Open Microchannels Research Articles

Spontaneous capillary flow in curved, open microchannels

Capillary flows are increasingly used in biotechnology, biology, chemistry, energy and space applications. Motivated by these new developments, designs of capillary channels have become more sophisticated. In particular, capillary microsystems often use winding channels for reasons such as compactness, or mixing. The behavior of capillary microflows in curved channels is still underdeveloped. In this work, we investigate this type of behavior. In the case of suspended capillary flows, is shown that the flow profile in the curved section is approximately analogous to that in a rectilinear section. In the case of open U-grooves where inner corners are present, the importance of the turn sharpness and of the presence of capillary filaments is pointed out. For sharp turns, and/or in the presence of precursor capillary filaments, we found the phenomenon that the inner filament precedes the outer filament in the channel. Analysis of the capillary flow in curved channels is performed experimentally using rectangular U-grooves and suspended channels. The experimental observations are compared to Surface Evolver numerical software results.

From square to circular polymeric microchannels by spin coating technology: a low cost platform for endothelial cell culture

Square microchannels are easy to fabricate by means of micromachining or lithographic techniques. However, in vitro vascular microcapillaries—as well as plug production and microparticle alignment—require mainly circular microchannels that can be used also in applications based on open microchannels. Nowadays, a simple, low cost, and versatile method to fabricate circular microchannels is still missing. Here, we report on a fast, inexpensive, flexible and reproducible method to fabricate circular microchannels by coupling spin coating with micromilled square microchannels. The proposed method is based on the balance between the displacement of liquid PDMS induced by centrifugal forces and the surface tension that tends to keep the liquid accumulated especially in the corners, which become therefore rounded. To show the versatility of the described experimental study we prepared a variety of rounded microchannels, including branched and PMMA–PDMS hybrid configuration microchannels. Finally, an endothelial cell layer was formed by culturing brain endothelial bEnd.3 cells inside the proposed circular microchannels. Results demonstrated a more successful adhesion, growth, and homogeneous distribution of the cells along the circular microchannel than those observed in the square microchannel used as a control.

Hierarchic micro-patterned porous scaffolds via electrochemical replica-deposition enhance neo-vascularization

Neo-vascularization is a key factor in tissue regeneration within porous scaffolds. Here, we tested the hypothesis that micro-patterned scaffolds, with precisely-designed, open micro-channels, might help endothelial cells to produce intra-scaffold vascular networks. Three series of micro-patterned scaffolds were produced via electrochemical replica-deposition of chitosan and cross-linking. All had regularly-oriented micro-channels (ϕ 500 μm), which differed for the inter-channel spacing, at 600, 700, or 900 μm, respectively. Random-pore scaffolds, using the same technique, were taken as controls. Physical-mechanical characterization revealed high water uptake and favorable elastic mechanical behavior for all scaffolds, slightly reduced in the presence of cross-linking and enhanced with the 700 μm-spaced micro-pattern. At MTT assay, mouse endothelial cell viability was >90% at day 1, 3 and 7, confirmed by visual examination with scanning electron microscopy (SEM). Intra-scaffold cell density, at fluorescence analysis, was higher for the 600 μm-spaced and the 700 μm-spaced micro-patterns over the others. The 700 μm-spaced scaffold was selected for the in vivo testing, to be compared to the random-pore one. Neither type produced an inflammatory reaction; both showed excellent tissue ingrowth. Micro-patterned scaffolds enhanced neo-vascularization, demonstrated by immunofluorescent, semi-quantitative analyses. These findings support the use of micro-patterned porous scaffolds, with adequately spaced micro-channels, to promote neo-vascularization.

Development of a new cellular solid breeder for enhanced tritium production

A new high-performance cellular solid breeder is presented that has several times the thermal conductivity and is substantially denser compared with sphere-packed breeder beds. The cellular breeder is fabricated using a patented process of melt-infiltrating ceramic breeder material into an open-cell carbon foam. Following solidification the carbon foam is removed by oxidation. This process results in a near 90% dense robust freestanding breeder in a block configuration with an internal network of open interconnected micro-channels for tritium release. The network of interconnected micro-channels was investigated using X-ray tomography. Aside from increased density and thermal conductivity relative to pebble beds, high temperature sintering is eliminated and thermal durability is increased. Cellular breeder morphology, thermal conductivity, specific heat, porosity levels, high temperature mechanical properties, and deuterium charging-desorption rates are presented.

