Theoretical development of in situ optical particle separator: cross-type optical chromatography

An in situ separation system, cross-type optical chromatography, is developed theoretically, and an analytic solution of the retention distance is derived. Particle trajectories in the cross-type optical chromatography are calculated for various sizes and materials of the particles and for flow velocities. Further, cross-type optical chromatography assisted by a particle beam generation system is designed.     

Cross-type optical particle separation in a microchannel

A continuous, real-time optical particle separation, which was previously delineated theoretically, is successfully implemented experimentally for the first time. In this method, particles suspended in a flowing fluid are irradiated with a laser beam propagating in a direction perpendicular to direction of fluid flow. Upstream of the laser beam, the particles move parallel to the direction of fluid flow. When the particles pass through the laser beam, the scattering force pushes them in the direction of laser beam propagation, causing the particles to be displaced perpendicular to the fluid flow direction. This displacement, known as the retention distance, depends on the particle size and the laser beam parameters. Finally, the particles escape from the laser beam and maintain their retention distances as they move downstream. In the present work, the trajectories and retention distances of polystyrene latex microspheres with three distinct diameters were monitored and measured using cross-type optical particle separation. The measured retention distances for different-sized particles were in good agreement with theoretical predictions.     

Real-time shape-based particle separation and detailed in situ particle shape characterization

Particle shape is an important attribute in determining particle properties and behavior, but it is difficult to control and characterize. We present a new portable system that offers, for the first time, the ability to separate particles with different shapes and characterize their chemical and physical properties, including their dynamic shape factors (DSFs) in the transition and free-molecular regimes, with high precision, in situ, and in real-time. The system uses an aerosol particle mass analyzer (APM) to classify particles of one mass-to-charge ratio, transporting them to a differential mobility analyzer (DMA) that is tuned to select particles of one charge, mobility diameter, and for particles with one density, one shape. These uniform particles are then ready for use and/or characterization by any application or analytical tool. We combine the APM and DMA with our single-particle mass spectrometer, SPLAT II, to form the ADS and demonstrate its utility to measure individual particle compositions, vacuum aerodynamic diameters, and particle DSFs in two flow regimes for each selected shape. We applied the ADS to the characterization of aspherical ammonium sulfate and NaCl particles, demonstrating that both have a wide distribution of particle shapes with DSFs from approximately 1 to 1.5.     

Measurement of particle separation by magnetic resonance imaging

Magnetic resonance imaging (MRI) of water in filtration devices made from glass and/or plastic not only visualizes in three dimensions all the structural components, but also the time course of the particle separation process.     

Axial particle trajectory measurements in high-gradient magnetic separation

A technique has been developed for studying the trajectories of paramagnetic particles in a high-gradient magnetic separator. Human red blood cells are used as the paramagnetic particle and their trajectories in relation to a single magnetized stainless-steel wire are imaged. The flow velocity is parallel to the wire and has a value of approximately 50 μm . s^-1. The magnetic field of 1.4 T is applied perpendicular to the wire. A phase modulation technique is used to image the red blood cells with a resolution of less than 0.5 μm. The measured particle trajectories confirm the main features of current theoretical models.     

Near-field optical sensors for particle shape measurements

Two new optical particle shape sensors are introduced. By placing them directly in the near-field of the particle projection (Fresnel region), no lenses or additional optical components are required to obtain particle images. Besides size information, accurate shape information is also obtained. Simulations show that distortion by diffraction is limited and can be reduced even further. Both static and dynamic measurements have been performed, which show that the sensors work as predicted by theory. The sensors have been developed to be applied in a microfluidic cytometer.     

Pseudo-time particle filtering for diffuse optical tomography

We recast the reconstruction problem of diffuse optical tomography (DOT) in a pseudo-dynamical framework and develop a method to recover the optical parameters using particle filters, i.e., stochastic filters based on Monte Carlo simulations. In particular, we have implemented two such filters, viz., the bootstrap (BS) filter and the Gaussian-sum (GS) filter and employed them to recover optical absorption coefficient distribution from both numerically simulated and experimentally generated photon fluence data. Using either indicator functions or compactly supported continuous kernels to represent the unknown property distribution within the inhomogeneous inclusions, we have drastically reduced the number of parameters to be recovered and thus brought the overall computation time to within reasonable limits. Even though the GS filter outperformed the BS filter in terms of accuracy of reconstruction, both gave fairly accurate recovery of the height, radius, and location of the inclusions. Since the present filtering algorithms do not use derivatives, we could demonstrate accurate contrast recovery even in the middle of the object where the usual deterministic algorithms perform poorly owing to the poor sensitivity of measurement of the parameters. Consistent with the fact that the DOT recovery, being ill posed, admits multiple solutions, both the filters gave solutions that were verified to be admissible by the closeness of the data computed through them to the data used in the filtering step (either numerically simulated or experimentally generated).     

