High-throughput microcapillary pump with efficient integrated low aspect ratio micropillars

Prediction and reduction of pressure drop and resistance flow in micropillar arrays are important for the design of microfluidic circuits used in different lab-on-a-chip and biomedical applications. In this work, a diamond microchannel-integrated micropillar pump (dMIMP) with a resistance flow 35.5 % lower than a circular-based micropillar pump (cMIMP) has been developed via the optimization of the fluid dynamic behavior of different pillar shapes in a low aspect ratio (H/D ranged from 0.06 to 0.2) integrated pillar microchannel. The effect of different geometrical parameters (such as pillar shape and its distribution) has been considered to minimize the microchannel resistance flow. Six-micrometer-depth polidimetilsiloxane (PDMS) channels have been fabricated using a modified soft lithography process, which prevents the PDMS deformation under high-pressure operation. Flow through the fabricated samples has been numerically solved and experimentally measured, with an agreement higher than 90 %. The results have been used to validate the derived analytical formulation to determine the flow resistance in this type of channels, a fast approach to obtain the resistance flow in the design stage of microdevices. The analysis of the results indicates that, although porosity can be a determinant parameter to predict the resistance flow of MIMP, other geometrical parameters such as side distance between pillars and pillar shape play a major role in this scenario. Finally, a high-throughput optimized diamond MIMP pump has been designed, tested and validated as a capillary pump, showing that it can provide a flow rate 73 % higher than a circular MIMP pump.     

Wicking through a confined micropillar array

This study considers the spreading of a Newtonian and perfectly wetting liquid in a square array of cylindric micropillars confined between two plates. We show experimentally that the dynamics of the contact line follows a Washburn-like law which depends on the characteristics of the micropillar array (height, diameter and pitch). The presence of pillars can either enhance or slow down the motion of the contact line. A theoretical model based on capillary and viscous forces has been developed in order to rationalize our observations. Finally, the impact of pillars on the volumic flow rate of liquid which is pumped in the microchannel is inspected.     

Valves for autonomous capillary systems

Autonomous capillary systems (CSs) are microfluidic systems inside which liquids move owing to capillary forces. CSs can in principle bring the high-performances of microfluidic-based analytical devices to near patient and environmental testing applications. In this paper, we show how wettable capillary valves can enhance CSs with novel functionalities, such as delaying and stopping liquids in microchannels. The valves employ an abruptly changing geometry of the flow path to delay a moving liquid filling front in a wettable microchannel. We show how to combine delay valves with capillary pumps, prevent shortcuts of liquid along the corners of microfluidic channels, stop liquids filling microchannels from a few seconds to over 30 min, trigger valves using two liquid fronts merging, and time a liquid using parallel microfluidic paths converging to a trigger valve. All together, these concepts should add functionality to passive microfluidic systems without departing from their initial simplicity of use. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Modeling, analysis and design of centrifugal force-driven transient filling flow into a circular microchannel

Recently, centrifugal pumping has been regarded as an excellent alternative control method for fluid flow inside microchannels. In this paper, we have first developed the physical modeling and carried out the analysis for the centrifugal force-driven transient filling flow into a circular microchannel. Two types of analytic solutions for the transient flow were obtained: (1) pseudostatic approximate solution and (2) exact solution. Analytic solutions include expressions for flow front advancement, detailed velocity profile and pressure distribution. The obtained analytic results show that the filling flow driven by centrifugal force is affected by two dimensionless parameters which combine fluid properties, channel geometry and processing condition of rotational speed. Effects of inertia, viscous and centrifugal forces were also discussed based on the parametric study. Furthermore, we have also successfully provided a simple and convenient analytic design tool for such microchannels, demonstrating two design application examples.     

Modeling, analysis and design of centrifugal force driven transient filling flow into rectangular microchannel

Recently, centrifugal pumping has been discovered to be an excellent alternative method for controlling the fluid flow inside microchannels. In this paper, we have developed the physical modeling and carried out the analysis for the centrifugal force driven transient filling flow into a rectangular microchannel. Two types of analytic solutions for the transient flow were obtained: (1) a pseudo-static approximate solution, and (2) an exact solution. Analytic solutions include expressions for flow front advancement, detailed velocity profile and pressure distribution. The obtained analytical results show that the filling flow driven by centrifugal force is affected by three dimensionless parameters which combine fluid properties, rectangular channel geometry and processing condition of rotational speed. Effects of inertia, viscous and centrifugal forces were also discussed based on the parametric study. Furthermore, we have also successfully provided a simple and convenient analytical design tool for such rectangular microchannels, demonstrating two design application examples.     

