Controlling capillary-driven surface flow on a paper-based microfluidic channel

This paper describes two methods for controlling capillary-driven liquid flow on microfluidic channels. Unlike flow driven by external forces, capillary-driven flow is dominated by interfacial phenomena and, therefore, is sensitive to the channel geometry and chemical composition (surface energy) along the channel. The first method to control fluid flow is based on altering surface energy along the channel through regulation of UV irradiation time, which enables adjusting the contact angle along the fluid path. The slowing down (delay) of the liquid flow depends on the stripe length and its position in the channel. Using this technique, we generated flow delays spanning from a second to over 3 min. In the second approach, we manipulated the flow velocity by introducing contractions and expansions in the channel. The methods used herein are inexpensive and can be incorporated to the microfluidic channel fabrication step. They are capable of controlling liquid flow with precise time delays without introducing the foreign matter in the fluidic device.     

Implications of variable fluid resistance caused by start-up flow in microfluidic networks

Electrical circuit analogies are often used to design microfluidic systems because they simplify device design, providing simple relationships between fluid flow rate, driving forces, and channel dimensions. However, such approximations often significantly overestimate flow rates in situations where start-up energy losses from establishing kinetic head are similar in magnitude to the energy required to overcome viscous shear stresses, as is often the case within complex microfluidic networks. These reduced flows can be more accurately predicted within an electrical analogy framework that accounts for the nonlinear flow resistance generated on the transient regime of start-up flow. In this work, standard flow resistance expressions are modified to account for such effects, and the onset of nonlinear resistance is predicted by a dimensionless parameter, $\xi = Re\frac{D}{L},$ which is dependent on the Reynolds number and the channel length. As a demonstration, variable fluid resistance is shown to dramatically affect the flow performance of common microfluidic units such as T-junctions and serpentine channels, and the change in performance is accurately predicted. Experimental and theoretical analysis of T-junctions further shows that variable flow resistance causes the ratio of flows through the junction to converge toward unity with respect to an increasing total flow rate. In addition, serpentine channels are shown to exaggerate these start-up effects, owing to compounded energetic demand associated with changing a flow’s direction. As a result, serpentine channels cause the ratio of flow rates exiting a T-junction to diverge from unity with respect to an increasing flow rate.     

Effects of heat source/sink,radiation and work done by deformation on flow and heat transfer of a viscoelastic fluid over a stretching sheet

This paper presents a study of the flow and heat transfer of an incompressible homogeneous second-grade fluid over a non-isothermal stretching sheet. The governing partial differential equations are converted into ordinary differential equations by a similarity transformation. The effects of viscous dissipation, work due to deformation, internal heat generation/absorption and thermal radiation are considered in the energy equation, and the variations of dimensionless surface temperature and dimensionless surface temperature gradient as well as the heat transfer characteristics with various physical parameters are graphed and tabulated. Two cases are studied, namely, (i) a sheet with prescribed surface temperature (PST case) and (ii) a sheet with prescribed heat flux (PHF case).     

Visualization and measurement of capillary-driven blood flow using spectral domain optical coherence tomography

Capillary-driven flow (CD-flow) in microchannels plays an important role in many microfluidic devices. These devices, the most popular being those based in lateral flow, are becoming increasingly used in health care and diagnostic applications. CD-flow can passively pump biological fluids as blood, serum or plasma, in microchannels and it can enhance the wall mass transfer by exploiting the convective effects of the flow behind the meniscus. The flow behind the meniscus has not been experimentally identified up to now because of the lack of high-resolution, non-invasive, cross-sectional imaging means. In this study, spectral-domain Doppler optical coherence tomography is used to visualize and measure the flow behind the meniscus in CD-flows of water and blood. Microchannels of polydimethylsiloxane and glass with different cross-sections are considered. The predictions of the flow behind the meniscus of numerical simulations using the power-law model for non-Newtonian fluids are in reasonable agreement with the measurements using blood as working fluid. The extension of the Lucas?CWashburn equation to non-Newtonian power-law fluids predicts well the velocity of the meniscus of the experiments using blood.     

