Open Microchannels Research Articles

Durable Janus membrane with on-demand mode switching fabricated by femtosecond laser

Despite their notable unidirectional water transport capabilities, Janus membranes are commonly challenged by the fragility of their chemical coatings and the clogging of open microchannels. Here, an on-demand mode-switching strategy is presented to consider the Janus functionality and mechanical durability separately and implement them by simply stretching and releasing the membrane. The stretching Janus mode facilitates unidirectional liquid flow through the hydrophilic micropores-microgrooves channels (PG channels) fabricated by femtosecond laser. The releasing protection mode is designed for the in-situ closure of the PG channels upon encountering external abrasion and impact. The protection mode imparts the Janus membrane robustness to reserve water unidirectional penetration under harsh conditions, such as 2000 cycles mechanical abrasion, 10 days exposure in air and other rigorous tests (sandpaper abrasion, finger rubbing, sand impact and tape peeling). The underlying mechanism of gridded grooves in protecting and enhancing water flow is unveiled. The Janus membrane serves as a fog collector to demonstrate its unwavering mechanical durability in harsh real-world conditions. The presented design strategy could open up new possibilities of Janus membrane in a multitude of applications ranging from multiphase separation devices to fog harvesting and wearable health-monitoring patches.

Precise control of liquid–liquid interface position and geometry in open microchannels

Accurate control of liquid–liquid interfaces is of great importance in many scientific fields. Currently, most studies on liquid–liquid interfaces are based on microfluidics in closed channels, and for open systems, it is difficult to form stable liquid–liquid interfaces in microchannels due to the interference of gas-phase molecules. Here, we introduce a new method to manipulate the motion of the liquid–liquid interface under an open microchannel. Under the effect of surface tension, the liquid–liquid interface moves in microchannels until it encounters microstructures in the microchannels, where the force equilibrates and remains stable to form a fixed shape. The shape of the interface is regulated by adjusting the dimensions of the microchannels and microstructures as well as the positions of the microstructures in the microchannels. This spontaneous liquid–liquid interface moves, stops, and remains stable in a very convenient way. The morphology and position of the interface are well manipulated using microstructures, and the liquid–liquid interface with well-defined geometry can be made to stay in different positions to play different roles.

CoO supported NiFe layered double hydroxide sandwich-like nanosheets on hierarchical carbon framework for efficient electrocatalytic oxygen evolution.

Exploration of greatly efficient and steady non-noble oxygen evolution reaction (OER) electrocatalysts is of great significance in improving the overall efficiency of energy density systems such as regenerative fuel cells, water electrolyzes, and metal-air batteries. Herein, inspired by hierarchical 3D porous structures with open microchannels of natural wood, CoO@NiFe LDH sandwich-like nanosheets were anchored on the carbonized wood (CW) via electrodeposition and calcination strategies. The strong interactions between CoO nanosheets and NiFe LDH nanosheets endow CoO@NiFe LDH/CW electrocatalyst with high catalytic properties toward the OER comparable to CoO/CW and NiFe LDH/CW. The optimized CoO@NiFe LDH/CW electrocatalyst demonstrates good OER catalytic performance with an overpotential of 230 mV at 100 mA cm-2. This work presents an innovative approach to utilize renewable resources for constructing advanced free-standing catalysts.

A modeling approach for capillary-driven flow of a wetting fluid in a rectangular open microchannel of arbitrary axisymmetric width profile: Application to heat pipes

A modeling approach for capillary-driven flow of a wetting fluid in a rectangular open microchannel of arbitrary axisymmetric width profile: Application to heat pipes

Capillary Filling in Open Rectangular Microchannels with a Spatially Varying Contact Angle.

