A general patterning approach by manipulating the evolution of two-dimensional liquid foams

Spatiotemporal-Dependent Confinement Effect of Bubble Swarms Enables a Fractal Hierarchical Assembly with Promoted Chirality

The submarine-confined bubble swarm is considered an important constraining environment for the early evolution of living matter due to the abundant gas/water interfaces it provides. Similarly, the spatiotemporal characteristics of the confinement effect in this particular scenario may also impact the origin, transfer, and amplification of chirality in organisms. Here, we explore the confinement effect on the chiral hierarchical assembly of the amphiphiles in the confined bubble array stabilized by the micropillar templates. Compared with the other confinement conditions, the assembly in the bubble scenario yields a fractal morphology and exhibits a unique level of the chiral degree, ordering, and orientation consistency, which can be attributed to the characteristic interfacial effects of the rapidly formed gas/water interfaces. Thus, molecules with a balanced amphiphilicity can be more favorable for the promotion. Not limited to the pure enantiomers, chiral amplification of the enantiomer-mixed assembly is observed only in the bubble scenario. Beyond the interfacial mechanism, the fast formation kinetics of the confined liquid bridges in the bubble scenario endows the assembly with the tunable hierarchical morphology when regulating the amphiphilicity, aggregates, and confined spaces. Furthermore, the chiral-induced spin selectivity (CISS) effect of the fractal hierarchical assembly was systematically investigated, and a strategy based on photoisomerization was developed to efficiently modulate the CISS effect. This work provides insights into the robustness of confined bubble swarms in promoting a chiral hierarchical assembly and the potential applications of the resulting chiral hierarchical patterns in solid-state spintronic and optical devices.

Dynamic Simulation Research on Microprofile Control Mechanism of Foam in Fractured Media Based on Level Set Method

Fractured reservoirs are an important source of oil and gas energy. After depletion of production, the production capacity of this reservoir decreases rapidly. Effective profile control is needed to improve the sweep efficiency and reservoir heterogeneity. Foam can solve such problems, but its profile control mechanism is not fully understood. Based on this, this paper uses the level set method to study the microscale control mechanism of foam in fractured media. The results show that artificial fractures and high-permeability microfractures are tighter than natural fractures, the Jamin effect of foam is stronger, and the secondary foaming ability is better. Therefore, the plugging ability of foam to natural fractures is far less than that of foam to artificial fractures and high-permeability microfractures. The larger the fracture opening, the larger the foam volume and the smaller the flow rate. As the opening ratio increases gradually, the generated foam flows more to the natural fractures with a large opening, and the effect of foam blocking large fractures becomes worse. The diversion rate curves of different opening ratios show that the foam has a good profile control effect when the opening ratios are 4:1 and 2:1, and even the diversion rate overturns, while the profile control diversion effect is poor when the opening ratio is 10:1, so it cannot play an effective role in profile control. The foam shows the profile control process of preferentially plugging the high-permeability area and allowing more subsequent fluids to enter the low-permeability area. The research reveals the profile control mechanism of foam on fractured reservoirs from the micro level, which is the supplement and verification of relevant macro research and provides a theoretical basis for the efficient development of fractured reservoirs.

Path-dependent morphology of CH4 hydrates and their dissociation studied with high-pressure microfluidics

Methane hydrates (MHs) have been considered a promising future energy source due to their vast resource volume and high energy density. Understanding the behavior of MH formation and dissociation at the pore-scale and the effect of MH distribution on the gas-liquid two phase flow is of critical importance for designing effective production strategies from natural gas hydrate (NGH) reservoirs. In this study, we devised a novel high-pressure microfluidic chip apparatus that is capable of direct observation of MH formation and dissociation behavior at the pore-scale. MH nucleation and growth behavior at 10.0 MPa and dissociation via thermal stimulation with gas bubble generation and evolution were examined. Our experimental results reveal that two different MH formation mechanisms co-exist in pores: (a) porous-type MH with a rough surface formed from CH4 gas bubbles at the gas-liquid interface and (b) crystal-type MH formed from dissolved CH4 gas. The growth and movement of crystal-type MH can trigger the sudden nucleation of porous-type MH. Spatially, MHs preferentially grow along the gas-liquid interface in pores. MH dissociation under thermal stimulation practically generates gas bubbles with diameters of 20.0-200.0 μm. Based on a custom-designed image analysis technique, three distinct stages of gas bubble evolution were identified during MH dissociation via thermal stimulation: (a) single gas bubble growth with an expanding water layer at an initial slow dissociation rate, (b) rapid generation of clusters of gas bubbles at a fast dissociation rate, and (c) gas bubble coalescence with uniform distribution in the pore space. The novel apparatus designed and the image analysis technique developed in this study allow us to directly capture the dynamic evolution of the gas-liquid interface during MH formation and dissociation at the pore-scale. The results provide direct first-hand visual evidence of the growth of MHs in pores and valuable insights into gas-liquid two-phase flow behavior during fluid production from NGHs.

Live Imaging of 3D Hanging Drop Arrays through Manipulation of Light-Responsive Pyroelectric Slippery Surface and Chip Adhesion

Three-dimensional (3D) hanging drop cell culture is widely used in organoid culture because of its lack of selection pressure and rapid cell aggregation. However, current hanging drop technology has limitations, such as a dependence on complex microfluidic transport channels or specific capillary force templates for drop formation, which leads to unchangeable drop features. These methods also hinder live imaging because of space and complexity constraints. Here, we have developed a hanging drop construction method and created a flexible 3D hanging drop construction platform composed of a manipulation module and an adhesion module. Their harmonious operation allows for the easy construction of hanging drops of varying sizes, types, and patterns. Our platform produces a cell hanging drop chip with small sizes and clear fields of view, thereby making it compatible with live imaging. This platform has great potential for personalized medicine, cancer and drug discovery, tissue engineering, and stem cell research.

