Advancements in the field of microfluidics has led to an increasing interest to study laminar flow in microchannel and its potential applications. Understanding mixing at a microscale can be useful in various biological, heating and industrial applications due to the space and time reduction that micro mixing permits. This work aims to study mixing enhancement due to curved microchannel and the influence of varying microchannel cross sectional shape through numerical and experimental investigations. Unlike prior studies which use channel dimensions in the lower microscale range, this work has been conducted on channels with dimensions in the higher end of micrometer range. Using a cross sectional hydraulic diameter of 600 μm enables introduction of flow into the curved channel at a Reynolds Number ranging from 0.15 to 75, the findings of which show considerable improvement in the mixing performance as compared to that of equivalent straight channels, due to the development of secondary flows known as Dean Vortices.

Predicting and controlling the flow regime transition of multiphase fluids in microchannels is essential for various energy applications, such as flow boiling, de-emulsification and oil recovery processes. This in turn requires a better understanding of multiphase flow behaviors in microchannels with various channel surface wettability, fluid interfacial tension and flow rates. In this paper, experiments and Lattice Boltzmann method (LBM) simulations are carried out to study complicated multiphase flow at micro or meso scales. With the Shan-Chen multiphase LBM model, the flow pattern transitions of adiabatic two phase flow in a microchannel were investigated. The effects of surface wettability and liquid/gas velocity ratio on the flow regime transition were further studied. A series of two-phase flow experiments were conducted on a PDMS microfluidic device under different gas/oil velocity ratios. Under various surface wettability conditions, our simulation results agree well with the flow visualization experiments equipped with a high speed camera (HSC). Our finding shows that the cross-section meniscus curve width, corresponding to the shadow in the HSC photo, increases with decreasing contact angle, which was confirmed by the simulated liquid/gas distribution. Besides the influence of surface wettability, the role of gas/liquid velocity ratio on two-phase flow regime transition was discussed in detail. The proposed approach paves the way to probe complicated physics of multiphase flows in microporous media.

The efficiency of conventional heat exchangers is restricted by many factors, such as effectiveness of convective heat transfer and the cost of their operation. The current research deals with these issues by developing a novel method for building a lower-cost yet more efficient heat sink. This method involves using a specially designed curved microchannel to utilise the enhanced fluid mixing characteristics of Dean vortices, and thus transferring heat efficiently. Numerical models have been employed to investigate the heat transfer enhancement of curved channels over straight equivalents, with the aim of optimising the heat exchanger design based on the parameters of maximising heat transfer whilst minimising pressure drop and unit cost. A range of cross-sectional geometries for the curved channels were compared, showing significantly higher Nusselt Numbers than equivalent straight channels throughout, and finding superior performance factors for square, circular and symmetrical trapezoidal profiles. Due the difficulty and expense in manufacturing circular microchannels, the relatively simple to fabricate square and symmetrical trapezoidal channels are put forward as the most advantageous designs. These results take into account both constant wall temperature and constant heat flux conditions. For a given set of channel dimensions, an optimal input flow rate condition is also determined.

Particulate fouling at elevated temperature is a crucial issue for microchannel heat exchangers. In this work, a microfluidic system is designed to experimentally study on the deposition of micro-particles suspended in microchannels, which simulates the working fluid in microscale heat exchangers. We have directly measured the deposition rate of microparticles and found that the number density of deposited particles was monotonically increased with solution temperature when constant flow rate of samples was maintained. Moreover, our results show that pulsatile flow, which was generated by a piezoelectric unit, could mitigate the particulate fouling in microchannels, and the deposition rate was decreased with increasing the frequency of pulsation within a low frequency region. Our findings are expected to gain better understanding of thermally driven particulate fouling as well as provide useful information for design and fabrication of microchannel heat exchangers.

