Microdroplet Emission Research Articles

Studies on laser induced emission of microdroplets containing Rhodamine 6G solutions in water doped with TiO2 nanoparticles

Studies on the emission spectra of microdroplets containing Rh6G solutions in water doped with TiO2 nanoparticles are reported. The excitation is made by the second harmonic of a pulsed Nd:YAG laser at 532nm, pulse duration at half maximum 6ns and energy varied between 6 and 10mJ. The laser induced emission spectra are analyzed function of TiO2 concentration and pumping laser energy. Comparison between fluorescence dispersed spectra emitted by pendant droplets containing TiO2 nanoparticles with respect to Rh6G water solutions droplets pumped in the same conditions is made. The surface tension measurements of the samples show that with variation of dye and nanoparticles concentrations, surface phenomena take place and influence the behavior of the droplet fluorescence.

Anisotropic Molecular Ionization at 1 V from Tellurium Nanowires (Te NWs).

Ionization of molecular species from one-dimensional (1D) tellurium nanowires (Te NWs) has been achieved at 1 V. Molecules with a range of chemical functional groups gave quality mass spectra with high signal/noise ratios and no fragment ions. Experiments suggest the possibility of emission of microdroplets of solution due to the intense fields at the ends or interfaces of nanostructures. It appears that electrolytic conduction of the solution wetting of the nanostructures and not the electronic conduction of the nanostructures themselves is involved in the ionization event. Anisotropy was seen when two-dimensionally aligned Te NWs were used for ionization. The orientation effect of aligned Te NWs on molecular ion intensity is demonstrated for many analytes including organic molecules and amino acids with experiments done using a silicon substrate having aligned Te NWs. These measurements suggest the possibility of creating a MS source that extends the applicability of mass spectrometry. Analysis of a variety of analytes, including amino acids, pesticides, and drugs, in pure form and in complex mixtures, is reported. These experiments suggest that 1D nanostructures in general could be excellent ionization sources.

Ultrasonic atomization of liquids in drop-chain acoustic fountains.

When focused ultrasound waves of moderate intensity in liquid encounter an air interface, a chain of drops emerges from the liquid surface to form what is known as a drop-chain fountain. Atomization, or the emission of micro-droplets, occurs when the acoustic intensity exceeds a liquid-dependent threshold. While the cavitation-wave hypothesis, which states that atomization arises from a combination of capillary-wave instabilities and cavitation bubble oscillations, is currently the most accepted theory of atomization, more data on the roles of cavitation, capillary waves, and even heat deposition or boiling would be valuable. In this paper, we experimentally test whether bubbles are a significant mechanism of atomization in drop-chain fountains. High-speed photography was used to observe the formation and atomization of drop-chain fountains composed of water and other liquids. For a range of ultrasonic frequencies and liquid sound speeds, it was found that the drop diameters approximately equalled the ultrasonic wavelengths. When water was exchanged for other liquids, it was observed that the atomization threshold increased with shear viscosity. Upon heating water, it was found that the time to commence atomization decreased with increasing temperature. Finally, water was atomized in an overpressure chamber where it was found that atomization was significantly diminished when the static pressure was increased. These results indicate that bubbles, generated by either acoustic cavitation or boiling, contribute significantly to atomization in the drop-chain fountain.

Molecular Ionization from Carbon Nanotube Paper

Ambient ionization is achieved by spraying from a carbon nanotube (CNT)-impregnated paper surface under the influence of small voltages (≥3 V). Organic molecules give simple high-quality mass spectra without fragmentation in the positive or negative ion modes. Conventional field ionization is ruled out, and it appears that field emission of microdroplets occurs. Microscopic examination of the CNT paper confirms that the nanoscale features at the paper surface are responsible for the high electric fields. Raman spectra imply substantial current flows in the nanotubes. The performance of this analytical method was demonstrated for a range of volatile and nonvolatile compounds and a variety of matrices.

