Charged Water Droplets Research Articles

The Charging of Micellar Nanoparticles in Electrospray Ionization

Charging of nanoparticles through electrospray has scarcely been explored. Spherical nanometer-sized amphiphilic block copolymer nanoparticles with diameters ranging from ∼65 to ∼150 nm were electrosprayed and analysed by charge detection spectrometry. Herein, we explore the charging of these micellar nano-objects by conducting a thorough study in different solvents, including pure water, and upon the addition of "supercharging" agents. The charge (z) of micellar nanoparticles electrosprayed from water solution is compared to the Rayleigh's limiting charge (z(R)) of a charged water droplet of the same dimensions. An average ratio (z/z(R)) of 0.6-0.65 is observed for the micellar macro-ions, supporting the charge residue mechanism, where the number of charges available to the micellar macro-ion is limited by the number of charges on the nanodroplet, which is a function of the surface tension of the solvent. Also we show the possibility of increasing the charging of micellar nanoparticles in the negative mode by adding organic bases (in particular piperidine) to water/methanol solutions.

Electrophoretic motion of a charged water droplet near an oil-air interface

The trajectory of a charged droplet near an oil-air interface is investigated. When a charged droplet is translated in oil by electrophoresis, the droplet follows a straight line between two electrodes. However, if an oil-air interface is close to a droplet, the droplet follows an upwardly concave parabolic pathway. By using the leaky dielectric model, we have verified that this phenomenon is caused by the distorted electric field due to the difference in permittivities. Furthermore, it is enhanced by the accumulated free charges on the oil-air interface due to the difference in the electric conductivities of oil and air. Finally, we suggest a complementary arrangement of electrodes for realizing a straight pathway of the droplet near the interface.

An experimental investigation on hydrodynamics of charged water droplets in dielectric liquid medium in the presence of electric field

An experimental study was carried out to investigate the motion, deformation and breakup of charged water droplets in the presence of electric field. A typical nozzle-ring electrode apparatus was conducted consisting of a nozzle employed as high voltage electrode and a metal ring as grounded electrode. Experiments were performed in silicone 100cSt oil as an insulating medium. Under influence of D.C. electric field the motion and deformation of conducting water droplet and its detachment from the tip of nozzle were experimentally studied. The droplet behavior was recorded by high speed camera precisely and its responses to deformation influenced by electric field were digitally extracted by image processing of the sequential frames. Three parameters were defined in order to characterize the detachment of drop from nozzle and its deformation result from electric and hydrodynamic forces. Results show that increase in applied voltage leads to an increase in frequency of drop formation and subsequently droplet size declining. Moreover the motion of produced drops occurs in particular branches for voltages more than 3.5kV. The number of these branches increases with applied voltage. The maximum amount of deformation increases in higher voltages leading to drop break-up in 4.5 and 5.5kV applied voltages. The results can be helpful for better understanding of the phenomena taking place during droplet detachment in dripping mode and reveal the drop behavior (e.g. deformation) in electrospraying apparatus geometry which has not been studied before.

Shape oscillations of an electrically charged diamagnetically levitated droplet

We use diamagnetic levitation to investigate the effect of electrical charge on the normal mode vibration frequencies of charged water droplets with radii 4.5-7.5 mm. This technique allows us to levitate almost spherical droplets, enabling us to directly compare the measured frequencies of the first seven modes with theoretical values calculated by Lord Rayleigh, with which we find good agreement.

Modeling the Behavior of Coarse-Grained Polymer Chains in Charged Water Droplets: Implications for the Mechanism of Electrospray Ionization