Capillary flow in microchannels is important for many technologies, such as microfluidic devices, heat exchangers, and fabrication of printed electronics. Due to a readily accessible interior, open rectangular microchannels are particularly attractive for these applications. Here, we develop modifications of the Lucas-Washburn model to explore how a spatially varying contact angle influences capillary flow in open rectangular microchannels. Four cases are considered: (i) different uniform contact angles on channel sidewalls and channel bottom, (ii) contact angles varying along the channel cross section, (iii) contact angle varying monotonically along the channel length, and (iv) contact angle varying periodically along the channel length. For case (i), it is found that the maximum filling velocity is more sensitive to changes in the wall contact angle. For case (ii), the contact angles can be averaged to transform the problem into that of case (i). For case (iii), the time evolution of the meniscus position no longer follows the simple square-root law at short times. Finally, for case (iv), the problem is well described by using a uniform contact angle that is a suitable average. These results provide insights into how to design contact-angle variations to control capillary filling and into the influence of naturally occurring contact-angle variations on capillary flow.

Spatial exclusion leads to "tug-of-war" ecological dynamics between competing species within microchannels.

Competition is ubiquitous in microbial communities, shaping both their spatial and temporal structure and composition. Classical minimal models of competition, such as the Moran model, have been employed in ecology and evolutionary biology to understand the role of fixation and invasion in the maintenance of population diversity. Informed by recent experimental studies of cellular competition in confined spaces, we extend the Moran model to incorporate mechanical interactions between cells that divide within the limited space of a one-dimensional open microchannel. The model characterizes the skewed collective growth of the cells dividing within the channel, causing cells to be expelled at the channel ends. The results of this spatial exclusion model differ significantly from those of its classical well-mixed counterpart. The mean time to fixation of a species is greatly accelerated, scaling logarithmically, rather than algebraically, with the system size, and fixation/extinction probability sharply depends on the species' initial fractional abundance. By contrast, successful takeovers by invasive species, whether through mutation or immigration, are substantially less likely than in the Moran model. We also find that the spatial exclusion tends to attenuate the effects of fitness differences on the fixation times and probabilities. We find that these effects arise from the combination of the quasi-neutral "tug-of-war" diffusion dynamics of the inter-species boundary around an unstable equipoise point and the quasi-deterministic avalanche dynamics away from the fixed point. These results, which can be tested in microfluidic monolayer devices, have implications for the maintenance of species diversity in dense bacterial and cellular ecosystems where spatial exclusion is central to the competition, such as in organized biofilms or intestinal crypts.

Design and analysis of a high sensitivity open microchannel PCF-based surface plasmon resonance refractometric sensor

This paper presents a high-sensitivity dual-open microchannel PCF-SPR refractometric sensor that was numerically analyzed using the Finite Element Method (FEM). A dual-open microchannel was created on both sides of the fiber to facilitate analyte infiltration and improve its detection performance. The microchannel of PCF was coated with gold plasmonic material to achieve practical sensing. The results revealed that the refractive index sensing range of the proposed refractometric sensor is between 1.37 and 1.43, with a highest amplitude sensitivity of 3743.64 RIU−1 and a wavelength sensitivity of 33000 nm/RIU. Additionally, it offers a maximum resolution of 5.56 × 10−6 RIU. All structure parameters, such as air-hole length, pitch, open-channel radius, and gold thickness, are optimized. The proposed sensor is a significant improvement in the detection of multiple analytes in bio-sensing applications.

Experimental study of flow boiling characteristics of open microchannels with elliptical cavities and elliptical ribs

