A size-dependent microparticle capture and release chip using concentric membrane ring barriers

We present a size-dependent microparticle separation chip where different sized slit gaps are formed around a central inlet by membrane ring barriers, concentrically arranged, and adjusted with a single pneumatic source. Previous microparticle separation methods, using multiple filters with different pore sizes, require additional structures and processes to release microparticles after they are captured in the filter. In addition, those methods often result in particle loss due to clogging of the fixed pore filters. We suggest a microparticle separation chip capable of size-dependent capture and release without the particle loss. The present separation chip has four concentrically arranged membrane rings (b₁ ∼ b₄) with a thickness of 90 μm and widths of 182 μm, 188 μm, 194 μm, and 200 μm, respectively, which form slit gaps estimated to be 11.2 μm, 9.5 μm, 7.6 μm, and 5.8 μm, respectively, at the pneumatic pressure of 80 kPa. In the experiment, we demonstrated microparticle capture and release using two different sizes of PS (polystyrene) beads (diameter = 6.51 ± 0.43 μm and 10.32 ± 0.39 μm) immersed in 0.5% BSA (Bovine Serum Albumin) solution at a flow rate of 100 μl/min. At a pressure of 80 kPa, 10.32 μm and 6.51 μm-diameter beads were captured at ring barriers b₃ and b₄, respectively. Subsequently, at pressures of 65 kPa and 50 kPa, the 6.51 μm and 10.32 μm-diameter beads were respectively released from the outermost barrier (b₄). The capture and release efficiencies of 10.32 μm-diameter beads at the b₃ barrier were 91.7 ± 16.7% and 90.9 ± 8.1%, respectively. The purity of 10.32 μm-diameter beads at the b₃ barrier was 80.2 ± 6.2%. The capture and release efficiencies of 6.51 μm-diameter beads at the b₄ barrier were 100.0 ± 0.0% and 97.1 ± 4.0%, respectively. The purity of 6.51 μm-diameter beads at the b₄ barrier was 91.8 ± 2.9%. We have verified that different sizes of captured microparticles were released sequentially by gradually reducing the pressure. The present chip, having concentric membrane ring barriers which can form different sized slit gaps using a single pressure source, is simply capable of not only size-dependent microparticle capture, but also release in size order without particle clogging.     

Nanometric laser trapping of microbubbles based on nanostructured substrates

Laser trapping near the surface of a nanostructured substrate is demonstrated. Stable microbubbles with radii of 1-20 μm have been created and manipulated with sub-micron precision by a focused laser beam in an immersion oil covering arrays of pairs of gold nanopillars deposited on a glass substrate. The threshold for bubble creation and trapping characteristics depended on near-field coupling of nanopillars. The nanometric laser tweezers showed giant trapping efficiency of Q ∼ 50 for the trapped microbubbles.     

Optical vortex beams for trapping and transport of particles in air

In this paper we show that laser beams containing phase singularity can be used for trapping and guiding light-absorbing particles in air. The experiments were performed with agglomerates of carbon nanoparticles with the size in the range 0.1–10 μm; the typical cw laser power was of a few mW. The stability of open-air three-dimensional trapping was within ±2 μm in both the transverse and the longitudinal directions. The particle position on the beams axis within the trap can be controlled by changing the relative intensity of two beams. The distinguishing feature of the trapping strategy is that particles are trapped at the intensity minimum of the beam, thus with minimum heating and intervention into the particle properties, which is important for direct studies of particle properties and for air-trapping of living cells.     

Trapping cavitation bubbles with a self-focused laser beam

We observed that laser-induced cavitation bubbles in water can be trapped in a self-focused laser beam. Both optical imaging and acoustic detection have been utilized to confirm bubble trapping. Transverse and longitudinal trapping forces were measured to be as large as 87 and 11 pN, respectively. This result is contrary to conventional wisdom, since the mechanism of trapping in conventional optical tweezers implies that a low-index particle (a bubble being the limiting case) should be antitrapped.     