The mechanism whereby macromolecular analytes are transferred into the gas phase during the final stages of electrospray ionization (ESI) remains a matter of debate. In this work, we employ molecular dynamics simulations to examine the temporal behavior of nanometer-sized aqueous ESI droplets containing a polymer chain and excess ammonium ions. The polymer is modeled using a coarse-grained framework where a bead-string backbone is decorated with side chains that can be nonpolar, cationic, or anionic. Polymers that adopt compact conformations and that carry a large number of charged side chains remain close to the droplet center, where the charges are extensively hydrated. The ESI process for these compact/hydrophilic macromolecules must involve solvent evaporation to dryness. This behavior is consistent with the charged residue model (CRM). A completely different scenario is encountered for disordered (extended) chains that carry a large number of nonpolar side chains. In this case, the macromolecule tends to be rapidly expelled from the droplet surface in a stepwise sequential fashion. This process produces metastable structures where one end of the extended polymer chain remains connected with the droplet via charge solvation. Disruption of these last interactions will then produce a free gas phase macromolecular ion. The data of this work imply that the ESI process for unfolded/hydrophobic polymers proceeds via an ion evaporation (IEM)-like mechanism that is facilitated by hydrophobic solute/solvent interactions. Our model predicts that the ESI efficiency of the latter scenario is considerably higher than for the CRM. This prediction is verified experimentally through ESI mass spectrometry measurements on myoglobin.

Study of the desorption/ionization mechanism in electrospray droplet impact secondary ion mass spectrometry

Electrospray droplet impact (EDI) secondary ion mass spectrometry (SIMS) is a desorption/ionization technique for mass spectrometry in which highly charged water clusters produced from an atmospheric-pressure electrospray are accelerated in vacuum by several kV and impact on the sample deposited on the metal substrate. The abundances of the secondary ions for C(60) and amino acids are measured as a function of the acceleration voltage of the primary charged water droplets. Two desorption/ionization mechanisms are suggested in the EDI ionization processes: low-energy and high-energy regimes. In the low-energy regime, the excess charges in the primary droplets play a role in the formation of secondary ions. In the high-energy regime, samples are ionized by the supersonic collision of the primary droplets with the sample. The yield of secondary ions increases by about three orders of magnitude with increase in the acceleration voltage of the primary droplets from 1.75 kV to 10 kV.

Surface analysis of polyvinyl chloride bombarded by Ar+ and charged water droplets

AbstractA comparative study of 500 eV Ar+ ion impact and the electrospray droplet impact (EDI) applied to polyvinyl chloride (PVC) was made. In EDI, the charged electrospray water droplets are introduced in vacuum, accelerated and allowed to impact the sample. The samples, after etching by Ar+ and EDI, were examined by XPS. While chlorine is selectively etched by Ar+ ion impact, the XPS spectra did not change with prolonged charged water droplets irradiation, i.e. EDI is capable of surface etching with little damage of the sample after irradiation. Copyright © 2010 John Wiley & Sons, Ltd.

X-ray Photoelectron Spectroscopy and Scanning Probe Microscopy Analysis of Polymethylmetacrylate Surface Etched by Charged Water Droplets

X-ray Photoelectron Spectroscopy and Scanning Probe Microscopy Analysis of Polymethylmetacrylate Surface Etched by Charged Water Droplets

Molecular depth profiling of polystyrene by electrospray droplet impact

AbstractThe molecular depth profiling of polymer by the electrospray droplet impact (EDI) has been performed. The multiply charged water droplets with kinetic energy of ∼106 eV were irradiated on the polystyrene spin coated on silicon wafer. With EDI etching, the ablation of the target is suppressed to a minimal, i.e. the shallow surface etching with nonrecognizable damage on the surface is realized. It was found that EDI/SIMS spectra and EDI/XPS spectra for polystyrene (PS) has shown no damage accumulation on the surface. This indicates that EDI is a unique technique for surface etching of polymer materials without causing any damage to the etched surface. Copyright © 2010 John Wiley & Sons, Ltd.

Millimeter-wave scattering from neutral and charged water droplets

We investigated 94 GHz millimeter-wave (MMW) scattering from neutral and charged water mist produced in the laboratory with an ultrasonic atomizer. Diffusion charging of the mist was accomplished with a negative ion generator (NIG). We observed increased forward- and backscattering of MMW from charged mist, as compared to MMW scattering from an uncharged mist. In order to interpret the experimental results, we developed a model based on classical electrodynamics theory of scattering from a dielectric sphere with diffusion-deposited mobile surface charge. In this approach, scattering and extinction cross-sections are calculated for a charged Rayleigh particle with effective dielectric constant consisting of the volume dielectric function of the neutral sphere and surface dielectric function due to the oscillation of the surface charge in the presence of applied electric field. For small droplets with radius smaller than 100 nm, this model predicts increased MMW scattering from charged mist, which is qualitatively consistent with the experimental observations. The objective of this work is to develop indirect remote sensing of radioactive gases via their charging action on atmospheric humid air.

