Continuous and precise particle separation by electroosmotic flow control in microfluidic devices

A new scheme has been described for continuous particle separation using EOF in microfluidic devices. We have previously reported a method for particle separation, called "pinched flow fractionation (PFF)", in which size-dependent and continuous particle separation can be achieved by introducing pressure-driven flows with and without particles into a pinched microchannel. In this study, EOF was employed to transport fluid flows inside a microchannel. By controlling the applied voltage to electrodes inserted in each inlet/outlet port, the flow rates from both inlets, and flow rates distributed to each outlet could be accurately tuned, thus enabling more effective separation compared to the pressure-driven scheme. In the experiment, the particle behaviors were compared between EOF and pressure-driven flow schemes. In addition, micrometer- and submicrometer-sized particles were accurately separated and individually collected using a microchannel with multiple outlet branch channels, demonstrating the high efficiency of the presented scheme.     

Continuous particle separation in a microchannel having asymmetrically arranged multiple branches

A new method for continuous size separation and collection of particles in microfabricated devices, asymmetric pinched flow fractionation (AsPFF), has been proposed and demonstrated. This method improves the separation scheme of pinched flow fractionation (PFF), which utilizes a laminar flow profile inside a microchannel. In this study, multiple branch channels with different channel dimensions were arranged at the end of the pinched segment, so that the flow rate distributions to each branch channel were varied, and a large part of the liquid was forced to go through one branch channel (drain channel). In the proposed channel system, the flow profile inside the microchannel was asymmetrically amplified, enabling the separation of one-order smaller particles compared with PFF. After introducing the method, we examined the effect of the asymmetric amplification by controlling the outlet of the drain channel. Also, a mixture of 1.0 approximately 5.0 microm particles was separated, and erythrocytes were successfully separated from blood. The results indicate that the AsPFF method could be applied to the separation of much smaller-size particles, since more precise separation can be achieved simply by changing the geometries of branch channels.     

Continuous and size-dependent sorting of emulsion droplets using hydrodynamics in pinched microchannels

In this report, a microfluidic system is presented for continuous and size-dependent separation of droplets utilizing microscale hydrodynamics. The separation scheme is based on laminar-flow focusing and spreading in a pinched microchannel, referred to as "pinched flow fractionation (PFF)", which was previously developed for the size-dependent separation of solid particles, such as polymer microparticles or cells. By simply introducing emulsion and the continuous phase into a microchannel, continuous separation could be achieved without using complicated operations or devices. We first examined whether this scheme could be applied for droplets, by using a pinched microchannel with one outlet, and observed the behaviors of monodisperse droplets generated at the upstream T-junction. Analysis via high-speed imaging revealed that the length of the pinched segment is critical for precise separation of droplets. Then, separation of a polydisperse oil-in-water emulsion that was prepared previously was demonstrated using a microfluidic device equipped with multiple outlets. These results showed the ability of the presented system to sort or select specific-sized droplets easily and accurately, which would be difficult to achieve using normal-scale schemes, such as centrifugation or filtration.     

Pinched flow fractionation devices for detection of single nucleotide polymorphisms

We demonstrate a new and flexible microfluidic based method for genotyping single nucleotide polymorphisms (SNPs). The method relies on size separation of selectively hybridized polystyrene microspheres in a microfluidic pinched flow fractionation (PFF) device. The microfluidic PFF devices with 13 mum deep channels were fabricated by thermal nanoimprint lithography (NIL) in a thin film of cyclic-olefin copolymer (mr-I T85) on a silicon wafer substrate, and the channels were sealed by thermal polymer bonding. Streptavidin coated polystyrene microspheres with a mean diameter of 3.09 microm and 5.6 microm were functionalized with biotin-labeled oligonucleotides for the detection of a mutant (Mt) or wild-type (Wt) DNA sequence in the HBB gene, respectively. Hybridization to functionalized beads was performed with fluorescent targets comprising synthetic DNA oligonucleotides or amplified RNA, synthesized using human DNA samples from individuals with point mutations in the HBB gene. Following a stringent wash, the beads were separated in a PFF device and the fluorescent signal from the beads was analyzed. Patients being wildtypes, heterozygotes or mutated respectively for the investigated mutation could reliably be diagnosed in the PFF device. This indicates that the PFF technique can be used for accurate and fast genotyping of SNPs.     