Open microchannel heat sinks (O-MCHS) stand out among many thermal management technologies due to their extra open gaps that reduce pressure drop and mitigate flow instability. Recently, there has been a growing trend of incorporating microstructures onto microchannels in order to enhance the hydrothermal performance of heat sinks. However, the heat transfer characteristics and bubble behavior of O-MCHS with microstructures are still rarely studied. In this study, open copper-based microchannel heat sinks with elliptical cavities and elliptical central ribs (O-EC-ECR) and elliptical cavities and elliptical side ribs (O-EC-ESR) were designed and manufactured; the flow patterns, heat transfer performance and pressure drop were systematically studied and compared with those of rectangular open microchannel heat sink (R O-MCHS). The working fluid was degassed DI water and the experiments were performed with the inlet subcooling of 75 ℃ at efficient heat fluxes of 668.00–2416.16 kW/m2 under various mass fluxes ranging from 421 to 801 kg/m2·s. The experimental results demonstrated that the O-EC-ECR exhibited the best two-phase heat transfer performance among three structures at all mass fluxes. The average two-phase heat transfer coefficient of O-EC-ECR was increased by 12.2 % compared to R O-MCHS at the mass flux of 801 kg/m2·s. The average wall temperature of O-EC-ECR was found to be 10.1 ℃ lower than that of R-O-MCHS and 6.7 ℃ lower than that of O-EC-ESR at the mass flux of 801 kg/m2·s. This should be attributed to the dramatically increased convective heat transfer area as well as the separating and breaking effect of the central ribs on the bubbles, which was manifested by the increase in heat transfer coefficient and critical heat flux (CHF). In addition, the central ribs improved the flow boiling stability by effectively suppressing the generation of confined bubbles and the formation of slugs. The arrangement of the side ribs and cavities made bubbles easier to condense, which facilitated the mixing of hot and cold fluid. At low to medium mass fluxes, the O-EC-ESR only showed superior heat transfer performance during the bubbly flow dominated nucleate boiling regime. Whereas high mass flux can improve the boiling heat transfer performance; the average heat transfer coefficient of O-EC-ESR was increased by 6.1 % compared to R O-MCHS. The average pressure drop of O-EC-ESR was decreased by 21.8 % compared to R O-MCHS and 25.6 % compared to O-EC-ECR at the mass flux of 801 kg/m2·s.

Glazing of 3D-Printed Silica to Reduce Surface Roughness and Permeability

The advantages that 3D printing brings to the development and production of customized structures make it suitable for use in the space industry, since spacecraft components are rarely produced in large series. This work explores the use of stereolithography printing of a silica resin for microfluidic applications, in particular small-scale microthrusters, where an impermeable high-temperature material with a smooth surface is required. The printing accuracy, firing shrinkage, surface roughness and permeability of 3D-printed ceramic samples were investigated. Furthermore, glazing of the ceramic material with a stoneware glaze was performed and evaluated with respect to its effect on surface roughness and gas permeability. Open microchannels with diameter down to 250 µm were obtained. However, the accuracy was poor. Surface roughness (Sa) of the unglazed material was between 2.4 and 20 µm in green state and 4.2-16 µm after firing, depending on the layer thickness and printing angle of the sample. Half of the unglazed samples were permeable, owing to porous areas at the interfaces between the printed layers. Two glazing methods were investigated: dip coating and airbrushing. For the latter, two amounts of coatings were explored. Dip coating and airbrushing with the larger amount of coatings resulted in uniform and smooth glaze layers. The smoothest surfaces, with Sa less than 0.2 µm, were obtained using airbrushing. Glazing made all samples impermeable, no matter the method used. Finally, the potential of the material in the suggested application was demonstrated through operation of a printed and glazed microthruster nozzle.

Study the Effect of Silicon Nanofluid on the Heat Transfer Enhancement of Triangular-Shaped Open Microchannel Heat Sinks

Study the Effect of Silicon Nanofluid on the Heat Transfer Enhancement of Triangular-Shaped Open Microchannel Heat Sinks

Dynamic contact angle measurement of hydrophilic open microchannels: The role of surface wettability

Wetting dynamics play a major role in many practical applications; however, many fundamental problems remain unresolved, especially in relation to the dynamic contact angle. The present study investigates the movement of a gas–liquid interface in an open microchannel with different materials and wettability. Using a high-speed microscope camera, the shape of the dynamic equilibrium of the gas–liquid interface in a flowing state was recorded, including the microscopic regions near the contact line. The results show that the effects of the surface wetting condition play a crucial role in altering the apparent dynamic contact angle. During the experiment, we observed an interesting phenomenon where the dynamic equilibrium contact angle under flow conditions is larger than the contact angle predicted by the Cox–Voinov law. This may be attributed to the existence of friction between the fluid and the wall surface under flow, which reduces the additional pressure at the gas–liquid interface, and the magnitude of friction is manifested through the hysteresis contact angle. The instability and periodic variation of the microscopic contact angle are caused by the surface heterogeneity near the contact line, namely, the spatial variation of solid–gas and solid–liquid interfacial tensions. Understanding the surface properties can help optimize the interface system's design and improve its efficiency for use.