Surface analysis of polyimide bombarded by charged water droplets

500 eV Ar+ ion impact and the electrospray droplet impact (EDI), in which the charged electrospray water droplets are introduced in vacuum, accelerated and allowed to impact the sample, are applied for etching of polyimide (PI). After the bombardment, x-ray photoelectron spectroscopy (XPS) was applied to the surface analysis. Although oxygen and nitrogen are selectively etched by Ar+ ion impact, the XPS spectra did not change with prolonged charged water droplets irradiation, i.e., EDI is capable of shallow surface etching for PI with little damage of the sample after irradiation.

XPS depth profiling of polystyrene etched by electrospray droplet impact

AbstractThe molecular‐level depth profiling of polystyrene (PS) has been performed by using a newly designed electrospray droplet impact (EDI) gun. The multiple charged water droplets with kinetic energy of ∼106 eV were irradiated to a bulk and a spin‐coated PS. When a PS target is etched by EDI, the ablation of the sample is suppressed to minimal, i.e. the shallow surface etching with nonrecognizable damage on the surface is realized. It was found that XPS spectra for PS were independent on the irradiation time by EDI. This indicates that EDI is a unique technique for the surface etching of synthetic polymer materials without resulting in any damage to the etched surface. Copyright © 2010 John Wiley & Sons, Ltd.

Enhancement of diesel particulate matter collection in an electrostatic water-spraying scrubber

Marine diesel engines are major sources of air pollution. They emit a large amount of pollutants, including diesel particulate matter (DPM), carbon oxide (CO x ), unburned hydrocarbons (HC), nitrogen oxide (NO x ), sulfur oxide (SO x ), and other products of combustion. Among these, DPM may cause respiratory and mutagenic diseases in humans, including lung and bladder cancer (see Donaldson et al. J Aerosol Sci 29:553–560, 1998). In this study, experiments were performed with an electrostatic water-spraying scrubber (EWSS) to evaluate its effectiveness for the collection of mass- and number-based DPM emissions from a marine diesel engine. The results show that the collection efficiency of the scrubber increases as the engine load increases due to an increase in the large DPM concentration and an increase in the amount of DPM collected by the charged droplets. The effect of water spraying performance on DPM collection at constant engine load was investigated experimentally. The strength of the electrical attraction between the charged water droplets and the charged DPM within the scrubber led to a significant increase in total DPM collection efficiency (up to 4–7 times). In addition, the total DPM collection efficiency was found to be directly related to the corona power, the electrical properties of water, water spraying performance and engine load. The EWSS appears to be a promising alternative method for controlling mass-based as well as number-based DPM emissions.

Scattering for Charged Multisphere Structure Located in Plane Wave/Gaussian Beam

Scattering characteristics for charged multisphere structure illuminated by a plane wave/a Gaussian beam are studied on the basis of generalized Lorenz-Mie theory. The incident, internal and scattered fields for each sphere in an arbitrarily incident Gaussian beam are expressed in terms of vector spherical wave functions (VSWFS) expansion form. Making use of the vector addition theorem of VSWFS, the scattering coefficients of total scattered fields for charged multisphere structure are deduced in detail. Taking charged carbon particles and water droplets for examples, numerical results of scattering intensity for different quantities of electric charge are shown and compared with uncharged case. A discussion of the effect of beam parameters on scattering intensity is presented.

Surface characterization of polymethylmetacrylate bombarded by charged water droplets

The electrospray droplet impact (EDI), in which the charged electrospray water droplets are introduced in vacuum, accelerated, and allowed to impact the sample, is applied to polymethylmetacrylate (PMMA). The secondary ions generated were measured by an orthogonal time-of-flight mass spectrometer. In EDI mass spectra for PMMA, fragment ions originating from PMMA could not be detected. This is due to the fact that the proton affinities of fragments formed from PMMA are smaller than those from acetic acid contained in the charged droplet. The x-ray photoelectron spectroscopy spectra of PMMA irradiated by water droplets did not change with prolonged cluster irradiation, i.e., EDI is capable of shallow surface etching for PMMA with a little damage of the sample underneath the surface.