Cluster formation and growth in microchannel flow of dilute particle suspensions

The lifetime of microfluidic devices depends on their ability to maintain flow without interruption. Certain applications require microdevices for transport of liquids containing particles. However, microchannels are susceptible to blockage by solid particles. Therefore, in this study, the phenomenon of interest is the formation and growth of clusters on a microchannel surface in the flow of a dilute suspension of hard spheres. Based on the present experiments, aggregation of clusters was observed for particle-laden flows in microchannels with particle void fraction as low as 0.001 and particle diameter to channel height ratio as low as 0.1. The incipience and growth of a single cluster is discussed, and the spatial distribution and time evolution of clusters along the microchannel are presented. Although the cluster size seems to be independent of location, more clusters are found at the inlet/outlet regions than in the microchannel center. Similarly as for an individual cluster, as long as particle–cluster interaction is the dominant mode, the total cluster area in the microchannel grows almost linearly in time. The effects of flow rate, particle size, and concentration are also reported.     

Liquid flow through a diverging microchannel

In this work, experiments and three-dimensional numerical calculations of fluid flow through diverging microchannels were carried out with the aim of bringing out differences between flow in uniform and nonuniform passages. Deionized water was used as the working fluid in the experiments where the effects of mass flow rate (8.33 × 10^?6 to 8.33 × 10^?5 kg/s), microchannel hydraulic diameter (118–177 µm), length (10–30 mm) and divergence angle (4°–16°) on pressure drop were studied. The results are analyzed in detail with the help of numerical data. The pressure drop exhibits a linear dependence on the mass flow rate, whereas it is inversely proportional to the divergence angle and square of the hydraulic diameter. The pressure drop increases anomalously at 16°, suggesting that flow reversal occurs between 12° and 16°, which agrees with the corresponding value at the conventional scale. For the purpose of predicting pressure drop using straight microchannel theory, an equivalent hydraulic diameter was defined. It is observed that the equivalent hydraulic diameter, located at one-third of the diverging microchannel length from the inlet, becomes mostly independent of the mass flow rate, microchannel hydraulic diameter, length and divergence angle. The pressure drop for a diverging microchannel becomes equal to an equivalent hydraulic diameter uniform cross-section microchannel, suggesting that conventional correlations for straight microchannels can also be applied to diverging microchannels. The data presented in this work are of fundamental importance and can help in optimization of diffuser design used for example in valveless micropumps.     

A novel technique for fabrication of micro- and nanofluidic device with embedded single carbon nanotube

This paper describes a novel technique for fabrication of micro- and nanofluidic device that consists of a carbon nanotube (CNT) and a polydimethylsiloxane (PDMS) microchannel. Single CNT was placed at desired locations using dielectrophoresis (DEP) and PDMS microchannel was constructed on the aligned CNT via photolithography and soft lithography techniques. This technique enables a CNT to be seamlessly embedded in a PDMS microchannel. Moreover, controlling the PDMS curing condition enables the construction of the device with or without a CNT (the device without CNT has a trace nanochannel in PDMS). Preliminary flow tests such as capillary effect and pressure-driven flow were performed with the fabricated devices. In the capillary effect tests, the flow stopped at the nanochannel in both devices. In the pressure-driven flow lower flow resistance was observed in the device with a CNT.     

Analysis and experiment of transient filling flow into a rectangular microchannel on a rotating disk

In order to predict the time-dependent behaviors of the moving front in lab-on-a-CD systems or centrifugal pumping, an analytical expression and experimental methods of centrifugal-force-driven transient filling flow into a rectangular microchannel in centrifugal microfluidics are presented in this paper. Considering the effect of surface tension, and neglecting the effect of Coriolis force, the velocity profile, flow rate, the moving front displacement and the pressure distribution along the microchannel are characterized. Experiments are carried out using the image-capturing unit to measure the shift of the flow in rectangular microchannels. The flow characteristics in rectangular microchannels with different cross-sectional dimensions (200, 300 and 400 μm in width and 140, 240 and 300 μm in depth) and length (18 and 25 mm) under different rotational speed are investigated. According to the experimental data, the model can be more reasonable to predict the flow displacement with time, and the errors between theoretical and the experimental will decrease with increasing the cross-section size of the microchannel.     

Femtosecond laser-induced modification of surface wettability of PMMA for fluid separation in microchannels

The modification of polymer surface wettability is receiving increasing interest in recent years. As surface wettability affects the flowing resistance, and thus the separation ratio and/or mixing ratio of samples in different microchannels, the controlled modification of surface wettability is highly desirable. In this study, microfluidic channels with controlled surface wettability were achieved and fabricated using femtosecond (fs) laser direct ablation of polymethyl methacrylate at various fluences. Varied flow velocities and separation ratio of water in microfluidic channels have been successfully obtained through fs laser-induced modification in wetting characteristics of the microchannel surfaces. A concave flow front was observed in a microchannel with hydrophilic surface. Correspondingly, a convex flow front was observed with hydrophobic surface. For an untreated channel, a straight flow front was observed. These results would be attractive for various microfluidic chip applications, such as control of the reagent reaction through controlling liquid medium separation or control of mixing ratio in different channels.     

