Microscale Liquid Transport in Polycrystalline Inverse Opals across Grain Boundaries

High-Performance Bioinspired Hierarchical Microgroove Wick for Ceramic Vapor Chambers Achieved by Nanosecond Pulsed Lasers

Directly integrating ceramic vapor chambers into the insulating substrate of semiconductor power devices is an effective approach to solve the problem of heat dissipation. Microgrooves that could be machined directly on the shell plate without contact thermal resistance and mechanical dislocation offer exciting opportunities to achieve high-performance ceramic vapor chambers. In this study, a bioinspired hierarchical microgroove wick (BHMW) containing low ribs via one-step nanosecond pulsed laser processing was developed, as inspired by the Sarracenia trichome. The superwicking behavior of microgrooves with different structural parameters was investigated using capillary rise tests and droplet-spreading experiments. The BHMW exhibited excellent capillary performance and anisotropic hemiwicking performance. At a laser scanning spacing of 30 μm, the BHMW achieved a capillary wicking height of 114 mm within 20 s. The optimized BHMW demonstrated a capillary parameter (ΔPc·K) and an anisotropic hemiwicking ratio of 4.46 × 10-7 N and 11.93, respectively, which were 1182 and 946% higher than references, as achieved through nanosecond pulsed laser texturing under identical parameters. This work not only develops a high-performance hierarchical alumina microgroove wick structure but also outlines design guidelines for high-performance ceramic vapor chambers for thermal management in semiconductor power devices.

Grain Crystallinity, Anisotropy, and Boundaries Govern Microscale Hydrodynamic Transport in Semicrystalline Porous Media

Polycrystallinity is often an unintended consequence of real manufacturing processes used to produce designer porous media with deterministic and periodic architectures. Porous media are widely employed as high-surface conduits for fluid transport; unfortunately, even small concentrations of defects in the long-range order become the dominant impediment to hydrodynamic transport. In this study, we isolate the effects of these defects using a microfluidic analogy to energy transport in atomic polycrystals by directly tracking capillary transport through polycrystalline inverse opals. We reveal─using high-fidelity florescent microscopy─the boundary-limited nature of flow motions, along with nonlinear impedance elements introduced by the presence of "grain boundaries" that are separating the well-ordered "crystalline grains". Coupled crystallinity, anisotropy, and linear defect density contribute to direction-dominated flow characteristics in a discretized manner rather than traditional diffusive-like flow patterns. Separating individual crystal grains' transport properties from polycrystals along with new probabilistic data sets enables demonstrating statistical predictive models. These results provide fundamental insight into transport phenomena in (poly)crystalline porous media beyond the deterministic properties of an idealized unit cell and bridge the gap between engineering models and the ubiquitous imperfections found in manufactured porous materials.

Capillary Transfer of Self-Assembled Colloidal Crystals

Colloidal self-assembly has attracted significant interest in numerous applications including optics, electrochemistry, thermofluidics, and biomolecule templating. To meet the requirements of these applications, numerous fabrication methods have been developed. However, these are limited to narrow ranges of feature sizes, are incompatible with many substrates, and/or have low scalability, significantly limiting the use of colloidal self-assembly. In this work, we study the capillary transfer of colloidal crystals and demonstrate that this approach overcomes these limitations. Enabled by capillary transfer, we fabricate 2D colloidal crystals with nano-to-micro feature sizes spanning 2 orders of magnitude and on typically challenging substrates including those that are hydrophobic, rough, curved, or structured with microchannels. We developed and systemically validated a capillary peeling model, elucidating the underlying transfer physics. Due to its high versatility, good quality, and simplicity, this approach can expand the possibilities of colloidal self-assembly and enhance the performance of applications using colloidal crystals.