Investigations on the flow behaviour in microfluidic device due to surface roughness: a computational fluid dynamics simulation

In the field of micro-fluidics device, as the cross section of micro-channel comes down to the scale of few tens of micro-meters, surface area to volume ratio increases significantly, and due to this, surface dependent phenomenon dominates during flow of the fluid. This surface dependent phenomenon is mainly governed by surface roughness as an important parameter which directly influences on flow and results in the loss of pressure head due to the building of localised pressure as well as eddy flow. To understand this mechanism, a computational fluid dynamics (CFD) simulation is carried out. In the present CFD simulation, fluid and solid interactions are modelled in two different types. The first is modelled as pure slip between them so that the effect of roughness can be investigated as a main source of friction factor. The second model consists the effect of the pure adhesion by maintain zero relative velocity on the surface of micro-channel. Behaviour of fluid flow and increase in pressure-drop are observed differently in the both types of model. It is observed that the rise in pressure-drop occurs exponentially as size of a channel reduces from 300 to 100 µm. This phenomenon reveals the science of the size effect on micro-channels. The surface roughness of micro-channel is simulated and it is also observed that the surface finish up to few tens of nanometers does not affect the fluid flow. However, the flow resistance increases as the surface roughness increases up to few hundreds of nanometers, and the pressure-drops along the channel length. In the present case, an elevated temperature of fluid mitigates the effect of surface roughness up to some extent for the efficient flow of fluid in a micro-fluidic device. Hence, micro-fluidic device with nano-finished micro-channel and elevated temperature of fluid is recommended for economic and efficient utilisation of the device.

     

Y- and T-junction microfluidic devices: effect of fluids and interface properties and operating conditions

Water-in-oil emulsions were produced in microchannels with Y- and T-junction geometries by individual droplet generation. For each microchannel configuration, the effect of the fluids and interface properties as well as of the process conditions was evaluated. The size of the droplets depended mainly on the relative velocity between continuous and dispersed phases and the relative fluid viscosity between phases. Those variables were related to the shear stress between the phases, which caused the droplet detachment. In addition, the interfacial forces played a minor role in Y-junction, and they had no effect in the droplets formation in T-junction microchannels. In Y-junction, a large variation in the droplet size was observed, depending on the system composition and the operating conditions. At low relative velocity and fluid viscosity, no droplets were generated. In contrast, the process in T-junction resulted in a lower variation of droplets size and the droplets were formed even at less favorable conditions. Such results indicate that the knowledge of the mechanism of droplets generation in each microchannel geometry makes it possible to choose the appropriate configuration according to the type of fluid, and the operating conditions can be adjusted to obtain the desired final emulsion.     

Effects of slip on sheet-driven flow and heat transfer of a non-Newtonian fluid past a stretching sheet

The flow and heat transfer of an electrically conducting non-Newtonian fluid due to a stretching surface subject to partial slip is considered. The constitutive equation of the non-Newtonian fluid is modeled by that for a third grade fluid. The heat transfer analysis has been carried out for two heating processes, namely, (i) with prescribed surface temperature (PST-case) and (ii) prescribed surface heat flux (PHFcase) in presence of a uniform heat source or sink. Suitable similarity transformations are used to reduce the resulting highly nonlinear partial differential equations into ordinary differential equations. The issue of paucity of boundary conditions is addressed and an effective second order numerical scheme has been adopted to solve the obtained differential equations. The important finding in this communication is the combined effects of the partial slip, magnetic field, heat source (sink) parameter and the third grade fluid parameters on the velocity, skin friction coefficient and the temperature field. It is interesting to find that slip decreases the momentum boundary layer thickness and increases the thermal boundary layer thickness, whereas the third grade fluid parameter has an opposite effect on the thermal and velocity boundary layers.     

Slip effects on MHD mixed convection stagnation point flow of a micropolar fluid towards a shrinking vertical sheet

In the present study, the effects of partial slip on steady boundary layer stagnation point flow of an electrically conducting micropolar fluid impinging normally towards a shrinking sheet in the presence of a uniform transverse magnetic field is investigated. A similarity transformation technique is adopted to obtain the self similar ordinary differential equations and then solved numerically using symbolic software MATHEMATICA 7.0. The features of the flow and heat transfer characteristics for different values of the governing parameters are analyzed and discussed through graphs and tables. Both cases of assisting and opposing flows are considered. The physical aspects of the problem are highlighted and discussed.     

