Flow-induced particle migration in microchannels for improved microfiltration processes

Microchannels can be used to induce migration phenomena of micron sized particles in a fluid. Separation processes, like microfiltration, could benefit from particle migration phenomena. Currently, microfiltration is designed around maximum flux, resulting in accumulation of particles in and on the membrane. In this paper it is shown that starting the design at the particle level will result in a new microfiltration process. The behaviour of suspensions between 9 and 38 volume% was studied by confocal scanning laser microscopy; migration as a result of shear-induced diffusion was observed in a rectangular microchannel with nonporous walls. Particles segregated on size within the first 10 cm of the channel. To illustrate this, at 20 volume% of small (1.53 μm) and large (2.65 μm) particles each, the larger particles migrated to the middle of the channel, while the small particles had high concentrations near the walls. The small particles could then be collected from their position close to the permeable walls, e.g. membranes, where the pore size of the membrane is no longer the determining factor for separation. Guidelines for using this phenomenon in a microfiltration process were derived and the selectivity of the process was experimentally evaluated. The small droplets could be removed from the mixtures with a membrane having pores 3.7 times larger than the droplets, thereby minimizing accumulation of droplets in and on the membrane. As long as the process conditions are chosen appropriately, no droplet deposition takes place and high fluxes (1.7 × 10³ L h^?1 m^?² bar^?1) can be maintained.     

Simulation of the filtration behaviour of dust filters

Periodically regenerable cake-forming filters have assumed a commercially dominant role in air purification due to their excellent dust collection capabilities. These filters are used to separate particles from gases with high dust concentrations, whereby the separation arises as the dust-gas mixture passes through the filter medium and the particles are retained. As soon as a certain maximum pressure drop over the filter medium is reached the filter medium is regenerated. In order to optimally design such filters, there are three important parameters for the economical evaluation of the filter: the correlating pressure drop increase of the filter during filtration which is mainly influenced by the compressibility of the dust cake, the dust separation efficiency especially at the beginning of the filtration and the rise of the residual pressure drop of the filter after several cycles of filtration and regeneration. In this work a 2-dimensional computer simulation model for the compressible dust cake build-up on widely used non-woven fabrics (needle-felts) is introduced. First this model is used to investigate the mechanism causing compression inside dust filter cakes and to clarify different compression phenomena. Second it is shown that this simulation program is capable of describing the particle penetration inside and through the filter medium, the clogging of the filter medium and the growing of the dust filter cake out of the filter medium, whereby the corresponding pressure drop and the number of particles in the clean gas as an function of the filtration time show good qualitative agreement with known experimental data.     

A high performance model predictive controller:: application on a polyethylene gas phase reactor

This paper describes the development of a new model predictive control technology INCA^® that enables a high performance demand driven operation in the chemical process industry. The technology sustains optimal grade changes, maintains tight quality control and leads to low application development and implementation costs. An application on a polyethylene gasphase reactor is discussed.     

Calculation of effective pore diameters in porous filtration membranes with image analysis

In filtration/separation applications, porous membranes separate particles by a sieving mechanism determined by pore size of membrane and particle size. However, since pores in the membranes have irregular shape and vary in size, the definition of the pore size has been complex and sometimes, even confusing. In this work, we introduced a novel geometric parameter called effective pore diameter to characterize particle permeation performance in porous filtration membranes. The effective diameter of a pore is defined as the maximum diameter of a spherical particle which can pass through the pore in the membrane. We for the first time applied advanced image analysis techniques to automatically measure effective pore diameters and their distribution from scanning electronic microscopic image of the membrane using Euclidean distance transform (EDT). It is found that the effective pore diameter and its distribution are more accurate than the parameters previously used, to evaluate the membrane's filtration and separation performance and especially, particle permeation performance.     

