Laser-induced Fluorescence Intensity Research Articles

A novel laser-induced fluorescence concept for thermometry measurements in machining lubricoolants

This study investigates a two-colour laser-induced fluorescence (LIF) technique for thermometry in cooling lubricants (CL) for machining. The diagnostic technique should enable a simultaneous determination of cooling film thickness and film temperature, which is relevant for typical machining processes like turning and milling processes. The temperature determination is based on a temperature-sensitive fluorescence intensity ratio of a special dye determined in suitable spectral regions representing common band pass filters. For this purpose, two CL emulsions CL6 (6 vol.% CL, 94 vol.% water; normal machining) and CL10 (10 vol.% CL, 90 vol.% water; heavy machining) and its individual components were doped with the fluorescence dye Eosin-Y. Temperature dependent spectral LIF intensities were recorded in the range of 273–313 K, which are relevant for milling and cutting operations. Furthermore, absorption spectra were analysed as well. The temperature-dependent fluorescence measurements revealed a temperature limit for the emulsion stability. The emission peaks of Eosin-Y are shifted with increasing temperature towards longer wavelengths. The sensitivities of CL10 (1.88%/K) and CL6 (1.58%/K) are comparable to or even higher than the sensitivities of other LIF dyes provided in the literature. The literature-based dyes were however not determined in those CL. The measurements showed, that the investigated tracer-CL/water emulsions mixed with the dye enable a reliable temperature determination.

Kinetic study of the vibrational relaxation of CH3Zn by collisions with He

Methyl zinc CH3Zn generated in the photolysis of dimethyl zinc (CH3)2Zn at 223 nm was detected with laser-induced fluorescence (LIF) via the A∼2E-X∼2A1 transition. The time-resolved LIF intensity of the level X∼2A1,00 grew with time, indicating that the nascent CH3Zn was highly vibrationally excited. The He-pressure dependence of the growth rates of the LIF intensity was analyzed by the kinetic scheme with a rate-determining step, giving the rate coefficient for vibrational relaxation to the X∼2A1,00 level to be [8.2±0.9(2σ)]×10-13 cm3 molecule−1 s−1. The probability of vibrational relaxation of CH3Zn per collision with He nicely correlates with those of non- or less-polar diatomic molecules.

Nascent Vibrational Energy Distribution of CS(X1Σ+) Generated in the S(1D) + CS2 Reaction.

The internal energy distributions of reaction products are important information in clarifying the mechanism of chemical reactions. There are few reports of the nascent vibrational energy distribution of CS(X1Σ+) generated in the S(1D) + CS2 reaction. As long as S(1D) is produced by photodissociation of CS2, CS(X1Σ+), as a product of the chemical reaction and as a photoproduct of CS2 is indistinguishable. In this study, S(1D) was generated by the photolysis of OCS at 248 nm, where CS2 hardly dissociates, and CS(X1Σ+) was generated only by the S(1D) + CS2 reaction. The vibrational levels v″ = 0-6 of CS(X1Σ+) were detected with laser-induced fluorescence (LIF) via the A1Π-X1Σ+ transition. The identical time profiles of the LIF intensities showed that all the vibrational levels were produced by the S(1D) + CS2 reaction. The relative nascent vibrational populations of CS(X1Σ+) determined from the area intensities of the excitation spectra are 1.00 ± 0.11/0.58 ± 0.06/0.31 ± 0.03/0.078 ± 0.009/0.013 ± 0.001/<0.002/<0.002 (the values for v″ = 5 and 6 are the upper limits) for v″ = 0/1/2/3/4/5/6. The distribution agrees well with the statistical (prior) distribution.

Characterization of Fluorescence Tracers for Thermometry and Film Thickness Measurements in Liquid Coolants Relevant for Thermal Management of Electric and Electronic Components.

