The microdroplet test: experimental and finite element analysis of the dependance of failure mode on droplet shape

The microdroplet technique is usually designed as a fibre embedded in a drop of resin and subsequently pulled out while the drop is being supported by two knife edges, resulting in either debonding of the droplets from the fibres, or breakage of the fibres before debonding can occur. In this study, the microdroplet technique was performed using a platinum ring with a 40 μm hole instead of the usual two knife edges, giving an axisymmetric geometry, load and stress distribution. Glass/phenolic and glass/polyester composite systems were tested experimentally and subsequent finite element modelling studies were performed to assess the variation of droplet size, and contact angle between the droplet and fibre. It was found that contact angle is of major influence in the proposed failure model. This study characterizes the influence of the contact angle between the droplet and the fibre on the subsequent stress distribution in the microdroplet specimen.     

Numerical investigation of acoustic vaporization threshold of microdroplets

A numerical model is presented for the acoustic vaporization threshold of a dodecafluoropentane (or perfluoropentane) microdroplet. The model is based on the Rayleigh-Plesset equation and is improved by properly treating the supercritical state that occurs when a bubble collapses rapidly and by employing the van der Waals equation of state to consider the supercritical state. The present computations demonstrate that the microdroplet vaporization behavior depends intricately on bubble compressibility, liquid inertia and phase-change heat transfer under acoustic excitation conditions. We present acoustic pressure-frequency diagrams for bubble growth regimes and the ADV threshold conditions. The effects of acoustic parameters, fluid properties and the droplet radius on the ADV threshold are investigated.     

Microdroplet evolution induced by a laser pulse

Interaction between high-power ultrashort laser pulse and giant clusters (microdroplets) consisting of 10⁹ to 10¹⁰ atoms is considered. The microdroplet size is comparable to the laser wavelength. A model of the evolution of a microdroplet plasma induced by a high-power laser pulse is developed, and the processes taking place after interaction with the pulse are analyzed. It is shown theoretically that the plasma is superheated: its temperature is approximately equal to the ionization potential of an ion having a typical charge. The microdroplet plasma parameters are independent of the pulse shape and duration. The theoretical conclusions are supported by experimental studies of x-ray spectra conducted at JAERI, where a 100-terawatt Ti-sapphire laser system was used to irradiate krypton and xenon microdroplets by laser pulses with pulse widths of 30 to 500 fs and intensities of 6×10¹⁶ to 2×10¹⁹W/cm².     

About diagnosis of submicron droplets in a gas-droplet flow

Possibilities and limitations of the optical diagnostic methods for small-size (about 1 m) droplets in the gasdroplet flows are discussed. Here we show the possibility for restoration of the functions of droplet distribution and droplet average size on the basis of measurements of ultradispersed particle parameters formed from microdroplet flows after droplet evaporation.     

Physics and Chemistry in Laser—Trapped Single Microparticles Studied by Light Scattering

In this lecture various physical and chemical aspects are discussed when single particles of the size of the faserwavelength are trapped in strongly focused laser beams.First,we describe varius possibilities for the trapping techniques and micro-Raman setups.Then we discuss Raman-Mie-scattering and morphology dependent resonances which occur in dielectric spherical microparticles.These particles react as microresonators and influence the Raman spectra senerating input and output resonances.The electric field distribution inside and outside the dielectric microsphere in and out of resonance have been calcuated quantitatively to explain the input resonances.We also report on observed fast temperature jumps when microdroplets undergo an input resonance.Applications of these combined inelastic/elastic light scattring studies will be shown for microdroplet evaporation and for chemical studies such as simple acid/base reactions in a microdroplet,Furthermore we show results of recent studies on the emulsion polymerization process of styrene by applying a gradient force trap in a microscope.     

微尺度下锥形微操作探针表面液滴形成的研究（英）

基于液滴的转移方法可实现微操作任务中微对象的拾取,锥形操作探针则常作为一种毛细力微操作执行工具。主要研究在空气冷凝模式下锥形探针端面的液滴形成。建立了微液滴形成的数学模型,主要包括初始液滴的形成、液滴的合并和液滴的移动,研究了影响操作液滴的关键参数,分析表明:过冷度决定最小液滴半径。对单液滴的生长机制进行理论分析,并通过数值求解的方法模拟了锥形操作探针端面的液滴形成。搭建实验测试平台,实验研究了微尺度下锥形微操作探针端面的液滴形成。实验结果表明:在空气冷凝模式下,操作探针端面能够形成微液滴。经过初始液滴的形成,液滴的合并和移动等过程最终可形成稳定的微液滴,且不同锥顶角下液滴的形成呈现多样化。     