Acoustophoresis of hollow and core-shell particles in two-dimensional resonance modes

Motivated by the applications of ultrasonic particle manipulation in a biotechnological context, a study on acoustophoresis of hollow and core-shell particles is presented with analytical derivations, numerical simulations and confirming experiments. For a long-wavelength calculation of the acoustic radiation forces, the Gor’kov potential of hollow, air-filled particles and particles with solid or fluid core and shell is derived. The validity as well as the applicable range of the long-wavelength calculation is evaluated with numerical simulations in Comsol Multiphysics^®. The results are experimentally verified in the acoustic field of an intrinsically two-dimensional fluid resonance mode, which allows for a more complex analysis than the common one-dimensional ultrasonic standing waves or their superposition to two-dimensional fields. Experiments were conducted with hollow glass particles (13.9 μm diameter) in a microfluidic chamber of 1.2 mm × 1.2 mm × 0.2 mm on a silicon-based device with piezoelectric excitation around 870 kHz. The described resonance mode is of additional interest for particle trapping and medium exchange on certain particle types, and it reveals a novel approach for particle characterization or separation.     

Mesoporous silicon-based miniature fuel cells for nomadic and chip-scale systems

We report here the fabrication of a new miniature fuel cell for nomadic applications and chip-scale power supply based on a Nafion^®-filled porous silicon self-supported membrane. Combining advantages of Nafion^® for its great proton conduction and silicon for an easier integration and standard microfabrication techniques, this solution enables the integration of gas feed and electrical contacts into the membrane etching process thanks to simple KOH wet etching processes and metal sputterings. The encapsulation is also possible. Compared to simple Nafion^® membranes, this technique may reduce the lateral water diffusion through the membrane. Experiments have been carried out at room temperature and gas feed H₂ is provided by the electrolysis of a NaOH solution. A long-term power density of 18 mW cm^?2 has been achieved after stabilization with a maximum current density of 101 mA cm^?2 and an open circuit voltage of 0.8 V.     

Quantitative characterization of the focusing process and dynamic behavior of differently sized microparticles in a spiral microchannel

In this paper, a spiral microchannel was fabricated to systematically investigate particle dynamics. The focusing process or migration behavior of different-sized particles in the outlet region was presented. Specifically, for focused microparticles, quantitative characterization and analysis of how particles migrate towards the equilibrium positions with the increase in flow rate (De = 0.31–3.36) were performed. For unfocused microparticles, the particle migration behavior and the particle-free region’s formation process were characterized over a wide range of flow rates (De = 0.31–4.58), and the emergence of double particle-free regions was observed at De ≥ 3.36. These results provide insights into the design and operation of high-throughput particle/cell filtration and separation. Furthermore, using the location markers pre-fabricated along with the microchannel structures, the focusing or migration dynamics of different-sized particles along the spiral microchannel was systematically explored. The particle migration length effects on focusing degree and particle-free region width were analyzed. These analyses may be valuable for the optimization of microchannel structures. In addition, this device was successfully used to efficiently filter rare particles from a large-volume sample and separate particles of two different sizes according to their focusing states.     

Finite element simulation of thermoforming processes for polymer sheets

The problem of modelling and the finite element simulation of thermoforming processes for polymeric sheets at various temperatures and for different loading regimes is addressed. In particular, the vacuum forming process for sheets at temperatures of approximately 200 °C and the Niebling process for sheets at temperature of 100 °C with high pressure loading are both described. Discussion is given to the assumptions made concerning the behaviour of the polymers and the physical happenings in the process in order that realistic models of the inflation part of each process may be produced. Stress–strain curves produced from experimental testing of BAYFOL^® at various strain rates and temperatures are presented. A model for the elastic–plastic deformation of BAYFOL^® is described and is used within the finite element framework to simulate the inflation part of the Niebling process. Numerical results for the deformation of sheets into a mould in the Niebling context are presented.     

An evaluation of asymmetric interfaces for bimanual virtual assembly with haptics

Immersive computing technology provides a human–computer interface to support natural human interaction with digital data and models. One application for this technology is product assembly methods planning and validation. This paper presents the results of a user study which explores the effectiveness of various bimanual interaction device configurations for virtual assembly tasks. Participants completed two assembly tasks with two device configurations in five randomized bimanual treatment conditions (within subjects). A Phantom Omni^® with and without haptics enabled and a 5DT Data Glove were used. Participant performance, as measured by time to assemble, was the evaluation metric. The results revealed that there was no significant difference in performance between the five treatment conditions. However, half of the participants chose the 5DT Data Glove and the haptic-enabled Phantom Omni^® as their preferred device configuration. In addition, qualitative comments support both the preference of haptics during the assembly process and comments confirming Guiard’s kinematic chain model.     

