Two-Phase Flow Characteristics in a Co-Flow Induced Microchannel With Microbubbles Generation

This paper presents a micro-fluidic device which generates micro-bubbles, ranging from 70μm to 160μm in diameter, and two-phase flow characteristics in the device were tested. The device is composed of three sub-channels: a centered gas channel (10μm×50μm) and two liquid channels (both with 85μm×50μm) on each side of the gas channel. Micro-bubbles are generated by co-flow of gas and liquid at the exit of the gas channel when the drag force becomes larger than the surface tension force as bubbles grow. Methanol and a gas mixture of CO2 and N2 were used as the working fluid. Since the flow rate of gas was very small, the gas momentum effect was considered negligible. Thus, in the present case, the controlling parameters were the liquid superficial velocity and the inlet pressure of the gas. A high speed camera was used to record two-phase flow patterns and micro-bubbles of the device. To confine the ranges of the micro-bubbles generation, two-phase flow patterns in the device is observed at first. Four different flow patterns were observed: annular, annular-slug, slug, and bubbly flow. In bubbly flows, uniform-sized micro-bubbles were generated, and the operating ranges of the liquid superficial velocity and the gas pressure were below 0.132 m/s and 0.7 bar, respectively. Diameters of the micro-bubbles appeared smaller with the higher superficial liquid velocity and/or with a lower gas pressure. Experimental results showed that, with the gas pressure lower than a certain level, the sizes of micro-bubbles were almost insensitive to the gas pressure. In such a ranges, the micro-bubble diameters could be estimated from a drag coefficient correlation, CDw = 31330/Re3, which is different from the correlations for macro-channels due to a larger wall effect with the micro-channels. In the latter part of the paper, as a potential of application of the micro-bubble generator to gas analysis, dissolution behavior of the gas components into the liquid flow was examined. The result shows that the micro-bubble generator can be adopted as a component of miniaturized gas analyzers if a proper improvement could be made in controlling the bubble sizes effectively.

Heat Transfer Model for Condensation in Non-Circular Microchannels

A model for predicting heat transfer during condensation of refrigerant R134a in horizontal noncircular microchannels is presented. The thermal amplification technique developed and reported in earlier work by the authors is used to measure condensation heat transfer coefficients for six non-circular microchannels (0.424 < Dh < 0.839 mm) of different shapes over the mass flux range 150 < G < 750 kg/m2-s. The channels included barrel-shaped, N-shaped, rectangular, square, and triangular extruded tubes, and a channel with a W-shaped corrugated insert that yielded triangular microchannels. Results from previous work by the authors on condensation flow mechanisms in microchannel geometries were used to interpret the results based on the applicable flow regimes. The effect of tube shape was also considered in deciding the applicable flow regime. A modified version of the annular flow based heat transfer model proposed recently by the authors for circular microchannels, with the required shear stress being calculated from a noncircular microchannel pressure drop model also reported earlier was found to best correlate the present data for square, rectangular and barrel-shaped microchannels. For the other microchannel shapes with sharp acute-angle corners, a mist flow based model from the literature on larger tubes was found to suffice for the prediction of the heat transfer data. These models predict the data significantly better than the other available correlations in the literature.

Temperature Dependence of Thermal Conductivity of Amorphous and Crystal Thin Film by Molecular Dynamics Simulation

For crystal materials, thermal conductivity (TC) is proportional to T3 at low temperatures and to T−1 at high temperatures. TCs of most amorphous materials decrease with the decreasing temperatures. If a material is thin film, boundary will influence the TC and then influence the temperature dependence. In this paper, we calculate the TC of crystal and amorphous SiO2 thin films, which is a commonly used material in micro devices and Integrated Circuits, by NEMD simulations. The calculation temperatures are from 100K to 700K and the thicknesses are from 2nm to 8nm. TCs of crystal thin films reach their peak values at different temperatures for different thicknesses. The smaller thickness the larger peak values obtained. But for amorphous thin films, the results show that the temperature dependence of thin films is the same as bulk materials and not relative to their thicknesses. The obtained temperature dependence of the thin films is consistent with some previous measurements and the theory predictions.