Printing Crack-Free Microporous Structures by Combining Additive Manufacturing with Colloidal Assembly

To date high printing resolution and scalability, i.e., macroscale component dimensions and fast printing, are incompatible characteristics for additive manufacturing (AM) processes. It is hereby demonstrated that the combination of direct writing as an AM process with colloidal assembly enables the breaching of this processing barrier. By tailoring printing parameters for polystyrene (PS) microparticle-templates, how to avoid coffee ring formation is demonstrated, thus printing uniform single lines and macroscale areas. Moreover, a novel "comb"-strategy is introduced to print macroscale, crack-free colloidal coatings with low viscous colloidal suspensions. The printed templates are transformed into ceramic microporous channels as well as photonic coatings via atomic layer deposition (ALD) and calcination. The obtained structures reveal promising wicking capabilities and broadband reflection in the near-infrared, respectively. This work provides guidelines for printing low viscous colloidal suspensions and highlights the advancements that this printing process offers toward novel applications of colloidal-based printed structures.

Computer vision-assisted investigation of boiling heat transfer on segmented nanowires with vertical wettability

The boiling efficacy is intrinsically tethered to trade-offs between the desire for bubble nucleation and necessity of vapor removal. The solution to these competing demands requires the separation of bubble activity and liquid delivery, often achieved through surface engineering. In this study, we independently engineer bubble nucleation and departure mechanisms through the design of heterogeneous and segmented nanowires with dual wettability with the aim of pushing the limit of structure-enhanced boiling heat transfer performances. The demonstration of separating liquid and vapor pathways outperforms state-of-the-art hierarchical nanowires, in particular, at low heat flux regimes while maintaining equal performances at high heat fluxes. A deep-learning based computer vision framework realized the autonomous curation and extraction of hidden big data along with digitalized bubbles. The combined efforts of materials design, deep learning techniques, and data-driven approach shed light on the mechanistic relationship between vapor/liquid pathways, bubble statistics, and phase change performance.

Shaping in the Third Direction; Synthesis of Patterned Colloidal Crystals by Polyester Fabric-Guided Self-Assembly

A polyester fabric with rectangular openings was used as a sacrificial template for the guiding of a sub-micron sphere (polystyrene (PS) and silica) aqueous colloid self-assembly process during evaporation as a patterned colloidal crystal (PCC). This simple process is also a robust one, being less sensitive to external parameters (ambient pressure, temperature, humidity, vibrations). The most interesting feature of the concave-shape-pattern unit cell (350 μm × 400 μm × 3 μm) of this crystal is the presence of triangular prisms at its border, each prism having a one-dimensional sphere array at its top edge. The high-quality ordered single layer found inside of each unit cell presents the super-prism effect and left-handed behavior. Wider yet elongated deposits with ordered walls and disordered top surfaces were formed under the fabric knots. Rectangular patterning was obtained even for 20 μm PS spheres. Polyester fabrics with other opening geometries and sizes (~300–1000 μm) or with higher fiber elasticity also allowed the formation of similar PCCs, some having curved prismatic walls. A higher colloid concentration (10–20%) induces the formation of thicker walls with fiber-negative replica morphology. Additionally, thick-wall PCCs (~100 μm) with semi-cylindrical morphology were obtained using SiO₂ sub-microspheres and a wavy fabric. The colloidal pattern was used as a lithographic mask for natural lithography and as a template for the synthesis of triangular-prism-shaped inverted opals.

Rational Fabrication of Nano-to-Microsphere Polycrystalline Opals Using Slope Self-Assembly

Self-assembly of artificial opals has garnered significant interest as a facile nanofabrication technique capable of producing highly ordered structures for optical, electrochemical, biomolecular, and thermal applications. In these applications, the optimum opal particle diameter can vary by several orders of magnitude because the properties of the resultant structures depend strongly on the feature size. However, current opal fabrication techniques only produce high-quality structures over a limited range of sphere sizes or require complex processes and equipment. In this work, the rational and simple fabrication of polycrystalline opals with diameters between 500 nm and 10 μm was demonstrated using slope self-assembly of colloids suspended in ethanol-water. The role of the various process parameters was elucidated through a scaling-based model that accurately captures the variations of opal substrate coverage for spheres of size 2 μm or smaller. For spheres of 10 μm and larger, capillary forces were shown to play a key role in the process dynamics. Based on these insights, millimeter-scale monolayered opals were successfully fabricated, while centimeter-scale opals were possible with sparse sphere stacking or small uncovered areas. These insights provide a guide for the simple and fast fabrication of opals that can be used as optical coatings, templates for high power density electrodes, molecule templates, and high-performance thermo-fluidic devices.