An integrated microfluidic device for characterizing chondrocyte metabolism in response to distinct levels of fluid flow stimulus

In this work, we presented a novel integrated microfluidic perfusion system to generate multiple parameter fluid flow-induced shear stresses simultaneously and investigated the effects of distinct levels of fluid flow stimulus on the responses of chondrocytes, including the changes of morphology and metabolism. Based on the electric circuit analogy, two devices were fabricated, each with four chambers to enable eight different shear stresses spanning over four orders of magnitude from 0.007 to 15.4 dyne/cm² with computational fluid dynamics analysis. Chondrocytes subjected to shear stresses (7.5 and 15.4 dyne/cm²) for 24 h reoriented their cytoskeleton to align with the direction of flow. Meanwhile, the collagen I, collagen II and aggrecan expression of chondrocytes increased in different ranges, respectively. Furthermore, interleukin-6 as a proinflammatory cytokine can be detected at shear stress of 7.5 and 15.4 dyne/cm² in mRNA level. These results indicated that fluid flow was beneficial for chondrocyte metabolism at interstitial levels (0.007 and 0.046 dyne/cm²), but induced an increase in fibrocartilage phenotype with increasing magnitude of stimulation. Moreover, a moderate level of flow stimulus (7.5 dyne/cm²) could also result in detrimental cytokine release. This work described a simple and versatile way to rapidly screen cell responses to fluid flow stimulus from interstitial shear stress level to pathological level, providing multi-condition fluid flow-induced microenvironment in vitro for understanding deeply chondrocyte metabolism, cartilage reconstruction and osteoarthritis etiology.     

Measuring the force of adhesion between multiple kinesins and a microtubule using the fluid force produced by microfluidic flow

We developed a method to measure the adhesion force between the motor protein, kinesin, and a microtubule. Compared with conventional methods that use optical tweezers, our method employs the fluid force that acts on the interaction between a kinesin-coated microbead and a microtubule in a microfluidic channel. When the fluid force just exceeds the kinesin-microtubule adhesion force, the beads are released from the microtubules. Having modeled the kinesins that are bound to the microtubules and the beads as mechanical springs, adhesion forces were measured as 31.3 or 362.9 pN for fluid containing 1 mM ATP or 0 M ATP, respectively. These forces are much larger than those measured when optical tweezers were used to measure the adhesion force between a single kinesin and a microtubule. For our multi-kinesin system we elucidated the relationship between the binding force of a single kinesin molecule and that of all kinesin molecules in a contact area by varying one of two parameters: either the contact area length or the kinesin density on a bead. This study provides insight into the behavior of a bead that is supported by several kinesins in a microfluidic system, which is essential knowledge if a motor protein is to be used as a nanoactuator for in vitro molecular transport.     

Investigation of wax and paper materials for the fabrication of paper-based microfluidic devices

Paper-based microfluidic devices hold great potential in today’s microfluidic applications. They offer low costs, simple and quick fabrication processes, ease of uses, etc. In this work, several wax and paper materials are investigated for the fabrication of paper-based microfluidic devices. A novel method of using wax as a suitable backing to a paper-based analytical device has been demonstrated. Governing equations for the mechanics of the fluid flow in paper-based channels with constant widths have been experimentally validated. Experimental results showing deviations from the governing equations have been verified using fluidic channels with varying widths. There lies the possibility of manipulation of the fluid flow in paper-based microfluidic devices solely using geometric factors. This opens up many potential applications that may require sequential delivery of reagents or samples. Lastly, properties of paper such as the average pore diameter and permeability can be deduced from experimental results.     