An open superhydrophilic microchannel heat sink for thin film boiling with a high coefficient of performance

Currently, the existing two-phase boiling heat transfer technologies have limitations in extremely narrow spaces and cannot satisfy the high heat flux dissipation requirements of highly integrated and ultrathin devices. In this study, a high-efficiency and low-cost open superhydrophilic microchannel heat sink with a micro/nanocomposite structure, negligible pumping energy, and a high coefficient of performance (COP) was achieved. The wettability and microscopic morphology of the different heat sinks as well as the variation in the critical heat flux (CHF) and heat transfer coefficient (HTC) at different mass fluxes were discussed. When the mass flux was 0.05 kg/m2⋅s, the CHF of the microgrooved composite nanostructured copper (MCNC) heat sink was 173.90 W/cm2, which was 182.80% higher than that of the smooth copper (SC) heat sink and 20% higher than that of the microgrooved copper (MC) heat sink. In addition, the highest CHF of 325.90 W/cm2 was obtained for the MCNC at a 0.15 kg/m2⋅s mass flux. The high CHF of thin film boiling was achieved at a low inlet water pressure. This resulted in a COP of 695,600 for this boiling method, which is a larger value than that in most reports thus far. This study provides a preliminary investigation of the thin film boiling performance for an open microchannel. This may provide a new solution to the challenge of high heat flux dissipation in extreme spaces.

A Multifunctional Wood‐Derived Separator Towards the Problems of Semi‐Open System in Lithium‐Oxygen Batteries

AbstractThe semi‐open system of lithium‐oxygen batteries (LOBs) results in electrolyte depletion, lithium anode corrosion, and by‐product deposition, and therefore represents a major challenge that hinders their application. Here, the aligned and open microchannel structures of wood are fabricated as separators to provide low‐tortuosity pathways for rapid ionic transport and serve as reservoirs for retaining the electrolyte by capillary forces to improve the electrochemical kinetics. In an open environment, the wood separator can hold 39% of the initial adsorption electrolyte capacity after 40 days, much higher than that of glass fiber (GF, 15%). The cellulose in the wood can confine the crossover effect of water thereby inhibiting the corrosion of lithium anode and reducing the deposition of by‐products. Density functional theory calculations certify that the abundant functional groups and uniform electron distribution in cellulose increase lithium‐ion concentration on the wood surface and promote lithium‐ion migration with a low diffusion barrier. LOBs composed of the wood‐derived separator displayed excellent anodic reversibility (over 1200 h) and effectively improved cathodic lifetime over 300 cycles (1.6 times longer than that of GF separator). These findings illustrate the significant potential of this candidate separator for high‐performance LOBs and are expected to be extended to metal‐air batteries.

Design and fabrication of bifurcated overlapping open microchannels

Si-based double-layer overlapping open microchannels have been designed and fabricated for microfluidic applications. The top channel has been ingeniously designed in both T-shaped and X-shaped configurations along the <100> direction, while the bottom channel has been prepared along the <110> direction, allowing it to be semi-buried into the top channel at a 45-degree angle. An undercut characteristic of 25% TMAH has been utilized to create a unique overlapping microchannel structure by removing material from the overlapping part of the two layers of channels. This results in intersecting microchannels that are not connected, providing significant structural advantages over traditional single-layer microchannels. With this innovative design, two different substances can be delivered independently and efficiently in the same area.