Humidity-Controlled Molecular Assembly and Photoisomerization Behavior with a Bubble-Assisted Patterning Approach

Precise control of molecular assembly is of great significance in the application of functional molecules. This work has systematically investigated the humidity effect in bubble-assisted molecular assembly. This work finds humidity is critical in the evolution of the soft confined space, leading to the formation of microscale liquid confined space under high humidity, and nanoscale liquid confined space under low humidity. It is also revealed that the differences in surface wettability and adhesion play the key role. Consequently, a flat pattern with thermodynamically favorable ordered structure and a sharp pattern with dynamically favorable disordered structure are achieved, which show different solid-state photoisomerization behaviors and photoresponsiveness. Interestingly, conductivity of sharp pattern with disordered structure is higher than that of flat pattern with layered ordered structure due to electronic transport mechanism of different spatial dimensions. This work opens a new way for manipulating the molecular self-assembly to control the morphology and function of molecular patterns.

Patterned Nanoparticle Arrays Fabricated Using Liquid Film Rupture Self-Assembly

Self-assembly is an important bottom-up fabrication approach based on accurate manipulation of solid-air-liquid interfaces to construct microscale structures using nanoscale materials. This approach plays a substantial role in the fabrication of microsensors, nanosensors, and actuators. Improving the controllability of self-assembly to realize large-scale regular micro/nano patterns is crucial for this approach's further development and wider applications. Herein, we propose a novel strategy for patterning nanoparticle arrays on soft substrates. This strategy is based on a unique process of liquid film rupture self-assembly that is convenient, precise, and cost-efficient for mass manufacturing. This approach involves two key steps. First, suspended liquid films comprising monolayer polystyrene (PS) spheres are realized via liquid-air interface self-assembly over prepatterned microstructures. Second, these suspended liquid films are ruptured in a controlled manner to induce the self-assembly of internal PS spheres around the morphological edges of the underlying microstructures. This nanoparticle array patterning method is comprehensively investigated in terms of the effect of the PS sphere size, morphological effect of the microstructured substrate, key factors influencing liquid film-rupture self-assembly, and optical transmittance of the fabricated samples. A maximum rupture rate of 95.4% was achieved with an optimized geometric and dimensional design. Compared with other nanoparticle-based self-assembly methods used to form patterned arrays, the proposed approach reduces the waste of nanoparticles substantially because all nanoparticles self-assemble around the prepatterned microstructures. More nanoparticles assemble to form prepatterned arrays, which could strengthen the nanoparticle array network without affecting the initial features of prepatterned microstructures.

Bubble wall confinement-driven molecular assembly toward sub-12 nm and beyond precision patterning

Patterning is attractive for nanofabrication, electron devices, and bioengineering. However, achieving the molecular-scale patterns to meet the demands of these fields is challenging. Here, we propose a bubble-template molecular printing concept by introducing the ultrathin liquid film of bubble walls to confine the self-assembly of molecules and achieve ultrahigh-precision assembly up to 12 nanometers corresponding to the critical point toward the Newton black film limit. The disjoining pressure describing the intermolecular interaction could predict the highest precision effectively. The symmetric molecules exhibit better reconfiguration capacity and smaller preaggregates than the asymmetric ones, which are helpful in stabilizing the drainage of foam films and construct high-precision patterns. Our results confirm the robustness of the bubble template to prepare molecular-scale patterns, verify the criticality of molecular symmetry to obtain the ultimate precision, and predict the application potential of high-precision organic patterns in hierarchical self-assembly and high-sensitivity sensors.

Orientation-Controlled Ultralong Assembly of Janus Particles Induced by Bubble-Driven Instant Quasi-1D Interfaces

Constructing precisely oriented assemblies and exploring their orientation-dependent properties remain a challenge for Janus nanoparticles (JNPs) due to their asymmetric characteristics. Herein, we propose a bubble-driven instant quasi-1D interfacial strategy for the oriented assembly of JNP chains in a highly controllable manner. It is found that the rapid formation of templated bubbles can promote the interfacial orientation of JNPs kinetically, while the confined quasi-1D interface in the curved liquid bridge can constrain the disordered rotation of the particles, yielding well-oriented JNP chains in a long range. During the evaporation process, the interfacial orientation of the JNPs can be transferred to the assembled chains. By regulating the amphiphilicity of the JNPs, both heteraxial and coaxial JNP assemblies are obtained, which show different polarization dependences on light scattering, and the related colorimetric logic behaviors are demonstrated. This work demonstrates the great potential of patterned interfacial assembly with a manageable orientation and shows the broad prospect of asymmetric JNP assembly in constructing novel optoelectronic devices.

Encapsulated salts in velvet worm slime drive its hardening

Slime expelled by velvet worms entraps prey insects within seconds in a hardened biopolymer network that matches the mechanical strength of industrial polymers. While the mechanic stimuli-responsive nature and building blocks of the polymerization are known, it is still unclear how the velvet worms' slime hardens so fast. Here, we investigated the slime for the first time, not only after, but also before expulsion. Further, we investigated the slime's micro- and nanostructures in-depth. Besides the previously reported protein nanoglobules, carbohydrates, and lipids, we discovered abundant encapsulated phosphate and carbonate salts. We also detected CO₂ bubbles during the hardening of the slime. These findings, along with further observations, suggest that the encapsulated salts in expelled slime rapidly dissolve and neutralize in a baking-powder-like reaction, which seems to accelerate the drying of the slime. The proteins' conformation and aggregation are thus influenced by shear stress and the salts' neutralization reaction, increasing the slime's pH and ionic strength. These insights into the drying process of the velvet worm's slime demonstrate how naturally evolved polymerizations can unwind in seconds, and could inspire new polymers that are stimuli-responsive or fast-drying under ambient conditions.

Manipulating Single-Walled Carbon Nanotube Arrays for Flexible Photothermoelectric Devices

Flexible photothermoelectric (PTE) devices possess great application prospects in the field of light energy and thermoelectric energy harvesting which are some of the cornerstones of modern green renewable energy power generation. However, the low efficiency of PTE materials and lack of suitable manufacturing processes remain an impediment to restrict its rapid development. Here, we designed a flexible PTE device by printing a highly integrated single-walled carbon nanotubes (SWCNTs) array at intervals that were surface-functionalized with poly(acrylic acid) and poly(ethylene imine) as p-n heterofilms. After the introduction of a mask to give a selective light illumination and taking advantage of the photothermal effect of SWCNTs, a remarkable temperature gradient along the printed SWCNTs and a considerable power density of 1.3 μW/cm² can be achieved. Meanwhile, both experimental data and COMSOL theoretical simulations were adopted to optimize the performance of our device, showing new opportunities for new generation flexible PTE devices.

