WALL DEPOSITION OF SMALL SUSPENDED PARTICLES IN A TURBULENT CHANNEL FLOW

The diffusion of small suspended particles in a turbulent channel flow is studied by solving the transport advection-diffusion equation. The mean flowfield in the channel is simulated using a two-equation k-ε turbulence model. Deposition velocity is evaluated at different sections in the channel for different particle sizes and flow Reynolds numbers. The effects of turbulence dispersion and Brownian diffusion on particle deposition velocity are discussed. The variation of particle deposition velocity with particle diameter, density and flow Reynolds number are analyzed. The wall deposition velocities for different size particles are compared with those obtained by other models.     

The analytical approach to polydimethylsiloxane microfluidic technology and its biological applications

This review article discusses PDMS (polydimethylsiloxane) microfluidic devices and their biological applications. First, the already developed devices are classified from the viewpoints of underlying technology within a common logical framework comprising single-layer, multilayer, and integrated devices, as well as surface chemistry modifications of PDMS. Combinatorial techniques are applied to re-derive existing devices within this framework. Next, the relevant scales of both microfluidics and biology are compared, obtaining the promise and limitations of PDMS microfluidics. Finally, the body of work is reclassified in terms of addressed biological applications and compared to the standard methods in cellular and molecular biology, to offer insights for future devices and applications.     

Incorporating domain growth into hybrid methods for reaction–diffusion systems

Reaction–diffusion mechanisms are a robust paradigm that can be used to represent many biological and physical phenomena over multiple spatial scales. Applications include intracellular dynamics, the migration of cells and the patterns formed by vegetation in semi-arid landscapes. Moreover, domain growth is an important process for embryonic growth and wound healing. There are many numerical modelling frameworks capable of simulating such systems on growing domains; however, each of these may be well suited to different spatial scales and particle numbers. Recently, spatially extended hybrid methods on static domains have been produced to bridge the gap between these different modelling paradigms in order to represent multi-scale phenomena. However, such methods have not been developed with domain growth in mind. In this paper, we develop three hybrid methods on growing domains, extending three of the prominent static-domain hybrid methods. We also provide detailed algorithms to allow others to employ them. We demonstrate that the methods are able to accurately model three representative reaction–diffusion systems accurately and without bias.     

Colloidal Crystals from Microfluidics

Colloidal crystals are of great interest to researchers because of their excellent optical properties and broad applications in barcodes, sensors, displays, drug delivery, and other fields. Therefore, the preparation of high quality colloidal crystals in large quantities with high speed is worth investigating. After decades of development, microfluidics have been developed that provide new choices for many fields, especially for the generation of functional materials in microscale. Through the design of microfluidic chips, colloidal crystals can be prepared controllably with the advantages of fast speed and low cost. In this Review, research progress on colloidal crystals from microfluidics is discussed. After summarizing the classifications, the generation of colloidal crystals from microfluidics is discussed, including basic colloidal particles preparation, and their assembly inside or outside of microfluidic devices. Then, applications of the achieved colloidal crystals from microfluidics are illustrated. Finally, the future development and prospects of microfluidic‐based colloidal crystals are summarized.     

New generation of amino coumarin methyl sulfonate-based fluorogenic substrates for amidase assays in droplet-based microfluidic applications

Droplet-based microfluidics is a powerful tool for biology and chemistry as it allows the production and the manipulation of picoliter-size droplets acting as individual reactors. In this format, high-sensitivity assays are typically based on fluorescence, so fluorophore exchange between droplets must be avoided. Fluorogenic substrates based on the coumarin leaving group are widely used to measure a variety of enzymatic activities, but their application in droplet-based microfluidic systems is severely impaired by the fast transport of the fluorescent product between compartments. Here we report the synthesis of new amidase fluorogenic substrates based on 7-aminocoumarin-4-methanesulfonic acid (ACMS), a highly water-soluble dye, and their suitability for droplet-based microfluidics applications. Both substrate and product had the required spectral characteristics and remained confined in droplets from hours to days. As a model experiment, a phenylacetylated ACMS was synthesized and used as a fluorogenic substrate of Escherichia coli penicillin G acylase. Kinetic parameters (k(cat) and K(M)) measured in bulk and in droplets on-chip were very similar, demonstrating the suitability of this synthesis strategy to produce a variety of ACMS-based substrates for assaying amidase activities both in microtiter plate and droplet-based microfluidic formats.     

