Interconnected Microchannels Research Articles

Porous three-dimensional carbon foams with interconnected microchannels for high-efficiency solar-to-vapor conversion and desalination

Carbon foams were fabricated from wood via alkali activation for solar steam generation.

Translucent packed bed structures for high throughput photocatalytic reactors

Translucent photocatalytic reactor structures are investigated as a possible alternative to numbering up as a method for the scale-up of microreactors. The structure and the light source design is elaborated to introduce this concept. The light field was characterized using a ray tracing algorithm. A rectangular reactor made from glass was designed using borosilicate spheres small enough to create an array of interconnected microchannels in the reactor. It was found that ray tracing can be used as a proper tool to easily design multiple-LED light sources and predict respective irradiance patterns. The performance of the reactor was assessed using the apparent rate constant, the space-time yield and the photocatalytic space-time yield, a recently introduced performance parameter which takes into account the lamp power and the reactor productivity. The apparent rate constant of the reactor for an incoming irradiance of 191 W m−2 was found to be 0.82 min−1, which is, to our knowledge, in the range of microreactors and 1–2 orders of magnitude higher than any high throughput immobilized reactor in literature. With a photocatalytic space-time yield of 0.657 m3 day−1 m−3 reactor kW−1 our reactor was amongst the best reported performers in terms of productivity and energy efficiency. This performance is related to the high specific illuminated surface area of 4267 m2 m−3 and high catalyst load of 1.9 g L−1.

Experimental investigations of heat transfer mechanisms of a pulsating heat pipe

Experimental work was carried out to clarify the heat transfer mechanism of a pulsating heat pipe (PHP). A micro pulsating heat pipe (MPHP) with five turns was fabricated by engraving an interconnected micro-channel on a 1.1 mm thick transparent glass wafer. The engraved glass wafer and a 500 μm thick silicon wafer were anodically bonded to form a closed-loop MPHP. Ethanol was charged into the MPHP as the working fluid. The MPHP was vertically oriented with a bottom-heating mode. Using infrared (IR) thermometry, the distributions of temperature and heat flux were measured at the fluid-wall interface for the first time over the entire MPHP with high spatial and temporal resolution. High-speed flow visualization, which was synchronized to heat transfer measurements, was utilized to identify the local flow patterns corresponding to the local temperature and heat flux. The heat transfer at the fluid–silicon interface in the channels could be divided into the two parts: sensible heat transfer and latent heat transfer, and quantitative analysis of the data clarified the contributions of sensible/latent heat transfer to overall heat transfer. The overall contribution of latent heat transfer was estimated to be between 66% and 74%. Latent heat transfer not only induces the oscillating flow but also contributes significantly to overall heat transfer, whereas sensible heat transfer is a byproduct of the oscillating flow.

Vascularized microfluidic platforms to mimic the tumor microenvironment.

Microfluidic technology has led to the development of advanced in vitro tumor platforms that overcome the challenges of in vivo animal and in vitro two dimensional models. This paper presents platform designs and methods used to develop complex vascularized in vitro models to mimic the tumor microenvironment. Features of these platforms include a continuous, aligned endothelium that allows for cell-cell interactions between vasculature and tumor cells. A novel platform for fabrication of a single endothelialized microchannel encased within a collagen platform hosting breast cancer cells was developed and utilized to study the influence of cellular interaction on transport phenomenon through vasculature in a hyperpermeable tumor microenvironment. This platform relies on subtractive tissue engineering fabrication techniques. Through confocal imaging we have demonstrated that the platform produces enhanced vessel leakiness recapitulating physiological features of the tumor microenvironment. The influence of tumor endothelial interactions on transport of particles was also demonstrated. Additionally, we designed two more complex and intricate endothelialized microfluidic networks by combining lithographic techniques with additive tissue engineering methods. We created a network platform consisting of interconnected microchannels to model a highly vascularized system and successfully perfused the system with fluorescent particles. Finally, we developed a physiologically representative in vitro microfluidic platform with vasculature patterned from in vivo data showing the versatility of these systems to replicate the complex geometries of tumor microvasculature and dynamically measured particle transport. Overall, we have shown the ability to develop functional microfluidic vascular tumor platforms of varying complexities and demonstrated their utility for studying spatial particle transport within these systems.