Controllable and Directional Transportation of Bubbles on Asymmetric Hexagonal Cage Substrate in Aqueous Environment

Controllable and directional bubble transport is usually the critical step in applications involving bubbles. However, current bubble transport strategies either are limited in controllability and transport distance or require the assistance of a specific external field. Here, we propose a strategy for bubble transport in an asymmetric hexagonal cage (ASHC), which works smoothly even under antibuoyancy conditions. The transport efficiency of bubbles can be greatly improved by adjusting the structural parameters of the cage. The control of the bubble depends only on the change of the bubble's volume, so there is no strict restriction on the driving force, which can be pressure, photothermal, electrothermal, and even acoustic-thermal forces. Moreover, we demonstrate that long-distance transport and controllable merging of bubbles can be easily achieved by cascading multistage ASHC structures. This investigation offers a simple, low-cost, extensible, and versatile construction for bubble transport for fundamental research and practical applications.

On-Demand Maneuvering of Diverse Prodrug Liquids on a Light-Responsive Candle-Soot-Hybridized Lubricant-Infused Slippery Surface for Highly Effective Toxicity Screening

At present, microscale high-throughput screening (HTS) for drug toxicity has drawn increased attention. Reported methods are often constrained by the inability to execute rapid fusion over diverse droplets or the inflexibility of relying on rigid customized templates. Herein, a light-responsive candle-soot-hybridized lubricant-infused slippery surface (CS-LISS) was reported by one-step femtosecond laser cross-scanning to realize highly effective and flexible drug HTS. Due to its low-hysteresis merits, the CS-LISS can readily steer diverse droplets toward arbitrary directions at a velocity over 1.0 mm/s with the help of tracing lateral near-infrared irradiation; additionally, it has the capability of self-cleaning and self-deicing. Significantly, by integrating the CS-LISS with a GFP HeLa cell chip, high-efficiency drug toxicity screening can be successfully achieved with the aid of fluorescence imaging. This work provides insights into the design of microscale high-throughput drug screening.

Rationally Tuning Phase Separation in Polymeric Membranes toward Optimized All-day Passive Radiative Coolers

The all-day passive radiative cooler has emerged as one of the state-of-the-art energy-saving cooling tool kits but routinely suffers from limited processability, high cost, and complicated fabrication processes, which impede large-scale applications. To address these challenges, this work exploits a polymer-based passive radiative cooler with optimized turbidity, reconfigurability, and recyclability. These cooling membranes are fabricated via selective condensation of octyl side chain-modified polyvinyl alcohol through a non-solvent-induced phase separation method. The rational tuning over spatial organization and distribution of the air-polymer interface renders optimized bright whiteness with solar reflectance at 96%. Meanwhile, the abundant -C-O-C- bonds endow such membranes with infrared thermal emittance over 90%. The optimized membrane realizes a subambient cooling of ∼5.7 °C with an average cooling power of ∼81 W m-2 under a solar intensity of ∼528 W m-2. Furthermore, the supramolecule nature of the developed passive radiative cooling membrane bears enhanced shape malleability and recyclability, substantially enhancing its conformability to the complex geometry and extending its life for an eco-friendly society.

Evaporation-Driven Coassembly of Hierarchical, Multicomponent Networks

Self-assembly is an increasingly popular approach to systematically control the formation of complex, multicomponent materials with structural features orders of magnitude larger than the constituent colloidal nanocrystals. Common approaches often involve templating via prefabricated patterns to control particle organization- or programming-specific interactions between individual building blocks. While effective, such fabrication methods suffer from major bottlenecks due to the complexity required in mask creation for patterning or surface modification techniques needed to program directed interactions between particles. Here, we propose an alternative strategy that aims to bypass such limitations. First, we design a ligand structure that can bridge two distinct nanocrystal types. Then, by leveraging the solvent's evaporative dynamics to drive particle organization, we direct a cross-linked, multicomponent system of nanocrystals to organize hierarchically into ordered, open-network structures with domain sizes orders of magnitude larger than the constituent building blocks. We employ simulation and theory to rationalize the driving forces governing this evaporation-driven process, showing excellent agreement across theory, simulations, and experiments. These results suggest that evaporation-driven organization can be a powerful approach to designing and fabricating hierarchical, multifunctional materials.

Microfluidic Investigation of Foam Coarsening Dynamics in Porous Media at High-Pressure and High-Temperature Conditions

Coarsening or Oswald ripening, induced by interbubble gas diffusion, is considered to dominate foam structure evolution in porous media. We present the first study of trapped foam coarsening dynamics under realistic deep reservoir conditions (up to 3200 psi/22 MPa of pore pressure and 100 °C of temperature) in a high-pressure and high-temperature microfluidic system. The findings are expected to help predict foam structure evolution in applications such as enhanced oil recovery and CO₂ geological sequestration. It is shown that, in porous media, larger bubbles grow at the expense of smaller bubbles. The growth rate of the average bubble area (⟨a⟩) over time shows a long-term linear increase when ⟨a⟩ is between 1/5 and 1/2 of the average pore size. The foam coarsening kinetics are determined by the liquid film permeability, gas-liquid interfacial tension, and the molar volume of the dispersed phase. In summary, foams prepared with less water-soluble gases (e.g., N₂ and air) and lower foam quality show slower coarsening kinetics due to a lower film permeability. Foam coarsening is more sensitive to surfactant concentration (than surfactant type), as it determines the interfacial tension that controls the mass transfer driving force (capillary pressure difference). The transport properties of the dispersed phase depend strongly on its density, which increases with increasing pore pressure and decreasing temperature. At the same experimental conditions, gas CO₂ foam shows a 10-fold faster coarsening rate than N₂ foam. However, dense (i.e., liquid and supercritical) CO₂ foams show a remarkable 20-500-fold reduction in coarsening kinetics compared with gas N₂ and CO₂ foams due to the significantly reduced mass transfer driving forces. In a sense, trapped CO₂ foam can be stronger than N₂ foam at high-pressure and high-temperature conditions.