Stochastic Theory of Nucleation in Nonequilibrium Bistable Reaction Systems

Nucleation is one of the fundamental types of forming stable dissipative nonequilibrium structures in metastable initial states. The nucleation process of a bistable chemical reaction system is investigated. First a microdroplet with radius R₀ of the new state imbedded in the parent one is formed by a local particle number fluctuation. The radius of the microdroplet R(t) fluctuates due to the interference of reaction and diffusion. If R(t) considerably exceeds a critical value R_k the droplet starts to grow deterministically. The mean nucleation time of one droplet in weakly supersaturated systems is calculated using the concept of the mean first passage time.     

Numerical simulation of particle-laden droplet evaporation with the Marangoni effect

The European Physical Journal Special Topics - A comprehensive numerical method for analysis of the evaporation of a particle-laden microdroplet is developed including the effects of heat and mass...     

Oscillatory Motion of Microdroplets of a Droplet Cluster in a Linearly Nonuniform Electric Field

The forces acting on freely levitating water microdroplets in the structure found 15 years ago and called the droplet cluster (A. A. Fedorets, 2004) are considered. It is shown that lowfrequency vertical damping microdroplet oscillations can take place in such a cluster near the equilibrium position, which also exist in the case of instantaneous turn-on of an external electric field. These oscillations are considered from the viewpoint of the Fourier analysis.     

Controlled observation of nondegenerate cavity modes in a microdroplet on a superhydrophobic surface

We demonstrate controlled lifting of the azimuthal degeneracy of the whispering gallery modes (WGMs) of single glycerol-water microdroplets standing on a superhydrophobic surface by using a uniform electric field. A good agreement is observed between the measured spectral positions of the nondegenerate WGMs and predictions made for a prolate spheroid. Our results reveal fewer azimuthal modes than expected from an ideal spherical microdroplet due to the truncation by the surface. We use this difference to estimate the contact angles of the microdroplets.     

Dual IR laser shattering of a water microdroplet

Ion desorption from the infrared (IR) laser shattering of water microdroplets (?90???m in diameter) was experimentally examined by ion current measurements coupled with time-resolved imaging by a charge-coupled-device camera. When a microdroplet was shattered by simultaneous illumination by two IR lasers (??=2.9???m) from both the left- and right-hand sides, the time-resolved imaging shows that a lot of small fragments of splash spread around the droplet. The spatial distributions of the small fragments were symmetrically compressed. The resulting fragment swarm was effectively introduced into a vacuum chamber through an inlet skimmer ?0.3?C0.4?mm in diameter. The ion current measured from a 10^?6 mol/m³ NaCl water solution microdroplet using two lasers was considerably enhanced compared to that by single IR laser shattering. When one of the two IR lasers was delayed by 0?C1000???s, the ion current gradually decreased with the delay time, and dropped substantially at delays longer than 100 ns. The results are ascribed to dynamical processes following the multi-photon excitation. The dual IR laser ablation of a liquid droplet can enhance the efficiency of ion formation with a lower dispersion velocity, which can be conveniently combined with time-of-flight mass spectrometry.     

Q switching by saturable absorption in microdroplets: elastic scattering and laser emission

Laser-induced modification of cavity Q's has been achieved in a microdroplet containing a saturable absorber. The elastic-scattering spectra from such droplets for higher incident intensities show that cavity Q's are increased when the absorption is bleached. The lasing spectra from a droplet containing a saturable absorber and laser dye are modified when an intense bleaching field is injected into the droplet cavity after the pump field has initiated the lasing.     

Spontaneous radiation of molecules in open cavities

Within the framework of classical electrodynamics, a formula is derived for the spontaneous radiation rate of molecules and atoms in an arbitrary open cavity in the weak coupling approximation with allowance made for radiation absorption or amplification by the cavity material. The formula agrees well with microdroplet luminescence data. The effect of suppression of the spontaneous resonance radiation rate by the active laser medium is predicted.     

Efficiency of frequency-pulsed excitation of a micron-sized spherical microcavity by chirped ultrashort laser radiation

Peculiarities of internal optical field resonant excitation inside micron-sized spherical transparent dielectric cavity illuminated by a train of ultrashort laser pulses are investigated. On an example of water microdroplet is shown that optimal tuning of incident radiation to a selected high-Q resonance electromagnetic cavity mode can be realized by varying temporal interpulse interval in a laser train together with linear frequency modulation of every pulse (chirp). Efficiency of resonant particle excitation by a chirped picosecond pulses train can be considerably increased as compared to unchirped pulse train and CW excitation also.     