Computational Fluid Dynamics Study of the Effects of Surface Roughness on Permeability and Fluid Flow-Induced Wall Shear Stress in Scaffolds.

In this work, we investigated surface roughness effects on bone scaffold permeability and fluid flow-induced wall shear stress (WSS) using computational fluid dynamics (CFD) analysis. Scaffolds are made of interconnected microchannels, whose fluid flow can be examined from the perspective of fluid flow dynamics. Given that the roughness of microchannel surfaces serves a non-negligible function in the fluid dynamics within the channels, it is believed that the wall roughness of scaffolds can play an important role in their permeability and WSS. Given the criticality of permeability and WSS in the effective biological functioning of scaffolds, we investigated manufacturing-induced surface roughness effects on the two aforementioned biocompatibility characteristics. To this end, three scaffolds with square pores of different sizes (300, 600, and 900µm) and identical porosity (63%) were designed. Six roughness levels (0, 4, 8, 12, 16, and 20µm) were established for the scaffold walls, thus enabling us to develop 18 scaffold models. The pressure drop and WSS in the scaffolds were then measured by CFD. Scaffold permeability was calculated using Darcy's law, with reference to geometrical parameters and the pressure drop derived from the CFD analysis. In all the scaffolds, high roughness decreased permeability and WSS. A significant difference in WSS reduction was found between the models with smooth scaffolds and the models with scaffolds that had a roughness of 20µm. Except for the scaffold with a pore size of 300µm, all the others showed no considerable change in permeability at different roughness levels.

Capillary-driven liquid film boiling heat transfer on hybrid mesh wicking structures

Thermal management of high power electronic systems such as advanced lasers, light emitting diodes, radars, microprocessors, electrical machines, and power inverters, is becoming critical in defense, space, and commercial applications. As a liquid-vapor phase-change process, capillary evaporation on wicking structures has received increased interest owing to its capability dissipating high heat flux by increasing the effective evaporation area and sustaining the liquid supply. However, the approaches based on capillary evaporation suffer from intrinsic tradeoffs between the low thermal resistance for heat flow and the high wicking flow rate for liquid supply, which make it challenging to optimize both heat transfer coefficient (HTC) and critical heat flux (CHF) on the same wicking structure. Here, we present a cost-effective hybrid wicking structure, that can be scalably manufactured using commercial copper micromeshes along with simple etching processes, to enable a novel capillary-driven liquid film boiling heat transfer by simultaneously improving liquid supply and increasing nucleation sites. The interconnected microchannels between woven mesh layers and the substrate serve as low flow resistance passages for high volumetric flow rate of the liquid. Through creating nanograsses and microcavities on the multilayer micromeshes, the liquid flow is enhanced for higher CHF due to the increased surface wettability. Moreover, a plural of microcavities on the mesh surface decrease the superheat for early onset of nucleate boiling (ONB), resulting in higher HTC. Enhanced CHF (198.6 W/cm2) and HTC (138.7 kW/m2 K) are observed on the hybrid mesh wicking structure with a 10 mm × 10 mm heated area due to more than twice the flow rate for liquid supply and more than one third reduced superheat at the ONB, compared to that on the plain mesh wicking structure. The insights gained from this study can be used to guide the design and manufacturing of highly efficient wicking structures in heat pipes, vapor chambers, and other high flux thermal systems.

Highly Reversible Li Plating Confined in Three-Dimensional Interconnected Microchannels toward High-Rate and Stable Metallic Lithium Anodes.