Unraveling hidden rules behind the wet-to-dry transition of bubble array by glass-box physics rule learner

A liquid–gas foam, here called bubble array, is a ubiquitous phenomenon widely observed in daily lives, food, pharmaceutical and cosmetic products, and even bio- and nano-technologies. This intriguing phenomenon has been often studied in a well-controlled environment in laboratories, computations, or analytical models. Still, real-world bubble undergoes complex nonlinear transitions from wet to dry conditions, which are hard to describe by unified rules as a whole. Here, we show that a few early-phase snapshots of bubble array can be learned by a glass-box physics rule learner (GPRL) leading to prediction rules of future bubble array. Unlike the black-box machine learning approach, the glass-box approach seeks to unravel expressive rules of the phenomenon that can evolve. Without known principles, GPRL identifies plausible rules of bubble prediction with an elongated bubble array data that transitions from wet to dry states. Then, the best-so-far GPRL-identified rule is applied to an independent circular bubble array, demonstrating the potential generality of the rule. We explain how GPRL uses the spatio-temporal convolved information of early bubbles to mimic the scientist’s perception of bubble sides, shapes, and inter-bubble influences. This research will help combine foam physics and machine learning to better understand and control bubbles.

Universal fluctuation of polygonal crack geometry in solidified lava

Outcrops of columnar joints made of solidified lava flows are often covered by semiordered polygonal cracks. The polygon diameters are fairly uniform at each outcrop, but their shapes largely vary in the number of sides and internal angles. Herein, we unveil that the statistical variation in the polygon shape follows an extreme value distribution class: the Gumbel distribution. The Gumbel law was found to hold for different columnar joints, regardless of the locality, lithologic composition, and typical diameter. A common distribution for columnar joints implies a universal class that may integrate the polygonal crack networks observed on the surface of various fractured brittle materials.

Printable Smart Materials and Devices: Strategies and Applications

Smart materials are a kind of functional materials which can sense and response to environmental conditions or stimuli from optical, electrical, magnetic mechanical, thermal, and chemical signals, etc. Patterning of smart materials is the key to achieving large-scale arrays of functional devices. Over the last decades, printing methods including inkjet printing, template-assisted printing, and 3D printing are extensively investigated and utilized in fabricating intelligent micro/nano devices, as printing strategies allow for constructing multidimensional and multimaterial architectures. Great strides in printable smart materials are opening new possibilities for functional devices to better serve human beings, such as wearable sensors, integrated optoelectronics, artificial neurons, and so on. However, there are still many challenges and drawbacks that need to be overcome in order to achieve the controllable modulation between smart materials and device performance. In this review, we give an overview on printable smart materials, printing strategies, and applications of printed functional devices. In addition, the advantages in actual practices of printing smart materials-based devices are discussed, and the current limitations and future opportunities are proposed. This review aims to summarize the recent progress and provide reference for novel smart materials and printing strategies as well as applications of intelligent devices.

Magnetic-actuated "capillary container" for versatile three-dimensional fluid interface manipulation

Fluid interfaces are omnipresent in nature. Engineering the fluid interface is essential to study interfacial processes for basic research and industrial applications. However, it remains challenging to precisely control the fluid interface because of its fluidity and instability. Here, we proposed a magnetic-actuated "capillary container" to realize three-dimensional (3D) fluid interface creation and programmable dynamic manipulation. By wettability modification, 3D fluid interfaces with predesigned sizes and geometries can be constructed in air, water, and oils. Multiple motion modes were realized by adjusting the container's structure and magnetic field. Besides, we demonstrated its feasibility in various fluids by performing selective fluid collection and chemical reaction manipulations. The container can also be encapsulated with an interfacial gelation reaction. Using this process, diverse free-standing 3D membranes were produced, and the dynamic release of riboflavin (vitamin B₂) was studied. This versatile capillary container will provide a promising platform for open microfluidics, interfacial chemistry, and biomedical engineering.

A Bubble-Assisted Approach for Patterning Nanoscale Molecular Aggregates

We demonstrate a new approach to pattern functional organic molecules with a template of foams, and achieve a resolution of sub 100 nm. The bubble-assisted assembly (BAA) process is consisted of two periods, including bubble evolution and molecular assembly, which are dominated by the Laplace pressure and molecular interactions, respectively. Using TPPS (meso-tetra(4-sulfonatophenyl) porphyrin), we systematically investigate the patterns and assembly behaviour in the bubble system with a series of characterizations, which show good uniformity in nanoscale resolution. Theoretical simulations reveal that TPPS's J-aggregates contribute to the ordered construction of molecular patterns. Finally, we propose an empirical rule for molecular patterning approach, that the surfactant and functional molecules should have the same type of charge in a two-component system. This approach exhibits promising feasibility to assemble molecular patterns at nanoscale resolution for micro/nano functional devices.

Preparation and performance of melamine-formaldehyde rigid foams with high closed cell content

Melamine-formaldehyde (MF)rigid foams with high closed cell content were prepared via oven heating process, using MF prepolymer prepared from melamine and paraformaldehyde as a matrix, cyclohexane as the foaming agent, dimethyl silicon oil as the foam stabilizers, hydrochloric acid as the catalyst. The effect of MF prepolymer viscosity, foaming temperature, amount of catalyst on morphology, closed cell content, apparent density, water absorption and compressive strength of MF rigid foams were systematically studied. The optimized foaming conditions are as follows: the viscosity of MF prepolymer ranges from 35 Pa·s to 45 Pa·s, the foaming temperature is 125°C and the content of the catalyst is 0.65 wt%. The as-prepared MF foams showed the best comprehensive performance with closed cell content of 83.5%, apparent density of 62 kg·m−3, water absorption of 12.0%, compressive strength of 292kPa, thermal conductivity of 0.033 W m−1 K−1 and limiting oxygen index (LOI) of 36%. Compared to conventional organic foams, MF rigid foams possess low water absorption, excellent thermal insulation and flame retardancy due to high closed cell content, and can be expected to be used as thermal insulation material for building exterior walls.

