Pinched flow fractionation: continuous size separation of particles utilizing a laminar flow profile in a pinched microchannel

A concept of "pinched flow fractionation" for the continuous size separation and analysis of particles in microfabricated devices has been proposed and demonstrated. In this method, particles suspended in liquid were continuously introduced into a microchannel having a pinched segment and were aligned to one sidewall in the pinched segment by another liquid flow without particles. The particles were then separated perpendicularly to the flow direction according to their sizes by the spreading flow profile inside the microchannel. Polymer microbeads were successfully separated, and the effects of the flow rate and channel shapes on the separation performance were examined. Also, separated particles were collected independently by making branches at the end of the pinched segment. Since this method utilizes only the laminar flow profile inside a microchannel, complicated outer field control could be eliminated, which is usually required for other kinds of particle separation methods such as field flow fractionation. Also, this method can be applied both for particle size analysis and for preparation of monodispersed particles, since separation can be rapidly and continuously performed.     

Size separation of single-wall carbon nanotubes by flow-field flow fractionation

Flow-field flow fractionation (flow-FFF) is used to separate single wall carbon nanotubes (SWNTs) dispersed in aqueous medium by the use of DNA. Online measurements are made of SWNT concentration, molar mass, and size by using UV-vis absorption and multiangle light scattering (MALS). Separations are made of both unfractionated SWNTs and SWNT fractions made by use of size exclusion chromatography (SEC). The SEC fractions are well resolved by flow-FFF. SWNT hydrodynamic volume from calibrations with polymer latex particles in flow-FFF are compared to calibrations of hydrodynamic volume from the SEC fractions derived from dissolved polymers. Rod lengths of the SWNTs are calculated from online measurements of MALS and those are compared to rod lengths from hydrodynamic models based on latex sphere calibrations. Samples with varied sizes were prepared by fracturing SWNTs through extended sonication. Flow-FFF of these fractured samples shows very broad size distributions compared to the original SEC and flow-FFF fractions.     

Circular asymmetrical flow field-flow fractionation for the semipreparative separation of particles

A new technique for the separation and characterization of particles and polymers based on asymmetrical flow field-flow fractionation was developed. The new circular asymmetrical flow field-flow fractionation instrument (CAFFFE) resembles a quasi-parallel arrangement of 12 individual flow channels. As compared to the classical asymmetrical flow field-flow fractionation (AF-FFF), which can be used so far only for analytical separation and characterization of particles and polymers, the CAFFFE allows the introduction of higher amounts of sample into the channel in a single run so that semipreparative to preparative separation becomes possible. This was demonstrated by the separation of polymer latex standards.     

Charged species transport, separation, and dispersion in nanoscale channels: autogenous electric field-flow fractionation

Numerical methods are employed to examine the transport of charged species in pressure-driven and electroosmotic flow along nanoscale channels having an electric double-layer thickness comparable to the channel size. In such channels, the electric field inherent to the double layer produces transverse species distributions that depend on species charge. Flow along the channel thus yields mean axial species speeds that also depend on the species charge, enabling species separation and identification. Here we characterize field-flow separations of this type via the retention and plate height. For pressure-driven flows, we demonstrate that mean species speeds along the channel are uniquely associated with a single species charge, allowing species separation based on charge alone. In contrast, electroosmotic flows generally yield identical speeds for several values of the charge, and these speeds generally depend on both the species charge and electrophoretic mobility. Coefficients of dispersion for charged species in both planar and cylindrical geometries are presented as part of this analysis.     

Correction for particle-wall interactions in the separation of colloids by flow field-flow fractionation

In the characterization of materials by field-flow fractionation (FFF), the experienced analyst understands the importance of incorporating additives in the carrier liquid that minimize or eliminate interactions between the analyte and accumulation wall, particularly in aqueous systems. However, as FFF is applied to more difficult samples, such as those with high surface energies, it is increasingly difficult to find additives that completely eliminate particle-wall interactions. Furthermore, the analyst may wish to use specific conditions that preserve the high surface energy of particles, to study their interaction with other materials through their behavior in the FFF channel. With this in mind, Williams and co-workers developed a model that quantifies the effect of particle-wall interactions in FFF using an empirically determined interaction parameter. In this work, the model is evaluated for the application of flow FFF in carrier liquids of low ionic strength, where particle-wall interactions are magnified. The retention of particles ranging in size from 64 to 1000 nm is measured using a wide range of field strengths and retention levels. The model is found to be generally valid over the entire range, except for minor discrepancies at lower levels of retention. Although retention levels are dramatically affected by particle-wall interactions, the point of steric inversion (500 nm), where the size-based elution order reverses, is not affected. When particle-wall interactions are not accounted for, they lead to a bias in particle sizes calculated from standard retention theory of up to 70%. The model can also be used to refine the measurement of channel thickness, which is important for the accurate conversion of retention parameters to particle sizes. In this work, for example, errors in channel thickness led to systematic errors on the order of 10% in particle diameter.     