Practical application of metallic Li anode in Li-ion batteries has been restricted because of dendrite growth of Li which induces poor stability and safety issues. Despite various hosts for Li having been developed to address these issues, it is still a challenge to achieve highly reversible and stable stripping/plating behavior of Li, especially at high-rate conditions. Herein, we propose a simple method of incorporating Li in commercial carbon fiber cloth (CFC) to realize high-rate and stable metallic Li anodes by confining stripping/plating of Li in microchannels of ZnO-decorated CFC (CFC/ZnO) and dissipating high current densities through conductive carbon fiber networks. The symmetrical cell using this novel anode can run stably for over 1800 h (900 cycles) under 1 mA cm-2 and even 320 h (800 cycles) at 5 mA cm-2, which has rarely been achieved previously through structural evolution of Li-metal anode. When it is paired with commercial activated carbon, the as-made Li-ion capacitor coin cell can deliver high-rate capability (up to 30 A g-1) and long-term cycling stability for over 5000 cycles at 10 A g-1, and a large pouch cell can operate as ultrafast charge (∼1 min) battery with high-energy density of ∼50 W h kg-1.

Interconnected Microchannels in Hydrogels to Control Cell Adhesion and Mechanotransduction

Cells respond to external mechanical stimuli through the activation of mechanotransduction, which influences cell proliferation, migration and differentiation, and adhesion. Likewise, diseases such as cancer and cardiac dysfunctions are strongly related to cellular mechanotransduction. Here we show an innovative 3D material to serve as a platform for controlling mechanotransduction by mimicking natural 3D cellular environments. Our material contains a novel form of microporous structures represented by micron-sized channels embedded in a hydrogel matrix of a well-defined stiffness and conductivity. The material guarantees pore interconnectivity independently of pore density and size, and different types of surface functionalization are possible. Furthermore, the material also provides a large and spatially controlled cell-surface contact area through the specific architecture of its pores, such that the mechanical properties of the environment have large impact on the cells. We here show data on how different cell types grow in the microporous materials and future applications where cellular mechanotransduction can be exploited.

3D printed microchannel networks to direct vascularisation during endochondral bone repair

Bone tissue engineering strategies that recapitulate the developmental process of endochondral ossification offer a promising route to bone repair. Clinical translation of such endochondral tissue engineering strategies will require overcoming a number of challenges, including the engineering of large and often anatomically complex cartilage grafts, as well as the persistence of core regions of avascular cartilage following their implantation into large bone defects. Here 3D printing technology is utilized to develop a versatile and scalable approach to guide vascularisation during endochondral bone repair. First, a sacrificial pluronic ink was used to 3D print interconnected microchannel networks in a mesenchymal stem cell (MSC) laden gelatin-methacryloyl (GelMA) hydrogel. These constructs (with and without microchannels) were next chondrogenically primed in vitro and then implanted into critically sized femoral bone defects in rats. The solid and microchanneled cartilage templates enhanced bone repair compared to untreated controls, with the solid cartilage templates (without microchannels) supporting the highest levels of total bone formation. However, the inclusion of 3D printed microchannels was found to promote osteoclast/immune cell invasion, hydrogel degradation, and vascularisation following implantation. In addition, the endochondral bone tissue engineering strategy was found to support comparable levels of bone healing to BMP-2 delivery, whilst promoting lower levels of heterotopic bone formation, with the microchanneled templates supporting the lowest levels of heterotopic bone formation. Taken together, these results demonstrate that 3D printed hypertrophic cartilage grafts represent a promising approach for the repair of complex bone fractures, particularly for larger defects where vascularisation will be a key challenge.

