Soap Film Transfer Printing for Ultrathin Electronics.

Flexible and stretchable electronics have attractive applications inaccessible to conventional rigid electronics. However, the mainstream transfer printing techniques have challenges for electronic films in terms of thickness and size and limitations for target substrates in terms of curvature, depth, and interfacial adhesion. Here a facile, damage-free, and contamination-free soap film transfer printing technique is reported that enables the wrinkle-free transfer of ultrathin electronic films, precise alignment in a transparent manner, and conformal and adhesion-independent printing onto various substrates, including those too topographically and adhesively challenging by existing methods. In principle, not only the pattern, resolution, and thickness of transferred films, but also the curvature, depth, and adhesion of target substrates are unlimited, while the size of transferred films can be as high as meter-scale. To demonstrate the capabilities of soap film transfer printing, pre-fabricated ultrathin electronics with multiple patterns, single micron resolution, sub-micron thickness, and centimeter size are conformably integrated onto the ultrathin web, ultra-soft cotton, DVD-R disk with the minimum radius of curvature of 131 nm, interior cavity of Klein bottle and dandelion with ultralow adhesion. The printed ultrathin sensors show superior conformabilities and robust adhesion, leading to engineering opportunities including electrocardiogram (ECG) signal acquisition and temperature measurement in aqueous environments.

Enrichment Effects Induced by Non-uniform Wettability Surfaces in the Presence of Non-condensable Gas: A Molecular Dynamics Simulation.

For vapor condensation, the control of heterogeneous nucleation and spatial distribution of nuclei are crucial for regulating droplet dynamics and improving condensation efficiency. However, due to the complex characteristics of multicomponent, multiphase, and multiscale, the underlying mechanism of mixed vapor condensation remains unclear, especially at the nucleation stage. In this paper, we focus on the enrichment effects of non-uniform wettability surfaces by molecular dynamics simulation, which could intensify the droplet nucleation and growth processes in a water-air mixed system. The results clarify the inhibitory effect of non-condensable gas on droplet nucleation and prove that only 1% of non-condensable gas could reduce one half of the condensation performance from a molecular perspective. Furthermore, non-uniform surfaces are designed to promote the efficient enrichment of vapor molecules on nucleation sites, and the synergistic effect of hydrophilic and hydrophobic regions is proposed. In addition, the non-uniform wettability surfaces are characterized by varying the proportion and dispersion of hydrophilic regions. The results reveal that an optimal proportion of hydrophilic region (R = 5/6) could furnish the non-uniform surface with the best transfer performance. Moreover, the enhancement of condensation performance can also be achieved through the dispersed arrangement of hydrophilic regions. The results provide guidance for the optimized design of functionalized surfaces with enhanced mixed vapor condensation.

Microscopic Observation of Preferential Capillary Pumping in Hollow Nanowire Bundles.

Numerous studies have focused on designing micro/nanostructured surfaces to improve wicking capability for rapid liquid transport in many industrial applications. Although hierarchical surfaces have been demonstrated to enhance wicking capability, the underlying mechanism of liquid transport remains elusive. Here, we report the preferential capillary pumping on hollow hierarchical surfaces with internal nanostructures, which are different from the conventional solid hierarchical surfaces with external nanostructures. Specifically, capillary pumping preferentially occurs in the nanowire bundles instead of the interconnected V-groove on hollow hierarchical surfaces, observed by confocal laser scanning fluorescence microscopy. Theoretical analysis shows that capillary pumping capability is mainly dependent on the nanowire diameter and results in 15.5 times higher capillary climbing velocity in the nanowire bundles than that in the microscale V-groove. Driven by the Laplace pressure difference between nanowire bundles and V-grooves, the preferential capillary pumping is increased with the reduction of the nanowire diameter. Capillary pumping of the nanowire bundles provides a preferential path for rapid liquid flow, leading to 2 times higher wicking capability of the hollow hierarchical surface comparing with the conventional hierarchical surface. The unique mechanism of preferential capillary pumping revealed in this work paves the way for wicking enhancement and provides an insight into the design of wicking surfaces for high-performance capillary evaporation in a broad range of applications.

Optimal Operation of an Oscillatory Flow Crystallizer: Coupling Disturbance and Stability.

In the process of industrial crystallization, it is always difficult to balance the secondary nucleation rate and metastable zone width (MSZW). Herein, we report an experimental and numerical study for the cooling crystallization of paracetamol in an oscillatory flow crystallizer (OFC), finding the optimal operating conditions for balancing the secondary nucleation rate and MSZW. The results show that the MSZW decreases with the increase of oscillation Reynolds number (Re o). Compared to the traditional stirring system, the OFC has an MSZW three times larger than that of the stirring system under a similar power density of consumption. With the numerical simulation, the OFC can produce a stable space environment and instantaneous strong disturbance, which is conducive to the crystallization process. Above all, a high Re o is favorable to produce a sufficient nucleation rate, which may inevitably constrict the MSZW to a certain degree. Then, the optimization strategy of the operating parameter (Re o) in the OFC is proposed.