Centimetre-scale crack-free self-assembly for ultra-high tensile strength metallic nanolattices

Nanolattices exhibit attractive mechanical, energy conversion and optical properties, but it is challenging to fabricate large nanolattices while maintaining the dense regular nanometre features that enable their properties. Here we report a crack-free self-assembly approach for fabricating centimetre-scale nickel nanolattices with much larger crack-free areas than prior self-assembled nanolattices and many more unit cells than three-dimensionally printed nanolattices. These nickel nanolattices have a feature size of 100 nm, a grain size of 30 nm and a tensile strength of 260 MPa, which approaches the theoretical strength limit for porous nickel. The self-assembly method and porous metal mechanics reported in this work may advance the fabrication and applications of high-strength multifunctional porous materials.

Heterostructured CoOx-TiO2 Mesoporous/Photonic Crystal Bilayer Films for Enhanced Visible-Light Harvesting and Photocatalysis

Heterostructured bilayer films, consisting of co-assembled TiO₂ photonic crystals as the bottom layer and a highly performing mesoporous P25 titania as the top layer decorated with CoO_x nanoclusters, are demonstrated as highly efficient visible-light photocatalysts. Broadband visible-light activation of the bilayer films was implemented by the surface modification of both titania layers with nanoscale clusters of Co oxides relying on the chemisorption of Co acetylacetonate complexes on TiO₂, followed by post-calcination. Tuning the slow photon regions of the inverse opal supporting layer to the visible-light absorption of surface CoO_x oxides resulted in significant amplification of salicylic-acid photodegradation under visible and ultraviolet (UV)–visible light (Vis), outperforming benchmark P25 films of higher titania loading. This enhancement was related to the spatially separated contributions of slow photon propagation in the inverse opal support layer assisted by Bragg reflection toward the CoO_x-modified mesoporous P25 top layer. This effect indicates that photonic crystals may be highly effective as both photocatalytically active and backscattering layers in multilayer photocatalytic films.

In situ investigation of particle clustering dynamics in colloidal assemblies using fluorescence microscopy

Colloidal self-assembly is a process in which dispersed matter spontaneously form higher-order structures without external intervention. During self-assembly, packed particles are subject to solvent-evaporation induced dynamic structuring phases, which leads to microscale defects called the grain boundaries. While it is imperative to precisely control detailed grain boundaries to fabricate well-defined self-assembled crystals, the understanding of the colloidal physics that govern grain boundaries remains a challenge due to limited resolutions of current visualization approaches. In this work, we experimentally report in situ particle clustering dynamics during evaporative colloidal assembly by studying a novel microscale laser induced fluorescence technique. The fluorescence microscopy measures the saturation levels with high fidelity to identify distinct colloidal structuring regimes during self-assembly as well as cracking mechanics. The techniques discussed in this work not only enables unprecedented levels of colloidal self-assembly analysis but also have potential to be used for various sensing applications with microscopic resolutions.

Thick Free-Standing Metallic Inverse Opals Enabled by New Insights into the Fracture of Drying Particle Films

Metallic inverse opals are porous materials with enhanced mechanical, chemical, thermal, and photonic properties used to improve the performance of many technologies, such as battery electrodes, photonic devices, and heat exchangers. Cracking in the drying opal templates used to fabricate inverse opals, however, is a major hindrance to the use of these materials for practical and fundamental studies. In this work, we conduct desiccation experiments on polystyrene particle opals self-assembled on indium-tin oxide coated substrates to study their fracture mechanisms, which we describe using an energy-conservation fracture model. The model incorporates film yielding, particle order, and interfacial friction to explain several experimental observations, including thickness-dependent crack spacings, cracking stresses, and order-dependent crack behavior. Guided by this model, we are the first to fabricate 120 μm thick free-standing metallic inverse opals, which are 4 times thicker than previously reported non-free-standing metallic inverse opals. Moreover, by controlling cracks, we achieve a crack-free single-crystal domain up to 1.35 mm², the largest ever reported in metallic inverse opals. This work improves our understanding of fracture mechanics in drying particle films, provides guidelines to reduce crack formation in opal templates, and enables the fabrication of free-standing large-area single-crystal inverse opals.