The diffusion coefficient of Brownian particles in a rapidly fluctuating periodic potential field

An exact formula describing the effective coefficient of diffusion for Brownian particles in a rapidly fluctuating periodic potential field is derived. It is shown that, irrespective of the particular form of the potential, the particle migration is accelerated as compared to the case of free diffusion. The values of diffusion coefficients for several particular potential profiles are calculated.     

A statistical study on nanoparticle movements in a microfluidic channel

Microfluidic channels have received much attention because they can be used to control and transport nanoscale objects such as nanoparticles, nanowires, carbon nanotubes, DNA and cells. However, so far, practical channels have not been easy to design because they require very expensive fabrication and sensitive experiments. Numerical approaches can be alternatives or supplementary measures to predict the performance of new channels because they efficiently explain nanoscale multi-physics phenomena and successfully solve nanowire alignment and cell adhesion problems. In this paper, a newly updated immersed finite element method that accounts for collision force and Brownian motion as well as fluid-solid interaction is proposed, and is applied to simulate nanoparticle movements in a microfluidic channel. As part of the simulation, Brownian motion effects in a single nanoparticle focusing lens system are examined under different temperature conditions, and the resulting transport efficiencies are discussed. Furthermore, nanoparticle movements in a double focusing lens system are predicted to show the enhancement of focusing efficiency.     

Stochastic model of ultrafine particle deposition and clearance in the human respiratory tract

Deposition and clearance of insoluble ultrafine particles, ranging from 1 to 100 nm, were simulated by stochastic models using Monte Carlo methods. Brownian motion is the dominant mode of deposition in human airways. The additional effects of convective diffusion in bifurcations and axial diffusion (convective mixing) primarily affect particle transport and deposition of particles in the 1-10 nm range. Regarding total deposition, the effects of both convective mechanisms are practically compensated by the concomitant effect of molecular radial diffusion (Brownian motion). During the first hours following inhalation, 1 nm particles are predicted to be cleared much faster than particles in the size range from 10 to 100 nm, with a retained fraction of about 80% after 24 h. For 1-10 nm particles, extracellular transfer to blood is the most likely mode of clearance, while uptake and subsequent accumulation in epithelial cells are assumed to be the preferential mechanisms for 10-100 nm particles.     

An equilibrium method for continuous-flow cell sorting using dielectrophoresis

Separations represent a fundamental unit operation in biology and biotechnology. Commensurate with their importance is the diversity of methods that have been developed for performing them. One important class of separations are equilibrium gradient methods, wherein a medium with some type of spatial nonuniformity is combined with a force field to focus particles to equilibrium positions related to those particles' intrinsic properties. A second class of techniques that is nonequilibrium exploits labels to sort particles based upon their extrinsic properties. While equilibrium techniques such as iso-electric focusing (IEF) have become instrumental within analytical chemistry and proteomics, cell separations predominantly rely upon the second, label-based class of techniques, exemplified by fluorescence-activated cell sorting (FACS) and magnetic-activated cell sorting (MACS). To extend the equilibrium techniques available for separating cells, we demonstrate the first implementation of a new microfluidic equilibrium separation method, which we call isodielectric separation (IDS), for sorting cells based upon electrically distinguishable phenotypes. IDS is analogous to isoelectric focusing, except instead of separating amphoteric molecules in a pH gradient using electrophoresis, we separate cells and particles in an electrical conductivity gradient using dielectrophoresis. IDS leverages many of the advantages of microfluidics and equilibrium gradient separation methods to create a device that is continuous-flow, capable of parallel separations of multiple (>2) subpopulations from a heterogeneous background, and label-free. We demonstrate the separation of polystyrene beads based upon surface conductance as well as sorting nonviable from viable cells of the budding yeast Saccharomyces cerevisiae.     