Experimental and numerical study of microchannel formation in rubber-assisted hot embossing with an open-channel mold

In comparison to conventional hot embossing with a precisely aligned pair of hard mold plates, the rubber-assisted hot embossing process with a modified embossing tool that employs only one hard mold plate containing through-thickness open microchannels was investigated both experimentally and numerically. The numerical process dynamics model was constructed using a hyperelastic constitutive model to describe the large deformation behavior of the embossing film at an embossing temperature near the glass transition temperature. Both experimental and numerical results showed that excellent replication of microchannel patterns can be achieved using the simplified tool setup and an embossing temperature slightly above Tg is suitable for achieving a uniform embossing thickness. Additional advantages from the use of open channels on the embossing mold include reduced diesel effect in the mold cavity and improved surface smoothness of the embossed film. The simulation results agreed very well with the experimental observations for the effects of the main process parameters on the replication quality, and this study demonstrated that the hyperelastic process model is capable of predicting the basic characteristics of the rubber-assisted embossing process with an open-channel embossing mold.

Ultra-high pool boiling performance and effect of channel width with selectively coated open microchannels

Recent developments in the microelectronics industry has placed increasing demand on developing high heat flux removal systems. Pool boiling offers a simple technique without introducing complicated header configurations and moving parts. Enhancement in pool boiling is achieved by delaying critical heat flux (CHF) and increasing heat transfer coefficient (HTC), which dictates the heat removal capability of a surface. This study focuses on the effect of channel width on the performance and heat transfer mechanisms on open microchannel surfaces with three coating configurations: (i) sintered-throughout, (ii) sintered-fin-tops, and (iii) sintered-channels. Pool boiling performance is obtained with water at atmospheric pressure for 300μm, 500μm and 762μm channel widths. The separate liquid–vapor pathways in narrow channels, with sintered coatings only inside the channel, yielded an unprecedented performance with a CHF of 420W/cm2 based on the 1cm2 projected area at a wall superheat of 1.7°C at the fin top surface, resulting in an HTC of 2.9MW/m2°C. High speed videos were taken to understand the underlying mechanism. Furthermore, liquid–vapor pathways were identified to explain the parametric trends observed for each selectively enhanced configuration set.

Optimization of capillary flow through open-microchannel and open-micropillar arrays

We report semi-analytical models for capillary flow across surfaces covered by an array of open micropillars or open microchannels that incorporate first-order gravitational effects as well as the reduction of permeability due to meniscus curvature. We experimentally validate our models by measuring the vertical rise of a column of liquid across a series of vertical microstructured surfaces with varying characteristic dimensions, using Newtonian and non-Newtonian liquids. The data show that arrays of open microchannels deliver higher flow rates than comparable arrays of micropillars in most practical configurations. Furthermore, we experimentally demonstrate a flow maximum for open microchannels with respect to channel width that closely agrees with our analytical predictions. In addition, as an application example of the microstructures used in this study, we report electrospray emission from the edge of a chip covered by a microstructured surface that transports the liquid from a reservoir to the emission sites; the supply-limited regime of the current-voltage characteristics of these devices is in agreement with the literature on electrospray droplet emission.

Capillary driven flow of polydimethylsiloxane in open rectangular microchannels.

The flow of liquid polydimethylsiloxane (PDMS, Dow Corning Sylgard 184, 10 : 1 base to cross-linker ratio) in open, rectangular silicon microchannels, with and without a coating (100 nm) of poly-tetra-fluoro-ethylene (PTFE), was studied. Photolithographic patterning and etching of silicon wafers was used to create microchannels with a range of widths (∼5-50 μm) and depths (5-20 μm). Experimental PDMS flow rates in both PTFE-coated and uncoated channels were compared to an analytical model based on the work of Lucas and Washburn. The experimental flow rates matched the predicted flow rates reasonably well when the channel aspect ratio (width to depth), p, was less than 2. For channels with p > 2, the observed flow rates progressively lagged model predictions with increasing p. The experimental data, including zero flow rates in certain high aspect ratio PTFE-coated channels, can largely be explained by changes in the front and upper meniscus morphology of the flow as the channel aspect ratio is varied. The results strongly suggest that meniscus morphology needs to be taken into account to accurately model capillary flow in microchannels, especially those with large aspect ratios.