Trapping multiple particles in single optical tweezers

Optical trapping of multiple particles in three dimensions in a single inverted optical tweezers is investigated. The effect of trapping pairs of beads on the voltage signal recorded by a quadrant photo diode detector is examined, with particular emphasise on its power spectral density. It is found that trapped pairs of beads could be mistaken for single beads in a trap of half the strength. Stokes drag measurements reveal that both beads of the pair are confined less than a single particle in the same trap and that the quadrant signal provides some information of their mean displacements.     

Ultrasonic measurement and characterization of a low concentration system of solid particles in liquid,in high shear flow

The objective of this paper is ultrasonic measurement and characterization of solid particles in liquid (20–40 μm glass beads in water) in high shear flow (1.5–2 m/s). Ultrasonic time dependent signals as well as frequency spectra are analyzed, for simultaneous determination of average particle concentration and average flow speed. As a result, the distribution of sound energy in such concentrated systems at given flow speeds is measured. Influence of flow turbulence is demonstrated in measurements. Also, characteristic behaviors of liquid–particle mixtures like particle clustering and influence of gas bubbles have been investigated. Experimental results are complemented with a discussion of factors that influence measurement uncertainty.     

Combustion and microexplosion of collision-merged methanol/alkane droplets

The combustion characteristics of freely falling droplets, individually generated by the merging of colliding methanol and alkane droplets, were investigated and compared with those for pure methanol and alkanes. The merging of the nominally immiscible methanol and alkanes was manifested in an apparently adhesive, but unmixed, manner in all test conditions. An air bubble was found to be trapped at the colliding interfaces where they were “adhered,” with the trapping favored for head-on or near head-on collision orientations. The trapped air bubble ostensibly induced heterogeneous nucleation of the methanol, being facilitated by the relatively low limit of superheat of methanol. Consequently, the droplet exploded almost immediately upon ignition, leading to an extremely short overall lifetime. For collision orientations that were more off-centered, bubble trapping and thereby heterogeneous nucleation were not favored. However, delayed, albeit strong, microexplosion occurred through homogeneous nucleation of methanol at the contacting interface. The global burning rate was therefore again augmented. In general, microexplosion was facilitated for high-boiling-point alkanes such as hexadecane and tetradecane. The co-vaporization of methanol and alkane from their respective hemispherical segments constituting the adhered droplet also led to flame colors that were more bluish than yellowish, indicating the reduction of soot from alkane burning in the presence of methanol vapor. In light of the difficulty of forming stable methanol/oil emulsions, the potential of separate injection of oil and methanol in opposed jet arrangement, in direct-injection engines to facilitate collision, is suggested.     

AlGaN barrier thickness dependent surface and interface trapping characteristics of AlGaN/GaN heterostructure

The aluminium gallium nitride (AlGaN) barrier thickness dependent trapping characteristic of AlGaN/GaN heterostructure is investigated in detail by frequency dependent conductance measurements. The conductance measurementsin the depletion region biases (−4.8 V to −3.2 V) shows that the Al_0.3Ga_0.7N(18 nm)/GaN structure suffers from both the surface (the metal/AlGaN interface of the gate region) and interface (the AlGaN/GaN interface of the channel region) trapping states, whereas the AlGaN/GaN structure with a thicker AlGaN barrier (25 nm) layer suffers from only interface (the channel region of AlGaN/GaN) trap energy states in the bias region (−6 V to −4.2). The two extracted time constants of the trap levels are (2.6–4.59) μs (surface) and (113.4–33.8) μs (interface) for the Al_0.3Ga_0.7N(18 nm)/GaN structure in the depletion region of biases (−4.8 V to −3.2 V), whereas the Al_0.3Ga_0.7N (25 nm)/GaN structure yields only interface trap states with time constants of (86.8–33.3) μs in the voltage bias range of −6.0 V to −4.2 V. The extracted surface trapping time constants are found to be very muchless in the Al_0.3Ga_0.7N(18 nm)/GaN heterostructure compared to that of the interface trap states. The higher electric field formation across the AlGaN barrier causes de-trapping of the surface trapped electron through a tunnelling process for the Al_0.3Ga_0.7N(18 nm)/GaN structure, and hence the time constants of the surface trap are less.     