Integration of a uniform and rapid heating source into microfluidic systems

The purpose of this study has been to demonstrate the possibilities of uniform heating of a cavity, with great accuracy, by means of an integrated resistor built with the same dimensions as the cavity, i.e., with a high level of integration. For application purposes, a compact resistor allows increasing the number of cavities in which temperature can be independently controlled on the same substrate, which can prove critical for high-throughput screening applications. Potential applications lie in the field of biology or chemistry. In order to achieve the desired result, an optimization procedure was performed on the shape of the resistor. The heater size reduction enables a high level of integration with a reduced heating source surface area. Resistor shape has been optimized to reduce the influence of boundary effects, using improvements introduced in genetic algorithms. An experimental validation of the temperature profile inside the cavity has been carried out using a dye whose fluorescence depends on temperature, i.e., Rhodamine B, it will be shown herein that the optimized resistor allows for temperature cycling, e.g., for PCR applications.     

A three-dimensional microfluidic tumor cell migration assay to screen the effect of anti-migratory drugs and interstitial flow

Most anti-cancer drug screening assays are currently performed in two dimensions, on flat, rigid surfaces. However, there are increasing indications that three-dimensional (3D) platforms provide a more realistic setting to investigate accurate morphology, growth, and sensitivity of tumor cells to chemical factors. Moreover, interstitial flow plays a pivotal role in tumor growth. Here, we present a microfluidic 3D platform to investigate behaviors of tumor cells in flow conditions with anti-migratory compounds. Our results show that interstitial flow and its direction have significant impact on migration and growth of hepatocellular carcinoma cell lines such as HepG2 and HLE. In particular, HepG2/HLE cells tend to migrate against interstitial flow, and their growth increases in interstitial flow conditions regardless of the flow direction. Furthermore, this migratory activity of HepG2 cells is enhanced when they are co-cultured with human umbilical vein endothelial cells. We also found that migration activity of HepG2 cells attenuates under hypoxic conditions. In addition, the effect of Artemisinin, an anti-migratory compound, on HepG2 cells was quantitatively analyzed. The microfluidic 3D platform described here is useful to investigate more accurately the effect of anti-migratory drugs on tumor cells and the critical influence of interstitial flow than 2D culture models.     

Acoustofluidic separation: impact of microfluidic system design and of sample properties

We describe the results of a numerical study about the separation of fluid-suspended microsamples (as microbeads and cells) by acoustophoresis. A microfluidic channel with rectangular cross section is considered, and we investigate the impact on particle separation of many different parameters, dividing them in two groups: sample’s intrinsic factors (own properties of the sample) and extrinsic factors (related to the microfluidic system design and operation). Differently from what is usually done, we include in our study the impact of the initial sample position, which always has a certain variability in real experiments, and we introduce several new parameters allowing to assess system performance and to optimize the microchip separation efficiency. The obtained results show the importance of two design parameters that are generally overlooked: the channel width and the offset between the channel border and the input of the target samples in the microchannel. Additionally, the analysis method we describe and the new parameters we introduce to study the system can be beneficially used in almost any study of acoustophoretic-based separation system.     

Influence of geometry on hydrodynamic focusing and long-range fluid behavior in PDMS microfluidic chips

Details of hydrodynamic focusing in a 2D microfluidic channel-junction are investigated experimentally and theoretically, especially the effect on the focusing width of volumetric flow ratio r between main and side channels, as well as angle θ between channels. A non-linear relationship is observed where the focus width decreases rapidly with increasing r and levels off at higher values. For the dependence on θ, results from both experiments and modeling show that an increased focusing effect is obtained as θ approaches 90°. Long-range focusing is explored along a 1 cm long channel and it is observed that in the middle section of the channel, a smaller θ induces less divergence. This effect is of importance for microfluidic systems utilizing hydrodynamic focusing in long, straight channels.     