New designs of ceramic hollow fibres toward broadened applications

Three structural designs for ceramic membranes have been achieved for the first time through the co-extrusion of polymeric and ceramic layers. During co-extrusion, micro-channels are initiated due to the Rayleigh–Taylor instability and they propagate through the different layers. The polymeric layer(s) is then calcined off during the heat treatment step, which opens the micro-channels and following sintering a ceramic membrane with open micro-channels ranging from a few to a few tens of micrometres in diameter can be formed. These long, straight and non-tortuous micro-channels can be controlled to be open at any or all of the surfaces. Design 1 has open micro-channels passing through the entire membrane wall, Design 2 has a separation layer at the lumen and open micro-channels at the shell side, and Design 3 has open micro-channels from both lumen and shell sides sandwiching a separation layer of sponge-like structure. Aside from having much improved mass transfer property due to the reduced effective membrane thickness, they can be easily incorporated into hybrid systems with anticipated improvements in unit compactness and performance. The pure water permeation of Design 2 reached up to 159,000L/m2hbar with pore sizes in the micro-filtration range. The micro-channels are easily accessible from the shell/lumen side; therefore catalysts or adsorbents can be easily deposited into the micro-channels. Examples of possible applications include a high-efficiency dispersing device realised with Design 1; a gas chromatography column for gas separation with very low pressure drop realised with Design 2 and a highly compact membrane micro-reactor for consecutive reactions proposed with Design 3.

Enhanced Flow Boiling of Ethanol in Open Microchannels With Tapered Manifolds in a Gravity-Driven Flow

There is a clear need for cooling high heat flux generating electronic devices using a dielectric fluid without using a pump. This paper explores the feasibility of employing ethanol as a dielectric fluid in a horizontal, open microchannel heat sink configuration with a tapered gap manifold to yield very low pressure head requirements. The paper presents experimental results for such a system utilizing ethanol as a working fluid under gravity-driven flow. A heat flux of 217 W/cm2 was dissipated with a pressure drop of only 9 kPa. The paper further presents parametric trends regarding flow rate and pressure drop characteristics that provide basic insight into designing high heat flux systems under a given gravity head requirement. Based on the results, interrelationships and design guidelines are developed for the taper, ethanol flow rate and imposed heat flux on heat transfer coefficient and gravity head requirement for electronics cooling. Reducing flow instability, reducing pressure drop, and enhancing heat transfer performance for a dielectric fluid will enable the development of pumpless cooling solutions in a variety of electronics cooling applications.

Enhanced pool boiling for electronics cooling using porous fin tops on open microchannels with FC-87

The miniaturization trend in microelectronics demands effective heat removal from the high energy density components. Pool boiling in vapor chambers is a cooling technique which uses phase change to remove heat directly from the surface to effectively manage the thermal heat transfer needs. The enhancement in pool boiling can be achieved by providing additional nucleation sites and/or increasing the heat transfer surface area. In this paper, an enhanced copper surface with porous fin tops on open micorchannels is used with FC-87 as the working fluid. A maximum Critical Heat Flux (CHF) of 37 W/cm2 is achieved here which translates to a 270% enhancement in CHF compared to a plain chip. High speed images obtained suggest that separate liquid–vapor pathways generated from nucleation on the porous fin tops surface was responsible for the enhancement. The cohesive effect of channel width and channel depth is also studied here.

Towards combinatorial mixing devices without any pumps by open-capillary channels: fundamentals and applications.

In chemistry, biology, medical sciences and pharmaceutical industries, many reactions have to be checked by transporting and mixing expensive liquids. For such purposes, microfluidics systems consisting of closed channels with external pumps have been useful. However, the usage has been limited because of high fabrication cost and need for a fixed setup. Here, we show that open-capillary channels, which can be fabricated outside a clean room on durable substrates and are washable and reusable, are considerably promising for micro-devices that function without pumps, as a result of detailed studies on the imbibition of open micro-channels. We find that the statics and dynamics of the imbibition follow simple scaling laws in a wide and practical range; although a precursor film obeying a universal dynamics appears in the vertical imbibition, it disappears in the horizontal mode to make the design of complex micro-channel geometry feasible. We fabricate micro open-channel devices without any pumps to express the green florescent protein (GFP) by transporting highly viscous solutions and to accomplish simultaneous chemical reactions for the Bromothymol blue (BTB) solution. We envision that open-capillary devices will become a simple and low-cost option to achieve microfluidic devices that are usable in small clinics and field studies.

