Use of a porous membrane for gas bubble removal in microfluidic channels: physical mechanisms and design criteria

We demonstrate and explain a simple and efficient way to remove gas bubbles from liquid-filled microchannels, by integrating a hydrophobic porous membrane on top of the microchannel. A prototype chip is manufactured in hard, transparent polymer with the ability to completely filter gas plugs out of a segmented flow at rates up to 7.4 μl/s/mm² of membrane area. The device involves a bubble generation section and a gas removal section. In the bubble generation section, a T-junction is used to generate a train of gas plugs into a water stream. These gas plugs are then transported toward the gas removal section, where they slide along a hydrophobic membrane until complete removal. The system has been successfully modeled, and four necessary operating criteria have been determined to achieve a complete separation of the gas from the liquid. The first criterion is that the bubble length needs to be larger than the channel diameter. The second criterion is that the gas plug should stay on the membrane for a time sufficient to transport all the gas through the membrane. The third criterion is that the gas plug travel speed should be lower than a critical value: otherwise a stable liquid film between the bubble and the membrane prevents mass transfer. The fourth criterion is that the pressure difference across the membrane should not be larger than the Laplace pressure to prevent water from leaking through the membrane.     

Synthesis of hollow, magnetic Fe/Ga-based oxide nanospheres using a bubble templating method in a microfluidic system

This paper describes a new approach to synthesize hollow nanospheres in a microfluidic system using air bubbles as templates. A new microfluidic system which integrates a micro-mixer, a micro-condenser channel, microvalves, a micro-heater, and a micro-temperature sensor, to form an automatic micro-reactor, is used to generate air bubbles that assist in the synthesis of hollow Fe/Ga-based oxide nanospheres. Experimental data show that Fe/Ga-based oxide nanoparticles with a diameter of 157 ± 26 nm can be successfully synthesized. The formation mechanism is that the seed nanoparticles are attaching themselves onto the bubbles to form a solid shell. The magnetic properties of the hollow Fe/Ga-based oxide nanospheres are also measured. This may be a promising platform to synthesize hollow nanoparticles for drug delivery applications.     

Experimental study of microbubble coalescence in a T-junction microfluidic device

This study presents the microbubble coalescence process in a confined microchannel. Triple T-junction microfluidic devices with different main channel size were designed to generate monodispersed microbubble pairs with air/n-butyl alcohol–glycerol solution as the working system. The head-on collision of microbubble pair was realized in the microfluidic devices. Three collision results including absolute coalescence, probabilistic coalescence, and non-coalescence were distinguished. The effects of liquid viscosities and two-phase superficial velocities on the coalescence behavior were determined. The results showed that microbubble coalescence process in the confined space was slightly faster than in the free space. Increasing liquid viscosity apparently prevents coalescence. In the probabilistic coalescence region, higher two-phase superficial velocity could reduce the percentage of coalescence events. Two characteristic parameters representing the bubble contact time and film drainage time have been introduced to analyze the microbubble coalescence behaviors and a linear correlation could clearly distinguish the coalescence and non-coalescence region.     

Experimental study of fabricating a four-layers Cantor fractal microfluidic chip by CO2 laser system

Microsystem Technologies - In this paper, we design a four-layers Cantor fractal microfluidic chip and study suitable materials and parameters to fabricate the microfluidic chip. The effect of...     

Pinch-off mechanism for Taylor bubble formation in a microfluidic flow-focusing device

The present work aims at studying the nonlinear breakup mechanism for Taylor bubble formation in a microfluidic flow-focusing device by using a high-speed digital camera. Experiments were carried out in a square microchannel with cross section of 600 × 600 μm. During the nonlinear collapse process, the variation of the minimum radius of bubble neck (r ₀) with the remaining time until pinch-off (τ) can be scaled by a power–law relationship: \(r_{0} \propto \tau^{\alpha } .\) Due to the interface rearrangement around the neck, the nonlinear collapse process can be divided into two distinct stages: liquid squeezing collapse stage and free pinch-off stage. In the liquid squeezing collapse stage, the neck collapses under the constriction of the liquid flow and the exponent α approaches to 0.33 with the increase in the liquid flow rate Q _l. In the free pinch-off stage, the value of α is close to the theoretical value of 0.50 derived from the Rayleigh–Plesset equation and is independent of Q _l.     