This study investigated a novel two-color LIF (laser-induced fluorescence) technique for thermometry in coolants relevant for electric components. In principle, this diagnostic enables thermometry in liquid flows but also a simultaneous determination of film thickness and film temperature, which is relevant, e.g., for jet impingement cooled electric components. Temperature measurements are based on a temperature-sensitive intensity ratio of special tracers realized by suitable band pass filters within the respective emission spectra. For this purpose, the heat transfer fluids Fragoltherm F12, Marlotherm LH, and a water-glycol mixture WG20 (80 vol.% water, 20 vol.% glycol) and its individual components were doped with suitable tracers. The tracer Eosin-Y was utilized for polar coolants (water, WG20, and glycol) and Nile red was utilized for non-polar solvents (Fragoltherm F12 and Marlotherm LH). The spectral LIF intensities were recorded for a wide range of temperatures (253-393 K), which are relevant for cooling of electric motors, batteries, and power electronics. Furthermore, absorption spectra were analyzed as well. The temperature-dependent fluorescence measurements revealed different behavior for the polar and non-polar solvents. A temperature increase in the polar solvents (water, WG20, glycol) led to a spectral shift of the emission peaks of Eosin-Y towards longer wavelengths (red-shifted), while the peaks of Nile red in the non-polar solvents (Fragoltherm F12 and Marlotherm LH) showed an opposite behavior and were blue-shifted. The highest average temperature sensitivity was achieved for Marlotherm LH (4.22%/K), followed by glycol (1.99%/K), WG20 (1.80%/K), water (1.62%/K), and Fragoltherm F12 (1.12%/K). These sensitivities are similar to or even much higher than the literature data of other LIF tracers, which were, however, not determined in those coolants. Consequently, the two novel proposed dyes for the studied heat transfer liquids enable a reliable temperature determination.

Rate coefficients for the CH(X2Π) + CHX3 (X = Cl and Br) reactions and the propensity of the reactions of CH with halomethanes

Methylidyne (CH) was generated in the ultraviolet multiphoton photolysis of CHCl3 and CHBr3 and detected with laser-induced fluorescence (LIF) via the A2Δ − X2Π (0,0) transition. The time-resolved LIF intensity recorded at varying pressures of CHCl3 and CHBr3 has given the overall rate coefficients for the reactions of CH with CHCl3 and CHBr3. The near gas kinetic rates indicate that CH inserts into the C − Cl and C − Br bonds. The correlation between the rate coefficients for the CH + CHnX4−n (X = F, Cl, and Br) reactions and the heats of reactions for H-, Cl-, Br-elimination is elucidated by the Evans − Polanyi principle.

Assay of honey freshness by a novel optical technique

Assay of Maillard reaction products (e.g. furosine) is a reputable method for determination of the honey freshness. In this report, novel optical technique is proposed for real-time measurement of the changes of furosine content in honey. The method is based on the unidirectional energy transfer between two peaks of the doubled-peak fluorescence spectrum as secondary inner filter effect (2nd-IFE) in a specific arrangement of the laser induced fluorescence (LIF) setup. Proper optical parameters are defined accordingly, and affirmed to be dependent on the content of furosine in honey. It is shown that the introduced parameters are not sensitive to the LIF intensity fluctuations induced by the ambient noises and particularly alter due to the 2nd-IFE. Furosine level of 8 honey types with different botanical origin were chemically determined before and after the 1 year storage, and compared with the values of the devised spectral parameters. Proofs conducted that the proposed technique can be utilized for evaluation of the honey freshness.

Nascent vibrational distributions of [formula omitted] generated in the S(1D) + OCS reaction and vibrational relaxation by collisions with He and CF4

Disulfur S2X3Σg- was generated by irradiation of the pulsed laser light at 248 nm to the gaseous OCS. Single vibrational level of S2X3Σg-,v=0-4 was detected with laser-induced fluorescence (LIF) via theB3Σu- − X3Σg-transition. The kinetic analysis of the time-resolved LIF intensities recorded at varying pressures of CF4 has given the rate coefficients kv for vibrational relaxation of S2X3Σg-,v to be kv/10−12 cm3 molecule−1 s−1 = 0.25 ± 0.07, 0.63 ± 0.06, and 1.2 ± 0.2 for v = 1, 2, and 3, respectively. The analysis also determined the relative detectabilities of the adjacent vibrational levels, deriving the nascent relative vibrational populations of the levels v = 0/1/2/3 to be 1/0.69 ± 0.05/0.47 ± 0.06/0.49 ± 0.07.

