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.

Interfacial Features Govern Nanoscale Jumping Droplets.

The solid surfaces with different profile levels impact the liquid-solid contact nature and hence wetting characteristics, showing a vital role in liquid droplets' mobility and dynamic behaviors. Therefore, engineering nanostructured features ultimately enables tuning and controlling the dynamic motion of droplets. In this study, we demonstrate an approach to manipulate nanodroplets' motion behaviors in contact with a surface through tailoring the surface morphological profile. By tracking the trajectories of water molecules at the interface, we find that the motions of a nanodroplet subjected to different levels of lateral force reveal various modes that are identified as creeping, rolling, and jumping motions. Interestingly, the elastic deformation of the droplet and sudden changes in the receding contact angle provide the mechanistic origin for droplet jumping. Guided by computational simulations, a regime map delineating the droplet motion modes with surface profile levels and applied forces is constructed, providing a design strategy for controlling droplet motions via surface engineering.

Enhanced Capillary Wicking through Hierarchically Porous Constructs Derived from Bijel Templates.

Liquid capillarity through porous media can be enhanced by a rational design of hierarchically porous constructs that suggest sufficiently large liquid pathways from an upper-level hierarchy as well as capillary pressure enabled by a lower hierarchy. Here, we demonstrate a material design strategy utilizing a new class of self-assembled soft materials, called bicontinuous interfacially jammed emulsion gels (bijels), to produce hierarchically porous copper, which enables the unique combination of unprecedented control over both macropores and mesopores in a regular, uniform, and continuous arrangement. The dynamic droplet topologies on the hierarchically copper pores prove the significant enhancement in liquid capillarity compared to homogeneous porous structures. The role of nanoscale morphology in liquid infiltration is further investigated through environmental scanning electron microscopy, in which wetting through the mesopores occurs at the beginning, followed by liquid transport through macropores. This understanding on capillary wicking will allow us to design better hierarchically porous media that can address performance breakthroughs in interfacial applications, ranging from battery electrodes, cell delivery in biomedical devices, to capillary-fed thermal management systems.

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.

A Deep Learning Perspective on Dropwise Condensation.

Condensation is ubiquitous in nature and industry. Heterogeneous condensation on surfaces is typified by the continuous cycle of droplet nucleation, growth, and departure. Central to the mechanistic understanding of the thermofluidic processes governing condensation is the rapid and high-fidelity extraction of interpretable physical descriptors from the highly transient droplet population. However, extracting quantifiable measures out of dynamic objects with conventional imaging technologies poses a challenge to researchers. Here, an intelligent vision-based framework is demonstrated that unites classical thermofluidic imaging techniques with deep learning to fundamentally address this challenge. The deep learning framework can autonomously harness physical descriptors and quantify thermal performance at extreme spatio-temporal resolutions of 300 nm and 200 ms, respectively. The data-centric analysis conclusively shows that contrary to classical understanding, the overall condensation performance is governed by a key tradeoff between heat transfer rate per individual droplet and droplet population density. The vision-based approach presents a powerful tool for the study of not only phase-change processes but also any nucleation-based process within and beyond the thermal science community through the harnessing of big data.

Mapping between Surface Wettability, Droplets, and Their Impacting Behaviors.

Droplet behaviors on solid surfaces will influence numerous droplet-based applications ranging from nonwetting-preferred water-repellent surfaces to wetting-preferred spray coatings. Understanding droplet behaviors is complicated and centered on integrating multiple parameters that include surface properties, droplet initial states, and other boundary conditions. Previous studies have observed that droplet impacting performance by showing their underlying mechanisms is sensitive to either droplet or surface boundary conditions. While the holistic view about droplet behaviors is still missing, here we study the droplet impacting and spreading behaviors by systemically varying surface conditions and droplet input parameters through the combination of optical experiments, simulations, and theoretical approaches. The observation defines three droplet behavior modes: bouncing, semibouncing, and spreading modes through their dynamic phases, where the most contributing parameters can be identified as the combination of initial Weber number and surface wettability. The We-θ phase diagram suggested here will provide a guideline for surface engineering with desired droplet dynamic behaviors.

Deep learning predicts boiling heat transfer.