不同起始形貌气泡在磁性液体中上升的实验与数值分析

研究了气泡在磁性液体中的上升过程.用VOF方法追踪了气泡的自由界面,并预测了气泡的变形.考虑了磁场强度对不同起始形貌的气泡的影响,并且发现气泡的最终形貌受磁场强度的影响,当磁场强度较弱时,动力学压力占主导,使气泡变扁,当磁场强度强时,磁性压力占主导,使气泡变圆.当磁场较弱时,起始形貌对气泡的最终形态有影响.     

Modeling strong compression of a gas bubble in fluid

A technique for calculating strong adiabatic compression of a gas bubble in fluid is proposed. The compression results from the pressure applied to the outer surface of the fluid. The motion of the fluid and gas is described by two-dimensional dynamic equations of compressed fluid and gas with realistic equations of its state. The effects of viscosity and thermal conductivity are not allowed for. The bubble surface is defined as a contact interface where there is a surface tension. Coupled Euler-Lagrange coordinates are used, with the bubble surface serving as a coordinate system. A spherical system of coordinates is used as a fixed reference. Equations of gas and fluid dynamics are solved by Godunov’s equations with second-order accuracy in space and time. The economic feasibility of the technique is illustrated by some model problems. The proposed method has been proven to be much more efficient than the classic first-order-approximation Godunov’s schemes traditionally used in solving problems of a highly compressed bubble. One of the scenarios is used to show the influence of slight spherical-shape distortions of the bubble on the evolution of the radially converged shock wave resulting from the strong compression.     

Hydrodynamic focusing for microfluidic impedance cytometry: a system integration study

We present the first in-depth system integration study of in-plane hydrodynamic focusing in a microfluidic impedance cytometry lab-on-a-chip. The method relies on constricting the detection volume with non-conductive sheath flows and characterizing particles or cells based on changes in impedance. This approach represents an avenue of overcoming current limitations in sensitivity with translating cytometers to the point of care for rapid, low-cost blood analysis. While examples of integrated devices are present in the literature, no systematic study of the interplay between hydrodynamics and electrodynamics has been carried out as of yet. We develop analytical and numerical models to describe the impedimetric response of the sensor as a function of cellular characteristics, physical flow properties, and device geometry. We fabricate a working prototype lab-on-a-chip for experimental validation using latex particles. We find that ionic diffusion can be a critical limiting factor even at high Péclet number. Moreover, we explore geometric variations, revealing that the ionic diffusion-related distance between the center of the hydrodynamic focusing junction and the impedance measurement electrodes plays a dominant role. With our device, we demonstrate over fivefold enhancement in impedance signals and population separation with in-plane hydrodynamic focusing. It is only through such in-depth system studies, in both models and experiments, that optimal utilization of microsystem capabilities becomes possible.     

Prediction of relative position of CT slices using a computational intelligence system

One of the most common techniques in radiology is the computerized tomography (CT) scan. Automatically determining the relative position of a single CT slice within the human body can be very useful. It can allow for an efficient retrieval of slices from the same body region taken in other volume scans and provide useful information to the non-expert user. This work addresses the problem of determining which portion of the body is shown by a stack of axial CT image slices. To tackle this problem, this work proposes a computational intelligence system that combines semantics-based operators for Genetic Programming with a local search algorithm, coupling the exploration ability of the former with the exploitation ability of the latter. This allows the search process to quickly converge towards (near-)optimal solutions. Experimental results, using a large database of CT images, have confirmed the suitability of the proposed system for the prediction of the relative position of a CT slice. In particular, the new method achieves a median localization error of 3.4 cm on unseen data, outperforming standard Genetic Programming and other techniques that have been applied to the same dataset. In summary, this paper makes two contributions: (i) in the radiology domain, the proposed system outperforms current state-of-the-art techniques; (ii) from the computational intelligence perspective, the results show that including a local searcher in Geometric Semantic Genetic Programming can speed up convergence without degrading test performance.     

Nonlinear radial oscillations and spatial translations of a nonspherical gas bubble in a liquid

A mathematical model of dynamics of a gas bubble in a liquid with non-small distortions of its spherical shape has been developed, with allowing for the spatial translations of the bubble, as well as the influence of the gravitational force and the liquid velocity. The liquid viscosity and compressibility are taken into account approximately. It has been shown that in some particular cases the derived equations are coincident with those obtained by the other authors. Some results of solving the problem of oscillations of a moving nonspherical bubble under periodic variation of liquid pressure are presented.     