Miniaturized flow fractionation device assisted by a pulsed electric field for nanoparticle separation

Electric field flow fractionation (EFFF) is a powerful separation technique based on an electrical field perpendicular to a pressure-driven flow. Previous studies of microelectric field flow fractionation (micro-EFFF) indicate that separation performance was limited due to a weak effective electric field caused by polarization layers on the electrode surfaces. In this work, we report on a micro-EFFF device that uses a pulsed voltage scheme to overcome these limitations. The device was fabricated in indium tin oxide (ITO)-coated glass with ITO as electrodes. The effective electric field for pulsed voltage operation was found to be 50-fold stronger when compared with constant voltage operation. A strong influence of pulsed voltage frequency on nanoparticle retention times was observed. Using pulsed voltage, improved separation of polystyrene particles of different surface charge and particle size is demonstrated. Pulsed voltage also offers more parameters compared to the constant voltage mode, e.g., pulse frequency, duty cycle, and waveform to optimize the retention behavior of analytes.     

Finite element simulation of pinched pressure-driven flow injection in microchannels

A pinched pressure-driven flow injection on a microchip is numerically simulated in order to optimize the relative values of the operational parameters. The geometry studied is a two-dimensional rectangular channel featuring a cross-junction with a large depth-over-width ratio. The hydrodynamic and convection-diffusion equations are solved for the two steps of the process: first, the sample solution is pinched into the transversal channel (injection channel), and then it is injected into the longitudinal one (separation channel), where the time evolution of the concentration is analyzed for different types of the detectors. Electroosmotic flow calculations have also been performed and have shown a good agreement with literature. The results for pressure-driven flow point out that the shape of the detection signal is strongly dependent on the velocity in the separation channel and on the position of the detection probes. The so-called double-humped peak, caused by the parabolic flow profile at high driving flow rate is analyzed. A tight pinch greatly decreases the amount of injected sample and, consequently, the signal sensitivity without increasing its quality. A proper pullback of the sample during the separation process can decrease the tailing due to the sample leakage from the injection channel. Although a high sample pullback causes a considerable decrease in the signal sensitivity, it also greatly enhances the peak resolution. Finally, it is shown that a wider injection channel with high sample pullback ensures an improved signal sensitivity with good resolution.     

Particle dispersion in a gas

The dispersion mechanism of particles in a gas is discussed from the viewpoint of nonequilibrium thermodynamics and kinetic theory. A heuristic equation, based on the quasi-Lorentz model developed by Mason and other workers, for the particle velocity in the presence of a gradient of pressure and temperature is proposed for all values of a parameters, wheres is defined as the ratio of the particle radiusr to a multiple of the mean free path, λ, of the gas medium. A schematic calculation demonstrates the conditions under which molecular diffusion could play a role in a realistic atmospheric pollution problem. Paper dedicated to Professor Edward A. Mason.     

DNA-assisted dispersion and separation of carbon nanotubes

Carbon nanotubes are man-made one-dimensional carbon crystals with different diameters and chiralities. Owing to their superb mechanical and electrical properties, many potential applications have been proposed for them. However, polydispersity and poor solubility in both aqueous and non-aqueous solution impose a considerable challenge for their separation and assembly, which is required for many applications. Here we report our finding of DNA-assisted dispersion and separation of carbon nanotubes. Bundled single-walled carbon nanotubes are effectively dispersed in water by their sonication in the presence of single-stranded DNA (ssDNA). Optical absorption and fluorescence spectroscopy and atomic force microscopy measurements provide evidence for individually dispersed carbon nanotubes. Molecular modelling suggests that ssDNA can bind to carbon nanotubes through pi-stacking, resulting in helical wrapping to the surface. The binding free energy of ssDNA to carbon nanotubes rivals that of two nanotubes for each other. We also demonstrate that DNA-coated carbon nanotubes can be separated into fractions with different electronic structures by ion-exchange chromatography. This finding links one of the central molecules in biology to a technologically very important nanomaterial, and opens the door to carbon-nanotube-based applications in biotechnology.     

Size separation of supermicrometer particles in asymmetrical flow field-flow fractionation. Flow conditions for rapid elution

The performance of lift-hyperlayer asymmetrical flow field-flow fractionation using rapid elution conditions was tested through the separation of standard polystyrene latex particles of diameters from 2 to 20 microm. Optimization of flowrates was studied not only in order to obtain efficient and rapid separation, but also to work under conditions of various shape and steepness of the axial flow velocity gradient. Using extreme flow conditions, the five widely spaced particle sizes, 20.5-, 15.0-, 9.7-, 5.0-, and 2.0-microm diameter, could be resolved in 6 min, whereas for the narrower size range of 20.5-5.0 microm, 1 min was enough. The size selectivity in the size range 9.7-2.0 microm was studied as a function of flowrates and particle size and was found to be constant. A particle trapping device made it possible to separate particles of sizes > 10 microm, which has previously proven to be difficult in asymmetrical channels.     