An open microfluidic design for contact angle measurement

Spontaneous capillary flow in open microchannels is a phenomenon driven by surface energies. The contact angle that the liquid forms with the channel's substrate material and the cross-section of the microchannel decide whether liquid from a connected reservoir will automatically fill the channel or not. In this work we show how this behavior can be used to design a passive contact angle measurement device (CAMD) based on parabolic open microgrooves. To that end, we present a theory of open capillary flow in such microgrooves and compare the results to minimal energy surface simulations. Additionally, we discuss that the condition for capillary flow of curved microchannels is essentially equal to the condition for their straight counterparts having the same cross-section. Lastly, we present two demonstrators of our CAMD made out of micromilled poly(methyl methacrylate). The devices consist of five open microchannels with different cross-sections which are connected to a common liquid reservoir. We show how the behavior of a liquid placed into that reservoir can be used to evaluate the contact angle between the liquid and the substrate material. A comparison to conventional contact angle goniometry shows that our approach is able to successfully estimate contact angles with an accuracy of 10° by design which can be improved by employing a greater number of microchannels. Since our devices were automatically designed and can be tuned to specific applications, this provides an easy approach to include contact angle measurement into existing lab-on-a-chip devices.

Hierarchical Electrode Architecture Enabling Ultrahigh-Capacity LiFePO4 Cathodes with Low Tortuosity.

Thickening electrodes are expected to increase the energy density of batteries. Unfortunately, the manufacturing issues, sluggish electrolyte infiltration, and restrictions on electron/ion transport seriously hamper the development of thick electrodes. In this work, an ultrathick LiFePO4 (LFP) electrode with hierarchically vertical microchannels and porous structures (I-LFP) is rationally designed by combining the template method and the mechanical channel-making method. By using ultrasonic transmission mapping technology, it is proven that the open and vertical microchannels and interconnected pores can successfully overcome the electrolyte infiltration difficulty of conventional thick electrodes. Meanwhile, both the electrochemical and simulation characterizations reveal the fast ion transport kinetics and low tortuosity (1.44) in the I-LFP electrode. As a result, the I-LFP electrode delivers marked improvements in rate performance and cycling stability even under a high areal loading of 180 mg cm-2. Moreover, according to the results of operando optical fiber sensors, the stress accumulation in the I-LFP electrode is effectively alleviated, which further confirms the improvement of mechanical stability.

Irregular-shaped nucleated bubbles induced enhancement of subcooled flow boiling heat transfer in leaf vein inspired three-tiered open microchannels (LTOMCs)

Nature is an important source of inspiration for developing desired structures for various industrial applications. The efficacy of water transport in leaf vein systems provides important insights for the structural design of microchannels. In addition, open microchannels (OMCs) have been reported to effectively reduce the two-phase pressure drop and suppress flow instabilities by providing extra space for bubble and vapor expansion. Thus, the synergistic cooperation of open microchannels with leaf-vein-like structures is expected to alter the flow patterns and improve the heat transfer performance in the flow boiling process. In this work, two leaf-vein inspired three-tiered open microchannels (LTOMC) with exactly the same structure but opposite orientations of vein-like channels, i.e., LTOMC-Ⅰ and LTOMC-Ⅱ, were manufactured, respectively. Flow boiling experiments were conducted at the inlet temperature of 25 °C under different mass fluxes. It was observed that the flow patterns of the LTOMC were divided into bubbly flow, slug flow, reverse and rewet flow; the shapes and movement trends of nucleated bubbles were mainly dominated by the vein-like structures. The required wall superheat of the onset of nucleated boiling for LTOMCs was 2–6 °C lower than ROMC; the one for LTOMC-Ⅱ was 2–3 °C lower than LTOMC-Ⅰ at various mass fluxes. The average two-phase heat transfer coefficient of LTOMC-Ⅱ was increased by 83.2% compared to ROMC at the mass flux of 234 kg/m2·s; the one of LTOMC-Ⅱ was 54.1% higher than LTOMC-Ⅰ. The pressure drop of three OMCs was comparable; however, the onset of two-phase flow instability occurred earlier for ROMC at the same heat flux. Besides, the liquid rewetting capability of ROMC was easier to get deteriorated at ultra-high heat flux. This work offers an avenue for structural optimization of microchannel heat sinks to enhance heat transfer performance without compromising the pressure loss.