Chitosan-based ion-imprinted cryo-composites with excellent selectivity for copper ions

An original strategy is proposed here to design chitosan-based ion-imprinted cryo-composites (II-CCs) with pre-organized recognition sites and tailored porous structure by combining ion-imprinting and ice-templating techniques. The cryo-composites showed a tube-like porous morphology with interconnected parallel micro-channels, the distance between the channel walls being around 15 μm. Both the entrapment of a natural zeolite and the presence of carboxylate groups, generated by partial hydrolysis of amide moieties, led to II-CCs with controlled swelling ratios (25–40 g/g, depending on pH) and enhanced overall chelating efficiency (260 mg Cu2+/g composite). To point out the importance of introducing Cu2+ recognition sites, sorption experiments using mixtures of Cu2+ and other competing ions (Co2+, Ni2+, Zn2+ or/and Pb2+) were also carried out. The higher values of selectivity coefficients obtained for the II-CCs compared to those of non-imprinted ones highlight the remarkable potential of our sorbents for decontamination of wastewaters and recycling of Cu2+ ions.

Enhancing wicking microflows in metallic foams

Metallic foams with high pore density have the potential to be used as highly wicking materials due to the improved capillarity caused by the high porosity and interconnected microchannels. The challenge for using them in their original state lies in their relatively large contact angle. To overcome this, a chemical blackening process for creating a superhydrophilic surface on copper foams is developed. The treatment resulted in a reduction of contact angle from about 91° to almost 0°, indicating a significantly improved wettability. The wicking capacity is further enhanced by controlling the porosity. Rate-of-rise experiments on the treated metal foams with different porosity were conducted. A wicking height of 140 mm in the treated metal foams was achieved compared to no wicking before treatment. A 3-month stability test in a vacuum showed that the contact angle and the wicking performance of the treated copper samples remained unchanged.

Effects of heat flux, mass flux and channel width on flow boiling performance of porous interconnected microchannel nets

This study presents an experimental investigation of flow boiling heat transfer performance in porous interconnected microchannel nets (PIMNs) as an effective cooling solution for microelectronic devices. Three PIMNs with different microchannel widths, i.e., 0.25, 0.4 and 0.55 mm, were fabricated via copper powder sintering and wire electric discharge machining. The effects of heat flux, mass flux and channel width on flow boiling characteristics, i.e., two-phase heat transfer, pressure drops and two-phase flow instabilities were evaluated for the optimization design. Flow boiling experiments were conducted using deionized water as the coolant with variation in the heat flux and mass flux of 200–500 kg m−2 s−1 under an inlet subcooling of 40 K. The high speed visualization showed the flow pattern transition from bubbly flow to annular flow was accompanied with the change of boiling heat transfer mechanisms. Both the heat flux and the mass flux have significant effect on the two-phase heat transfer performance of the PIMNs. The PIMN-2 with the medium channel width of 0.4 mm presented the highest heat transfer coefficients and best capability to mitigate the severe two-phase flow instability, as well as the favorable pressure drop penalty, which achieved the best overall flow boiling performance in this study.

Continuous fabrication of polymer microfiber bundles with interconnected microchannels for oil/water separation

Recently, the development of methods for the cleanup of large-scale oil spills or organic pollutants from water has been attracting global attention. However, traditional techniques suffer from the limits of low efficiency and high operation cost. Herein, we propose a high efficiency, large-scale and easy operation approach to fabricate a hydrophobic and oleophilic high density polyethylene microfiber bundles with interconnected microchannels for oil/water separation. The microfiber bundles with high porosity, excellent absorption capacity and large water contact angle are continuously fabricated by a melt extrusion and leaching technique. More importantly, they are readily cleaned by centrifugation and exhibit excellent shape-recoverable property, which are very promising for the cleanup of large-scale oil spills. This study provides a great promising way to fabricate multifunctional polymer materials by using common polymer molding method.