Microparticles manipulation and enhancement of their separation in pinched flow fractionation by insulator‐based dielectrophoresis

The separation and manipulation of microparticles in lab on a chip devices have importance in point of care diagnostic tools and analytical applications. The separation and sorting of particles from biological and clinical samples can be performed using active and passive techniques. In passive techniques, no external force is applied while in active techniques by applying external force (e.g. electrical), higher separation efficiency is obtained. In this article, passive (pinched flow fractionation) and active (insulator‐based dielectrophoresis) methods were combined to increase the separation efficiency at lower voltages. First by simulation, appropriate values of geometry and applied voltages for better focusing, separation, and lower Joule heating were obtained. Separation of 1.5 and 6 μm polystyrene microparticles was experimentally obtained at optimized geometry and low total applied voltage (25 V). Also, the trajectory of 1.5 μm microparticles was controlled by adjusting the total applied voltage.     

Atomic force microscopy colloid-probe measurements with explicit measurement of particle-solid separation

We describe the use of evanescent wave scattering to measure the separation between the surface of a solid and a particle that is attached to an atomic force microscope (AFM) cantilever. Termed evanescent wave atomic force microscopy, our approach involves measuring the intensity of the light scattered from an evanescent field formed by the total internal reflection of a laser beam at a solid/fluid interface. In a conventional AFM "colloid probe" measurement, this separation must be inferred from an examination of the surface forces. Direct measurement of this separation with an evanescent wave atomic force microscope (EW-AFM) removes some ambiguity in the surface force measurement and, in addition, allows new types of measurements. For example, the force can be monitored at a constant separation. Our evanescent scattering apparatus is essentially identical to that used in total internal reflection microscopy (TIRM), except that we collect the light that scatters back into the incident medium, because the AFM partly obscures the forward scattered light (i.e., light scattered into the transmitted region). Compared to a conventional TIRM measurement, where the particle moves freely, attaching the particle to the cantilever in an EW-AFM gives much greater control of the particle position.     

光色谱研究进展

对光色谱分离原理、理论研究及实验应用等方面的进展进行了评述,对光色谱分离技术的发展方向和应用前景进行了展望。     

Separation of micro and sub-micro diamagnetic particles in dual ferrofluid streams based on negative magnetophoresis

In the present study, we numerically demonstrate an approach for separation of micro and sub-micro diamagnetic particles in dual ferrofluid streams based on negative magnetophoresis. The dual streams are constructed by an intermediate sheath flow, after which the negative magnetophoretic force induced by an array of permanent magnets dominates the separation of diamagnetic particles. A simple and efficient numerical model is developed to calculate the motions of particles under the action of magnetic field and flow field. Effects of the average flow velocity, the ratio of sheath fluid flow to sample fluid flow, the number of the magnet pair as well as the position of magnet pair are investigated. The optimal parametric condition for complete separation is obtained through the parametric analysis, and the separation principle is further elucidated by the force analysis. The separation of smaller micro and sub-micro diamagnetic particles is finally demonstrated. This study provides an insight into the negative magnetophoretic phenomenon and guides the fabrication of feasible, low-cost diagnostic devices for sub-micro particle separation.     

Behavior of double emulsions in a cross-type optical separation system

The behavior of double emulsions in a cross-type optical particle separation system was studied for different combinations of refractive indices and different inner and outer layer radii. The radii and refractive indices of the double emulsions were easily adjusted by taking advantage of the coflowing geometry of a cross-type optical particle separation device. An analytical expression of the optical forces on a pair of concentric spheres was derived using the photon stream method in the ray optics regime. The predicted trajectories of the double emulsions by the optical force agreed well with the experimental data. This work has potential uses in cell separation by morphometry, drug delivery vehicle, and emulsion-based biomedical applications.     

"Vector Chromatography": Modeling Micropatterned Separation Devices

A repetitive sequence of quiescent fluid layers of differing viscosities through which small spherical Brownian particles move is analyzed in order to illustrate in a simple context how the theory of macrotransport processes, a generalization of Taylor dispersion theory, may be employed to rigorously analyze spatially periodic micropatterned chromatographic separation devices for circumstances in which the solute species to be separated are animated by the action of species-specific external forces oriented asymmetrically relative to the body-fixed pattern. In the generic "vector" separation scheme, illustrated by our elementary example, the different species undergoing separation move, on average, in different directions relative to pattern-fixed axes, whence their chromatographic sorting is effected according to their different mean angular trajectories through the device. This scheme differs fundamentally from traditional "scalar" chromatographic separation schemes, wherein all species move on average parallel to the animating force (including circumstances in which they are passively entrained in a unidirectional solvent flow) and hence for which the sorting is effected by the relative speeds of the several species through the chromatographic column. Vector chromatography is quantified by two global "macrotransport coefficients", namely the solute mobility dyadic M* (representing the tensor proportionality coefficient between the mean solute velocity vector U* and the external force vector F acting upon the solute molecules) and the dispersivity dyadic D* (resulting from the deviation of the instantaneous position of the particle from its mean position based upon its mean velocity vector). In the present example these coefficients are studied parametrically as functions of: (i) the orientation of the external force relative to the symmetry axis of the fluid layers; (ii) the local viscosity distribution within a layer; (iii) the vector particle Peclet number (constructed from the vector force, the length of the viscosity period, and the Boltzmann factor kT); and (iv) the thermodynamic interphase solute partition distribution coefficient between the two fluid layers comprising a unit cell. Copyright 2001 Academic Press.     