Effect of the excitation line on hydroxyl radical imaging by laser induced fluorescence in hydrogen detonations

The present study aims to evidence the effect of the excitation line on the planar laser-induced fluorescence of hydroxyl radical (OH-PLIF) imaging in H2-fueled detonation wave. We experimentally validated an updated laser-induced fluorescence (LIF) model, called KAT-LIF, which simulates spectrally-resolved fluorescence spectra, using a recently developed optical detonation duct. We numerically investigated the effects of the excitation line, the initial pressure (20–100 kPa), and the diluent (N2/Ar) on the fluorescence spectrum, the spectrally- and one-dimensionally-averaged LIF intensity, and the quantitative capabilities of the OH-LIF measurements for different 2H2-O2-3.76diluent detonable mixtures. The investigated excitation lines were (0,0)Q1(7), both (1,0)Q2(8) and (1,0)Q1(9), and (1,0)Q1(6), which all belong to the A2Σ+←X2Π transition. The main findings are the following: (i) considering the commercially available OH filters, Q2(8)+Q1(9) excitation scheme has the highest LIF intensity for all the investigated H2-fueled detonations, while Q1(7) could provide the strongest intensity with better (custom) collection optics; (ii) the maximum LIF intensity decreases with increasing pressure for all the excitation schemes; (iii) using a single point calibration, at the fluorescence peak, it is not possible to perform quantitative measurements of OH radicals for any H2-fueled detonation, using the conventional excitation schemes. Finally, we experimentally evidenced more favorable excitation schemes to obtain qualitative information far behind the front, by employing the saturated regime of fluorescence or the optically thin linear regime with appropriate laser configuration. These two excitation schemes correspond to more appropriate LIF strategies that will enable better detonation flow visualization in future studies.

Laser-induced fluorescence and its effect on the damage resistance of fluoride-containing phosphate-based glasses

A series of multi-component fluoride-containing phosphate-based glasses prepared in a reducing atmosphere showed improved resistance to high-energy ultraviolet (UV) laser-induced damage and strong laser-induced fluorescence (LIF) within the glass bulk. The UV optical absorption, photoluminescence, and fluorescence decay properties of these glasses were investigated to explore the defect-related LIF mechanism and its underlying effect on the glasses’ laser-induced damage threshold (LIDT). Seven laser wavelengths ranging from 253 nm to 532 nm were used to excite the LIF, and two characteristic fluorescence bands peaking at approximately 414 nm and 780–800 nm occurred in all three types of glasses. The LIF band at 414 nm was attributed to PO3-EC defects in the second harmonic frequency (2ω) absorptive glass and third harmonic frequency (3ω) transparent glass, but Fe2+ ions in the fundamental frequency (1ω) absorptive glass. A later fluorescence band at 780–800 nm occurred due to POHC defects in the 2ω absorptive and 3ω transparent glasses and Fe3+ ions in the 1ω absorptive glass. A detailed study on the dynamic decay processes of two additional dominant fluorescence peaks at 450 nm and 780 nm under 266 nm excitation revealed the potential effect of LIF on LIDT improvement. The relatively longer LIF lifetime, higher LIF intensity, and larger LIF peak area corresponded with and contributed to a higher LIDT, especially in the 3ω transparent glass with a low UV absorption coefficient. This study provides strong evidence for the prior hypothesis between strong LIF and LIDT.