Effect of operational parameters on flow boiling heat transfer performance for porous interconnected microchannel nets

A porous interconnected microchannel net (PIMN) is proposed as an effective structure in this study. As a combination of IMN and sintered porous material, the PIMN was fabricated via solid-phase sintering and wire electric discharge machining with orthogonally interconnected microchannels on the both side of the porous matrix. Two-phase heat transfer performance of the PIMN was investigated and evaluated through flow boiling experiments utilizing deionized water as the coolant at different operation parameters (mass flux and inlet subcooling). The results showed that the heat transfer enhancement by the PIMN was outstanding comparing to the reentrant porous microchannels (RPM) thanks to the interconnected microchannels and porous structure. The PIMN under the condition of higher mass flux or inlet subcooling yielded better two-phase heat transfer performance, including the higher heat transfer coefficients and lower pressure drop. Visualization images captured by high speed camera revealed that the flow pattern inside the PIMN transformed from bubbly flow to annular flow as the boiling mechanism developing from nucleate boiling into convective boiling. Further study in two-phase flow instabilities showed that more stable two-phase flow for the PIMN leads to better pressure drop and boiling heat transfer performance.

Synthesis of Biocompatible Liquid Crystal Elastomer Foams as Cell Scaffolds for 3D Spatial Cell Cultures.

Here, we present a step-by-step preparation of a 3D, biodegradable, foam-like cell scaffold. These scaffolds were prepared by cross-linking star block co-polymers featuring cholesterol units as side-chain pendant groups, resulting in smectic-A (SmA) liquid crystal elastomers (LCEs). Foam-like scaffolds, prepared using metal templates, feature interconnected microchannels, making them suitable as 3D cell culture scaffolds. The combined properties of the regular structure of the metal foam and of the elastomer result in a 3D cell scaffold that promotes not only higher cell proliferation compared to conventional porous templated films, but also better management of mass transport (i.e., nutrients, gases, waste, etc.). The nature of the metal template allows for the easy manipulation of foam shapes (i.e., rolls or films) and for the preparation of scaffolds of different pore sizes for different cell studies while preserving the interconnected porous nature of the template. The etching process does not affect the chemistry of the elastomers, preserving their biocompatible and biodegradable nature. We show that these smectic LCEs, when grown for extensive time periods, enable the study of clinically relevant and complex tissue constructs while promoting the growth and proliferation of cells.

On the flow boiling enhancement in interconnected reentrant microchannels

Interconnected reentrant microchannels (IRM) were developed to explore the feasibility of flow boiling enhancement and applications in high heat flux dissipations. They consisted of 14 parallel semi-closed Ω-shaped microchannels and 8 transverse ones each with a hydraulic diameter of 781μm. These microchannels were fabricated in copper by wire electrical discharge machining process. One-to-one comparative studies between the IRM and parallel reentrant microchannels (PRM) with the same reentrant configurations were conduced in flow boiling experiments. Two coolants tests of deionized water and ethanol were performed at inlet subcoolings of 10 and 40°C and mass fluxes of 125–300kg/m2s. Significant flow boiling heat transfer augmentation, i.e., an enhancement ratio of 40–280% in water tests, and 28–61% in ethanol tests, were achieved for the interconnected reentrant microchannels. This can be attributed to that the addition of transverse microchannels provided more flow passages, diminished the confinement effects for vapor flow and introduced intense disruption and re-initializations of the liquid thin film. Moreover, the two-phase flow instabilities were mitigated considerably, and the stream-wise temperature uniformity was also improved for the IRM. All these features of interconnected reentrant microchannels indicate that they could be a promising option for the application in high heat flux dissipations.

Effects of structural parameter on flow boiling performance of interconnected microchannel net

Developed thermal management solutions for microelectronic fields have got extensive attentions these years. In this study, an interconnected microchannel net (IMN) was developed via micro wire electrical discharge machining process to explore the feasibility of flow boiling heat transfer enhancement. The experiments for the IMNs with different structural parameters were conducted using deionized water as the coolant with variation in the heat flux and under an inlet subcooling of 40°C and constant mass flow rate of 180kgm−2s−1. Four IMN samples with different channel width, i.e., 0.25, 0.4, 0.55 and 0.7mm, were investigated to study the effects of channel size and heat flux on flow boiling characteristics, i.e., two-phase heat transfer, pressure drops and two-phase flow instabilities, were proceeded in order to optimize the structural design of the IMN. Test results indicate that the sample IMN-2 with the channel width of 0.4mm manifests the overall best flow boiling performance, which exhibits more favorable two-phase heat transfer and pressure drop characteristics, as well as the prominent superiority for the mitigation of two-phase instability. This reveals a highlight for the potential application in the flow boiling enhancement of microchannel heat transfer.