Investigation of Three-Dimensional Flow in Microchannels With Patterned Surfaces

A three-dimensional numerical simulation of flow in patterned microchannel with alternate layers of hydrophilic and hydrophobic surfaces at the bottom wall is studied here. Surface characteristics of the microchannel are accounted by specifying the contact angle and the surface tension of the fluid. Meniscus profiles with varying amplitude and shapes are obtained under the different specified surface conditions. Flow instability increases as the fluid at the bottom wall traverses alternately from hydrophilic region to hydrophobic region. To understand the surface tension effect of the side walls, a two-dimensional numerical study has also been carried out for the microchannel and the results are compared with three-dimensional simulation. The surface tension effect of the side walls enhances the capillary effect for three-dimensional case.

Towards Single-Molecule Diagnostics Using Microfluidic Manipulation and Quantum Dot Nanosensors

In this report, we review several single-molecule detection (SMD) methods and newly developed nanocrystal-mediated single-fluorophore strategies for ultrasensitive and specific analysis of genomic sequences. These include techniques, such as quantum dot (QD)-mediated fluorescence resonance energy transfer (FRET) technology and dual-color fluorescence coincidence and colocalization analysis, which allow separation-free detection of low-abundance DNA sequences and mutational analysis of oncogenes. Microfluidic approaches developed for use with single-molecule detection to achieve rapid, low-volume, and quantitative analysis of nucleic acids, such as electrokinetic manipulation of single molecules and confinement of sub-nanoliter samples using microfluidic networks integrated with valves, are also discussed.

Liquid Bridges: A Novel Approach for Dispensing Biofluids, Characterisation and Correlations

Control of fluids at the microscale represents an important point of interest in the widely studied field of Microfluidics. In fact, most of the biological and medical research undergone would benefit from Microfluidic solutions. One of the engineering challenges brought about by this technologic evolution involves the dispensing of fluids at these scales. The study presented in this paper concerns the development of a novel dispenser of biofluids, which would find its first application in the measurement of multi-gene expression levels as part of cancer diagnosis. The studied geometry is termed “two-way liquid bridge” and consists of injecting a continuous fluid to be segmented via an inlet PFE tubing in a microgravity environment until an isothermal mass of liquid is held by surface tension between the inlet and outlet tubings, parallel and opposite. Due to constant pressurisation of the microgravity environment, this mass eventually ruptures delivering a segmented volume of biofluids on which an analysis such as PCR can be performed. Experimental investigations were conducted in a backlighted transparent PMMA device in which fluids were injected using Harvard Apparatus syringe pumps. A CMOS colour camera recorded the images which were automatically analysed using a Canny edge detection algorithm. A dimensional analysis was conducted highlighting the main dimensionless groups for a complete understanding of the occurring phenomena. Experimental observations showed good repeatability and consistency in the dispensing process. It was also shown that fluids flowrates, tubings sizes and length of separation between inlet and outlet tubings have a direct impact on the size and frequency of the produced droplets. The present paper addresses the complete characterisation of the geometry as well as the establishment of correlations in order to provide a useful engineering design tool.

Optimal Shape Design of Pressure-Driven Microchannels Using Adjoint Variable Method

The shape of microchannels is an important design variable to achieve the desired performance. Since most microchannels are, at present, designed by trial and error, a systematic optimal shape design method needs to be established. Computational fluid dynamics (CFD) is often used to rigorously examine the influence of the shape of microchannels on heat and mass transport phenomena in the flow field. However, the rash combination of CFD and the optimization technique based on evaluating gradients of the cost function requires enormous computation time when the number of design variables is large. Recently, the adjoint variable method has attracted the attention as an efficient sensitivity analysis method, particularly for aeronautical shape design, since it allows one to successfully obtain the shape gradient functions independently of the number of design variables. In this research, an automatic shape optimization system based on the adjoint variable method is developed using C language on a Windows platform. To validate the effectiveness of the developed system, pressure drop minimization problems of a 180° curved microchannel and a branched microchannel in incompressible flows under constant volume conditions are solved. These design examples illustrate that the pressure drop of the optimally designed microchannels is decreased by about 20% ∼ 40% as compared with that of the initial shape.