Optically induced electrohydrodynamic instability-based micro-patterning of fluidic thin films

Projected light patterns are used to induce electrohydrodynamic instabilities in a polymer thin film sandwiched between two electrodes. Using this optically induced electrohydrodynamic instability (OEHI) phenomenon, we have successfully demonstrated rapid, microscale patterning of polydimethylsiloxane (PDMS) pillar arrays on a thin-film hydrogenated amorphous silicon layer on top of an indium titanium oxide glass substrate. This glass substrate is the bottom electrode in a two-electrode, parallel-plate capacitor configuration with a micron-scale gap. Within this gap are a thin film of spin-coated PDMS and a thin layer of air. Primary pillar growth is first observed within 5–90 s in the dark regions of the projected patterns and pillar growth eventually spreads to the illuminated regions when the initial PDMS thickness is <2 μm. Experimental data characterizing the change in pillar diameters (between 15 and 30 μm in diameter) show that they can be decoupled from the inter-pillar spacing (maintaining a constant ~84 μm pitch between pillar centers) by controlling the applied DC voltage (between 110 and 210 V). Experimental results also show the importance of the optically induced lateral electric field on controlling pillar formation. This OEHI method of rapid pillar generation, with voltage control of the pillar diameter and control of pillar position via projected light patterns, presents new opportunities for low cost, efficient, and simple fabrication of micro, and perhaps nanoscale, polymer structures that could be used in many bioMEMS applications.     

Flow-induced deformation of compliant microchannels and its effect on pressure–flow characteristics

We report theoretical and experimental investigations of flow through compliant microchannels in which one of the walls is a thin PDMS membrane. A theoretical model is derived that provides an insight into the physics of the coupled fluid–structure interaction. For a fixed channel size, flow rate and fluid viscosity, a compliance parameter \(f_{\text{p}}\) is identified, which controls the pressure–flow characteristics. The pressure and deflection profiles and pressure–flow characteristics of the compliant microchannels are predicted using the model and compared with experimental data, which show good agreement. The pressure–flow characteristics of the compliant microchannel are compared with that obtained for an identical conventional (rigid) microchannel. For a fixed channel size and flow rate, the effect of fluid viscosity and compliance parameter \(f_{\text{p}}\) on the pressure drop is predicted using the theoretical model, which successfully confront experimental data. The pressure–flow characteristics of a non-Newtonian fluid (0.1 % polyethylene oxide solution) through the compliant and conventional (rigid) microchannels are experimentally measured and compared. The results reveal that for a given change in the flow rate, the corresponding modification in the viscosity due to the shear thinning effect determines the change in the pressure drop in such microchannels.     

Enhanced wetting properties of silicon mesh microchannels coated with SiO2/SnO2 nanoparticles through layer-by-layer self assembly

The enhanced wetting property of silicon mesh microchannels coated with SiO₂/SnO₂ nanoparticles is presented in this paper. The SiO₂/SnO₂ bi-layers are prepared using layer-by-layer nano self assembly. It is found that the silicon mesh microchannels are super hydrophilic and demonstrated powerful capillary. The capillary rise rate is characterized by measuring the front location of liquid on the silicon mesh surface, laid on a 45° inclined platform. For a silicon mesh sample with an overall dimension of 25 mm × 25 mm, when the microchannel width is 0.5 mm, the liquid front can reach the top edge of the sample in approximately 30 s. The mesh silicon surface with a SiO₂/SnO₂ multilayer film presented in this paper has better wettability and higher capillary pressure than other hydrophilic surfaces reported. The results provide a new way to improve the capillary in microchannels with enhanced super hydrophilic surfaces in microchannels for variety of micro/nanofluidic applications.     

Analysis and experiment of capillary valves for microfluidics on a rotating disk

This paper presents an analytical expression of the pressure barrier in a capillary-burst valve for flow regulation in centrifugal microfluidics. The analysis considers variations of the interfacial energies at the meniscus of three-dimensional (3D) configuration in a rectangular microchannel with a sudden expansion in cross-section. We derive a simple expression that predicts the critical burst pressure or rotational speed to overcome the capillary valve. Experiments were carried out for capillary valves that were integrated with microchannels on a rotating disk having various cross-sectional dimensions (300 and 400 μm in width and 80–600 μm in depth) and wedge angles (30°–100°) of sudden expansion. The flow visualization of the meniscus development across the capillary valve supports the assumptions made for the present analysis. The measurements of burst rotational speeds for the capillary valves are in good agreement with the predictions by the simple expression except that those with a larger channel width and wider wedge angles are nearly 10% lower than the predicted values.     