Long-Term Stability of Different Kinds of Gas Nanobubbles in Deionized and Salt Water

Nanobubbles have many potential applications depending on their types. The long-term stability of different gas nanobubbles is necessary to be studied considering their applications. In the present study, five kinds of nanobubbles (N₂, O₂, Ar + 8%H₂, air and CO₂) in deionized water and a salt aqueous solution were prepared by the hydrodynamic cavitation method. The mean size and zeta potential of the nanobubbles were measured by a light scattering system, while the pH and Eh of the nanobubble suspensions were measured as a function of time. The nanobubble stability was predicted and discussed by the total potential energies between two bubbles by the extended Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. The nanobubbles, except CO₂, in deionized water showed a long-term stability for 60 days, while they were not stable in the 1 mM (milli mol/L) salt aqueous solution. During the 60 days, the bubble size gradually increased and decreased in deionized water. This size change was discussed by the Ostwald ripening effect coupled with the bubble interaction evaluated by the extended DLVO theory. On the other hand, CO₂ nanobubbles in deionized water were not stable and disappeared after 5 days, while the CO₂ nanobubbles in 1 mM of NaCl and CaCl₂ aqueous solution became stable for 2 weeks. The floating and disappearing phenomena of nanobubbles were estimated and discussed by calculating the relationship between the terminal velocity of the floating bubble and bubble size.

A Wetting-Enabled-Transfer (WET) Strategy for Precise Surface Patterning of Organohydrogels

The ability to manipulate water and oil phases in a designable manner is of great significance in widespread fields from art paintings to materials science. However, achieving precise and stable surface patterns for two immiscible phases of water and oil remains a challenge. Herein, a general wetting-enabled-transfer (WET) strategy is reported to construct discretionary shape-defined surface patterns of organohydrogels along with their monolithic formation either from flat to curved surfaces or from the microscale to the macroscale. Locally differentiated wettability induces hydrophilic monomers and hydrophobic monomers from an emulsion system onto the wettability-matching regions of the prepatterned substrates, subsequently forming corresponding hydrogel and organogel patterns on the organohydrogel surface after in situ photopolymerization. The precision of the surface patterns can be controlled by optimizing the gel monomers, emulsion droplet size, and surface chemical composition of the prepatterned substrates. This finding may provide a feasible strategy for precisely patterning functional materials from two-immiscible-phase systems.

A fluid-guided printing strategy for patterning high refractive index photonic microarrays

High refractive index (HRI, n > 1.8) photonic structures offer strong light confinement and refractive efficiencies, cover the entire visible spectrum and can be tuned by designing geometric arrayed features. However, its practical applications are still hindered by the applicability and material limitation of lithography-based micro/nano fabrication approaches. Herein, we demonstrate a fluid-guided printing process for preparing HRI selenium microarrays. The microstructured flexible template is replicated from the diced silicon wafer without any lithography-based methods. When heated above the glass transition temperature, the flow characteristics of selenium endows the structure downsizing and orientation patterning between the target substrate and the template. Near 10 times narrowing selenium microarrays (1.9 μm width) are patterned from the non-lithography template (18 μm width). HRI selenium microarrays offer high refractive efficiencies and strong optical confinement abilities, which achieve angle-dependent structurally coloration and polarization. Meanwhile, the color difference can be recognized under the one degree distinction of the angle between incident and refracted light. This printing platform will facilitate HRI optical metasurfaces in a variety of applications, ranging from photonic sensor, polarization modulation to light manipulation.

Fabrication of Silver Mesh/Grid and Its Applications in Electronics

With the development of flexible electronics, researchers have endeavored to improve the characteristics of the commonly used indium tin oxide such as brittleness, poor mechanical or chemical stability, and scarcity. Currently, many alternative materials have been considered such as conductive polymers, graphene, carbon nanotubes, metallic nanoparticles (NPs), nanowires (NWs), or nanofibers. Among them, silver (Ag) mesh/grid NPs or NWs have been considered as an excellent substitute due to the good transmittance, excellent electrical conductivity, outstanding mechanical robustness, and cost competitiveness. So far, much effort has been devoted to the fabrication of Ag mesh/grid, and many methods such as printing technology, self-assembly, electrospun, hot-pressing, and atomic layer deposition have been reported. Here printing technologies include jet printing, gravure printing, screen printing, nanoimprint lithography, microcontact printing, and flexographic printing. The solution-based self-assembly usually combines with coating, template, or mask assistance. This review summarizes the characteristics of these fabrication methods for the Ag mesh/grid with its related applications in electronics. Then the prospect and challenges of the fabrication methods are discussed, and the new preparation approaches and applications of the Ag mesh/grid are highlighted, which will be of significance for the applications in electronics such as transparent conducting electrodes, organic light-emitting diode, energy harvester, strain sensor, cells, etc.

Inhibiting Ostwald Ripening by Scaffolding Droplets

Nanoemulsions as colloidal dispersions of deformable nanodroplets promise wide range of applications in pharmaceuticals, cosmetics, and agriculture. The main limitation that reduces their industrial applications is stability, with Ostwald ripening acting as the main destabilization mechanism. Different from the conventional methods by functionalizing nanoemulsions with adequate ripening inhibitors, here we propose an alternative strategy to stabilize nanoemulsions by inhibiting Ostwald ripening. We report via Lattice Boltzmann method (LBM) and theoretical analysis that the evolution of droplets can be manipulated with the help of solid substrates, either along or against the direction of Ostwald ripening. It turns out that through pinning contact line of sessile droplets, heterogeneous substrates or solid nanoparticles can behave as a scaffold to suppress Ostwald ripening, to regulate droplet morphology and to enhance droplet stability. The identical curvature and unexpected stability of scaffolding droplets are then interpreted with free energy analysis. In addition, by simulating substrates with various heterogeneities and solid particles of different shapes, we demonstrate that it is a common phenomenon that scaffolding droplets can evolve beyond Ostwald ripening.