X-ray photoelectron spectroscopy analysis of organic materials etched by charged water droplet impact

Electrospray droplet impact (EDI) has been developed for matrix-free secondary ion mass spectrometry for surface analysis. When a target is etched by EDI, the physical etching on the target is suppressed to minimal, i.e., the occurrence of shallow surface etching. A novel approach to shallow surface etching of polystyrene (PS) by EDI was investigated. The charged water droplets were irradiated to a bulk and a spin coated PS. After irradiation, these samples were analyzed by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy. It was found that XPS spectra for PS were independent on the irradiation time by EDI. This indicates that EDI is a unique technique for the surface etching of the organic materials without leaving any damage on the etched surface.

Depth Profiling of Polystyrene Using Charged Water Droplet Impact

Depth Profiling of Polystyrene Using Charged Water Droplet Impact

Characteristics of a Dry Fog Ionizer

The newly developed ‘Dry Fog Ionizer’ generates charged dry fog. The dry fog consists of very fine water droplets 8μm in mean diameter. This system consists of a dry fog nozzle (H.Ikeuchi & Co., LTD.), a ring electrode for induction charging (50mm outside diameter, and 10mm thick) in front of the nozzle, and a fan for dissipating charged dry fog. The ring electrode is DC or AC-biased and fine droplets ejected from the nozzle are electrified by induction charging. The particle size of the charged water droplets are reduced through evaporation during the transporting process by air flow, and completely evaporate approximately 2m from the nozzle under normal atmospheric conditions (25 °C, 60%R.H.) leaving high density ions. Using this system, high density ionic space charge can be realized in a remote spot from the ionizer. By this principle, the Dry Fog Ionizer shows strong charge-eliminating ability in the region away from the ionizer. When a dc bias of 5kV was applied to a ring electrode with the rate of water flow from the nozzle being 21/h, an ionic space-charge density of 1200nC /m3 was able to be obtained at a distance 2m away from the ionizer, which was 102 times the value produced by an ordinary corona-type ionizer with an air blower.

Charge Separation in the Aerodynamic Breakup of Micrometer-Sized Water Droplets

Charged water droplets generated by electrospray, sonic spray, and a vibrating orifice aerosol generator (VOAG) have been studied by digital macrophotography and image charge detection mass spectrometry. Image charge detection mass spectrometry provides information on the droplet size, charge, and velocity after transmission through a capillary interface. The digital images provide the droplet size distribution before they enter the capillary. Droplets with 10-100 microm radii generated by sonic spray and VOAG are reduced to 2-3 microm radii by transmission through the capillary interface. The droplets from sonic spray and VOAG are much more highly charged than expected for random charging, and positive droplets are much more prevalent than negative. For positive mode electrospray, >99% of the detected droplets carry a positive charge, whereas for negative mode electrospray, <30% of the detected droplets carry a negative charge (i.e., >70% carry a positive charge). These observation can all be accounted for by the aerodynamic breakup of the droplets in the capillary interface. This breakup reduces the droplets to a terminal size at which point further breakup does not occur. Charge separation during droplet breakup is responsible for the relatively high charges on the sonic spray and VOAG droplets and for the preference for positively charged droplets. The charge separation can be explained using the bag mechanism for droplet breakup and the electrical bilayer at the surface of water.

Molecular‐level etching by electrospray droplet impact

AbstractA novel ionization method, electrospray droplet impact (EDI), has been developed for matrix‐free SIMS. The charged water droplets used in EDI are extremely large cluster ions with masses of about a few 106 u. When the charged water droplets impact on a target, the physical sputtering of the target is suppressed to minimal and an atomic‐and molecular‐level layer‐by‐layer etching is realized. Copyright © 2008 John Wiley &amp; Sons, Ltd.