Boiling is arguably Nature's most effective thermal management mechanism that cools submersed matter through bubble-induced advective transport. Central to the boiling process is the development of bubbles. Connecting boiling physics with bubble dynamics is an important, yet daunting challenge because of the intrinsically complex and high dimensional of bubble dynamics. Here, we introduce a data-driven learning framework that correlates high-quality imaging on dynamic bubbles with associated boiling curves. The framework leverages cutting-edge deep learning models including convolutional neural networks and object detection algorithms to automatically extract both hierarchical and physics-based features. By training on these features, our model learns physical boiling laws that statistically describe the manner in which bubbles nucleate, coalesce, and depart under boiling conditions, enabling in situ boiling curve prediction with a mean error of 6%. Our framework offers an automated, learning-based, alternative to conventional boiling heat transfer metrology.

Solid-like Behaviors Govern Evaporative Transport in Adsorbed Water Nanofilms.

The thermophysical attributes of water molecules confined in a sub-nanometer thickness significantly differ from those in bulk liquid where their molecular behaviors start governing interfacial physics at the nanoscale. In this study, we elucidate nanothin film evaporation by employing a computational approach from a molecular perspective. As the liquid thickness decreases, the solid-like characteristics of adsorbed water nanofilms make the resistance at solid-liquid interfaces or Kapitza resistance significant. Kapitza resistances not only show a strong correlation with the surface wettability but also dominate the overall thermal resistance during evaporation rather than the resistance at evaporating liquid-vapor interfaces. Once the liquid thickness reaches the critical value of 0.5-0.6 nm, the evaporation kinetics is suppressed due to the excessive forces between the liquid and solid atoms. The understanding of molecular-level behaviors explains how a hydrophilic surface plays a role in determining evaporation rates from an atomistic perspective.

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.

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.

Fabrication of patterned graphitized carbon wires using low voltage near-field electrospinning, pyrolysis, electrodeposition, and chemical vapor deposition.

We herein report a high-resolution nanopatterning method using low voltage electromechanical spinning with a rotating collector to obtain aligned graphitized micro and nanowires for carbon nanomanufacturing. A small wire diameter and a small inter-wire spacing were obtained by controlling the electric field, the spinneret-to-collector distance, the pyrolysis parameters, the linear speed of the spinneret, the rotational speed of the collector. Using a simple scaling analysis, we show how the straightness and the diameter of the wires can be controlled by the electric field and the distance of the spinneret to the collector. A small inter-wire spacing, as predicted by a simple model, was achieved by simultaneously controlling the linear speed of the spinneret and the rotational speed of the collector. Rapid drying of the polymer nanowires enabled the facile fabrication of suspended wires over various structures. Patterned polyacrylonitrile wires were carbonized using standard stabilization and pyrolysis to obtain carbon nanowires. Suspended carbon nanowires with a diameter of <50 nm were obtained. We also established a method for making patterned, highly graphitized structures by using the aforementioned carbon wire structures as a template for chemical vapor deposition of graphite. This patterning technique offers high throughput for nano writing, which outperforms other existing nanopatterning techniques, making it a potential candidate for large-scale carbon nanomanufacturing.

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/ Reff = 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.

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/ Reff = ∼5 × 10-3 µm) is shown to be higher than that of a typical crystalline inverse opal ( K/ Reff = ∼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.

Porous micropillar structures for retaining low surface tension liquids.