Fabrication of Waterproof Artificial Compound Eyes with Variable Field of View Based on the Bioinspiration from Natural Hierarchical Micro-Nanostructures

Planar and curved microlens arrays (MLAs) are the key components of miniaturized microoptical systems. In order to meet the requirements for advanced and multipurpose applications in microoptical field, a simple manufacturing method is urgently required for fabricating MLAs with unique properties, such as waterproofness and variable field-of-view (FOV) imaging. Such properties are beneficial for the production of advanced artificial compound eyes for the significant applications in complex microcavity environments with high humidity, for instance, miniature medical endoscopy. However, the simple and effective fabrication of advanced artificial compound eyes still presents significant challenges. In this paper, bioinspired by the natural superhydrophobic surface of lotus leaf, we propose a novel method for the fabrication of waterproof artificial compound eyes. Electrohydrodynamic jet printing was used to fabricate hierarchical MLAs and nanolens arrays (NLAs) on polydimethylsiloxane film. The flexible film of MLAs hybridized with NLAs exhibited excellent superhydrophobic property with a water contact angle of 158°. The MLAs film was deformed using a microfluidics chip to create artificial compound eyes with variable FOV, which ranged from 0° to 160°.

Nanobubble-Assisted Nanopatterning Reveals the Existence of Liquid Quasi-Two-Dimensional Foams Pinned to a Water-Immersed Surface

This paper reports on our observation of a quasi-two-dimensional (quasi-2D) liquid nanofoam spontaneously appearing on a submersed solid surface. Unlike common liquid foams existing on top of the liquid, the quasi-2D liquid nanofoam is pinned to a water-immersed solid surface. The foam imaging was performed by a nanobubble imprint technique, which allows recording the positions of the surface nanobubbles by their imprints in a polystyrene film, as described in our previous papers [Tarábková et al. Langmuir 2014, 30, 14522; Tarábková et al., Langmuir 2016, 32, 11221]. Nanobubble imprints are then examined by ex situ atomic force microscopy. Besides randomly distributed nanoprotrusions corresponding to solitary nanobubbles, quasi-periodic arrangements of a tight cellular structure and more spaced round-shaped patterns, corresponding to "dry" and "wet" quasi-2D micro- and nanofoams, respectively, are identified. Although randomly spread solitary nanobubbles can occupy up to 30% of an immersed solid surface, their self-organization in a quasi-2D nanofoam leads to surface gas coverage reaching up to 80%, which implies significantly lowered surface wetting. Existence of a submersed quasi-2D nanofoam thus opens the novel question on the impact of dense surface nanobubble assemblies on heterogeneous processes at the solid-liquid interface.

Retarding Ostwald Ripening to Directly Cast 3D Porous Graphene Oxide Bulks at Open Ambient Conditions

Graphene aerogels (GAs) with attractive properties have shown tremendous potentials in energy- and environment-related applications. Unfortunately, current assembly methods for GAs such as sol-gel and freeze-casting processes must be conducted in enclosed spaces with unconventional conditions, thus being literally inoperative for in situ and continuous productions. Herein, a direct slurry-casting method at open ambient conditions is established to arbitrarily prepare three-dimensional (3D) porous graphene oxide (GO) bulks without macroscopic dimension limits on a wide range of solid surfaces by retarding Ostwald ripening of 3D liquid GO foams when being dried in air. A subsequent fast thermal reduction (FTR) of GO foams leads to the formation of graphene aerogels (denoted as FTR-GAs) with hierarchical closed-cellular graphene structures. The FTR-GAs show outstanding high-temperature thermal insulation (70% decrease for 400 °C), as well as superelasticity (>1000 compression-recovery cycles at 50% strain), ultralow density (10-28 mg cm^-3), large specific surface area (BET, 206.8 m² g^-1), and high conductivity (ca. 100 S m^-1). This work provides a viable method to achieve in situ preparations of high-performance GAs as multifunctional structural materials in aircrafts, high-speed trains, or even buildings for the targets of energy efficiency, comfort, and safety.

Cross-scale additive direct-writing fabrication of micro/nano lens arrays by electrohydrodynamic jet printing

High-quality micro/nanolens arrays (M/NLAs) are becoming irreplaceable components of various compact and miniaturized optical systems and functional devices. There is urgent requirement for a low-cost, high-efficiency, and high-precision technique to manufacture high-quality M/NLAs to meet their diverse and personalized applications. In this paper, we report the one-step maskless fabrication of M/NLAs via electrohydrodynamic jet (E-jet) printing. In order to get the best morphological parameters of M/NLAs, we adopted the stable cone-jet printing mode with optimized parameters instead of the micro dripping mode. The optical parameters of M/NLAs were analyzed and optimized, and they were influenced by the E-jet printing parameters, the wettability of the substrate, and the viscosity of the UV-curable adhesive. Thus, diverse and customized M/NLAs were obtained. Herein, we realized the fabrication of nanolens with a minimum diameter of 120 nm, and NLAs with different parameters were printed on a silicon substrate, a cantilever of atomic force microscopy probe, and single-layer graphene.

Bioinspired programmable wettability arrays for droplets manipulation

The manipulation of liquid droplets demonstrates great importance in various areas from laboratory research to our daily life. Here, inspired by the unique microstructure of plant stomata, we present a surface with programmable wettability arrays for droplets manipulation. The substrate film of this surface is constructed by using a coaxial capillary microfluidics to emulsify and pack graphene oxide (GO) hybrid N-isopropylacrylamide (NIPAM) hydrogel solution into silica nanoparticles-dispersed ethoxylated trimethylolpropane triacrylate (ETPTA) phase. Because of the distribution of the silica nanoparticles on the ETPTA interface, the outer surface of the film could achieve favorable hydrophobic property under selective fluorosilane decoration. Owing to the outstanding photothermal energy transformation property of the GO, the encapsulated hydrophilic hydrogel arrays could shrink back into the holes to expose their hydrophobic surface with near-infrared (NIR) irradiation; this imparts the composite film with remotely switchable surface droplet adhesion status. Based on this phenomenon, we have demonstrated controllable droplet sliding on programmable wettability pathways, together with effective droplet transfer for printing with mask integration, which remains difficult to realize by existing techniques.