Morpholinos (MOs) are synthetic nucleic acids analogues with a non-charged backbone of morpholine rings. To enhance the MO-DNA hybridization assay speed, we propose the integration of a MO microarray with an ion concentration polarization (ICP) based microfluidic concentrator. The ICP concentrator collects target biomolecules from a ∼μL fluidic DNA sample and concentrates them electrokinetically into a ∼nL plug located in the vicinity of the MO probes. ICP preconcentration not only reduces the analyte diffusion length but also increases the binding reaction rate, and as a result, ICP-enhanced MO microarrays allow much faster hybridization than standard diffusion-limited MO microarrays.

The efficiency of conventional heat exchangers is restricted by many factors, such as effectiveness of convective heat transfer and the cost of their operation. The current research deals with these issues by developing a novel method for building a lower-cost yet more efficient heat sink. This method involves using a specially designed curved microchannel to utilise the enhanced fluid mixing characteristics of Dean vortices, and thus transferring heat efficiently. Numerical models have been employed to investigate the heat transfer enhancement of curved channels over straight equivalents, with the aim of optimising the heat exchanger design based on the parameters of maximising heat transfer whilst minimising pressure drop and unit cost. These studies examined the variation of Nusselt Number over the length of the channel, for a range of different curvatures (and hence Dean numbers). The results showed significantly higher heat transfer occurring in curved channels, especially in areas where the generated Dean vortices are strongest, with the variation in Nusselt Number forming the shape of an ‘arc’. In this way, a relationship between the Dean Number and the Nusselt Number is characterised and discussed, leading to suggestions regarding optimal microfluidic heat transfer design.

The use of liquids for the manipulation of light has many advantages over their conventional solid counterparts. With the emergence of microfluidic technologies to control fluid interfaces, various devices capable of replacing conventional optical components have been developed. Because liquids are intrinsically smooth and can change shape or form, they have been utilized for highly versatile components to manipulate light with high degrees of control using optofluidic technologies. Liquid lenses and beam steering devices are among the typical optofluidic devices that have gained much interest over recent years. In this work, we present high-performance tunable liquid prisms capable of wide beam steering of incoming light. By using the electrowetting phenomena, we are able to modulate the fluid-fluid interface at which beam steering occurs. Optical analyses were conducted to study the effect of liquid selection in the effectiveness of our prisms. Furthermore, the double-stacked prism configuration is proposed to achieve wide beam steering and its performance is compared with that of a single prism for different liquid selections. Finally, our analytical studies have been experimentally demonstrated. We successfully fabricated the tunable liquid prism filled with water and 1-bromonaphthalene (1-BN). Due to large refractive index difference between two liquids ( n water = 1.33 and n 1− BN = 1.65 at λ = 532 nm), high-performance beam steering was enabled. With an apex angle of 25°, we were able to experimentally demonstrate a beam steering of β ≤ 8.82° with the single prism configuration. It was significantly improved up to β ≤ 17.04° for the double-stacked prism.

Microchannel heat exchangers have become widely employed in modern systems, found within aerospace applications, waste heat recovery, water treatment processes, air conditioning, biomedical treatments and various industrial process applications. The microchannels increase the ratio of heat transfer surface to volume, thus improving the heat transfer performance significantly whilst reducing the overall weight and size. Moreover, by utilizing secondary flow from Dean Vortices induced by curved microfluidic channels, the fluid flow and heat transfer performance can be enhanced even further beyond conventional straight channels. However, since pressure drops found in microchannels are often quite high, channel lengths must be kept relatively short to balance the friction loss and energy consumption. Due to this, the developing region length at the microchannel entrance area has a greater impact than for macroscale channels, in terms of hydrodynamic and thermal performance over the remaining full developed region. The thermo-hydraulic design for heat transfer microchannel surfaces is strongly dependent on several dimensionless performance indicators, namely Nusselt number ‘Nu’ for heat transfer, and Poiseuille number ‘Po’, which is the product of Fanning friction factor ‘f’ and Reynolds number ‘Re’. These parameters are used to characterize and optimize the performance of microchannel surfaces and heat exchangers in general, also can be used to determine both the thermal and hydraulic developing region lengths at the channel entrance area. Whilst many such studies exist for theoretical analysis and experimental verifications, currently there is little literature on the developing region lengths and impacts researched through the method of Computational Fluid Dynamics (CFD). As such, this paper identifies and explores via quantitative analysis the hydraulic and thermal performance changes created by the relevant developing region lengths at the entrance area of spiral microchannels, as well as determinations and comparisons of these effects over straight channels. The numerical results, generated via COMSOL Multiphysics and contrasted with previous literature on the subject, also compared with the effect of the developing region on the effectiveness and efficiency of both spiral and straight microchannels, finding an improved heat transfer performance but an increased impact of hydraulic friction as well for spiral channels against straight counterpart. Furthermore, significant differences between thermal developing region length and hydraulic developing region length can be observed throughout, which illustrates high challenge and the need for compromise in microchannel design. In this way, implications for the configuration and design of industrial microchannels and micro heat exchangers are self-evident. All the key factors given in this paper are dimensionless, and thus the generated results can be utilized for a variety of flow conditions. Hence, this work should permit an increased understanding for and boost the curved microchannel and micro heat exchanger designs subsequently, through reducing the required numbers of tests and experiments and expediting the development for similar applications followed.