Erosion of carbon arc cathodes operating in the thermo-field electron emission mode

Steady-state modelling and experimental results are given on the electric arc attachment on cold carbon cathodes working at low pressure. The modelling results are compared with the case of copper cathodes and with experimental data on vacuum arc erosion characteristics for graphite materials. A region of existence of a physically meaningful solution for self-sustained operation of the steady-state cathode spot is given in the electron temperature–cathode spot plasma pressure space. A solution domain comprised between Te ≈ 1.2–1.5 eV and p≈ 2–45 atm corresponding to carbon surface temperatures in the range 4200–4900 K is found. Values of the local heat flux to the cathode surface are evaluated in the range 1–20 × 1010 W m−2, and ratios of the various contributions to this flux and current density are given. Also given are the cathode spot radii and upper/lower limits for the erosion rate through vapourization, these being compared with experimental data. It is shown that the cathode spot pressure conditions can provide a mechanism for the control of macroparticle emission on carbon. This effect is used experimentally through cathode spot plasma confinement for the reduction of the microdroplet emission in arc sources used for diamondlike film deposition. Experimental data obtained on graphite materials are in agreement with the model-based design guidelines.

On the mechanism of ion formation from the aqueous solutions irradiated with 3 µm IR laser pulses under atmospheric pressure

The mechanism of atmospheric pressure (AP) laser ionization of water and water/glycerol liquid samples at a 3-microm wavelength is studied experimentally. For the ion desorption, an in-house built Yb : YAG-pumped optical parametric oscillator (OPO) infrared (IR) laser has been coupled with AP MALDI ion source interfaced to an ion trap mass spectrometer (MS). It has been shown that water is primarily responsible for ion generation in water/glycerol samples, while glycerol increases the solution viscosity and decreases the water evaporation rate and sample losses. In contrast to AP UV-MALDI, the electric field in the case of AP IR-MALDI does not assist in ion production. It was found that the absence of the electrical field provides the optimum ionization condition both for water and water/glycerol liquid samples at the 3-microm laser irradiation. A two-stage ion formation mechanism, which includes the initial emission of microdroplets and release of molecular ions at the second stage, can explain the experimentally observed ion signal dependencies upon the voltage applied between MS inlet and the MALDI sample plate. Postionization using additional corona discharge APCI increases the observed signal by approximately 50%, which indicates that some portion of the analyte is desorbed in the form of neutral molecules.

Experimental validation of a mass- efficiency model for an indium liquid-metal ion source

A model is derived linking microdroplet emission of a liquid-metal ion source (LMIS) to the actual current–voltage characteristic and operating temperature. All parameters were experimentally investigated using an indium LMIS, confirming the relationships found. The model allows for the first time the optimisation of a LMIS for low droplet emission at high emission currents. This is very important for application as a thruster, which has been developed at ARC Seibersdorf research. It can be also used to extrapolate droplet emission values along the current–voltage characteristic.

Shape of a Co–Nd Liquid Alloy Ion Source

Co–Nd-liquid alloy ion sources (LAIS) were operated of a special specimen holder in situ in a 1 MeV transmission electron microscope. The dynamics of the tip shape in dependence of the ion emission current, spatial shifts of the liquid cone and the process of double cone formation have been observed. This results in a better understanding of the ion emission processes from Co–Nd LAISs in a large range of ion current. A linear dependence of the decrease of the cone half-angle α and of the increase of the jet length l and of the diameter d on the increased ion emission current was found. No emission of microdroplets at the shanks of the Taylor cone and behind the cone has been detected.

In situ HV TEM observation of the tip shape of lead liquid metal ion sources

Lead liquid metal ion sources (LMISs) were operated using a special specimen holder in situ in a 1 MeV TEM. The dynamics of the tip shape have been observed as a function of the ion emission current. At currents there was a jet-like protrusion on the vertex of the Taylor cone. Linear dependences were found for the decrease of the cone half-angle and the increase of the jet length l as functions of increase in ion emission current. Microdroplet emission was observed at the shank of the Taylor cone over time intervals of 0.1 s and more. The diameter of these microdroplets varied between 40 and 340 nm. This work has improved our understanding of the ion and microdroplet emission processes from the Pb LMIS.