Effect of the stagnation plate on PAHs, soot and OH distributions in partially premixed laminar flames fueled with a blend of n-heptane and toluene

The stagnant plate influence on spatial distributions of polycyclic aromatic hydrocarbons (PAHs), soot and hydroxyl radical (OH) in the surrogate fuel (80% n-heptane and 20% toluene in volume) partially premixed flame on a McKenna burner were originally investigated in terms of different plate heights above the burner (Hp) and plate temperatures (Tp). Both PAHs and OH distributions were qualitatively measured by laser-induced fluorescence (LIF) and soot volume fraction field was quantitatively measured by two-color laser-induced incandescence (2C-LII). It is found that the LIF intensity of 2&3-ring aromatics (A2&A3) and 4-ring aromatics (A4) and soot volume fraction firstly increase and then reduce when the Hp is set from 35 to 10 mm. The variation tendency transition happens between Hp = 20 and 25 mm. In contrary, OH-LIF intensity gradually decreases as the Hp decreases. On the other hand, the Tp has an opposite effect on the A2&A3-LIF, A4-LIF and soot volume fraction variation tendency at two Hp. At Hp = 15 mm, A2&A3-LIF, A4-LIF and soot volume fraction increase as the Tp increases. However, A2&A3-LIF, A4-LIF and soot volume fraction decrease with the rise of the Tp at Hp = 35 mm. Interestingly, OH-LIF intensity always increases as the Tp rises at any Hp. Therefore, it can be concluded that the PAHs-LIF intensity and soot volume fraction depend on the Hp and Tp, and the maximums of them would be reached at the specific Hp and Tp. While both the Hp and Tp have a monotonic effect on the OH-LIF intensity.

The branching ratio of intercombination [formula omitted]transitions in the RbCs molecule: Measurements and calculations

We observed the A1Σ+∼b3Π→a3Σ+/X1Σ+laser-induced fluorescence (LIF) of the RbCs molecule excited from the ground X1Σ+state by the Ti:Sapphire laser. The LIF spectra from the common perturbed levels of the singlet-triplet A ∼ b complex was recorded by the Fourier-transform (FT) spectrometer with the instrumental resolution of 0.03 cm−1. The relative intensity distribution in the rotationally resolved A∼b→a3Σ+(va)/X1Σ+(vX)progressions was measured, and their branching ratio was found to be about of 1÷5×10−4in the bound region of the a3Σ+and X1Σ+states. The experiment was complemented with the scalar- and full-relativistic calculations of the A/b−X/atransition dipole moments (TDMs) as functions of internuclear distance. The relative systematic error in the resulting ab initio TDM functions evaluated for the strong A−Xtransition was estimated as few percent in the energy region, where the experimental LIF intensities are relevant. The relative spectral sensitivity of the FT registration system, operated with the InGaAs diode detector and CaF beam-splitter, was calibrated in the range [6 500, 12 000] cm−1by a comparison of experimental intensities in the long A ∼ b → X(vX) LIF progressions of the K2 and KCs molecules with their theoretical counterparts evaluated using the ab initioA−XTDMs. Both experimental and theoretical transition probabilities can be employed to improve the stimulated Raman adiabatic passage process, a → A ∼ b → X, which is exploited for a laser assembling of ultracold RbCs molecules.

The role of DME addition on the evolution of soot and soot precursors in laminar ethylene jet flames

Dimethyl ether (DME) has received considerable attention as a fuel additive to reduce the emission of particulate matter (PM) due to its low-temperature chemistry, molecularly bound oxygen atom and the absence of CC bonds. However, the effect of DME addition on the evolution of soot and particularly soot precursors is not entirely understood. This study aims to shed light on this issue by blending different proportions of DME with diffusion, E60, and partially premixed, PP12, base cases of laminar ethylene flames using the Yale benchmark burner. Laser-induced fluorescence (LIF) intensity and decay time are used to characterize the structure and evolution of soot precursors, while laser-induced incandescence (LII) is utilized to determine the soot volume fraction (SVF) and the effective primary particle diameter (Dp). For the diffusion flames, the addition of 10% DME increases the concentrations of both soot and soot precursors. With the further addition of DME to 20%, the SVF decreases to levels similar to those of E60 and then decreases further with 30% DME addition. All diffusion flames with DME addition exhibit higher concentrations of soot precursors than those of the reference E60 case. For PP12, the addition of 10% DME shows similar concentrations of soot precursors and a slight reduction in the SVF which continues to decrease with further increases in DME additions to the PP12 flame. The addition of DME seems to have little effect on the soot particle diameters for all the studied flames. Overall, the PP flames result in smaller mean particle diameters than the diffusion flame counterparts.