Mathematical modeling of microbubble cavitation at 70 kHz and the importance of the subharmonic in drug delivery from micelles

In order to gain insight into the experimental observation of ultrasound-induced release of drugs from micelles, we modeled the dynamic oscillations of a 10-μm-diameter bubble insonated at 70 kHz. The Parlitz modification of the Keller–Miksis model was employed to generate bubble dynamics over a wide range of mechanical index values. The resulting Poincaré maps and bifurcation diagram show that bubble oscillations bifurcate at a MI value of 0.32, then return apparently to a single mode before displaying a sudden onset of chaotic behavior at 0.35. The experimental release of drug from micelles occurs at a MI value of 0.37 and correlates with the intensity of the subharmonic in (μW/cm²) of the acoustic spectrum. The dynamic model shows the return to single mode at a MI value of 0.43, and bifurcation leading to chaos at values above 0.5. The correlation between the chaotic behavior predicted by the model and drug release hints at insonation conditions that could facilitate drug delivery.     

Smoke triggered corona discharge sensor

This paper presents an ionic smoke sensor working without a radioactive ionization source. The presence of smoke particles reduces significantly the effective corona discharge threshold of air by a factor greater than 5. The smoke sensor consists of a wire under an intermediate continuous voltage which generates a current only in presence of smoke. The sensor electric consumption is therefore very low and can operate for a long time. Results of a prototype operating under 600 V with a 25-μm-diameter wire are shown.     

Theoretical investigation and experimental support for the cavitation bubble dynamics near a spherical particle based on Weiss theorem and Kelvin impulse

In the present paper, the laser-induced cavitation bubble dynamics near a fixed spherical particle is comprehensively investigated based on the Weiss theorem, the Kelvin impulse theory and the high-speed photography experiment. Firstly, the applicability range of the theoretical model in the time and the space is statistically obtained based on sufficient experimental results. Then, the in-depth theoretical analysis is carried out in terms of the liquid flow field and the bubble Kelvin impulse with the corresponding experimental results as the reasonable support. In addition, the theoretical prediction model of the bubble movement is established and experimentally fitted from the analytic expression of the Kelvin impulse. Through our research, it is found that: (1) the applicability range of the Kelvin impulse theory for the bubble near the spherical particle is approximately the dimensionless distance between the bubble and particle (γ) greater than 0.50. (2) The effect of the particle on the liquid velocity between the bubble and the particle is mainly manifested in the form of the image bubble, which always causes the liquid velocity in this region to be significantly lower than other surrounding regions. (3) The average movement velocity of the bubble centroid can be reasonably predicted by establishing a directly proportional function between the Kelvin impulse and the velocity with the relationship constant (α) equal to 3.57×10⁻⁶ ± 1.63×10⁻⁷ kg.     

Realization of an advanced nozzle concept for compact chemical oxygen iodine laser

Conventional supersonic chemical oxygen–iodine lasers (SCOIL) are not only low-pressure systems, with cavity pressure of 2–3 Torr and Mach number of approximately 1.5, but also are high-throughput systems with a typical laser power per unit evacuation capacity of nearly 1 J/l, thus demanding high capacity vacuum systems which mainly determine the compactness of the system. These conventional nozzle-based systems usually require a minimum of a two-stage ejector system for realization of atmospheric pressure recovery in a SCOIL. Typically for a 500 W class SCOIL, a first stage requires a motive gas flow (air) of 120 gm/s to entrain a laser gas flow of 3 g/s and is capable of achieving the pressure recovery in the range of 60–80 Torr. On the other hand, the second stage ejector requires 4.5 kg/s of motive gas (air) to achieve atmospheric pressure recovery. An advanced nozzle, also known as ejector nozzle, suitable for a 500 W-class SCOIL employing an active medium flow of nearly 12 g/s, has been developed and used instead of a conventional slit nozzle. The nozzle has been tested in both cold as well as hot run conditions of SCOIL, achieving a typical cavity pressure of nearly 10 Torr, stagnation pressure of approximately 85 Torr and a cavity Mach number of 2.5. The present study details the gas dynamic aspects of this ejector nozzle and highlights its potential as a SCOIL pressure recovery device. This nozzle in conjunction with a diffuser is capable of achieving pressure recovery equivalent to a more cumbersome first stage of the pressure recovery system used in the case of a conventional slit nozzle-based system. Thus, use of this nozzle in place of a conventional slit nozzle can achieve atmospheric discharge using a single stage ejector system, thereby making the pressure recovery system quite compact.     