Henequen/poly(butylene succinate) biocomposites: electron beam irradiation effects on henequen fiber and the interfacial properties of biocomposites

Henequen natural fiber-reinforced poly(butylene succinate) biocomposites were prepared through a resin microdroplet formation on a single fiber and also fabricated by a compression molding technique using chopped henequen fibers, surface-treated with electron beam irradiation (EBI) at various dosages. The effect of EBI treatment on the surface characteristics and dynamic mechanical properties of henequen fibers was investigated using SEM, XPS and DMA methods, respectively. Also, the interfacial behavior of biocomposites was explored through a single fiber microbonding test and fracture surface observations. The result indicates that the interfacial shear strength (IFSS) of biocomposites greatly depends on the EBI treatment level on the henequen fiber surface. This study also suggests that appropriate modification of natural fiber surfaces at an optimum EBI dosage significantly contributes to improving the interfacial properties of biocomposites.     

Numerical study of the effect of liquid compressibility on acoustic droplet vaporization

In acoustic droplet vaporization (ADV), a cavitated bubble grows and collapses depending on the pressure amplitude of the acoustic pulse. During the bubble collapse, the surrounding liquid is compressed to high pressure, and liquid compressibility can have a significant impact on bubble behavior and ADV threshold. In this work, a one-dimensional numerical model considering liquid compressibility is presented for ADV of a volatile microdroplet, extending our previous Rayleigh-Plesset based model [Ultrason. Chem. 71 (2021) 105361]. The numerical results for bubble motion and liquid energy change in ADV show that the liquid compressibility highly inhibits bubble growth during bubble collapse and rebound, especially under high acoustic frequency conditions. The liquid compressibility effect on the ADV threshold is quantified with varying acoustic frequencies and amplitudes.     

New biochemical material laser emission in microdroplets using ribo-flavin and its enhancement by mixing highly scattering intralipid medium

New laser emission from a biochemical material, ribo-flavin (vitamin B₂), at 570 nm was observed for the first time in liquid microdroplets pumped at 475 nm using a pulsed optical parametric oscillator. Laser emission spectra from microdroplets containing ribo-flavin in water and glycerol solution exhibit periodic mode structures depending on the microdroplet diameter demonstrating the morphology-dependent resonances. Magnitude enhancement of lasing emission is confirmed experimentally from liquid microdroplets of ribo-flavin solution with appropriate mixture of the fat emulsion Intralipid-10% suspension as highly scattering medium. We believe that such biochemical-soft multiple light scatterer systems may allow a wide range lasing of other bio-materials and even lead to the discovery of some novel species as well as highly sensitive analysis and diagnosis for future applications.     

Lasing emission of dye-doped cholesteric liquid crystal microdroplet wrapped by polyglycerol in hollow glass microsphere

In this Letter,a dye-doped cholesteric liquid crystal(DDCLC)-filled hollow glass microsphere is demonstrated to be a resonator with good temperature response.A diglycerol layer is used to wrap the DDCLCs microdroplet to keep it steady and control its orientation.The whispering gallery mode(WGM)lasing and photonic band gap(PBG)lasing caused by two different mechanisms were obtained under the pump of a pulsed laser,and the temperature response of these two kinds of lasing was studied.For the liquid crystal and chiral material used in this Letter,both the WGM lasing and the PBG lasing have a blue shift in wavelength with increasing temperature.     

Angular distribution of non-linear optical emission from spheroidal microparticles

We measured the angular distribution of the near-backward multiphoton-excited fluorescence emission by ellipsoidal-shaped, dye-doped ethanol microdroplets deformed perpendicularly to the direction of the incident laser beam. The high-intensity region in the backward direction is elongated in the same direction as the emitting microdroplet. Simulations based on ray tracing agree well with the experimental pattern and show that the droplet aspect ratio ϱ may be deduced from the fluorescence pattern of both oblate and prolate microparticles with ϱ varying from 0.9 to 1.3. PACS 42.68.Jg; 33.80.Wz; 42.15.Dp     

White-light nanosource with directional emission

We report the first observation of white-light emission from femtosecond laser-induced plasma in a water droplet. Such emission is not observed with water in a cell. The microdroplet acts as a lens, focusing the incident light to nanosized regions within itself and directing the emission from these regions primarily back toward the laser source. This focusing increases the intensity so that multiphoton ionization generates plasma and causes it to reach the critical density during the initial part of the pulse, enabling the rest of the pulse to heat the plasma enough to emit in the visible.