Study of PAH formation in laminar premixed toluene and C8H10 aromatics flames

Toluene and C8H10 aromatic fuels are widely proposed as components of surrogate mixtures and usually employed as the prototypical fuels to investigate the combustion properties of aromatic fuels. In this study, the PAH formation of pure C8H10 aromatics (xylenes and ethylbenzene) and toluene was studied in a laminar premixed flame at atmospheric pressure using laser-induced fluorescence (LIF) diagnostic. The experimental results indicated the aromatic structure has a significant influence on the PAH tendency, and the observed LIF intensities were sequenced by toluene < xylenes < ethylbenzene. A detailed kinetic model was employed to validate against the experimental results based on the ROP analysis. The predicted mole fraction profiles of PAHs were consistent with the measured PAH tendency for toluene, o-xylene, and ethylbenzene. The chemical kinetic analysis indicated that benzyl is the most dominant intermediate product in toluene decomposition and precursor in PAH formation, corresponding to methylbenzyl in o-xylene flame, styrene and benzyl in ethylbenzene flame. In the comparison of o-xylene and toluene, the increased methylated groups on the benzenoid ring make the yield of benzylic radicals (mostly methylbenzyl) enhanced, increasing the probability of radical–radical recombination on PAH formation. In the ethylbenzene flame, two secondary CH bonds in the ethyl site are much easier to be abstracted than primary CH bonds in methyl sites of o-xylene. The different bond dissociation energies (BDEs) depended on the aromatic structure can change the distribution of the intermediate species in fuel decomposition, which can result in either an increase or decrease of PAH formation. Additionally, the weak benzylic CC bond makes the initiation of ethylbenzene decomposition easier, which could also explain the fated PAH formation in contrast with toluene and o-xylene. The experimental and kinetic analysis for C8H10 aromatics and toluene conducted in the present study contributed a deep insight into the PAH formation of aromatics fuels.

Fourier-transform spectroscopy, relativistic electronic structure calculation, and coupled-channel deperturbation analysis of the fully mixed A1Σu+ and b3Πu states of Cs2

The 4503 rovibronic term values belonging to the mutually perturbed $A^1\Sigma^+_u$ and $b^3\Pi_u$ states of Cs$_2$ were extracted from laser induced fluorescence (LIF) $A\sim b\rightarrow X^1\Sigma^+_g$ Fourier transform spectra with the 0.01 cm$^{-1}$ uncertainty. The experimental term values of the $A^1\Sigma^+_u\sim b^3\Pi_u$ complex covering the rotational levels $J\in [4,395]$ in the excitation energy range $[9655,13630]$ cm$^{-1}$ were involved into coupled-channel (CC) deperturbation analysis. The deperturbation model takes explicitly into account spin-orbit coupling of the $A^1\Sigma^+_u(A0^+_u)$ and $b^3\Pi^+_{0_u}(b0^+_u)$ states as well as spin-rotational interaction between the $\Omega=0$, $1$ and $2$ components of the $b^3\Pi^+_{\Omega_u}$ state. The \emph{ab initio} relativistic calculations on the low-lying electronic states of Cs$_2$ were accomplished in the framework of Fock space relativistic coupled cluster (FSRCC) approach to provide the interatomic potentials of the interacting $A0^+_u$ and $b0^+_u$ states as well as the relevant $A\sim b$ spin-orbit coupling function. To validate the present CC deperturbation analysis solely obtained by energy-based data, the $A\sim b \to X(v^{\prime\prime}_X)$ LIF intensity distributions were measured and compared with their theoretical counterparts obtained by means of the non-adiabatic vibrational wave functions of the $A\sim b$ complex and the FSRCC $A\sim b \to X$ transition dipole moments calculated by the finite-field method.