Applications of textured surfaces on bubble trapping and degassing for microfluidic devices

This paper introduces a passive degassing mechanism using textured surfaces to trap and transport bubbles, and then using hydrophobic porous membranes to vent out bubbles in a microfluidic system. The bubble trapping ability is achieved by creating nanostructures to promote bubble nucleation and coalescence on the sidewalls of KOH-etched concave pits in a silicon substrate. The substrate, which is bonded with a porous membrane, is placed in a liquid system with chemically generated CO₂ bubbles to examine the degassing ability. The results validate that the bubbles can be easily trapped on the surfaces with nanostructures, and then vented through the porous membrane. Our proposed approach possesses the advantage of simple fabrication, great structure robustness, and effective bubble trapping and removing abilities, which show their great potential as economic, passive means of preventing the gas byproducts from blocking surfaces and improving the efficiency of microfluidic systems during operations.     

Fabrication and in situ characterization of microcapsules in a microfluidic system

We have designed a microfluidic system that enables both the fabrication of calibrated capsules and the in situ characterization of their mechanical properties. The fabrication setup consists of a double flow-focusing system. A human serum albumin aqueous solution is introduced in the central channel of a first Y-junction. Intercepted by the lateral flows of a hydrophobic phase, it is dispersed into microdroplets. A cross-linking agent is then introduced at a second Y-junction allowing a membrane to form around the droplets. The time of cross-linking is controlled by the length of a wavy channel located downstream of the second junction. A cylindrical microchannel finally enables to deform and characterize the capsules thus formed. The mechanical properties of the capsule membrane are obtained by inverse analysis. The results show that the drop size increases with the flow rate ratio between the central and lateral channels. The mean shear modulus of the capsules fabricated after 23 s of cross-linking is of the order of the surface tension between the two phases indicating that a reaction time of 23 s is too short for an elastic membrane to form around the droplet. When the cross-linking time is increased to 60 s, the microcapsules surface is wrinkled, thus confirming that a solid membrane is formed around the drop. The mean shear modulus of the capsule membrane increases with the cross-linking time, which is in agreement with our previous chemical results and proves that a fine control of the mechanical properties is possible by choosing adequately the control parameters of the system.     

一种新型微流体系统设计与合理性分析

微流体系统作为一种可对流体进行精密控制、操作与检测的技术,其发展为细胞体外培养提供了新的平台,而且可与生物传感器结合构成微流体传感测试系统,大大提高细胞传感检测的精确性、一致性和稳定性。在微系统设计与制造的基础上,提出了一种新型微流体系统结构,应用COMSOL软件建立了微流体系统模型,通过对其流动特性的分析对比,优化了系统结构,系统地研究了其对细胞培养与检测的影响。结果表明:该结构既能实现细胞的长期培养;又能通过精确的微流控制,结合生物传感器,对不同时期或不同病态的细胞进行实时检测和分析。该研究对医用药物测试芯片与微流传感测试系统有着重要的意义。     

Metaheuristics for protecting critical components in a service system: A computational study

Deliberate sabotages and terrorist attacks are major threats to the safety of modern societies. These attacks often target at important infrastructures such as energy production and transmission systems, food and water supply networks, telecommunications networks, transportation networks, etc. In such systems, some components are critical as their malfunction may adversely affect the operations of the whole systems. This research examines several models based on the median problem for identifying these components in a service system. In addition to the existing exact solution methods, we propose meta-heuristics to tackle this computationally hard problem. Our hybrid approach combines the strengths of both meta-heuristics and exact solution methods. The experiment shows that the combination of solution methods significantly cut down the computational requirement for finding the critical components in a service network for protection.     