The ability to manipulate fluid interfaces, e.g., to retain liquid behind or within porous structures, can be beneficial in multiple applications, including microfluidics, biochemical analysis, and the thermal management of electronic systems. While there are a variety of strategies for controlling the disposition of liquid water via capillarity, such as the use of chemically modified porous adhesive structures and capillary stop valves or surface geometric features, methods that work well for low surface tension liquids are far more difficult to implement. This study demonstrates the microfabrication of a silicon membrane that can retain exceptionally low surface tension fluorinated liquids against a significant pressure difference across the membrane via an array of porous micropillar structures. The membrane uses capillary forces along the triple phase contact line to maintain stable liquid menisci that yield positive working Laplace pressures. The micropillars have inner diameters and thicknesses of 1.5-3 µm and ∼1 µm, respectively, sustaining Laplace pressures up to 39 kPa for water and 9 kPa for Fluorinert™ (FC-40). A theoretical model for predicting the change in pressure as the liquid advances along the porous micropillar structure is derived based on a free energy analysis of the liquid meniscus with capped spherical geometry. The theoretical prediction was found to overestimate the burst pressure compared with the experimental measurements. To elucidate this deviation, transient numerical simulations based on the Volume of Fluid (VOF) were performed to explore the liquid pressure and evolution of meniscus shape under different flow rates (i.e., Capillary numbers). The results from VOF simulations reveal strong dynamic effects where the anisotropic expansion of liquid along the outer micropillar edge leads to an irregular meniscus shape before the liquid spills along the micropillar edge. These findings suggest that the analytical prediction of burst Laplace pressure obtained under quasi-static condition (i.e., equilibrium thermodynamic analysis under low capillary number) is not applicable to highly dynamic flow conditions, where the liquid meniscus shape deformation by flow perturbation cannot be restored by surface tension force instantaneously. Therefore, the critical burst pressure is dependent on the liquid velocity and viscosity under dynamic flow conditions. A numerical simulation using Surface Evolver also predicts that surface defects along the outer micropillar edge can yield up to 50% lower Laplace pressures than those predicted with ideal feature geometries. The liquid retention strategy developed here can facilitate the routing and phase management of dielectric working fluids for application in heat exchangers. Further improvements in the retention performance can be realized by optimizing the fabrication process to reduce surface defects.

Controlled Wetting Properties through Heterogeneous Surfaces Containing Two-level Nanofeatures.

Addressing the direct control of surface wettability has been a significant challenge for a variety of applications from self-cleaning surfaces to phase-change applications. Surface wettability has been traditionally modulated by installing surface nanostructures or changing their chemistry. Among numerous nanofabrication efforts, the chemical oxidation method is considered a promising approach because it allows cost-effective, quick, and direct control of the morphologies and chemical compositions of the grown nanofeatures. Despite the wide applicability of the surface oxidation method, the precise control of wetting behaviors through the growth of nanostructures has yet to be addressed. Here, we investigate the wetting characteristics of heterogeneous surfaces that contain two-level features (i.e., nanograsses and nanoflowers) with different petal shapes and structural chemistry. The difference in growth rates between nanograsses and nanoflowers creates a time-evolving morphology that can be classified by grass-dominated or flower-dominated regimes, which induces a wide range of water contact angles from 120 to 20°. The following study systematically quantifies the structural details and chemistry of nanostructures associated with their wetting characteristics. This investigation of heterogeneous surfaces will pave the way for selective growth of copper nanostructures and thus a direct control of surface wetting properties for use in future copper-based thermal applications.

Burst behavior at a capillary tip: Effect of low and high surface tension.

Liquid retention in micron and millimeter scale devices is important for maintaining stable interfaces in various processes including bimolecular separation, phase change heat transfer, and water desalination. There have been several studies of re-entrant geometries, and very few studies on retaining low surface tension liquids such as fluorocarbon-based dielectric liquids. Here, we study retention of a liquid with very low contact angles using borosilicate glass capillary tips. We analyzed capillary tips with outer diameters ranging from 250 to 840 µm and measured Laplace pressures up to 2.9 kPa. Experimental results agree well with a numerical model that predicts burst pressure (the maximum Laplace pressure for liquid retention), which is a function of the outer diameter (D) and capillary exit edge radius of curvature (r).

Zipping, entanglement, and the elastic modulus of aligned single-walled carbon nanotube films.

Reliably routing heat to and from conversion materials is a daunting challenge for a variety of innovative energy technologies--from thermal solar to automotive waste heat recovery systems--whose efficiencies degrade due to massive thermomechanical stresses at interfaces. This problem may soon be addressed by adhesives based on vertically aligned carbon nanotubes, which promise the revolutionary combination of high through-plane thermal conductivity and vanishing in-plane mechanical stiffness. Here, we report the data for the in-plane modulus of aligned single-walled carbon nanotube films using a microfabricated resonator method. Molecular simulations and electron microscopy identify the nanoscale mechanisms responsible for this property. The zipping and unzipping of adjacent nanotubes and the degree of alignment and entanglement are shown to govern the spatially varying local modulus, thereby providing the route to engineered materials with outstanding combinations of mechanical and thermal properties.