In situ observation of the tip shape of AuGe liquid alloy ion sources using a high voltage transmission electron microscope

A AuGe liquid alloy ion source was observed in situ under various operating conditions in a 1 MeV transmission electron microscope. The formation of a field-stabilized liquid cone, the change in the cone shape as a function of ion emission current, spatial shifts of the liquid cone, and microdroplet emission were investigated. Below the onset voltage the shape of the tip covered with liquid AuGe alloy is spherical. At the onset voltage a Taylor cone is formed. At ion emission currents Ie larger than 4 μA a jetlike protrusion at the Taylor cone vertex can definitively be seen. At lower emission currents possible jet formation cannot be identified unambiguously. The cone half-angle α decreases linearly with the ion emission current, while the jet length and the jet diameter increase linearly with Ie. Emission of microdroplets was observed from the surroundings of the Taylor cone region. Microdroplets with a radius between 80 and 2300 nm are emitted in time intervals of about 0.1 s and more.

In situ observation of the tip shape of Co–Ge liquid alloy ion sources in a high-voltage transmission electron microscope

For in situ observation in a 1 MeV TEM Co–Ge liquid alloy ion sources (LAISs) were mounted to a special specimen holder revealing the formation of the field-stabilized Co–Ge liquid alloy cone, the change in the tip shape as a function of the ion emission current, spatial shifts of the liquid alloy cone, and microdroplet emission from Co–Ge LAISs. This has facilitated the better understanding of the ion and microdroplet emission process from Co–Ge LAISs in a large range of ion current. Below the onset voltage the shape of the tip covered with the liquid Co–Ge alloy is spherical. At the onset voltage the Taylor cone is formed. A jetlike protrusion at the cone vertex may or may not exist. At higher ion emission currents there is a jetlike protrusion at the Taylor cone vertex. The cone half-angle α decreases and the jet length l increases with increasing emission current Ie. Linear dependences of α and l on Ie are found. Emission of microdroplets is observed at the shanks behind the Taylor cone. The radius of microdroplets varied between 0.035 and 10 μm. These microdroplets are emitted in time intervals of about 0.1 s and more.

Microdroplet emission on liquid metal surface at the development of Rayleigh instabilities — applications in cosmos

It has been shown that at charging and melting of the cosmic garbage particles by electron beam its intensive disintegration into very small droplets, whose dimension is 10 2–10 4 times smaller than the dimension of an original droplet, is possible. This process occurs only in the case, when the particle charge significantly exceeds the Rayleigh limit, determining the instability threshold for charged droplets. The consideration was carried out for garbage particles, having dimensions 0.1-1 cm. The possibility for the creation of a reactive engine of small thrust on a base of microdroplet emission phenomenon in LMIS was studied. It has been shown that such thrusters may have an advantage over the ion thruster for neutralization of the cosmic object body. For the solution of both these problems we have taken into account the compensating ion current, coming from the cosmic plasma to the negatively charged particle (satellite) surface.

In situ HVTEM observation of the tip shape of tin liquid metal ion sources

Tin liquid metal ion sources (LMISs) were mounted to a special specimen holder for in situ observation in a 1 MeV TEM revealing the formation of the field-stabilized tin liquid cone, the change in the tip shape as a function of the ion emission current, spatial shifts of the liquid cone, and microdroplet emission from a tin liquid metal ion source. This has facilitated the better understanding of the ion and microdroplet emission process from tin LMISs in a large range of ion current. Below the onset voltage the shape of the tip covered with the liquid tin is spherical. In a small voltage range above the onset value (∼ 20 V) oscillations of the Taylor cone between two well defined positions appear (with and without cone). At a slightly higher voltage the cone appears stable. A jet at the cone vertex may or may not exist. At higher ion emission currents there was a jet-like protrusion at the Taylor cone vertex. A linear dependence was observed of the decrease of the cone half-angle α and of the increase of the length l of the jet-like protrusion on the increased ion emission current. Emission of microdroplets was observed at the surroundings of the Taylor cone. The radius of microdroplets varied between 30 and 900 nm. These microdroplets are emitted in time intervals of about 0.1 s and more.