Bioinspired Patterned Bubbles for Broad and Low-Frequency Acoustic Blocking

Bubble crystals in water are expected to achieve the broad and low-frequency acoustic band gaps that are crucial for acoustic blocking. However, preparing patterned bubble crystals in water remains a challenge because of the instability of bubbly liquids. Here, inspired by biological superhydrophobic systems, we report a simple and rapid approach to prepare patterned bubble arrays in water and their applications in low-frequency acoustic blocking. Patterned bubbles with the desired size, shape, and position can be prepared. Single-layer bubble arrays can block the sounds at low frequencies because of local resonance. By varying the size and distance of the bubbles without changing the thickness, the operating frequency can change from 9 to 1756 kHz. Besides, by preparing multilayer bubbles, broad and low-frequency acoustic band gaps can be achieved, with the generalized width of γ (ratio of the bandgap width to its start frequency) reaching 1.26. This method provides a feasible strategy to control acoustic waves at low frequencies for applications such as acoustic blocking, focusing, imaging, and detecting.

Fully Printed Geranium-Inspired Encapsulated Arrays for Quantitative Odor Releasing

Olfactory is an extremely fine way of perception. However, the process of smelling is prone to various interference factors. Further development to enhance the communication desires an odor-releasing strategy, which could quantitatively offer a variety of fragrances. Here, we report a fully printing strategy to heterogeneously integrate odor-containing materials and protective coating films. Inspired from the fragrance-containing drum structure on the geranium leaf, encapsulated arrays are fully printed on the flexible or rigid substrates with more than 20 spices. Quantitative concentrations of odor molecules can be released from the encapsulated arrays after scraping the protective poly(lactic-co-glycolic) acid (PLGA) shells. Importantly, various odor-based arrays are printed on the same flexible substrate, which permits selective releasing and arbitrary mixing of the spices. Effective odor-releasing properties of encapsulated arrays make them promising for food security and anticounterfeiting, investigating olfactory discrimination abilities, and strengthening olfactory communication.

How Cellulose Nanofibrils Affect Bulk, Surface, and Foam Properties of Anionic Surfactant Solutions

We study the generation and decay of aqueous foams stabilized by sodium dodecyl sulfate (SDS) in the presence of unmodified cellulose nanofibrils (CNF). Together with the rheology of aqueous suspensions containing CNF and SDS, the interfacial/colloidal interactions are determined by quartz crystal microgravimetry with dissipation monitoring, surface plasmon resonance, and isothermal titration calorimetry. The results are used to explain the properties of the air/water interface (interfacial activity and dilatational moduli determined from oscillating air bubbles) and of the bulk (steady-state flow, oscillatory shear, and capillary thinning). These properties are finally correlated to the foamability and to the foam stability. The latter was studied as a function of time by monitoring the foam volume, the liquid fraction, and the bubble size distribution. The shear-thinning effect of CNF is found to facilitate foam formation at SDS concentrations above the critical micelle concentration (c_SDS ≥ cmc). Compared with foams stabilized by pure SDS, the presence of CNF enhances the viscosity and elasticity of the continuous phase as well as of the air/water interface. The CNF-containing foams have higher liquid fractions, larger initial bubble sizes, and better stability. Due to charge screening effects caused by sodium counter ions and depletion attraction caused by SDS micelles, especially at high SDS concentrations, CNF forms aggregates in the Plateau borders and nodes of the foam, thus slowing down liquid drainage and bubble growth and improving foam stability. Overall, our findings advance the understanding of the role of CNF in foam generation and stabilization.

Anomalous Ostwald Ripening Enables 2D Polymer Crystals via Fast Evaporation

We demonstrate by molecular simulations that the Ostwald ripening of crystalline polymer nuclei within the fast-evaporation-induced 2D skin layer is retarded at suitable temperatures and evaporation rates. Such an anomalous ripening can be attributed to the interplay between the thermodynamically driven diffusion of noncrystalline fragments toward the growing nuclei and the diffusive current away from the free surface caused by the densification in the nonequilibrium skin layer. The growth orientation of the nuclei inside the skin plane can be adjusted during this anomalous ripening process, which is beneficial for fabricating 2D polymer crystals.

Bubbles nucleating on superhydrophobic micropillar arrays under flow

When a supersaturated aqueous solution flows over a microstructured, hydrophobic surface, bubbles tend to nucleate. Here, we control heterogeneous nucleation of gas bubbles from supersaturated CO₂ solution. By designing the shape, size, and arrangement of hydrophobic micropillars and by adjusting the flow we obtain uniform nucleation patterns. It is possible to selectively turn nucleation on and off. We use laser scanning confocal microscopy to resolve nucleation in early stages at the micropillar-substrate intersection. Numerical simulations show a correlation between minute pressure drops behind micropillars and nucleation sites. Bubbles nucleate uniformly behind pillars of the same size. The flow profile further contributes to the uniform growth of the bubbles. We control heterogeneous nucleation by varying micropillar geometry or size, flow direction and rate. While nucleation behind square pillars is independent of the flow direction, nucleation behind round micropillars is coupled with the direction. Nucleation behind triangular micropillars is bifurcated. These observations pave the way for the replenishment of the gas layer entrapped in between hydrophobic surface features, needed for superhydrophobicity.

Encoded Microneedle Arrays for Detection of Skin Interstitial Fluid Biomarkers

Skin interstitial fluid (ISF) is considered as an emerging source of biomarkers with physiological and medical significance. Microneedle arrays (MNs) provide a promising means for painless, noninvasive detection of these biomarkers. Here, novel MNs integrated with photonic crystal (PhC) barcodes are presented, and multiplex specific detection of ISF biomarkers is realized for the first time. The PhC barcodes-loaded flexible MNs are simply fabricated by replicating dynamic ferrofluid-cast micromoldings. When the prepared MNs are inserted into skin, they can enrich specific biomarkers to their probes-decorated PhC barcodes. Thus, by adding corresponding fluorescent probes to form sandwich immunocomplexes, the relative content of the biomarkers can be read out through the fluorescence intensity of the barcodes; meanwhile, the species of these biomarkers can be clearly distinguished by the reflection peaks of the PhC barcodes. Based on the encoded MNs, their sensitivity, flexibility, and versatility of capturing and detecting three inflammatory cytokines are demonstrated in a sepsis mice model. Compared with existing MNs for ISF detection, the encoded MNs not only possess equivalent detection effects with less post-processing and simplified procedures, but can also detect multiple biomarkers simultaneously, which makes them ideal in many clinical and biomedical detection areas.