Soot inception in laminar coflow diffusion flames

This paper focuses on the soot inception region in laminar coflow diffusion flames of methane and ethylene stabilised on the Yale diffusion burner. Earlier studies of these flames have focused on the downstream regions where soot has already developed. Laser-induced fluorescence (LIF) and elastic scattering measurements from 266 nm excitation are combined with laser-induced incandescence (LII) excited at 1064 nm. The structure and evolution of the soot precursor particles are characterised using the LIF intensity, decay time and the relative spectral emission in the ultraviolet and visible. The LIF decay times indicate that the majority of 266 nm excited LIF originates from nanostructures rather than gas phase polycyclic aromatic hydrocarbons (PAH). A similarity in the particle evolution for the low and high sooting flames in the upstream regions is found, indicating a general transition towards larger structures with more aromatic features as the nanostructures advect downstream in the fuel rich pyrolytic conditions. Higher nanostructure concentrations are found to precede the higher soot volume fractions (SVF) found in fuel rich sootier flames, although not proportionally, suggesting that surface growth strongly contributes to SVF in the high sooting flames. In the heavier sooting flames the majority of particle formation shifts from the centreline to the wings at the outer edges of the flame closer to stoichiometry. Particle formation in the wings of the flames occurs in the presence of oxygen and higher temperatures, resulting in particles with spectroscopic properties resembling those formed toward the oxidiser side of counter-flow diffusion flames. In the heavier sooting flames, particles produced in the wings appear to mix with particles formed along the centreline of the flame at the flame tip, resulting in a broad range of nanostructures and soot occurring in this region.

Introduction of 5-aminolevulinic acid as a theranostics agent in dentistry.

The aim of this paper is to study the theranostic potential of 5-Aminolevulinic Acid (5-ALA)in dentistry. Photodynamic inactivation (PDI) and fluorescence spectroscopy of Streptococcus sanguis, and laser induced fluorescence (LIF) of several decayed teeth were performed using 5-ALA. In the absence of 5-ALA, 15 min illumination of the bacteria by the means of an LED light source led to only 1.16% viability reduction. On the other hand, 5-ALA revealed remarkable dark toxicity at concentrations above 20 μM. Furthermore, the synergistic effects of 10 μM 5-ALA and illumination by the light source for 5 and 15 min intervals led respectivelyto0.74log10 and 1.69log10 reduction of viability. Also, fluorescence spectroscopy of the bacteria showed a direct relationship between emission line intensity at 620 nm and the concentration of 5-ALA. In dental experiments, following exposing tooth with 40 mM 5-ALA, a significant autofluorescence growth was observed just in the decayed parts. Based on the strong dual modality of 5-ALA to annihilate cariogenic bacteria through photodynamic inactivation and enhancing LIF intensity for identification of dental caries, 5-ALA is proposed as a theranostic agent in dentistry.

Simultaneously calibrated two-line atomic fluorescence for high-precision temperature imaging in sooting flames

Simultaneously calibrated, non-linear two-line atomic fluorescence (SC-nTLAF) thermometry for application in turbulent sooting flames has been developed to increase the precision of single-shot, planar measurements of gas temperature. The technique has been demonstrated in both steady and turbulent sooting flames, showing good agreements with previous optical measurements. The SC-nTLAF involves imaging simultaneously laser-induced fluorescence (LIF) of atomic indium in both the target flame and a non-sooting calibration flame for which the temperature distribution is known. The LIF intensities from the reference flame enable correction for fluctuations, not only in the laser power, but also in the laser mode. The resulting precision was found to be ±67 K and ±75 K (based on one standard deviation) in the rich and oxidizing regions of a steady sooting flame for which the measured temperature was 1610 K and 1854 K, respectively, with a spatial resolution of 550 × 550 µm2. This corresponds to a relative precision of ∼ 4.1%. The resulting precision in the single-shot temperature images for a well-characterized, lifted ethylene jet diffusion flame (fuel jet Reynolds number = 10,000) compares favorably with previously reported data obtained with shifted-vibrational coherent anti-Stokes Raman spectroscopy (CARS), together with increased spatial resolution. The planar imaging also provides more details of the temperature distribution, particularly in the flame brush region, which offers potential for measurement of more parameters, such as gradients and spatial corrections. The new calibration method has also achieved a significant time-saving in both data collection and processing, which is an estimated total of ∼ 60%–70% compared with conventional nTLAF.