Flow patterns and heat transfer mechanisms during flow boiling over open microchannels in tapered manifold (OMM)

The ever increasing demand of high heat removal from compact form factor devices has generated considerable interest in advanced thermal management techniques. Flow boiling in microchannels has the ability to provide high heat dissipation due to the utilization of the latent heat of vaporization, while maintaining a uniform coolant temperature. Recently, a number of studies have introduced variable flow cross-sectional area to augment the thermal performance of microchannels. The open microchannel with manifold (OMM) configuration provides stable high heat transfer performance with very low pressure drop. In the current work, high speed images are obtained to gain an insight into the nucleating bubble behavior and flow patterns at high heat fluxes including critical heat flux (CHF). The flow patterns are plotted as a function of superficial gas and liquid velocity. The resulting map indicates significant departure from the earlier work on macroscale tubes and confined microchannels. A mechanistic description of the heat transfer mechanism is also presented and the underlying differences between flow boiling in closed microchannels and open microchannels with tapered manifold configuration are highlighted. Furthermore, bubble ebullition cycle in pool boiling is compared with the tapered geometry utilizing plain and microchannel surfaces.

On-Chip Transportation and Measurement of Mechanical Characteristics of Oocytes in an Open Environment

We propose a system that transports oocytes and measures their mechanical characteristics in an open environment using a robot integrated microfluidic chip (chip). The cells are transported through a micropillar array in the chip, and their characteristics are measured by a mechanical probe and a force sensor. Because the chip has an open microchannel, important cells such as oocytes are easily introduced and collected without the risk for losing them. In addition, any bubbles trapped in the chip, which degrade the measurement precision, are easily removed. To transport the oocytes through the open microchannel, we adopt a transportation technique based on a vibration-induced flow. Under this flow, oocytes arrive at the measurement point, where their mechanical characteristics are determined. We demonstrate the introduction, transportation, measurement of mechanical characteristics, and collection of oocytes using this system.

Enhanced pool boiling heat transfer mechanisms for selectively sintered open microchannels

Pool boiling performance can be enhanced significantly by generating separate liquid–vapor pathways through selectively coating different regions of a heat transfer surface. In this paper, heat transfer mechanisms are explored by depositing porous coatings on an open microchannel with channel width 762μm, channel depth 400μm and fin width 200μm. Three surfaces were fabricated with sintered porous coatings on: (i) entire microchannel surface (sintered-throughout), (ii) only the fin tops (sintered-fin-top), and (iii) only the channel walls (sintered-channel). Their pool boiling performance with degassed water at atmospheric pressure was experimentally obtained. A critical heat flux (CHF) of 313W/cm2 at a wall superheat of 7.5°C was obtained for a sintered-throughout surface with a 2.4 fold enhancement in CHF over a plain chip. Highest heat transfer coefficient (HTC) of 565kW/m2°C was obtained for this surface which translated into a 6.5 fold enhancement when compared to a plain surface. Three enhancement mechanisms were identified: (i) Area Augmented Enhanced Nucleation, (ii) Bubble Induced Liquid Jet Enhancement – Type-1 and (iii) Bubble Induced Liquid Jet Enhancement – Type-2. These mechanisms were responsible for the enhancement in HTC and CHF for sintered-throughout, sintered-fin-tops and sintered-channels, respectively. Although the current testing indicated the sintered-throughout surface to provide the highest CHF and HTC enhancement for the selected microchannel dimensions, changing the microchannel dimensions is expected to influence the relative merits of these configurations as the liquid and vapor flow mechanisms are influenced in fundamentally different ways.

Fiber in-line Mach-Zehnder interferometer based on an inner air-cavity for high-pressure sensing.

We demonstrate a fiber in-line Mach-Zehnder interferometer based on an inner air-cavity with open micro-channel for high-pressure sensing applications. The inner air-cavity is fabricated by combining femtosecond laser micromachining and the fusion splicing technique. The micro-channel is drilled on the top of the inner air-cavity to allow the high-pressure gas to flow in. The fiber in-line device is miniature, robust, and stable in operation and exhibits a high pressure sensitivity of ∼8,239 pm/MPa.

Tuning 3D topography on biomimetic surface for efficient self-cleaning and microfluidic manipulation

Currently, micro-/nanotopography on polymeric replica is generally limited to 2D when a mechanical demolding approach is applied. In this work, one-step replication of bio-inspired 3D topography is achieved using microinjection compression molding with novel dual-layer molds. Using a proposed flexible template, the replica topography and wettability are highly tunable during molding. Moreover, dual-scale topography on the mold is developed by coating the micropatterned insert with submicron silica particles. Contact angle and roll-off angle measurements indicate the lotus leaf, rose petal and rice leaf effects on biomimetic surfaces. Among the three kinds of surfaces, the petal-inspired surface possesses the superior performance in self-cleaning submicron contaminants and mechanical robustness, which is highly correlated to the low roughness-induced adhesive superhydrophobicity and the absence of fragile submicron-/nanostructure, respectively. Furthermore, a multi-layer mold structure is proposed for fabricating the open microfluidic devices. The embedment of the hydrophilic and hydrophobic silica particles in the microstructured open channel and the hydrophobic silica particles in the background area during replication renders the wettability contrast sharp, realizing the self-driven flow of microfluid confined within the open microchannel.