Controlled open-cell two-dimensional liquid foam generation for micro- and nanoscale patterning of materials

Liquid foam consists of liquid film networks. The films can be thinned to the nanoscale via evaporation and have potential in bottom-up material structuring applications. However, their use has been limited due to their dynamic fluidity, complex topological changes, and physical characteristics of the closed system. Here, we present a simple and versatile microfluidic approach for controlling two-dimensional liquid foam, designing not only evaporative microholes for directed drainage to generate desired film networks without topological changes for the first time, but also microposts to pin the generated films at set positions. Patterning materials in liquid is achievable using the thin films as nanoscale molds, which has additional potential through repeatable patterning on a substrate and combination with a lithographic technique. By enabling direct-writable multi-integrated patterning of various heterogeneous materials in two-dimensional or three-dimensional networked nanostructures, this technique provides novel means of nanofabrication superior to both lithographic and bottom-up state-of-the-art techniques.

Surface Activity and Foaming Capacity of Aggregates Formed between an Anionic Surfactant and Non-Cellulosics Leached from Wood Fibers

This study relates to the release of non-cellulosic components (cell wall heteropolysaccharides, lignin, and extractives) from swollen wood fibers in the presence of an anionic surfactant (sodium dodecyl sulfate, SDS) at submicellar concentrations. Highly surface-active aggregates form between SDS and the leached, non-cellulosic components, which otherwise do not occur in the presence of cationic or nonionic surfactants. The in situ and efficient generation of liquid foams in the presence of the leached species is demonstrated. The foaming capacity and foam stability, as well as the foam's structure, are determined as a function of the composition of the aqueous suspension. The results indicate that naturally occurring components bound to wood fibers are extractable solely with aqueous solutions of the anionic surfactant. Moreover, they can form surface-active aggregates that have a high foaming capacity. The results further our understanding of residual cell wall components and their role in the generation of foams.

Domino Patterning of Water and Oil Induced by Emulsion Breaking

Patterning of water or oil is of great significance in microfluidics and printing; however, it is still a great challenge to achieve the synchronous patterning of water and oil. Here, we report a distinct Domino patterning of water and oil, simultaneously originating from O/W emulsions. The O/W emulsions first form various emulsion patterns on the textured substrates due to local capillary differences. With the evaporation, the emulsion breaking in emulsion patterns propagates from the boundary toward the center along the radial direction, followed by the formation of water droplet arrays like a Domino. Combining patterned substrates with the Domino patterning, a variety of polymer patterns, including circle-dot arrays, quadrangle-dot arrays, and triangle-dot arrays on tens of micrometers, are successfully produced on the substrates. The Domino patterning may provide a new view for phase separation patterning, liquid directional transport, and block polymer patterns.

Micro-/Nanofluidics for Liquid-Mediated Patterning of Hybrid-Scale Material Structures

Various materials are fabricated to form specific structures/patterns at the micro-/nanoscale, which exhibit additional functions and performance. Recent liquid-mediated fabrication methods utilizing bottom-up approaches benefit from micro-/nanofluidic technologies that provide a high controllability for manipulating fluids containing various solutes, suspensions, and building blocks at the microscale and/or nanoscale. Here, the state-of-the-art micro-/nanofluidic approaches are discussed, which facilitate the liquid-mediated patterning of various hybrid-scale material structures, thereby showing many additional advantages in cost, labor, resolution, and throughput. Such systems are categorized here according to three representative forms defined by the degree of the free-fluid-fluid interface: free, semiconfined, and fully confined forms. The micro-/nanofluidic methods for each form are discussed, followed by recent examples of their applications. To close, the remaining issues and potential applications are summarized.

Droplet Microarray on Patterned Butterfly Wing Surfaces for Cell Spheroid Culture

Three-dimensional (3D) cell spheroids have a demonstrated value for in vitro biological research and therapeutics development. Attempts to this technique focus on the development of effective methods for fabricating cell spheroids. Here, inspired by the heterogeneously textured wettability bumps (with hydrophilic peaks and hydrophobic bases) of Stenocara beetle, we present a biotemplated substrate with wettable hydrogel arrays for culturing the cell spheroids. The biotemplates were Morpho butterfly wings with chitin and protein components, which could provide a natural superhydrophobic surface without any modification. The droplet microarrays could be formed for cell spheroid culture on this bioinspired wing substrate by using the hydrogel patterns to hanging droplets. The hanging drop culture method on hydrogel-covered wings has the advantages of high speed, uniform size, and controllable diameter for the formation of 3D cell spheroids. It was demonstrated that drugs produced distinct responses in the 3D cell spheroids compared to conventional two-dimensional cell cultures. As the presented system does not require complex instruments and chemical modifications, our method can simply construct the desired wettability substrates with high biocompatibility for cell culture, drug testing, and other biomedical applications.

Heterogeneous Integration of Three-Primary-Color Photoluminescent Nanoparticle Arrays with Defined Interfaces

Minimized photoluminescent devices require both high-density fluorescent arrays and minimal cross-talk between neighboring pixels on the limited area. However, the challenges to achieve the overall integration of nanomaterial-based devices hinder the development of microscale full-color displays, including micro/nanoarray density, orientation control, multimaterial interface morphology, and uniform colors. Here, we report a heterogeneous integration approach to control the orientation, combination, and density of fluorescent micro/nanoarrays on flexible substrates. By controlling the defined interface and critical shrinkage width of liquid bridges, the width of three-primary-color micro/nanolines reached 100 nm. The interval between two parallel luminous lines is down to 40 μm, and the optical cross-talk effect is remarkably reduced. This work provides a facile approach to prepare high-performance micro-photoluminescent and imaging arrays for next-generation flexible display and lighting technology.

Bioinspired transfer method for the patterning of multiple nanomaterials

Patterned nanomaterials have promising applications in various fields, particularly for microfluidic analysis and functional surfaces.