Investigation of plasma dynamics and spatially varying O and OH concentrations in atmospheric pressure plasma jets impinging on glass, water and metal substrates

Atmospheric pressure plasma jets (APPJs) have attracted considerable attention over the last decade, specifically for use in surface engineering. A comparative study of an APPJ, driven by pulsed DC voltage, is conducted in order to examine the plasma impingement onto different surfaces. In this paper, the effect of gas flow rate and composition is investigated using three kinds of substrates: dielectric glass, distilled water and metal plate using fast imaging. Alongside discharges associated with rising and falling voltage, a so-called third discharge is observed during the pulse for water and metal surfaces which corresponds to a restrike breakdown from surfaces to nozzle. The differences in plasma dynamics observed are mainly attributed to the differences in substrate conductivity. In addition, spatial and temporal distributions of OH and O density are investigated by means of laser induced fluorescence (LIF). The OH/O LIF intensity is found to be much higher for metal and water substrates compared to the glass plate. We attribute this effect to the differences in power dissipation associated with the presence and intensity of the third discharge. Effects of gas flow rate and seed gas (H2O and O2) mixing on the LIF enhancement are also studied. The related results provide additional insights for optimizing the generation of reactive species.

OH Radicals Distribution and Discharge Dynamics of an Atmospheric Pressure Plasma Jet Above Water Surface

In recent years, plasma in contact with water has received much attention because of their biological and medical applications. Hydroxyl radicals (OH) are believed to play an important role in these applications. In this paper, the dynamic of a nanosecond atmospheric-pressure plasma jet (APPJ) above water surface and the OH radicals produced by the APPJ were investigated experimentally. The distributions of OH both in free space and above the water surface were recorded by a laser-induced fluorescence (LIF) technique. It was observed that for cases above water surface, OH LIF intensity was 2–3 times stronger using the same operating parameters. Specially, a second OH intensity peak would appear above the liquid surface, potentially leading to a significant increase of OH radical flux diffused from the gas phase to dissolve in the liquid. Discharge dynamic recorded by high speed intensified charge-couple device camera showed that after the primary discharge, a secondary discharge would form and propagate backward from the gas-liquid interface toward the nozzle. This phenomenon was previously believed to exist only above conductive surfaces, such as metal. This paper showed the importance of using in situ diagnostics for plasma jet biomedical applications, as the spatial distribution of the reactive species produced during the discharge process can be significantly influenced by the introduction of wet surfaces, such as cultivated bacteria or human tissue.

Development of laser-induced fluorescence to quantify in-cylinder fuel wall films

Laser-induced fluorescence of a fuel tracer is a very sensitive technique to image in-cylinder liquid fuel films, but quantification of the measured film thickness has proven difficult so far. This article describes improvements in the quantification procedure and presents an example application in a motored, optically accessible spark-ignition engine with direct injection. We designed a calibration tool that could be pressurized and heated, allowing investigation of the laser-induced fluorescence intensities at temperatures exceeding the liquid’s standard-pressure boiling point. The fluorescence intensity of liquid toluene and 3-Pentanone dissolved in isooctane upon excitation with a pulsed laser at 266 nm was investigated as a function of temperature and pressure. Consistent with the literature results on gas-phase laser-induced fluorescence, the signal from toluene was much stronger than from 3-Pentanone, about two orders of magnitude for films thinner than 50 μm. Laser-induced fluorescence from both t...