Photocatalytic degradation of 4-chlorophenol in a photochemical reactor with an interconnected microchannel and a light guide network

Abstract Photocatalytic microchannel reactor with a novel light guide of fused glass beads was developed. Anatase nanoparticle photocatalyst was deposited on the inner surface of the porous glass monolith of the reactor. Activity of the reactor was evaluated based on photocatalytic degradation of 4-chlorophenol. All the glass beads in a glass column were fused with each other and with the inner peripheral of the column to form a glass network, which construct a network of light guide to deliver the incident light from side wall to each bead in the column. In contrast, almost all the voids among the beads connect with each other to form a network of microchannels for substrate solution, where many parallel flow of the substrate solution can pass through the reactor. Microchannel reactors composed of 5 mm or 1 mm glass beads in diameter were investigated, so that the dimension of the channels are evaluated as approximately 750 μm and 150 μm, respectively. The glass web monolith serves as a support of TiO 2 photocatalyst and a light guide to deliver the incident light to activate the photocatalyst on each bead. Photocatalytic activity of the reactor constructed with the 1 mm or the 5 mm glass beads was improved 13 and 3 times as much as that of the simple glass column without the beads, respectively. The ratio in the reactivity was proportional to the ratio in internal surface area of the reactors, supporting the effect of the glass web as light guide and the network of the voids as microchannel.

Porous shape-memory NiTi-Nb with microchannel arrays

Porous NiTi-Nb containing a 3D array of orthogonally interconnected microchannels was created via a novel powder metallurgy process combining: (i) Mg ribbon scaffold construction, (ii) slip casting of NiTi + Nb powder blend within the scaffold, (iii) Mg scaffold vacuum evaporation, and (iv) NiTi + Nb liquid phase sintering. The later stage was achieved by creating small amounts of quasi-binary NiTi-Nb liquid eutectic, which wicked between NiTi particles and bonded them together while leaving 28 vol.% of residual pores. These hierarchical porous structures have a total porosity of 30–53%, an effective stiffness of 5–9 GPa, and a yield strength of 20–80 MPa. They exhibit the shape memory effect, with 3% strain recovery after 7% compressive deformation upon multiple load-unload cycles. Finite element modeling (FEM) is used to model the anisotropy of these structures, as well as to probe the strain distributions on a microscopic level. Mechanical anisotropy was present in FEM of all structures, though more pronounced in structures with high microchannel volume fraction. With mechanical properties between those of trabecular and cortical bone, these structures are of great interest for bone implant applications.

Synthesis of 1D Polymer/Zeolite Nanocomposites under High Pressure

Recently, simple carbon based polymers have been synthesized at high pressures in silicalite, a pure SiO2 zeolite with a 3D system of mutually interconnected microchannels. These protocols permitted otherwise unstable polymers to be stabilized and protected from the atmosphere and to obtain an entirely novel class of nanocomposites with modified physical properties. In these 3-D systems, channel interconnection may prevent ideal, isolated polymer chains to be obtained. In this work, the high pressure (5–10 GPa) synthesis of two archetypal, linear polymers polyacetylene (PA) and polycarbonyl (pCO) in the 1D channel system of the pure SiO2 zeolite ZSM-22 (TON) has been performed. The two resulting nanocomposites PA/TON and pCO/TON are organic/inorganic composite materials, which are good candidates as highly directional semiconductors and high energy density materials, respectively. The synthesis was performed in diamond anvil cells, starting from dense C2H2 and CO, confined in ZSM-22, and the nanocomposite...