Design,optimization and simulation of a low-voltage shunt capacitive RF-MEMS switch

This paper presents the design, optimization and simulation of a radio frequency (RF) micro-electromechanical system (MEMS) switch. The capacitive RF-MEMS switch is electrostatically actuated. The structure contains a coplanar waveguide, a big suspended membrane, four folded beams to support the membrane and four straight beams to provide the bias voltage. The switch is designed in standard 0.35 µm complementary metal oxide semiconductor process and has a very low pull-in voltage of 3.04 V. Taguchi method and weighted principal component analysis is employed to optimize the geometric parameters of the beams, in order to obtain a low spring constant, low pull-in voltage, and a robust design. The optimized parameters were obtained as w = 2.5 µm, L1 = 30 µm, L2 = 30 µm and L3 = 65 µm. The mechanical and electrical behaviours of the RF-MEMS switch were simulated by the finite element modeling in software of COMSOL Multiphysics 4.3^® and IntelliSuite v8.7^®. RF performance of the switch was obtained by simulation results, which are insertion loss of ?5.65 dB and isolation of ?24.38 dB at 40 GHz.     

Fast Fluid Simulations with Sparse Volumes on the GPU

We introduce efficient, large scale fluid simulation on GPU hardware using the fluid‐implicit particle (FLIP) method over a sparse hierarchy of grids represented in NVIDIA^® GVDB Voxels. Our approach handles tens of millions of particles within a virtually unbounded simulation domain. We describe novel techniques for parallel sparse grid hierarchy construction and fast incremental updates on the GPU for moving particles. In addition, our FLIP technique introduces sparse, work efficient parallel data gathering from particle to voxel, and a matrix‐free GPU‐based conjugate gradient solver optimized for sparse grids. Our results show that our method can achieve up to an order of magnitude faster simulations on the GPU as compared to FLIP simulations running on the CPU.     

Inertial microfluidics for continuous particle filtration and extraction

In this paper, we describe a simple passive microfluidic device with rectangular microchannel geometry for continuous particle filtration. The design takes advantage of preferential migration of particles in rectangular microchannels based on shear-induced inertial lift forces. These dominant inertial forces cause particles to move laterally and occupy equilibrium positions along the longer vertical microchannel walls. Using this principle, we demonstrate extraction of 590 nm particles from a mixture of 1.9 μm and 590 nm particles in a straight microfluidic channel with rectangular cross-section. Based on the theoretical analysis and experimental data, we describe conditions required for predicting the onset of particle equilibration in square and rectangular microchannels. The microfluidic channel design has a simple planar structure and can be easily integrated with on-chip microfluidic components for filtration and extraction of wide range of particle sizes. The ability to continuously and differentially equilibrate particles of different size without external forces in microchannels is expected to have numerous applications in filtration, cytometry, and bioseparations.     

Application of fuzzy logic for predicting roof fall rate in coal mines

Roof fall is one of the serious hazards associated with underground coal mining. Roof fall can cause fatal and non-fatal injuries on miners, stoppages in mining operations and equipment breakdowns. Therefore, accurate prediction of roof fall rate is very important in controlling and eliminating of related problems. In this study, the fuzzy logic was applied to predict roof fall rate in coal mines. The predictive fuzzy model was implemented on fuzzy logic toolbox of MATLAB^® using Mamdani algorithm and was developed based on experts’ knowledge and also a database including 109 datasets of roof performance from US coal mines. 22 datasets of this database were used to assess the performance of this fuzzy model. The comparison between obtained results from model and actual roof fall rate showed that the fuzzy model can predict roof fall rate very well.

     

Calculating the effect of natural attenuation during bank filtration

A modelling concept is presented that enables a quantitative evaluation of transport and natural attenuation processes during bank filtration. The aim is to identify ranges of degradation rates for which bank filtration is effective or ineffective. Such modelling should accompany experimental work, as otherwise the meaning of determined degradation rates for a field situation remains uncertain. The presented concept is a combination of analytical and numerical methods, solving differential equations directly for the steady state. It is implemented using FEMLAB^® code and demonstrates a typical idealized situation with a single well near a straight bank boundary. The method can be applied to confined, to unconfined and to partially confined/unconfined aquifers and may be extended for applications in more complex situations, including a clogging layer, galleries of pumping and recharge wells, etc.     