Effects of channel surface finish on blood flow in microfluidic devices

The behaviour of blood flow in relation to microchannel surface roughness has been investigated. Special attention was focused on the techniques used to fabricate the microchannels and on the apparent viscosity of the blood as it flowed through these microchannels. For the experimental comparison of smooth and rough surface channels, each channel was designed to be 10 mm long and rectangular in cross-section with aspect ratios of ≥100:1 for channel heights of 50 and 100 μm. Polycarbonate was used as the material for the device construction. The shims, which created the heights of the channels, were made of polyethylene terephthalate. Surface roughnesses of the channels were varied from R _z of 60 nm to 1.8 μm. Whole horse blood and filtered water were used as the test fluids and differential pressures ranged from 200 to 5,000 Pa. The defibrinated horse blood was treated further to prevent coagulation. The results indicate that a surface roughness above an unknown value lowers the apparent viscosity of blood dramatically due to boundary effects. Furthermore, the roughness seemed to influence both water and whole blood almost equally. A set of design rules for channel fabrication is also presented in accordance with the experiments performed.     

Design and implementation of fluidic micro-pulleys for flow control on centrifugal microfluidic platforms

Microfluidic discs have been employed in a variety of applications for chemical analyses and biological diagnostics. These platforms offer a sophisticated fluidic toolbox, necessary to perform processes that involve sample preparation, purification, analysis, and detection. However, one of the weaknesses of such systems is the uni-directional movement of fluid from the disc centre to its periphery due to the uni-directionality of the propelling centrifugal force. Here we demonstrate a mechanism for fluid movement from the periphery of a hydrophobic disc towards its centre that does not rely on the energy supplied by any peripheral equipment. This method utilizes a ventless fluidic network that connects a column of working fluid to a sample fluid. As the working fluid is pushed by the centrifugal force to move towards the periphery of the disc, the sample fluid is pulled up towards the centre of the disc analogous to a physical pulley where two weights are connected by a rope passed through a block. The ventless network is analogous to the rope in the pulley. As the working fluid descends, it creates a negative pressure that pulls the sample fluid up. The sample and working fluids do not come into direct contact, and it allows the freedom to select a working fluid with physical properties markedly different from those of the sample. This article provides a demonstration of the “micro-pulley” on a disc, discusses underlying physical phenomena, provides design guidelines for fabrication of micro-pulleys on discs, and outlines a vision for future micro-pulley applications.     

Micro-magnetofluidics: interactions between magnetism and fluid flow on the microscale

Micro-magnetofluidics refers to the science and technology that combines magnetism with microfluidics to gain new functionalities. Magnetism has been used for actuation, manipulation and detection in microfluidics. In turn, microfluidic phenomena can be used for making tunable magnetic devices. This paper presents a systematic review on the interactions between magnetism and fluid flow on the microscale. The review rather focuses on physical and engineering aspects of micro-magnetofluidics, than on the biological applications which have been addressed in a number of previous excellent reviews. The field of micro-magnetofluidics can be categorized according to the type of the working fluids and the associated microscale phenomena of established research fields such as magnetohydrodynamics, ferrohydrodynamics, magnetorheology and magnetophoresis. Furthermore, similar to microfluidics the field can also be categorized as continuous and digital micro-magnetofluidics. Starting with the analysis of possible magnetic forces in microscale and the impact of miniaturization on these forces, the paper revisits the use of magnetism for controlling fluidic functions such as pumping, mixing, magnetowetting as well as magnetic manipulation of particles. Based on the observations made with the state of the art of the field micro-magnetofluidics, the paper presents some perspectives on the possible future development of this field. While the use of magnetism in microfluidics is relatively established, possible new phenomena and applications can be explored by utilizing flow of magnetic and electrically conducting fluids.     

掺Er光纤宽带光源的研究和应用

简述了近年来国内外掺Er光纤(EDF)光源的最新发展,详细分析了EDF光源的工作原理;介绍了EDF的基本结构,并概述了其各自的特点;总结了当前几种重要的EDF光源及其研究状况;指出了未来EDF光源发展方向。     

Comparison of flow and dispersion properties of free and wall turbulent jets for source dynamics characterisation

The objective of this paper is to provide an investigation, using large eddy simulations, into the dispersion of aircraft jets in co-flowing take-off conditions. Before carrying out such study, simple turbulent plane free and wall jet simulations are carried out to validate the computational models and to assess the impact of the presence of the solid boundary on the flow and dispersion properties. The current study represents a step towards a better understanding of the source dynamics behind an airplane jet engine during the take-off and landing phases. The information provided from these simulations can be used for future improvements of existing dispersion models.