Experimental investigations of liquid flow in rib-patterned microchannels with different surface wettability

The effects of rib-patterned surfaces and surface wettability on liquid flow in microchannels were experimentally investigated in this study. Microchannels were fabricated on single-crystal silicon wafers by photolithographic and wet-etching techniques. Rib structures were patterned in the silicon microchannel, and the surface was chemically treated by trichlorosilane to create hydrophobic condition. Experiments with water as the working fluid were performed with these microchannels over a wide range of Reynolds numbers between 110 and 1914. The results for the rib-patterned microchannels showed that the friction factor with the hydraulic diameter based on the rib-to-upper-wall height was lower than that predicted from incompressible theory with the same height. The friction factor-Reynolds number products for the hydrophobic condition increased as Reynolds number increased in the laminar flow regime. The experimental results were also compared with the predictive expressions from the literature, and it was found that the experimental data for the small rib/cavity geometry was in good agreement with those in the literature.     

A comprehensive theoretical model of capillary transport in rectangular microchannels

A detailed theoretical model of capillary transport in rectangular microchannels is proposed. Two important aspects of capillary transport are revisited, which are considered with simplified assumption in the literature. The capillary flow is assumed as a low Reynolds number flow and hence creeping flow assumptions are considered for majority of analyses. The velocity profile used with this assumption results into a steady state fully developed velocity profile. The capillary flow is inherently a transient process. In this study, the capillary flow analysis is performed with transient velocity profile. The pressure field expression at the entrance of the microchannel is another aspect which is not often accurately represented in the literature. The approximated pressure field expression at the entrance of the rectangular microchannel is widely used in the literature. An appropriate entrance pressure field expression for a rectangular microchannel is proposed. For both analyses, the governing equation of the capillary transport in rectangular microchannel is derived by applying the momentum equation to the fluid control volume along the microchannel. The non-dimensional governing equations are obtained, each for a transient velocity profile and a newly proposed pressure field, for analyzing the importance of such velocity profile and pressure field expression.     

Flow characteristics of polar liquids in microfluidic immunosensors

There is an interest in microfluidic devices for disease detection. In microfluidic immunosensors, the microchannel surfaces are functionalized with a stack of intermediate linker molecules to the specific antibodies. The efficiency of these immunosensors depends on the effective capture of antigens flowing in the carrier fluid by the surface-immobilized antibodies. The diffusion of these antigens to these antibody-immobilized surfaces is governed by the velocity profile, which in turn is governed by the interaction of the carrier fluid molecules with the surface antibodies. We report a systematic study to characterize fluid flow of different polar liquids (water, methanol and isopropyl alcohol) in trapezoidal Si microchannels, of about 100 μm hydraulic diameter, functionalized with intermediate molecular layers along with three different antibodies immobilized via these molecular layers. The friction constants were calculated from the pressure drop measurements. We attempted to understand the solid–liquid interactions in terms of the friction constants as a function of the solid surface free energies of the terminal antibody layers (which are affected by the energetics of the underlying layers) immobilized on to the microchannels, and the polarities of the liquids flowing through these microchannels. Correlations of liquid polarities with the friction constants were seen for almost all the functionalized surfaces. A reasonable correlation of the surface energies with the friction constants was seen for most of the surfaces studied. Possible reasons for the behaviors are discussed. The measured friction constants and the knowledge of the solid–liquid interactions could facilitate improved designs of microfluidic immunosensors.     

Design of shrinkage microvalve in microchannel with hydrophilic and hydrophobic walls

This paper presents a simple design of shrinkage microvalves which can be used to effectively stopping capillary flow inside a microchannel with hydrophilic and hydrophobic walls. Based on the relationship between capillary pressure and cross-section geometry of a microchannel, the microvalve is designed with a critical ratio of rectangular section. In order to verify the feasibility of the design rule, a couple of shrinkage microvalves with different aspect ratios of cross-section are fabricated by using PDMS bonded with glass wafer. The experiment demonstrates the stopping effect of the proposed design of shrinkage microvalve.     

Capillary flow in microchannels

The surface of microchannels, especially polymer channels, often needs to be treated to acquire specific properties. This study investigated the capillary flow and the interface behavior in several glass capillaries and fabricated microchannels using a photographic technique and image analysis. The effect of air plasma treatment on the characteristics of capillary flow in three types of microfluidic chips, and the longevity of the acquired surface properties were also studied. It was observed that the dynamic contact angles in microchannels were significantly larger than those measured from a flat substrate and the angle varied with channel size. This suggests that dynamic contact angle measured in situ must be used in the theoretical calculation of capillary flow speed, especially for microfabricated microchannels since the surface properties are likely to be different from the native material. This study also revealed that plasma treatment could induce different interface patterns in the PDMS channels from those in the glass and PC channels. The PDMS channel walls could acquire different level of hydrophilicity during the plasma treatment, and the recovery to hydrophobicity is also non-homogeneous.