Concentration fluctuations and particle self-diffusion in monodispersed systems

The dispersion of a random field of the volume concentration is calculated on the basis of the thermodynamic theory of flucturations in limiting situations of finely and coarsely dispersed systems. A representation for the spectral density of this field is written, and concentration dependences of Brownian and isotropic pseudoturbulent self-diffusion of particles in the indicated situations are found.Ural State University, Ekaterinburg. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 65, No. 1, pp. 39–47, July, 1993.     

A numerical study on dynamic inertial focusing of microparticles in a confined flow

Inertial microfluidics is regarded as a promising approach to facilitate precise, robust and continuous manipulation of particles through inertial focusing of particles in microchannels. Although there is a need to gain rich insights into the focusing dynamics of particles, it has been hardly studied numerically. In this study, the complex focusing dynamics of particles is simulated numerically for multi-particle suspensions in confined microchannels. To this end, we develop a new method that couples the discrete element method (DEM) with the direct numerical simulation (DNS). This method is referred to as the DEM–DNS method. In order to validate the DEM–DNS method, we then investigate complex dependence of particle behaviour on Reynolds number and channel geometries. With good agreement between the numerical results and existing observations, it is shown for the first time that the DEM–DNS method can simulate the counterintuitive focusing dynamics of particles. This study thus establishes that the DEM–DNS method is a powerful tool to examine the focusing dynamics of particles in inertial microfluidics.     

Computer simulations of electrokinetic transport in microfabricated channel structures

A mathematical model describing electrokinetically driven mass transport phenomena in microfabricated chip devices is presented in this paper. The model accounts for principal material transport mechanisms such as electrokinetic migration (electrophoresis and electroosmosis) and diffusion. A computer code that implements the model is capable of simulating transport of materials during electrokinetic manipulation in 2-D channel structures. The computer code allows arbitrary channel geometries with various boundary conditions for the electric field and the sample concentration. Two fundamental microfluidic chip elements, a cross and a mixing tee, are of particular interest. An electrokinetic focusing experiment using a cross structure and mixing in a tee structure are simulated. Simulations revealed an optimum focusing voltage for which the ratio of sample concentration to sample width is maximized. They also verified that the mixing tee provides very accurate dilution/mixing characteristics for both charged and neutral samples. Good agreement between simulated and experimental data verified the accuracy of the mathematical model.     

Enhanced Two‐Photon Absorption of Organic Chromophores: Theoretical and Experimental Assessments

Functional organic materials with enhanced two‐photon absorption lead to new technologies in the fields of chemistry, biology, and photonics. In this article we review experimental and theoretical methodologies allowing detailed investigation and analysis of two‐photon absorption properties of organic chromophores. This includes femtosecond two‐photon excited fluorescence experimental setups and quantum‐chemical methodologies based on time‐dependent density functional theory. We thoroughly analyze physical phenomena and trends leading to large two‐photon absorption responses of a few series of model chromophores focusing on the effects of symmetric and asymmetric donor/acceptor substitution and branching.     

Quantitative determination of dynamical properties using coherent spatial frequency domain imaging

Laser speckle imaging (LSI) is a fast, noninvasive method to obtain relative particle dynamics in highly light scattering media, such as biological tissue. To make quantitative measurements, we combine LSI with spatial frequency domain imaging, a technique where samples are illuminated with sinusoidal intensity patterns of light that control the characteristic path lengths of photons in the sample. We use both diffusion and radiative transport to predict the speckle contrast of coherent light remitted from turbid media. We validate our technique by measuring known Brownian diffusion coefficients (D(b)) of scattering liquid phantoms. Monte Carlo (MC) simulations of radiative transport were found to provide the most accurate contrast predictions. For polystyrene microspheres of radius 800 nm in water, the expected and fit D(b) using radiative transport were 6.10E-07 and 7.10E-07 mm2/s, respectively. For polystyrene microspheres of radius 1026 nm in water, the expected and fit D(b) were 4.7E-07 and 5.35 mm2/s, respectively. For scattering particles in water-glycerin solutions, the fit fractional changes in D(b) with changes in viscosity were all found to be within 3% of the expected value.     