Numerical study for the use of different nozzle shapes in microscale channels for producing clean energy

Nowadays, most rural and hilly areas use the small and microscale plants to produce electricity; it is cheap, available and effective. Utilizing hydrokinetic turbines in flow of rivers, canal or channel to produce power has been a topic of considerable interest to researchers for past years. Many countries that are surrounded by irrigation or rainy channels have a great potential for developing this technology. Development of open flow microchannels that suit these countries has a main problem, which is low velocity of current appears, hence deploying nozzle in-stream open channels flow is the brilliant method for increasing the channels current flow systems’ efficiency. The nozzle is believed to have an ability of concentrating the flow direction whilst increasing the flow velocity. In this study, the effects of nozzle geometrical parameters such as diameter ratio, nozzle configuration and nozzle edges shape on the characteristics of the flow in the microscale rectangular channels have been investigated numerically, using a finite volume RANSE code ANSYS CFX. The physical parameters were reported for a range of diameter ratio (d2/d1) from 5/6 to 1/6 and nozzle length (Ln) of 0.8 m for various nozzle shapes. We also proposed a new approach which is the use of NACA 0025 aerofoils as a deploying nozzle in channels. The results of the current study showed that, although the decrease in the nozzle diameter ratio led to an increase of the flow velocity through the channel but it can affect drastically on the flow pattern, especially the free surface, at the nozzle area, which may reduce the amount of the generated power, thus the study concluded with optimum diameter ratio, which was 2/3. The flow patterns improved with the curved edges shape; the NACA shape gave the most preferable results.

Osteogenic Evaluation of Porous Calcium Phosphate Granules with Drug Delivery System Using Nanoparticle Carriers.

Novel porous biphasic calcium phosphate (BCP) granules incorporated with drug-releasing poly(lactic-co-glycolic acid) (PLGA) nanoparticles were developed as a drug delivery platform for bone regeneration. The charge interaction between the BCP and PLGA nanoparticle surfaces was manipulated to create this combination system. Spherical BCP granules with open micro-channels and PLGA nanoparticles loaded with dexamethasone (DEX) as a model drug were fabricated using a liquid nitrogen method and standard emulsion method, respectively. Polyethyleneimine was coated on the DEX-loaded PLGA nanoparticle surfaces, resulting in a net positively charged surface. Such modified nanoparticles were immobilized physically on the negatively charged BCP granule surfaces. An in vitro evaluation of MG 63 cells cultured for 1 and 2 weeks on the BCP granules containing DEX-loaded PLGA nanoparticles showed greater cell proliferation, differentiation, and a more extensively connected-tissue network than those cultured on the BCP granules alone. This innovative platform for bioactive molecule delivery more potently induced osteogenesis in vitro, which might be exploited in implantable bioceramic bone graft materials for stem cell therapy or improved in vivo performance.

Effect of taper on pressure recovery during flow boiling in open microchannels with manifold using homogeneous flow model

Flow boiling in microchannels is considered an attractive cooling option due to its small hydraulic diameter, latent heat effect and relative uniformity of coolant temperature. Although various techniques have been successfully employed to provide stable boiling operation in microchannels, it suffers from poor heat transfer performance with low critical heat flux (CHF) and low heat transfer coefficient (HTC). The tapered manifold with open microchannel (OMM) geometry provides stable operation with high heat transfer performance coupled with a very low pressure drop (less than 10kPa). In this study, the pressure drop components from friction, acceleration and area changes during flow boiling of water at atmospheric pressure in the OMM geometry are evaluated using homogeneous flow model. Pressure recovery resulting from area change due to the taper is identified as a major factor in the extremely low pressure drops observed in this geometry. Seven viscosity averaging schemes are used to predict the frictional pressure drop. The best performing viscosity models are able to predict the pressure drop for the tapered configurations with a microchannel chip within an average MAE of less than ±30%. High speed visualization of the boiling phenomenon supports the applicability of the homogeneous flow model in the open microchannel geometry.