Universal Microcarriers for Microfluidic Assays

Bead and cell suspension based flow-through assays are popular for high throughput biological analysis. Several technologies incorporate a tagging scheme with beads to enable multiplexing. Modern flow-through systems such as flow cytometers and cell sorters are large, bulky and expensive; consequently, much research has been performed using microfluidics to miniaturize these systems. However, several problems remain with these systems, notably it remains difficult to perform manipulations on the beads (or cells), and in the case of multiplexed systems, it remains difficult to read the tags quickly. In this paper, we present a micromachined micro-carrier, referred to as a ‘micropallet’, designed to move through a microfluidic device, which helps to solve several of these problems. Micropallets are small carrier structures, micromachined out of plastic or other materials, that are used to carry attached biological or chemical samples through a microfluidic system (e.g., DNA, RNA, proteins, antibodies, adherent cells, organisms). Similar to conventional factory pallets that carry a product through an automated manufacturing line, micropallets are engineered to carry their cargo through a micro-scale system. Thus micropallets may contain shapes, structures and materials designed to interact with and work in a microfluidic system, such as for docking, sorting, manipulation and readout. Additionally, micropallets may include bar codes or other markings, and be engineered to optimally suit the cargo they carry (for example, a micropallet might contain 3-D structures and treated sections for cells, molecules or organisms to attach). Results are presented for the use of micropallets in cell assays, DNA assays and antibody assays. Micropallets may be designed to carry a sample through a microfluidic system or for use in a static assay system, enabling versatile customisation of the micropallets and flow system for design of a programmable system that interacts with the micropallets for detection, control and manipulation.

Effect of Non Uniform Flow Distribution on Single Phase Heat Transfer in Parallel Microchannels

The continuum assumption has been widely accepted for single phase liquid flows in microchannels. There are however a number of publications which indicate considerable deviation in thermal and hydrodynamic performance during laminar flow in microchannels. In the present work, experiments have been performed on six parallel microchannels with varying cross-sectional dimensions. A careful assessment of friction factor and heat transfer in is carried out by properly accounting for flow area variations and the accompanying non-uniform flow distribution in individual channels. These factors seem to be responsible for the discrepancy in predicting friction factor and heat transfer using conventional theory.

Improved Serpentine Laminating Micromixer: Numerical Investigation and Experimental Verification

It is a difficult task to achieve an efficient mixing inside a microchannel since the flow is characterized by low Reynolds number (Re). Recently, the serpentine laminating micromixer (SLM) was reported to achieve an efficient chaotic mixing by introducing ‘F’-shape mixing units successively in two layers such that two chaotic mixing mechanisms, namely splitting/recombination and chaotic advection, enhance the mixing performance in combination. The present study describes an improved serpentine laminating micromixer (ISLM) with a novel redesign of the ‘F’-shape mixing unit. Reduced cross-sectional area at the recombination region of ISLM locally enhances advection effect which helps better vertical lamination, resulting in improved mixing performance. Flow characteristics and mixing performances of SLM and ISLM are investigated numerically and verified experimentally. Numerical analysis system is developed based on a finite element method and a colored particle tracking method, while mixing entropy is adopted as a quantitative mixing measure. Numerical analysis result confirms enhanced vertical lamination performance and consequently improved mixing performance of ISLM. For experimental verification, SLM and ISLM were fabricated by polydimethylsiloxane (PDMS) casting against SU-8 patterned masters. Mixing performance is observed by normalized red color intensity change of phenolphthalein along the downchannel. Flow characteristics of SLM and ISLM are investigated by tracing the red interface of two streams via optical micrograph. The normalized mixing intensity behavior confirms improved mixing performance of ISLM, which is consistent with numerical analysis result.