Accelerated particle electrophoretic motion and separation in converging-diverging microchannels

Accelerated particle electrophoretic motions were visualized in converging-diverging microchannels on poly(dimethylsiloxane) chips. The accelerated particle electrophoretic separation is highly desirable in on-chip flow cytometry and high-speed electrophoresis. The effects of electric field, particle size, particle trajectory, and channel structure on the particle electrophoretic motion are examined. We find that the ratio of the particle velocity in the throat to that in the straight channel is significantly lower than their cross-sectional area ratio. This discrepancy may be attributed to the locally higher electric field around the two poles of a particle, as compared to other regions inside the microchannel. We also find that the particle velocity ratio is increased for smaller particles moving through symmetric converging-diverging channels under lower electric fields. These variations may be attributed to the negative dielectrophoretic force that is generated by the nonuniform electric field in the converging-diverging section. In addition, we find that particle trajectory has insignificant influences on the maximum velocity ratio obtained in the throat.     

Gravity-driven microfluidic particle sorting device with hydrodynamic separation amplification

This paper describes a simple microfluidic sorting system that can perform size profiling and continuous mass-dependent separation of particles through combined use of gravity (1 g) and hydrodynamic flows capable of rapidly amplifying sedimentation-based separation between particles. Operation of the device relies on two microfluidic transport processes: (i) initial hydrodynamic focusing of particles in a microchannel oriented parallel to gravity and (ii) subsequent sample separation where positional difference between particles with different mass generated by sedimentation is further amplified by hydrodynamic flows whose streamlines gradually widen out due to the geometry of a widening microchannel oriented perpendicular to gravity. The microfluidic sorting device was fabricated in poly(dimethylsiloxane), and hydrodynamic flows in microchannels were driven by gravity without using external pumps. We conducted theoretical and experimental studies on fluid dynamic characteristics of laminar flows in widening microchannels and hydrodynamic amplification of particle separation. Direct trajectory monitoring, collection, and post-analysis of separated particles were performed using polystyrene microbeads with different sizes to demonstrate rapid (<1 min) and high-purity (>99.9%) separation. Finally, we demonstrated biomedical applications of our system by isolating small-sized (diameter <6 microm) perfluorocarbon liquid droplets from polydisperse droplet emulsions, which is crucial in preparing contrast agents for safe, reliable ultrasound medical imaging, tracers for magnetic resonance imaging, or transpulmonary droplets used in ultrasound-based occlusion therapy for cancer treatment. Our method enables straightforward, rapid, real-time size monitoring and continuous separation of particles in simple stand-alone microfabricated devices without the need for bulky and complex external power sources. We believe that this system will provide a useful tool to separate colloids and particles for various analytical and preparative applications and may hold potential for separation of cells or development of diagnostic tools requiring point-of-care sample preparation or testing.     

尘埃粒子计数器全粒径范围内计数效率的校准

为实现对尘埃粒子计数器全粒径范围内的计数效率校准,分别对尘埃粒子计数器(OPC)-凝结核粒子计数器(CPC)-气溶胶静电计(FCAE)的逐级溯源方法和光学显微镜计数方法进行技术研究,建立一套完整的校准方法和装置。结果表明:装置具有很好的溯源性,能保证国内关于尘埃粒子计数器计数效率的计量技术的准确性。     

Supercurrent versus temperature characteristics of cross-type Josephson junctions nearT c

The temperature dependence of the maximum Josephson supercurrent is studied in ranges nearT _c . The results are discussed in the framework of an approximate analysis for proximity systems based on the de Gennes theory and its application to several junctions.     