Plasma is a host of various analytes such as proteins, metabolites, circulating nucleic acids (CNAs), pathogens. The key process of plasma extraction is to eliminate the contamination from blood cells. Conventional methods, such as centrifugation and membrane filtration, are generally lab-intensive, time consuming and even dangerous. In this study, we report an integrated microfluidic device that combines inertial microfluidics and membrane filter. The integrated microfluidic device was evaluated by the diluted (x1/10, x1/20) whole blood, and the quality of the extracted blood plasma was tested. It was found that quality of extracted blood plasma from integrated device was equivalent to that obtained by the centrifugation. This study demonstrates a significant progress towards the practical application of inertial microfluidics with membrane filter for high-throughput and high efficient blood plasma extraction.

We report on the development of a curved microfluidic channel that allows rapid and continuous size-based rare tumor cell separation from blood in a label-free manner by exploiting the hydrodynamic effects. The separated tumor cells are trapped and enriched on an integrated polycarbonate filter glued on top of the outlet reservoir of microchannels. CK19 mRNA of MCF-7 cells are detected by loop-mediated isothermal amplification (LAMP).

In modern microprocessors, thermal management has become one of the main hurdles in continued performance enhancement. Cooling schemes utilizing single phase microfluidics have been investigated extensively for enhanced heat dissipation from microprocessors. However, two-phase fluidic cooling devices are becoming a promising approach, and are less understood. This study aims to examine two-phase flow and heat transfer within a pin-fin enhanced micro-gap. The pin-fin array covered an area of 1cm × 1cm and had a pin diameter, height and pitch of 150μm, 200μm and 225μm, respectively, (aspect ratio of 1.33). Heating from two upstream heaters was considered. The working fluid used was R245fa. The average heat transfer coefficient was evaluated for a range of heat fluxes and flow rates. Flow regime visualization was performed using high-speed imaging. Results indicate a sharp transition to convective flow boiling mechanism. Unique, conically-shaped two-phase wakes are recorded, demonstrating 2D spreading capability of the device. Surface roughness features are also discussed.

Room temperature liquid metal has been widely used in many MEMS applications, such as integrated heaters, sensors, electrodes and stretchable wires. Injecting the liquid metal into microchannels provides a simple, rapid and low-cost way to fabricate micro heaters and sensors. The liquid-metal-filled microstructures can be designed in any shape and easily integrated into micro devices. In this study, a liquid-metal based thermal micro-system was proposed for on-chip cell culture purpose. The thermal micro-system consisted of two same microchannels filled with the liquid metal as electrical heaters. At the same time, the heater also worked as a resistance temperature sensor to control the heating process. The temperature sensor was calibrated from 20 °C to 70 °C to give an accurate temperature control for the microsystem. To justify whether this micro-system is capable of providing a uniform temperature distribution, Rhodamine B was filled into the micro cell culture chamber of interest to monitor the temperature distribution. Thermal analysis was numerically carried out to reveal the temperature field of the chip. This thermal micro-system has great potential use in many microfluidic applications, such as on-chip PCR, temperature gradient focusing, protein crystallization or chemical synthesis.