Boiling Heat Transfer with a Well-Ordered Microporous Architecture

Boiling heat transfer through a porous medium offers an attractive combination of enormous liquid-vapor interfacial area and high bubble nucleation site density. In this work, we characterize the boiling performances of porous media by employing the well-ordered and highly interconnected architecture of inverse opals (IOs). The boiling characterization identifies hydrodynamic mechanisms through which structural characteristics affect the boiling performance of metallic microporous architecture by validating empirical measurements. The boiling performances can be optimized through the rational design of both the structural thicknesses and pore diameters of IOs, which demonstrate up to 336% enhancement in boiling heat-transfer coefficient (HTC) over smooth surfaces. The optimal HTC and critical heat flux occur at approximately 3-4 μm in porous structure thickness, which is manifested through the balance of liquid-vapor occupation within the spatial confinement of the IO structure. The optimization of boiling performances with varying pore diameters (0.3-1.0 μm) can be attributed to the hydraulic competitions between permeability and viscous resistance to liquid-vapor transport. This study unveils thermophysical understandings to enhance multiphase heat transfer in microporous media for ultrahigh heat flux thermal management.

The Control of Colloidal Grain Boundaries through Evaporative Vertical Self-Assembly

Self-assembly continuously gains attention as an excellent method to create novel nanoscale structures with a wide range of applications in photonics, optoelectronics, biomedical engineering, and heat transfer applications. However, self-assembly is governed by a diversity of complex interparticle forces that cause fabricating defectless large scale (>1 cm) colloidal crystals, or opals, to be a daunting challenge. Despite numerous efforts to find an optimal method that offers the perfect colloidal crystal by minimizing defects, it has been difficult to provide physical interpretations that govern the development of defects such as grain boundaries. This study reports the control over grain domains and intentional defect characteristics that develop during evaporative vertical deposition. The degree of particle crystallinity and evaporation conditions is shown to govern the grain domain characteristics, such as shapes and sizes. In particular, the grains fabricated with 300 and 600 nm sphere diameters can be tuned into single-column structures exceeding ≈1 mm by elevating heating temperature up to 93 °C. The understanding of self-assembly physics presented in this work will enable the fabrication of novel self-assembled structures with high periodicity and offer fundamental groundworks for developing large-scale crack-free structures.

Capillary Wicking in Hierarchically Textured Copper Nanowire Arrays

Capillary wicking through homogeneous porous media remains challenging to simultaneously optimize due to the unique transport phenomena that occur at different length scales. This challenge may be overcome by introducing hierarchical porous media, which combine tailored morphologies across multiple length scales to design for the individual transport mechanisms. Here, we fabricate hierarchical nanowire arrays consisting of vertically aligned copper nanowires (∼100 to 1000 nm length scale) decorated with dense copper oxide nanostructures (∼10 to 100 nm length scale) to create unique property sets that include a large specific surface area, high rates of fluid delivery, and the structural flexibility of vertical arrays. These hierarchical nanowire arrays possess enhanced capillary wicking ( K/ R_eff = 0.004-0.023 μm) by utilizing hemispreading and are advantageous as evaporation surfaces. With the advent and acceleration of flexible electronics technologies, we measure the capillary properties of our freestanding hierarchical nanowire arrays installed on curved surfaces and observe comparable fluid delivery to flat arrays, showing the difference of 10-20%. The degree of effective inter-nanowire pore and porosity is shown to govern the capillary performance parameters, thereby this study provides the design strategy for capillary wicking materials with unique and tailored combinations of thermofluidic properties.

Droplets on Slippery Lubricant-Infused Porous Surfaces: A Macroscale to Nanoscale Perspective

A recent design approach in creating super-repellent surfaces through slippery surface lubrication offers tremendous liquid-shedding capabilities. Previous investigations have provided significant insights into droplet-lubricant interfacial behaviors that govern antiwetting properties but have often studied using macroscale droplets. Despite drastically different governing characteristics of ultrasmall droplets on slippery lubricated surfaces, little is known about the effects at the micro- and nanoscale. In this investigation, we impregnate a three-dimensionally, well-ordered porous metal architecture with a lubricant to confirm durable slippery surfaces. We then reduce the droplet size to a nanoliter range and experimentally compare the droplet behaviors at different length scales. By experimentally varying the lubricant thickness levels, we also reveal that the effect of lubricant wetting around ultrasmall droplets is intensely magnified, which significantly affects the transient droplet dynamics. Molecular dynamics computations further examine the ultrasmall droplets with varying lubricant levels or pore cut levels at the nanoscale. The combined experimental and computational work provides insights into droplet interfacial phenomena on slippery surfaces from a macroscale to nanoscale perspective.