Particle size fraction monitoring of dispersion liquids

Scientific and technical principles are considered for building photometric analyzers for monitoring the particle size fraction parameters of dispersion liquids, i.e., numerical concentration and sizes of particles suspended in liquids. __________ Translated from Izmeritel’naya Tekhnika, No. 7, pp. 67–72, July, 2008.     

颗粒弥散强化复相陶瓷临界颗粒尺寸的优化设计

本文应用弹性力学和Ｇｒｉｆｆｉｔｈ微裂纹理论，对颗粒弥散强化陶瓷材料内的热应力进行了分析，导出了临界粒尺寸的计算公式，为颗粒弥散强化复相陶瓷材料的界面设计和颗粒尺寸选择提供了依据。     

Particle dispersion in turbulent sedimentary duct flows

A simplified geometric model of a circular tube with a sedimentary layer is proposed and named as sedimentary duct. Based on h = 1/4d (h is the thickness of sedimentary layer and d is the pipe diameter), the flow field (Re = 40000) and particle distribution (5, 10, 50 μm and St = 0.6, 2.5, 63) in the sedimentary duct are simulated using Large Eddy Simulation (LES) coupled with Lagrange Particle Tracking (LPT) method. As a result, four streamwise eddies are found in the sedimentary duct as distributed in pairs near the corner. The eddy center near ceiling is found to be farther from the corner than that from the floor. Small particles (5 μm, St = 0.6) tend to move with the secondary flow as their upward movements distribute in both sides. Their centripetal movement is near the floor and preferential distribution near the top. For large particles (50 μm, St = 63), it is the drag force that dominates their motion while for medium particles (10 μm, St = 2.5) lift force may have significant influence on their motion. This study is the first work to investigate the characteristics of particle behavior in turbulent sedimentary duct flows.     

Development of non-gel-based two-dimensional separation of intact proteins by an on-line hyphenation of capillary isoelectric focusing and hollow fiber flow field-flow fractionation

A rapid, non-gel-based, on-line, two-dimensional separation method is introduced for proteome analysis. Protein fractionation was carried out by first exploiting the differences in their respective isoelectric points (pI) in a Teflon capillary using isoelectric focusing (IEF), followed by a molecular weight (MW)-based separation in a hollow fiber by flow field-flow fractionation (FlFFF). The method developed here (CIEF-HFFlFFF) may be a powerful alternative to two-dimensional polyacrylamide gel electrophoresis, which is currently used for the separation and purification of proteins. In CIEF-HFFlFFF, proteins can be collected as a fraction of a certain pI and MW interval without being denatured. Additionally, the ampholyte solution is simultaneously removed during separation in the hollow fiber, and the overall process time is significantly reduced. This method was applied to a human urinary proteome sample, leading to the identification of 114 proteins with the subsequent off-line use of nanoflow liquid chromatography-tandem mass spectrometry after the tryptic digestion of each collected protein fraction.     

Global attractors of pinched skew products

A class of skew products over irrational rotations of the circle is defined which contains some systems which have strange non-chaotic attractors. The global attractor of these systems is characterized: it lies between an upper semi-continuous curve and a lower semi-continuous curve. With additional assumptions on the class of maps considered more detail of the attractor can be given.     

Miniaturization of frit inlet asymmetrical flow field-flow fractionation

A miniaturized frit inlet asymmetrical flow field-flow fractionation (mFI-AFlFFF) channel has been constructed and tested for the separation of proteins. By scaling down the geometrical channel dimension of a conventional FI-AFlFFF system, flow rate ranges that can be manipulated were decreased to 20-30 microL/min, which reduces the injection amount of sample materials. The end effect contribution to plate height was evaluated by varying the inner diameter of the connection tubing between the injector and the channel inlet at various injection flow rates, and the results showed that the use of silica capillary tubing of the shortest possible distance is essential in reducing the initial band broadening prior to the sample injection to the microscale channel. The capability of the microFI-AFlFFF system was demonstrated with the separation of protein standards, polystyrenesulfonates, and ssDNA strains and for the characterization of replication protein A-ssDNA binding complex regulated by redox status.     

Capillary magnetic field flow fractionation and analysis of magnetic nanoparticles

This paper reports the purification and analysis of magnetic nanoparticles using capillary magnetic field flow fractionation, which utilizes an applied magnetic field oriented orthogonal to the capillary flow. To validate this approach as a separation method for nanometer-scale particles, samples of magnetic nanoparticles composed of either gamma-Fe2O3 (maghemite) or CoFe2O4 with average diameters ranging from 4 to 13 nm were prepared and characterized by transmission electron microscopy and SQUID magnetometry. Retention of the samples on the capillary was investigated as a function of solvent flow rate and the nanoparticle size and composition; the elution times of the nanoparticles are strongly dependent on their magnetic moments. We demonstrate the use of this method to separate a mixture of nanoparticles into size-monodisperse fractions. The magnetic moments of the particles are calculated based on analysis of the retention parameters and correlate with values obtained in separate SQUID magnetometry measurements.