Free flow acoustophoresis: microfluidic-based mode of particle and cell separation

A novel method, free flow acoustophoresis (FFA), capable of continuous separation of mixed particle suspensions into multiple outlet fractions is presented. Acoustic forces are utilized to separate particles based on their size and density. The method is shown to be suitable for both biological and nonbiological suspended particles. The microfluidic separation chips were fabricated using conventional microfabrication methods. Particle separation was accomplished by combining laminar flow with the axial acoustic primary radiation force in an ultrasonic standing wave field. Dissimilar suspended particles flowing through the 350-microm-wide channel were thereby laterally translated to different regions of the laminar flow profile, which was split into multiple outlets for continuous fraction collection. Using four outlets, a mixture of 2-, 5-, 8-, and 10-microm polystyrene particles was separated with between 62 and 94% of each particle size ending up in separate fractions. Using three outlets and three particle sizes (3, 7, and 10 microm) the corresponding results ranged between 76 and 96%. It was also proven possible to separate normally acoustically inseparable particle types by manipulating the density of the suspending medium with cesium chloride. The medium manipulation, in combination with FFA, was further used to enable the fractionation of red cells, platelets, and leukocytes. The results show that free flow acoustophoresis can be used to perform complex separation tasks, thereby offering an alternative to expensive and time-consuming methods currently in use.     

Slurry particle size evolution during the polishing of optical glass

The particle size distribution of aqueous metal-oxide slurries can evolve during the polishing of optical glass in response to changes in mechanical and chemical process factors. The size-evolution phenomenon and its consequences were systematically studied in a planar continuous-polishing process. The concurrent application of electrokinetic techniques to characterize common optical shop materials has contributed new insight into the nature of silicate glass polishing by demonstrating the pivotal role of fluid chemistry, particularly pH, in maintaining electrokinetically favorable conditions for a welldispersed polishing agent. According to the proposed slurry-charge-control effect, a well-dispersed polishing agent is the key to obtaining the smoothest possible glass surfaces, especially when a recirculated slurry is used.     

Evaluation of a digital optical ionizing radiation particle track detector

An outline of an ionizing radiation particle track detector is presented which can in principle, determine the three-dimensional spatial distribution of all the secondary electrons produced by the passage of the ionizing radiation through a low-pressure (0.1–10 kPa) gas. The electrons in the particle track are excited by the presence of a high-frequency ac electric field, and two digital cameras image the optical radiation produced in electronic excitation collisions of the surrounding gas by the electrons. The specific requirements of the detector for neutron dosimetry and microdosimetry are outlined (i.e., operating conditions of the digital cameras, high voltage fields, gas mixtures, etc.) along with an estimate of the resolution and sensitivity achievable with this technique. The proposed detector is shown to compare favorably with other methods for obtaining the details of the track structure, particularly in the quality of the information obtainable about the particle track and the comparative simplicity and adaptability of the detector for measuring the secondary electron track structure for many forms of ionizing radiation over a wide range of energies.     

Phase separation in polyaniline with near-field scanning optical microscopy

We report the studies of conjugated polymers, polyaniline thin films, with a near-field scanning optical microscope. Because of the absorption variation in different oxidation states, transmission-mode near-field scanning optical microscope images were employed to map out the distribution of the oxidation states on a submicrometer scale. When the near-field wavelength is varied (between 632.8 and 543.5 nm), the phase separation between the oxidized and the reduced repeated units, with domain sizes on a nanometer-length scale, is observed.     

Fatigue crack separation and merging simulation using the smoothed particle hydrodynamics

International Journal of Fracture - The present paper proposes a new computational method for the fatigue crack propagation in a metallic structure using the smoothed particle hydrodynamics (SPH)....     

Limits of optical transmission measurements with application to particle sizing techniques

Considerable confusion exists regarding the applicability limits of the Bouguer-Lambert-Beer law of optical transmission. We review the derivation of the law and discuss its application to the optical thickness of the light-scattering medium. We demonstrate the range of applicability by presenting a method for determining particle size by measuring optical transmission at two wavelengths.     

Use of optical scattering to discriminate particle types in coastal waters

The particulate scattering characteristics of coastal waters were examined at nine locations around the United States, including near-shore sites in the Gulf of Mexico and the Atlantic and Pacific oceans. The scattering measurements were used in conjunction with inversion models to estimate particle size distributions and bulk refractive indices of the suspended particles. The relationships between various scattering properties and chlorophyll concentration were also investigated and compared with previous relationships described for case I waters. Although the general patterns of scattering and particle characteristics in coastal waters were fairly consistent, fine-scale variability within the water column was substantial. Combining optical measurements with inversion techniques provided a more informative view of the environment and a better understanding of the nature of particle populations in the coastal ocean.