Hydrophilic Slippery Surface Promotes Efficient Defrosting.

Frost accretion occurs ubiquitously in various industrial applications and causes tremendous energy and economic loss, as manifested by the Texas power crisis that impacted millions of people over a vast area in 2021. To date, extensive efforts have been made on frost removal by micro-engineering surfaces with superhydrophobicity or lubricity. On such surfaces, air or oil cushions are introduced to suspend the frost layer and promote the rapid frost sliding off, which, although promising, faces the instability of the cushions under extreme frosting conditions. Most existing hydrophilic surfaces, characterized by large interfacial adhesion, have long been deemed unfavorable for frost shedding. Here, we demonstrated that a hydrophilic and slippery surface can achieve efficient defrosting. On such a surface, the hydrophilicity gave rise to a highly interconnected basal frost layer that boosted the substrate-to-frost heat transfer; then, the resulting melted frost readily slid off the surface due to the superb slipperiness. Notably, on our surface, the retained meltwater coverage after frost sliding off was only 2%. In comparison to two control surfaces, for example, surfaces lacking either hydrophilicity or slipperiness, the defrosting efficiency was 13 and 19 times higher and the energy consumption was 2.3 and 6.2 times lower, respectively. Our study highlights the use of a hydrophilic surface for the pronounced defrosting in a broad range of industrial applications.

Rapid and Persistent Suction Condensation on Hydrophilic Surfaces for High-Efficiency Water Collection.

Water collection by dew condensation emerges as a sustainable solution to water scarcity. However, the transient condensation process that involves droplet nucleation, growth, and transport imposes conflicting requirements on surface properties. It is challenging to satisfy all benefits for different condensation stages simultaneously. By mimicking the structures and functions of moss Rhacocarpus, here, we report the attainment of dropwise condensation for efficient water collection even on a hydrophilic surface gated by a liquid suction mechanism. The Rhacocarpus-inspired porous surface (RIPS), which possesses a three-level wettability gradient, facilitates a rapid, directional, and persistent droplet suction. Such suction condensation enables a low nucleation barrier, frequent surface refreshing, and well-defined maximum droplet shedding radius simultaneously. Thus, a maximum ∼160% enhancement in water collection performance compared to the hydrophobic surface is achieved. Our work provides new insights and a design route for developing engineered materials for a wide range of water-harvesting and phase-change heat-transfer applications.

Coupling droplets/bubbles with a liquid film for enhancing phase-change heat transfer.

Evaporation, boiling, and condensation are fundamental liquid-vapor phase-change heat transfer processes and have been utilized in many conventional and emerging energy systems. Recent advances in the manipulation of interface wetting and heterogeneous nucleation using micro/nano-structured surfaces have enabled exciting two-phase flow dynamics and heat transfer enhancement. However, independently manipulating droplets, bubbles, or liquid films through surface modification has encountered bottlenecks. In this Perspective, we discuss an emerging strategy where droplets/bubbles are coupled with a liquid film to control fluid dynamics for minimizing the thermal resistance between the liquid-vapor interface and solid substrate, thus significantly enhancing the heat transfer performance beyond the state of the art.

Laser-Induced Patterned Photonic Crystal Heterostructure for Multimetal Ion Recognition.

In this work, a new method of direct laser writing patterned photonic crystal heterostructure on a glass surface is proposed. A multi-heterostructure photonic crystal (MHPC) is predeposited on the glass surface and then the laser spot is focused on it and moves according to the set program, leading to the formation of patterned MHPC. Scanning electron microscopy (SEM) and finite element simulation show that the patterning is caused by the local thermal annealing of the polymer colloidal spheres through the thermal conduction effect of the substrate on the laser energy. The patterned area presents a function of the water confinement effect and can be used as a high-performance droplet analysis chip. By integrating the patterned MHPC array and seven fluorescent dyes, nine metal ions can be successfully recognized and discriminated. This approach is quite facile and fast for designing colloidal photonic crystals with controllable patterns. Moreover, it is of considerable significance for the practical application of photonic crystal heterostructure in the detection, sensing, anti-counterfeiting, and display fields.

Preferential Vapor Nucleation on Hierarchical Tapered Nanowire Bunches.

Controlling vapor nucleation on micro-/nanostructured surfaces is critical to achieving exciting droplet dynamics and condensation enhancement. However, the underlying mechanism of nucleation phenomena remains unclear because of its nature of nanoscale and transience, especially for the complex-structured surfaces. Manipulating vapor nucleation via the rational surface design of micro-/nanostructures is extremely challenging. Here, we fabricate hierarchical surfaces comprising tapered nanowire bunches and crisscross microgrooves. Nanosteps are formed around the top of the nanowire bunches, where the nanowires all around agglomerate densely because of surface tension. The theoretical analysis and molecular dynamics simulation show that nanostep morphologies that are around the top of the nanowire bunches can enable a lower energy barrier and a higher nucleation capability than those of the sparsely packed nanowires at the center and bottom of the nanowire bunches. Vapor condensation experiments demonstrate that the nucleation preferentially occurs around the top of the nanowire bunches. The results provide guidelines to design micro-/nanostructures for promoting vapor nucleation and droplet removal in condensation.