Electrostatic polarization is crucial for reproducing pKa shifts of carboxylic residues in Turkey ovomucoid third domain

We have computed pKa shifts for carboxylic residues of the serine protease inhibitor turkey ovomucoid third domain (residues Asp7, Glu10, Glu19, Asp27, and Glu43). Both polarizable and nonpolarizable empirical force fields were employed. Hydration was represented by the surface generalized Born and Poisson-Boltzmann continuum model. The calculations were carried out in the most physically straightforward fashion, by directly comparing energies of the protonated and deprotonated protein forms, without any additional parameter fitting or adjustment. Our studies have demonstrated that (i) the Poisson-Boltzmann solvation model is more than adequate in reproducing pKa shifts, most likely due to its intrinsically many-body formalism; (ii) explicit treatment of electrostatic polarization included in our polarizable force field (PFF) calculations appears to be crucial in reproducing the acidity constant shifts. The average error of the PFF results was found to be as low as 0.58 pKa units, with the best fixed-charges average deviation being 3.28 units. Therefore, the pKa shifts phenomena and the governing electrostatics are clearly many-body controlled in their intrinsic nature; (iii) our results confirm previously reported conclusions that pKa shifts for protein residues are controlled by the immediate environment of the residues in question, as opposed to long-range interactions in proteins. We are confident that our confirmation of the importance of explicit inclusion of polarization in empirical force fields for protein studies will be useful far beyond the immediate goal of accurate calculation of acidity constants.     

Orthogonal optical force separation simulation of particle and molecular species mixtures under direct current electroosmotic driven flow for applications in biological sample preparation

Presented here are the results from numerical simulations applying optical forces orthogonally to electroosmotically induced flow containing both molecular species and particles. Simulations were conducted using COMSOL v4.2a Multiphysics® software including the particle tracking module. The study addresses the application of optical forces to selectively remove particulates from a mixed sample stream that also includes molecular species in a pinched flow microfluidic device. This study explores the optimization of microfluidic cell geometry, magnitude of the applied direct current electric field, EOF rate, diffusion, and magnitude of the applied optical forces. The optimized equilibrium of these various contributing factors aids in the development of experimental conditions and geometry for future experimentation as well as directing experimental expectations, such as diffusional losses, separation resolution, and percent yield. The result of this work generated an optimized geometry with flow conditions leading to negligible diffusional losses of the molecular species while also being able to produce particle removal at near 100% levels. An analytical device, such as the one described herein with the capability to separate particulate and molecular species in a continuous, high‐throughput fashion would be valuable by minimizing sample preparation and integrating gross sample collection seamlessly into traditional analytical detection methods.     

Optical observation of gas bridging between hydrophobic surfaces in water

The very strong and long-ranged interaction force between hydrophobic surfaces in water has been a debated issue for a long time. Recent studies suggest that the long-range attraction is attributable to the bridging of nanoscopic bubbles attached on the surfaces. However, it is still unclear at present whether such a bridging is able to exit stably or not. To clarify the existence of the gas bridge, we conducted the optical observation and the force measurement between the hydrophobic glass particle and plate in water simultaneously, using a combined apparatus of an atomic force microscope and an optical inverted microscope. It is found that (i) the image of the dark ring of ca. 1 mum in diameter appears in the region where the existence of the bridge is confirmed by force curves, but disappears when the separation between surfaces becomes shorter than the low limit of the wavelength of visible light, and (ii) the sudden disappearance of the image coincides well with the breakage of the bridge estimated from the separating force curve. The results obtained here are consistent with the above-mentioned mechanism for the long-range attraction between hydrophobic surfaces in water.     

Simultaneous investigation of sedimentation and diffusion of a single colloidal particle near an interface

We describe here a new procedure for the simultaneous investigation of sedimentation and diffusion of a colloidal particle in close proximity to a solid, planar wall. The measurements were made using the optical technique of total internal reflection microscopy, coupled with optical radiation pressure, for dimensionless separation distances (gap width/radius of particle) ranging from 0.01 to 0.05. In this region, the hydrodynamic mobility and diffusion coefficient are substantially reduced below bulk values. The procedure involved measuring the mean and the variance of vertical displacements of a Brownian particle settling under gravity toward the plate. The spatially varying diffusion coefficient was calculated from the displacements at small times (where diffusive motion was dominant). The mobility relationship for motion normal to a flat plate was tested by measuring the average distance of travel versus time as the particle settled under the constant force of gravity. For the simple Newtonian fluid used here (aqueous salt solution), the magnitude of the diffusion coefficient and mobility, plus their dependence on separation distance, showed excellent agreement with predictions. This new technique could be of great value in measuring the mobility and diffusion coefficient for near-contact motion in more complex fluids for which the hydrodynamic correction factors are not known a priori, such as shear-thinning fluids.     