A microfluidic device for thermal particle detection

We demonstrate the use of heat to count microscopic particles. A thermal particle detector (TPD) was fabricated by combining a 500-nm-thick silicon nitride membrane containing a thin-film resistive temperature detector with a silicone elastomer microchannel. Particles with diameters of 90 and 200 μm created relative temperature changes of 0.11 and ?0.44 K, respectively, as they flowed by the sensor. A first-order lumped thermal model was developed to predict the temperature changes. Multiple particles were counted in series to demonstrate the utility of the TPD as a particle counter.     

Flow rate-insensitive microparticle separation and filtration using a microchannel with arc-shaped groove arrays

Inertial microfluidics can separate microparticles in a continuous and high-throughput manner, and is very promising for a wide range of industrial, biomedical and clinical applications. However, most of the proposed inertial microfluidic devices only work effectively at a limited and narrow flow rate range because the performance of inertial particle focusing and separation is normally very sensitive to the flow rate (Reynolds number). In this work, an innovative particle separation method is proposed and developed by taking advantage of the secondary flow and particle inertial lift force in a straight channel (AR = 0.2) with arc-shaped groove arrays patterned on the channel top surface. Through the simulation results achieved, it can be found that a secondary flow is induced within the cross section of the microchannel and guides different-size particles to the corresponding equilibrium positions. On the other hand, the effects of the particle size, flow rate and particle concentration on particle focusing and separation quality were experimentally investigated. In the experiments, the performance of particle focusing, however, was found relatively insensitive to the variation of flow rate. According to this, a separation of 4.8 and 13 µm particle suspensions was designed and successfully achieved in the proposed microchannel, and the results show that a qualified particle separation can be achieved at a wide range of flow rate. This flow rate-insensitive microfluidic separation (filtration) method is able to potentially serve as a reliable biosample preparation processing step for downstream bioassays.     

Modeling of micro/nano particle separation in microchannels with field-flow fractionation

The cyclical electrical field-flow fractionation (CyElFFF) is a very promising separation technique for particles and biological molecules such as proteins, nucleic acids, viruses, bacteria, yeast cells, mammalian cells. But a clear understanding of the mechanism and performance prediction of this system under different operating parameters is far from completed. This research focuses on a computational investigation of particle behavior in a CyElFFF system by taking into account both electrokinetic effects and particle dynamics. The model was validated with both theory and experimental results. The effects of key parameters such as applied electric field strength and frequency, solution fluid flow rate, particle size, particle shape on separation process are addressed in a systematic way. The developed model can also be utilized in studying the behavior of spherical or non-spherical particles (such as nanowire, nanorod, and nanofiber) in other microfluidic systems.     

自适应群体结构的粒子群优化算法

粒子群优化算法中,群体结构的组织模式直接决定了粒子间信息的共享和交流方式.根据复杂网络形成过程中的动力学原理,提出了一种自适应群体结构的粒子群优化算法.算法初期粒子空间分布分散,搜索过程中不断产生新的连接,群体的搜索模式由L_best? 模型逐渐进化为G_best? 模型,群体结构的这种进化方式有利于算法早期的“勘探”和后期的“开采”.实验结果表明,新算法在收敛性能上获得了较大提高.     

Embedding off-the-shelf filter in PDMS chip for microbe sampling

Filtration for microfluidic sample-collection devices is desirable for sample selection, concentration, preprocessing, and manipulation, but microfabricating the required sub-micrometer structures is an elaborate process. This article presents a simple method to integrate filters in polydimethylsiloxane (PDMS) devices to sample microorganisms in aqueous environments. An off-the-shelf membrane filter with 0.22-μm pores was embedded in a PDMS layer and sequentially bound with other PDMS channel layers. No leakage was observed during filtration. This device was validated by concentrating a large amount of biomass, from 15 × 10⁷ to 3 × 10⁸ cells/ml of cyanobacterium Synechocystis in simulated sample water with consistent performance across devices. The major advantages of this method are low cost, simple design, straightforward fabrication, and robust performance, enabling wide-utility of chip-based devices for field-deployable operations in environmental microbiology.