Stability analysis of optofluidic transport on solid-core waveguiding structures

Optofluidic transport involves the use of electromagnetic energy to transport nanoparticles through the exploitation of scattering, adsorption and gradient (polarization) based forces. This paper presents a new approach to stability analysis for a system of broad applicability to such transport, namely the optical trapping of dielectric particles in the evanescent field of low index (polymer) and high index (silicon) solid-core waveguide structures integrated with microfluidics. Three-dimensional finite element based simulations are used to determine the electromagnetic and hydrodynamic field variables for the system of interest. The net force acting on particles is determined through evaluation of the full Maxwell and flow shear stress tensors, and a trapping stability number is obtained by comparing the work required to remove a particle from the waveguide with available random thermal energy. These forces are correlated to controllable experimental parameters such as particle size, fluid velocity, and channel height, and a series of trapping stability diagrams is produced which detail the conditions under which optofluidic transport is possible.     

Microfluidics for Manipulating Cells

Microfluidics, a toolbox comprising methods for precise manipulation of fluids at small length scales (micrometers to millimeters), has become useful for manipulating cells. Its uses range from dynamic management of cellular interactions to high‐throughput screening of cells, and to precise analysis of chemical contents in single cells. Microfluidics demonstrates a completely new perspective and an excellent practical way to manipulate cells for solving various needs in biology and medicine. This review introduces and comments on recent achievements and challenges of using microfluidics to manipulate and analyze cells. It is believed that microfluidics will assume an even greater role in the mechanistic understanding of cell biology and, eventually, in clinical applications.     

Computer Simulation of Pollutant Transport and Deposition Near Peace Bridge

Using computer simulations, transport, dispersion, and deposition of particulate pollutants near the Peace Bridge in the city of Buffalo, New York, are studied. An unstructured computational grid of Peace Bridge and its vicinity is generated, and the wind flow in the area is simulated. The Reynolds stress transport (RST) and the k-ε models of FLUENT code are used for simulating the mean airflow condition. The instantaneous turbulence fluctuating velocity is simulated by a discrete random walk (DRW) model. A Lagrangian particle-tracking model is used, and dispersion and deposition of particulate emissions from the motor vehicle exhaust on the bridge and in the Peace Bridge Plaza (customs area) are analyzed. The pollutant transport model accounts for the drag and Brownian forces acting on the particles, in addition to the gravitational sedimentation effects. For the case of a northwesterly wind of 7.7 m/s and particulate emissions in the size range of 0.01 to 50 μm, the corresponding deposition rates on various surfaces are studied. The importance of wind turbulence and gravity on particle deposition are evaluated. It is found that the Peace Bridge Plaza provides for a large fraction of the pollutants.     

Desalination of porous building materials by electrokinetics: an NMR study

This article presents the first non destructive measurements of salt ions transport through fired-clay brick during electrokinetic desalination using nuclear magnetic resonance technique. The effect of the strength of an applied electric field on the migration of salt ions is examined by varying the electrical potential gradients from 0.75–2 V cm^?1 across the specimens. The measurements show that for electrokinetic to exceed ion transport by diffusion a minimum level of applied voltage is necessary. Below this threshold salt transport by diffusion is dominant over electromigration. The effect of advection on the salt transport is studied by introducing a hydraulic gradient across the specimen. The results show that advection is a major transport process in the materials studied. To assess the relative magnitude of the various active transport processes during electrokinetic desalination, a scale analysis on the basis of dimensionless numbers is presented. The value of these numbers determines which transport mechanism will dominate the desalination process in a given sample length and time scale.     

SUBMICRON MAGNETIC PARTICLE SEPARATION

The control, collection or separation of particles on the basis of Their magnetic moment relative Co the carrier fluid has been demonstrated in many applications. Usually the particle sizes are larger than one micron and the magnetic susceptibility at least moderately paramagnetic. Recently, particle separation techniques have been developed for both diamagnetic and submicron particles. These techniques have found application in mineral beneficiation, nuclear reactor coolants, biology and medicine. Such developments require an understanding of flow forces in liquids and gases, diffusion and Brownian motion, and of magnetic properties which range from the strong magnetic moments of ferromagnetic and superparamagnetic particles down orders of magnitude to those of diamagnetism.