Cell separation in a microfluidic channel using magnetic microspheres

Magnetophoretic isolation of biological cells in a microfluidic environment has strong relevance in biomedicine and biotechnology. A numerical analysis of magnetophoretic cell separation using magnetic microspheres in a straight and a T-shaped microfluidic channel under the influence of a line dipole is presented. The effect of coupled particle–fluid interactions on the fluid flow and particle trajectories are investigated under different particle loading and dipole strengths. Microchannel flow and particle trajectories are simulated for different values of dipole strength and position, particle diameter and magnetic susceptibility, fluid viscosity and flow velocity in both the microchannel configurations. Residence times of the captured particles within the channel are also computed. The capture efficiency is found to be a function of two nondimensional parameters, α and β. The first parameter denotes the ratio of magnetic to viscous forces, while the second one represents the ratio of channel height to the distance of the dipole from the channel wall. Two additional nondimensional parameters γ (representing the inverse of normalized offset distance of the dipole from the line of symmetry) and σ (representing the inverse of normalized width of the outlet limbs) are found to influence the capture efficiency in the T-channel. Results of this investigation can be applied for the selection of a wide range of operating and design parameters for practical microfluidic cell separators.     

Variable time step methods for the dissolution of a gas bubble in a liquid

Two variable time step methods, one due to Douglas and Gallie and the other proposed by the authors earlier, are presented in improved forms. An unconventional moving boundary problem, dissolution of a gas bubble in a liquid, is solved using these methods. The results obtained are compared with those of other authors and agreement is found to be very good. The importance of the variable time step methods lies in the fact that they require a significantly smaller number of time steps in comparison to other methods.     

Computer study of bubble motion in a rotating liquid

The motion of a small, shape-preserving gas bubble through a rotating, homogeneous, incompressible, viscous liquid is investigated numerically be means of the Runge-Kutta-Nyström method. The fluid (liquid-gas) system spins about a fixed horizontal axis with constant angular velocity. Computer solutions are compared with experimental observations, and numerical experiences concerning step widths and computer accuracy are described.     

Quantitative analysis of sperm rheotaxis using a microfluidic device

Quite puzzling issue in biology is how sperm cells are selected naturally where human sperm has to maintain a correct swimming behavior during the various stages of reproduction process. In nature, sperm has to compete a long journey from cervix to oocyte to stand a chance for fertilization. Although various guidance mechanisms such as chemical and thermal gradients are proposed previously, these mechanisms may only be relevant as sperm reaches very close to the oocyte. Rheotaxis, a phenomenon where sperm cells swim against the flow direction, is possibly the long-range sperm guidance mechanism for successful fertilization. A little is known quantitatively about how flow shear effects may help guide human sperm cells over long distances. Here, we have developed microfluidic devices to quantitatively investigate sperm rheotaxis at various physiological flow conditions. We observed that at certain flow rates sperm actively orient and swim against the flow. Sperm that exhibit positive rheotaxis show better motility and velocity than the control (no-flow condition), however, rheotaxis does not select sperm based on hyaluronic acid (HA) binding potential and morphology. Morphology and HA binding potential may not be a significant factor in sperm transport in natural sperm selection.     

Prototyping of microfluidic systems using a commercial thermoplastic elastomer

This article describes the fabrication of microfluidic networks (μFNs) from a commercially available (styrene)–(ethylene/butylene)–(styrene) (SEBS) block copolymer (BCP). The unique combination of hard and elastomeric properties provided by this material promotes high-throughput replication of fluidic structures using thermoforming technologies, while retaining the advantage of quick and easy assembly via conformal contact, as commonly achieved for devices fabricated from poly(dimethylsiloxane (PDMS). We employ Versaflex CL30, which is optically transparent, available at low cost (e.g., $2.50/Lbs), and likely to be compatible with a broad range of biological species. We demonstrate excellent fidelity in replication of fluidic structures using hot embossing lithography in conjunction with a photolithographically prepared Si/SU-8 master mold. Moreover, we introduce rapid prototyping of high-quality structures using an approach that we call soft thermoplastic lithography (STPL). Thanks to the rheological characteristics of the SEBS copolymer, STPL enables thermoforming on a heated master at temperatures around 170°C. Using this approach, replication can be completed within a very short period of time (e.g., less than 3 min) without the need of resorting to pressure- or vacuum-assisted instrumentation. Serving as a proof-of-concept, we devise a μFN that is suitable for the formation of miniaturized arrays comprising fluorescently labeled oligonucleotides and proteins on hard plastic substrates. Resultant spots are characterized by high fluorescent contrast, excellent edge definition, and uniform distribution of probes within the modified areas.