Hierarchical Superhydrophobic Surfaces with Micropatterned Nanowire Arrays for High-Efficiency Jumping Droplet Condensation.

Self-propelled droplet jumping on nanostructured superhydrophobic surfaces is of interest for a variety of industrial applications including self-cleaning, water harvesting, power generation, and thermal management systems. However, the uncontrolled nucleation-induced Wenzel state of condensed droplets at large surface subcooling (high heat flux) leads to the formation of unwanted large pinned droplets, which results in the flooding phenomenon and greatly degrades the heat transfer performance. In this work, we present a novel strategy to manipulate droplet behaviors during the process from the droplet nucleation to growth and departure through a combination of spatially controlling initial nucleation for mobile droplets by closely spaced nanowires and promoting the spontaneous outward movement of droplets for rapid removal using micropatterned nanowire arrays. Through the optical visualization experiments and heat transfer tests, we demonstrate greatly improved condensation heat transfer characteristics on the hierarchical superhydrophobic surface including the higher density of microdroplets, smaller droplet departure radius, 133% wider range of surface subcooling for droplet jumping, and 37% enhancement in critical heat flux for jumping droplet condensation, compared to the-state-of-art jumping droplet condensation on nanostructured superhydrophobic surfaces. The excellent water repellency of such hierarchical superhydrophobic surfaces can be promising for many potential applications, such as anti-icing, antifogging, water desalination, and phase-change heat transfer.

Wetting Transition of Condensed Droplets on Nanostructured Superhydrophobic Surfaces: Coordination of Surface Properties and Condensing Conditions.

Nanostructured superhydrophobic surfaces have been actively explored to promote favorable droplet dynamics for a wide range of technological applications. However, the tendency of condensed droplets to form as pinned states greatly limits their applicability in enhancing condensation heat transfer efficiency. Despite recent progresses, the understanding of physical mechanisms governing the wetting transition of condensed droplets is still lacking. In this work, a nanostructured superhydrophobic surface with tapered nanogaps is fabricated to demonstrate the coordination of surface wetting property, topography, and the condensing condition on the wetting state and dynamic behavior of condensed droplets. Combining the environmental scanning electron microscopy and optical visualization methods, we systematically show the morphology of nucleated droplets in nanostructures and the droplet dynamic evolution throughout the growth stages, which provides the direct evidence of condensing condition-induced droplet wetting transition. When the surface subcooling is smaller than 0.3 K, the droplets formed as the Cassie-Baxter state, followed by coalescence-induced droplet jumping. With the increase of surface subcooling up to 0.6 K, however, droplet formation occurs randomly inside nanogaps, resulting in the loss of superhydrophobicity. These new observations along with the new insights about the coordination of surface properties and condensing conditions on droplet wetting transition are useful for guiding the development of novel surfaces for improving droplet removal and phase-change heat transfer.

Atomic Layer Deposited Coatings on Nanowires for High Temperature Water Corrosion Protection.

Two-phase liquid-cooling technologies incorporating micro/nanostructured copper or silicon surfaces have been established as a promising thermal management solution to keep up with the increasing power demands of high power electronics. However, the reliability of nanometer-scale features of copper and silicon in these devices has not been well investigated. In this work, accelerated corrosion testing reveals that copper nanowires are not immune to corrosion in deaerated pure hot water. To solve this problem, we investigate atomic layer deposition (ALD) TiO2 coatings grown at 150 and 175 °C. We measured no difference in coating thickness for a duration of 12 days. Using a core/shell approach, we grow ALD TiO2/Al2O3 protective coatings on copper nanowires and demonstrate a preservation of nanoengineered copper features. These studies have identified a critical reliability problem of nanoscale copper and silicon surfaces in deaerated, pure, hot water and have successfully demonstrated a reliable solution using ALD TiO2/Al2O3 protective coatings.

Directional Movement of Droplets in Grooves: Suspended or Immersed?

The behavior of droplets trapped in geometric structures is essential to droplet manipulation applications such as for droplet transport. Here we show that directional droplet movement can be realized by a V-shaped groove with the movement direction controlled by adjusting the surface wettability of the groove inner wall and the cross sectional angle of the groove. Experiments and analyses show that a droplet in a superhydrophobic groove translates from the immersed state to the suspended state as the cross sectional angle of the groove decreases and the suspended droplet departs from the groove bottom as the droplet volume increases. We also demonstrate that this simple grooved structure can be used to separate a water-oil mixture and generate droplets with the desired sizes. The structural effect actuated droplet movements provide a controllable droplet transport method which can be used in a wide range of droplet manipulation applications.