High-frequency electric-field trap for micron and submicron particles

Summary Four e-beam-processed, planar electrodes with gaps between 0.5 and 4 μm were used to create quadrupole electric-field trap. The electrodes were immersed in an aqueous particle suspension and driven by kHz to MHz signals of several volts amplitude. Micron and submicron particles could be stably trapped by negative dielectrophoresis. Latex beads of 1000, 600, 100 and 14 nm diameter could be concentrated between the electrodes (positive dielectrophoresis) or levitated as condensed cloud (negative dielectrophoresis). The results are surprising since polarisation forces depend on the volume of the particle and, up to now, it was expected that thermal forces would dominate the behaviour of particles with diameters <100 nm. However, micron-scaled electrode configurations allow the application of extremely strong fields (up to 20 MV/m) and open up new perspectives for microparticle handling and macromolecule trapping.     

Experimental investigation on charging characteristics and penetration efficiency of PM2.5 emitted from coal combustion enhanced by positive corona pulsed ESP

The electrostatic precipitator (ESP) has been extensively used for collecting aerosol particles emitted from coal combustion, but its collection efficiency of PM2.5 (Particulate matter whose aerodynamic diameter is less than 2.5 μm) is relatively low due to insufficient particle charging. The positive pulsed ESP is considered to enhance particle charging and improve collection efficiency. A laboratory-scale pulsed ESP with wire-plate electrode configuration was established to investigate the particle charging and penetration efficiency under controlled operating conditions of different applied impulse peak voltages, impulse frequencies, dust loadings and residence times. The results show that most particles larger than 0.2 μm are negatively charged, while most particles smaller than 0.2 μm are positively charged. For a given operating condition, the particle penetration efficiency curve has the highest penetration efficiency for particles with a diameter near 0.2 μm, and there is always a negative correlation between the particle penetration efficiency and the average number of charges per particle. Under the same operating conditions, the particle penetration efficiency decreases with increasing impulse peak voltage and impulse frequency, but increases as the dust loading increases. The results imply that residence time of 4 s is optimum for particle charging and collection. PM2.5 number reduction exceeding 90% was achieved in our pulsed ESP.     

Destabilization of ultrafine bubbles in water using indirect ultrasonic irradiation

Ultrafine bubble (UFB) is a bubble with a diameter of less than 1 μm. Little attention has been paid to the defoaming and removal of UFBs. This study proposes a method to destabilize UFBs by using indirect ultrasonic irradiation. Besides, the destabilization mechanism of UFB was investigated. The ultrasonic frequency was 1.6 MHz and the dissipated power was 30 W. UFB dispersions were prepared using two different types of bubble generators: pressurized dissolution method and swirling liquid flow method. The effects of ultrasonic irradiation on the stability of UFBs were evaluated by particle tracking analysis (PTA) and electrophoretic zeta potential measurement. Results showed that the indirect ultrasonic irradiation for 30 min reduced the number concentration of UFBs by 90% regardless of the generation method. This destabilization was attributed to a decrease in the magnitude of zeta potential of UFBs due to the changes in pH and electrical conductivity. These changes in the electrochemical properties were caused by the formation of nitric acid. To study the destabilization mechanism, the pH of the UFB dispersions were modified by titration; the chemical and mechanical effects of ultrasound were separately examined. It was found that not only the chemical effect caused by the formation of nitric acid but also the mechanical effect contributed to the destabilization of UFB. Feasibility studies were also performed for UFBs in an aqueous surfactant solution and UFBs in a solid particle dispersion. The proposed method selectively destabilized UFBs in the solutions.     