Importance of electrostatic polarizability in calculating cysteine acidity constants and copper(I) binding energy of Bacillus subtilis CopZ

CopZ is a copper chaperone from Bacillus subtilis. It is an important part of Cu(I) trafficking. We have calculated pK_a values for the CXXC motif of this protein, which is responsible for the Cu(I) binding, and the Cu(I) binding constants. Polarizable and fixed‐charges formalisms were used, and solvation parameters for the both models have been refitted. We had to partially redevelop parameters for the protonated and deprotonated cysteine residues. We have discovered that the polarizable force field (PFF) is qualitatively superior and allows a uniformly better level of energetic results. The PFF pK_a values for cysteine are within about 0.8–2.8 pH units of the experimental data, while the fixed‐charges OPLS formalism yields errors of up to tens of units. The PFF magnitude of the copper binding energy is about 10 kcal/mol or 50% higher than the experimental value, while the using the refitted OPLS parameters leads to an overall positive binding energy, thus predicting no thermodynamically stable complex. At the same time, the agreement of the polarizable S···Cu(I) distances with the experimental results is within 0.08 Å range, and the nonpolarizable calculations lead to an error of about 0.4 Å. Moreover, the accuracy of the PFF has been achieved without any explicit fitting to either pK_a or CopZ···Cu(I) binding energies. We believe that this makes our polarizable technique a choice method in reproducing protein—copper binding and further supports the notion that explicit treatment of electrostatic polarization is crucial in many biologically relevant studies, especially ion binding and transport. © 2012 Wiley Periodicals, Inc.     

Electroviscous Forces on a Charged Cylinder Moving Near a Charged Wall

The refined theory of the electroviscous lift forces is presented for the case when the separation distance between the particle and the wall is larger than the double-layer thickness. The theory is based on the lubrication approximation for motion of a long cylinder near a solid wall in creeping flow. The approximate analytical formula for the lift force valid for Pe相似文献   

Continuous separation of particles using a microfluidic device equipped with flow rate control valves

We propose herein an improved microfluidic system for continuous and precise particle separation. We have previously proposed a method for particle separation called "pinched flow fractionation." Using the previously reported method, particles can be continuously separated according to differences in their diameters, simply by introducing liquid flows with and without particles into a specific microchannel structure. In this study, we incorporated PDMS membrane microvalves for flow rate control into the microfluidic device to improve the separation accuracy. By adjusting the flow rates distributed to each outlet, target particles could be precisely collected from the desired outlet. We succeeded in separating micron and submicron-size polymer particles. This method can be used widely for continuous and precise separation of various kinds of particles, and can function as an important part of microfluidic systems.     

The liquid flow force on a particle in the bubble-particle interaction in flotation

In this paper the problem of calculating the liquid flow force on a particle in interaction with an air bubble with a mobile surface in flotation as a function of the separation distance was solved. The force equation was obtained by first deriving the disturbed flow confined between the surfaces. The model for the force includes the separation distance between the bubble and the particle, the particle size, the bubble's Reynolds number, the bubble rise velocity, and the polar position of the particle on the bubble surface. The proposed equations provide an exact solution to the situation where the particle and the bubble are very close together. The attractive flow force and the surface forces are of similar orders of magnitude. Consequently, the models presented in this paper should provide a better estimate for calculating the forces on particles interacting with air bubbles in mineral flotation and other separation operations involving colloidal interactions.     

Direct measurement of forces between a colloidal particle and a phospholipid bilayer

Colloidal interaction forces between a silica particle and a solid-supported Langmuir-Schaefer phospholipid bilayer were directly measured using a gradient optical trap and evanescent wave light scattering. A small custom-built Langmuir trough was integrated with an optical trapping microscope to allow force measurements on a single particle within the subphase of the trough after the dip of the substrate was completed. The novel method allows the force measurements to be conducted without transferring the substratum across an air/water interface. The fluctuating particle position near the bilayer was tracked by evanescent wave light scattering to determine the deflection due to surface forces, and the relaxation time of particle fluctuations was measured to simultaneously determine the viscous forces. Measured equilibrium and viscous force-distance profiles of silica microspheres with diameters of 1 and 5 microm on bilayers of dipalmitoyl phosphatidyl choline (DPPC) were markedly different than force-distance on bare mica and DPPC monolayers under the same electrolyte conditions.