total organic carbon, the ex situ discharge system also proved to be more effective than the in situ system. The measurement of nitrogen oxides in both gaseous and liquid phases indicated that high energy in situ discharges produced a large amount of NO_x. These species contributed to decreased pH and significantly slowed the CBZ oxidation rate, due to their competition with ozone. Production of NO_x should be avoided when using the DBD technique for wastewater treatment.     

Barium strontium titanate nanocrystalline thin films prepared by soft chemical method

Barium strontium titanate (Ba_0.65Sr_0.35TiO₃) nanocrystalline thin films, which were produced by the soft chemical method, were crystallized at low temperature using a domestic microwave oven. A SiC susceptor were used to absorb the microwave energy and rapidly transfer the heat to the film. Low microwave power and short time have been used. The films obtained are crack-free, well-adhered, and fully crystallized. The microstructure displayed a polycrystalline nature with nanograin size. The metal-BST-metal structure of the thin films treated at 700 °C show good electric properties. The ferroelectric nature of the BST35 thin film was indicated by butterfly-shaped C–V curves. The capacitance–frequency curves reveal that the dielectric constant may reach a value up to 800 at 100 kHz. The dissipation factor was 0.01 at 100 kHz. The charge storage density as function of applied voltage graph showed that the charge storage densities are suitable for use in trench type 64 Mb (1–5 μC/cm²) and 265 Mb (2–11 μC/cm²) DRAMs.     

Electrohydrodynamic nano-spraying of ethanolic natural plant extracts

Aerosol techniques have recently been used to process natural products for medical, pharmaceutical, and environmental health applications. In particular, electrohydrodynamic spraying, or electrospraying, which is a method of atomizing liquids by means of electrical forces, is a promising aerosol technology because it generates non-agglomerated particles due to repulsive electrical forces between particles with unipolar charges. We investigated the characteristics of natural-product nanoparticles generated via electrospraying. A plant extract containing a natural-product (Sophora flavescens) was sprayed in steady cone-jet mode using a specially designed electrospray system with a point-to-orifice-plate configuration. The electrosprayed natural-product particles maintained their bimodal size distribution with good stability and uniformity for longer than 1 h. Compared to generation characteristics observed using a conventional nebulization process, the electrospray technique produced non-agglomerated, spherical particles and resulted in a narrow size range for both peaks. The size distribution of electrosprayed particles was controlled by varying the suspension flow rate of S. flavescens extract. Also, they had a high average charge per particle and positive polarity. The nanoparticles maintained the major chemical composition of the original S. flavescens ethanolic extract during electrospraying. Our investigation demonstrated that the electrospray process, driven by high-intensity electric fields, can be used to generate nanoscale particles from natural products.     

Preparation of strontium-based fibers via electrospinning technique

The Sr-based fiber was prepared by electrospinning process. The effect of electric field, precursor viscosity, and calcination temperature (T_cal) was investigated at required conditions. Then the Sr-based fiber was characterized by TGA, nitrogen adsorption, SEM, TEM, and XRD to elucidate thermal transition, specific surface area, morphology, crystal structure, and crystalline phase of the samples, respectively. The result showed that the smooth fiber sample could be spun at electric field of 1.5 kV cm^?1 with the suitable viscosity of ca. 5 Pa s. Finally the crystalline phase could be controlled by properly adjusting T_cal, i.e. when the T_cal was risen from 400 to 1000 °C, the crystalline phase was transformed from SrCO₃ to Sr(OH)₂H₂O, and eventually to SrO.