Numerical study of the synergistic effect of cavitation and micro-abrasive particles

In ultrasonic-assisted machining, the synergistic effect of the cavitation effect and micro-abrasive particles plays a crucial role. Studies have focused on the investigation of the micro-abrasive particles, cavitation micro-jets, and cavitation shock waves either individually or in pairs. To investigate the synergy of shock waves and micro-jets generated by cavitation with micro-abrasive particles in ultrasonic-assisted machining, the continuous control equations of a cavitation bubble, shock wave, micro-jet, and micro-abrasive particle influenced by the dimensionless amount (R/R₀), a particle size-velocity–pressure model of the micro-abrasive particle was established. The effects of ultrasonic frequency, sound pressure amplitude, and changes in particle size on micro-abrasive particle velocity and pressure were numerically simulated. At an ultrasonic frequency of 20 kHz and ultrasonic sound pressure of 0.1125 MPa, a smooth spherical SiO₂ micro-abrasive particle (size = 5 µm) was obtained, with a maximum velocity of 190.3–209.4 m/s and pressure of 79.69–89.41 MPa. The results show that in the range of 5–50 μm, smaller particle sizes of the micro-abrasive particles led to greater velocity and pressure. The shock waves, micro-jets, and micro-abrasive particles were all positively affected by the dimensionless amount (R/R₀) of cavitation bubble collapse, the larger the dimensionless quantity, the faster their velocity and the higher their pressure.     

Three-dimensional dynamic optical manipulation by combining a diffractive optical element and a spatial light modulator

Since the introduction of computer-controlled spatial light modulators (SLMs), holographic optical tweezers have become an important tool for dynamic parallel optical manipulation. In this paper we clarify the usefulness of a new configuration for optical trapping that creates light patterns using the combination of a diffractive optical element (DOE) and an SLM. This configuration not only enables the use of the higher part of the SLM’s diffraction efficiency curve, because a simple hologram can be chosen for the SLM, but also achieves three-dimensional dynamic optical manipulation over a large spatial range. By switching blaze-like holograms displayed on the SLM, we demonstrated simultaneous transportation of three 6-μm-diameter polystyrene beads over a range of 90 μm in the vertical direction and 37.5 μm in the horizontal direction. Compared with the same manipulation executed using only the SLM, the range of this method is extended four-fold in the vertical direction and three-fold in the horizontal direction.     

Individual particle handling in a microfluidic system based on parallel laser trapping

We present an optical trapping system combining individually addressable multiple laser traps with fluorescence spectroscopy. An in-line set of 64 near-IR laser diodes is used to create a line of individually addressable traps inside a microfluidic chip. This system is completed by an excitation/detection line for spectrally resolved fluorescence imaging of trapped particles. Highly parallel trapping in a constant flow (up to a few millimeters per second), fast particle handling rates (up to a few particles per second), and the possibility of recording fluorescence spectra of trapped objects lead to a performing bioanalytical platform, e.g., for highly parallel analysis and sorting.     

Electrohydrodynamic flow patterns and collection efficiency in narrow wire-cylinder type electrostatic precipitator

Recently, narrow electrostatic precipitators (ESPs) have become a subject of interest because of their possible application for the cleaning of the exhaust gases emitted by diesel engines. Diesel engines emit fine particles, which are harmful to human and animal health. There are several methods for decrease particulate emission from a diesel engines, but up to now, these methods are not enough effective or very expensive. Therefore, an electrostatic precipitation was proposed as an alternative method for control of a diesel particulate emission.In this work, results of electrohydrodynamic (EHD) secondary flow and particle collection efficiency measurements in a narrow wire-cylinder type ESP are presented. The ESP was a glass cylinder (300 mm × 29 mm) equipped with a wire discharge electrode and two collecting cylinder-electrodes. A 0.23 mm in diameter and 100 mm long stainless-steel discharge wire electrode was mounted in the center of the cylinder, parallel to the main flow direction. The collecting electrodes were made of stainless steel cylinders, each with a length of 100 mm and inner diameter of 25.5 mm. An air flow seeded with a cigarette smoke was blown along the ESP duct with an average velocity of 0.9 m/s.The EHD secondary flow was measured using 2-dimensional particle image velocimetry (PIV) method. The PIV measurements were carried out in the wire electrode mid-plane, perpendicularly to the wire and the collecting electrodes. The results show similarities and differences of the particle flow in the wire-cylinder type ESP for a negative and a positive DC voltage polarity.The collection efficiency was calculated from the fractional particle concentration. The fractional particle concentration was measured using the optical aerosol spectrometer. The results of the fractional collection efficiency confirmed the common view that the collection efficiency of fine particles in the ESP increases with increasing voltage and it is higher for negative voltage polarity and decreases when decreasing particle diameter.