Washing, separation and concentration of bioparticles are key operations for many biological and chemical analyses. In this study, the simulation of an integrated microfluidic device is studied. The proposed device has the capability to wash the bioparticles (transferring the bioparticles from one buffer solution to another), to separate the particles based on their dielectric properties and to concentrate the bioparticles. Washing and concentration of bioparticles are performed by acoustophoresis and the separation is performed by dielectrophoresis. For simulating the flow within the microchannel, a computational fluid dynamics model using COMSOL Multiphysics software is implemented. In order to simulate the particle trajectories under ultrasonic and electric field, point-particle assumption is chosen using MATLAB software. To account for the size variation of the bioparticles, particles with normal size distributions are used inside the microchannel. The effect of the key design parameters such as flow rate, applied voltage etc. on the performance of the device is discussed.

Microfluidic particle separation technologies are useful for enriching rare cell populations for academic and clinical purposes. In order to separate particles based on size, deterministic lateral displacement (DLD) arrays are designed assuming that the flow profile between posts is parabolic or shifted parabolic (depending on post geometry). The design process also assumes the shape of the normalized flow profile is speed-invariant. The work presented here shows flow profile shapes vary, in arrays with circular and triangular posts, from this assumption at practical flow rates (10 < Re < 100). The root-mean-square error (RMSE) of this assumption in the circular post arrays peaked at 0.144. The RMSE in the triangular post array peaked at 0.136. Flow development occurred more rapidly in circular post arrays when compared to triangular post arrays. Additionally, the changes in critical bumping diameter (D CB ) the DLD design metric used to calculate the size-based separation threshold were examined for 10 different row shift fractions (F RS ). These errors correspond to a D CB that varies as much as 11.7% in the circular post arrays and 15.1% in the triangular post arrays.

Microfluidics and its applications to Lab-on-a-Chip have attracted a lot of attention. Because of the small length scale, the flow is characterized by a low Re number. The governing equations become linear. Boundary element method (BEM) is a very good option for simulating the fluid flow with high accuracy. In this paper, we present a 2D numerical modeling of the electrothermal flow using BEM. In electrothermal flow the volumetric force is caused by electric field and temperature gradient. The physics is mathematically modeled by (i) Laplace equation for the electrical potential, (ii) Poisson equation for the heat conduction caused by Joule heating, (iii) continuity and Stokes equation for the low Reynolds number flow. We begin by solving the electrical potential and electric field. The heat conduction is caused by the Joule heating as the heat generation term. Superposition principle is used to solve for the temperature field. The Coulomb and dielectric forces are generated by the electrical field and temperature gradient of the system. We analyze the Stokes flow problem by superposition of fundamental solution for free-space velocity caused by body force and BEM for the corresponding homogeneous Stokes equation. It is well known that a singularity integral arises when the source point approaches the field point. To overcome this problem, we solve the free-space velocity analytically. For the BEM part, we also calculate all the integral terms analytically. With this effort, our solution is more accurate. In addition, we improve the robustness of the matrix system by combining the velocity integral equation with the traction integral equation. Our purpose is to design a pump for the microfluidics system. Since the system is a long channel, the flow is fully developed in the area far away from the electrodes. With this assumption, the velocity profile is parabolic at the inlet and outlet of the channel. So we can get appropriate boundary conditions for the BEM part of Stokes equation. Consequently, we can simulate the electrothermal flow in an open channel. In this paper, we will present the formulation and implementation of BEM to model electrothermal flow. Results of electrical potential, temperature field, Joule heating, electrothermal force, and velocity field will be presented.

Micro water droplet splitting and merging in a parallel-plate electrowetting-on-dielectric (EWOD) device has been studied numerically. The transient governing equations for the microfluidic flow are solved by a finite volume scheme with a two-step projection method on a fixed computational domain. The interface between liquid and gas is tracked by a coupled Level Set and Volume-of-Fluid (CLSVOF) method. A Continuum Surface Force (CSF) model is employed to model the surface tension at the interface. The physics of the fluid dynamics within the EWOD device has been examined. Contact angle hysteresis, which is an essential component in EWOD, is implemented together with a simplified model for the viscous stresses exerted by the two plates at the solid-liquid interface. The results of the numerical model have been validated with published experimental data. A parametric study has been performed in which the effects of channel height and several other parameters on the fluid motion have been studied.