Tailoring Permeability of Microporous Copper Structures through Template Sintering

Microporous metals are used extensively for applications that combine convective and conductive transport and benefit from low resistance to both modes of transport. Conventional fabrication methods, such as direct sintering of metallic particles, however, often produce structures with limited fluid transport properties due to the lack of control over pore morphologies such as the pore size and porosity. Here, we demonstrate control and improvement of hydraulic permeability of microporous copper structures fabricated using template-assisted electrodeposition. Template sintering is shown to modify the fluid transport network in a manner that increases permeability by nearly an order of magnitude with a less significant decrease (∼38%) in thermal conductivity. The measured permeabilities range from 4.8 × 10^-14 to 1.3 × 10^-12 m² with 5 μm pores, with the peak value being roughly 5 times larger than the published values for sintered copper particles of comparable feature sizes. Analysis indicates that the enhancement of permeability is limited by constrictions, i.e., bottlenecks between connecting pores, whose dimensions are highly sensitive to the sintering conditions. We further show contrasting trends in permeability versus conductivity of the electrodeposited microporous copper and conventional sintered copper particles and suggest these differing trends to be the result of their inverse structural relationship.

Hierarchical and Well-Ordered Porous Copper for Liquid Transport Properties Control

Liquid delivery through interconnected pore network is essential for various interfacial transport applications ranging from energy storage to evaporative cooling. The liquid transport performance in porous media can be significantly improved through the use of hierarchical morphology that leverages transport phenomena at different length scales. Traditional surface engineering techniques using chemical or thermal reactions often show nonuniform surface nanostructuring within three-dimensional pore network due to uncontrollable diffusion and reactivity in geometrically complex porous structures. Here, we demonstrate hierarchical architectures on the basis of crystalline copper inverse opals using an electrochemistry approach, which offers volumetric controllability of structural and surface properties within the complex porous metal. The electrochemical process sequentially combines subtractive and additive steps-electrochemical polishing and electrochemical oxidation-to improve surface wetting properties without sacrificing structural permeability. We report the transport performance of the hierarchical inverse opals by measuring the capillary-driven liquid rise. The capillary performance parameter of hierarchically engineered inverse opal ( K/ R_eff = ∼5 × 10^-3 μm) is shown to be higher than that of a typical crystalline inverse opal ( K/ R_eff = ∼1 × 10^-3 μm) owing to the enhancement in fluid permeable and hydrophilic pathways. The new surface engineering method presented in this work provides a rational approach in designing hierarchical porous copper for transport performance enhancements.

Integrated nanomaterials for extreme thermal management: a perspective for aerospace applications

Nanomaterials will play a disruptive role in next-generation thermal management for high power electronics in aerospace platforms. These high power and high frequency devices have been experiencing a paradigm shift toward designs that favor extreme integration and compaction. The reduction in form factor amplifies the intensity of the thermal loads and imposes extreme requirements on the thermal management architecture for reliable operation. In this perspective, we introduce the opportunities and challenges enabled by rationally integrating nanomaterials along the entire thermal resistance chain, beginning at the high heat flux source up to the system-level heat rejection. Using gallium nitride radio frequency devices as a case study, we employ a combination of viewpoints comprised of original research, academic literature, and industry adoption of emerging nanotechnologies being used to construct advanced thermal management architectures. We consider the benefits and challenges for nanomaterials along the entire thermal pathway from synthetic diamond and on-chip microfluidics at the heat source to vertically-aligned copper nanowires and nanoporous media along the heat rejection pathway. We then propose a vision for a materials-by-design approach to the rational engineering of complex nanostructures to achieve tunable property combinations on demand. These strategies offer a snapshot of the opportunities enabled by the rational design of nanomaterials to mitigate thermal constraints and approach the limits of performance in complex aerospace electronics.