Study of microdroplet generation from vacuum arcs on graphite cathodes

The emission of microdroplets from the cathode surface in the vacuum arc ion plating deposition technique is the major drawback to the technique’s industrial use. The generation of these particles from graphite cathodes is studied in this article and correlated to the local thermal load in the cathode spot area. A pulse discharge was used for a precise control of this load. Increases in the arc current level, arc duration time, and, more generally, the local temperature of the cathode were found to increase the number and the average size of the emitted particles. Particles under these conditions also show an increase in the width of their size distributions. Increasing the distance between cathode and substrate was found to decrease the number density of particles observed on the substrate according to the solid angle covered. The microdroplets show a graphite structure and diameters between 0.2 and 2.0 μm. Conditions needed to decrease the number of particles emitted to the substrate are given.

HV-TEM in-situ investigations of the tip shape of indium liquid metal ion emitter

An indium liquid metal ion emitter has been observed in-situ in the Halle 1 MeV electron transmission microscope. The investigations have shown deviations from the theoretical shape of the Taylor cone. A linear dependence of the opening angle and the extension of the apex from the emission current was stated. With emission currents greater than 10 μA an extension of the cone's apex had been observed. For the first time the dynamical behaviour of a liquid metal ion emitter had been investigated. Microdroplet emission had been observed in the cone's surrounding. The radii of microdroplets show a dependence from the field strength and vary between 0.04 and 1.4 μm. Some spatial instabilities of the Taylor cone had been noticed.

Liquid metal microdroplet source for deposition purposes

The phenomenon of microdroplet emission in unstable regimes of liquid metal ion sources is studied. The spectrum of small droplets of the radius of 10 to 200 Å is shown to be of continuous character (microdroplet chaos), and that of larger ones to be discrete. Both the charge-to-mass ratio and the intensity of droplets as function of the current magnitude are measured. The generation of small droplets is shown to be caused by the development of the Rayleigh instability on the surface of jets ejected from the Taylor cone vertex. High- and low-frequency oscillations of the electric field develop on the jet apex in the process of emission of small droplets. These oscillations cause the liquid pressure modulation and lead to the parametric excitation of capillary waves (Faraday effect) on the surface of the Taylor cone. The crests of the capillary relief serve as an additional generator of droplets. The parameters of droplets (radius, charge, intensity) are calculated along the frequency of low-frequency oscillations of the ion current. The calculations are in agreement with the measured values of these quantities. The experiments were carried out in the tin source using a Wien mass-analyzer and a transmission electron microscope. The results may have a practical interest for deposition of different kinds of coatings.

Microdroplet emission and instabilities in liquid-metal ion sources

When the liquid metal ion sources (LMIS) operate in the high-current mode, an instability arises which is accompanied by the comparatively low-frequency ion current oscillations and microdroplet emission. Two distinct groups of microdroplets are observed, the large ones (with radius higher than 10 3 Å). and the smaller ones (10–100 Å). The rate of formation of small-scale droplets proves to be two or three orders higher than that of the large droplets. In this work it is shown that the appearance of the above groups of microdroplets is due to two kinds of instability developed in an LMIS. The smaller droplets are due to the Rayleigh instability on the surface of thin cylindrical jets ejected from the tip of the Taylor cone at higher currents. This instability results in jet fragmentation and formation of small droplets (with sizes of the order of the jet radius) till the jet completely disappears. Following this the jet ejection reappears and the jet fragmentation process is repeated. Thus, the Rayleigh instability is accompanied by the comparatively low-frequency ion current oscillations ( f = 1–10 MHz) which are due to the process of fragmentation and recovery of the jets. This process leads to the liquid-metal pressure modulation and causes the comparatively large-scale capillary wave extension on the Taylor cone surface as a result of the Faraday parametrical effect. It is this effect that leads to formation of the large droplets generated on the Taylor cone surface. We have made measurements of the angular distributions of the large and small droplets, as well as their intensities in the instability mode in tin LMIS. The measurements were carried out with transmission electron microscopy (TEM). The higher harmonic dependence of low frequencies on the current value have been defined. Also, the comparison between the computed and the experimental data have been carried out.