Influence of liquid height on pool boiling heat transfer over open rectangular microchannels

Open rectangular microchannels surface (ORMS) has been extensively studied in pool boiling heat transfer because of lower cost of fabrication, ease of application and better heat transfer performance. Researchers have examined liquid height effect on heat transfer performance at various heat fluxes for copper plain surface (CPS). However, few studies have revealed the underlying mechanisms of liquid height for ORMS. This study tested different liquid heights of 1 mm, 3 mm, 5 mm, 7 mm, 9 mm, 12 mm, 15 mm, 20 mm and 50 mm over both ORMS and CPS with deionized water at atmospheric pressure. Critical heat flux (CHF) and heat transfer coefficient (HTC) were determined and considered as functions of liquid height for each liquid level. For ORMS, three different HTC variation trends versus liquid height were evaluated. The results show that the distinction is primarily determined by heat flux, as follows: Type-I has critical liquid level, Type-II has both critical liquid level and optimum liquid level, and Type-III has optimum liquid level. Type-II, however, did not appear for CPS. This research also showed that ORMS and CPS have a critical liquid level of 5 mm, whereas ORMS may be able to achieve a higher heat flux in shallow liquid levels of 1 mm and 3 mm due to its improved liquid replenishment capability.

Experimental study of heat transfer in a microchannel with pin fins and sintered coatings

ABSTRACT Increasing processing capacity without modifying the size of electronic devices has made thermal management important in the electronic industry. Commercialized thermal management, such as in a conventional air cooling system, is insufficient for electronic devices with high heat flux dissipation. Pool boiling is a better method for heat transfer because it can dissipate a substantial amount of heat at low wall superheats. This study focused on heat transfer enhancement using passive approaches, including nanostructures and microporous sintered surfaces over open microchannel surfaces and microchannels with pin-fins. In the present work, seven structures were studied in pool boiling, wherein experiments elucidated the effects of microchannels, sintered, and pin fins (micropillar) on boiling heat transfer from a copper chip in a pool of degassed water. Boiling performance is ascertained via critical heat flux (CHF) and heat transfer coefficient (HTC). The best heat transfer performance showed a heat flux of 243.75W/cm2 at 15.46°C on the pin-fins chip, which was 1.9 times the heat flux of the plain chip. The highest HTC was 181.03 kW/(m2 oC) at a heat flux of 172.61 W/cm2 for the microchannel with single pin-fins. The HTC enhancement was 2.8 times greater than the plain surface. It was found experimentally that HTC and CHF improved on all modified surfaces compared to the plain copper chip baseline.

Heat Transfer of Water Flow Boiling in Nanostructured Open Microchannels

In recent years, the open microchannel has drawn increasing interest, but severe local dryout limited the heat transfer capability of flow boiling. It was anticipated that nanostructures with exceptional capillary wicking abilities would overcome this problem. In this study, blade-like CuO nanostructures were created in the copper open microchannels to experimentally investigate water flow boiling. Experiments were carried out in nanostructured open microchannels (NMCs), and smooth-surface open microchannels (SMCs), as a comparison, were examined under identical operating conditions. Four main flow patterns, including bubbly flow, slug flow, and two kinds of stratified flow, dominated successively in NMCs and SMCs. Although the flow patterns were similar in NMCs and SMCs, the heat transfer coefficient (HTC) of flow boiling was greatly enhanced by nanostructures under conditions of medium and high heat flux, while the nanostructures’ influence on HTC was unnoticeable at low heat flux. At medium and high heat fluxes, the dependence of HTC on heat flux and flow rate indicated the joint contribution of nucleate boiling mechanism and convective evaporation mechanism to heat transfer. The enhanced effect of nanostructures on nucleate boiling and convective evaporation became more prominent as heat flux increased, leading to a higher HTC in NMCs than in SMCs at higher heat flux conditions.