Metal wear debris is an important component of oil particle contamination and is also an essential information carrier in hydraulic oil. Based on inductive Coulter counting principle, a microfluidic device to detect wear debris in oil is presented. The proposed device has the advantage that in theory the distance between wear particle in oil and an embedded coil is 0, which can greatly improve the sensitivity of detection. The relationship between coil parameters and inductive change of a coil is analyzed through the related experimental statistics. The result indicates it can distinguish effectively ferrous and nonferrous metal particles in oil, and the size of 19 μm iron particles and 40 μm copper particles can be detected.

In biomedical microdevices and medical applications there is a need to analyze fluid transport of solid structures with sizes comparable to channel dimensions. Examples include manipulation of biological cells in microfluidic devices or transport of thrombin particles in blood vessels. Computational modeling of such macroparticles is very difficult when the particle size is bigger than the size of the computational control volume (mesh element). In performing such simulations, conventional Lagrangian model of micro particles is not suitable since this approach doesn’t account particle’s volume blockage of the supporting Eulerian computational mesh. Other approaches such as deforming mesh or volume of fluid are either impractical of computationally very intensive or limited to structured meshes. We have developed a ‘macroparticle’ methodology where the large particle is represented as a large cluster of smaller particles (marker particles) that is “embedded” on a background computational grid. The macroparticle is then represented by blocking the cells in the background mesh that are overlapped by individual micro-particles. The discrete surface of the macroparticle is represented by partially or fully blocked cells of the background computational mesh. The translation /rotation/deformation motion of the macroparticle is calculated using a 6-DOF model with fluid pressure and shear forces acting on the particle surface used as forces and moments in calculating macroparticle position, velocity, acceleration and rotation. The size of the background grid determines the accuracy of the particle shape definition and the flow solution. The relevant physics and chemical conservation laws for each macroparticle are solved in a coupled, iterative method with the equation systems governing the background fluid domain. This methodology has been successfully used for simulations of macroparticle-laden fluids in micro channels in biochips. As an application of this novel method, we have applied this technology to simulate a moving clot in blood flow and process of clot mechanical dissolution (thrombolysis).

Temperature gradient focusing (TGF) is a highly efficient focusing technique for the concentration and separation of charged analytes in microfluidic channels. Design and control of an appropriate temperature gradient are very important for protein concentration and separation. In this study, we propose a new technique to generate the temperature gradient for the focusing purpose. This technique utilizes a microchannel filled with liquid-metals as a heater in the microfluidic chip. By applying electrical current, the liquid-metal microchannel generates Joule heat to form temperature gradient in the microfluidic chip. To optimize the temperature gradient, several typical designs were investigated. The results show the best design which provides the largest linear temperature gradient. The parametric studies present a clear guideline for designing aTGF microfluidic chip.

Shewanella oneidensis is a metal reducing bacterium, which is of interest for bioremediation and clean energy applications. S. oneidensis biofilms play a critical role in several situations such as in microbial energy harvesting devices. Here, we use a microfluidic device to quantify the effects of hydrodynamics on the biofilm morphology of S. oneidensis . For different rates of fluid flow through a complex microfluidic device, we studied the spatiotemporal dynamics of biofilms, and we quantified several morphological features such as spatial distribution, cluster formation and surface coverage. We found that hydrodynamics resulted in significant differences in biofilm dynamics. The baffles in the device created regions of low and high flow in the same device. At higher flow rates, a non-uniform biofilm develops, due to unequal advection in different regions of the microchannel. However, at lower flow rates, a more uniform biofilm evolved. This depicts competition between adhesion events, growth and fluid advection. Atomic force microscopy (AFM) revealed that higher production of extra-cellular polymeric substances (EPS) occurred at higher flow velocities.