Topography Optimization for Sustainable Dropwise Condensation: The Critical Role of Correlation Length

An effective pathway to enhance the heat transfer process is to induce the formation of highly mobile condensate droplets, employing micro-nanoengineered superhydrophobic surfaces. However, the design of the topography of these surfaces for sustained high performance constitutes a significant scientific and technological challenge. Herein, the critical role of the correlation length of topography is demonstrated as an important factor when designing superhydrophobic surfaces for heat transfer applications. Specifically, it is shown that a) a high correlation length value corresponds to increased space between surface structures and higher lateral distances between nucleating droplets, which results in lower droplet departure diameter and significantly delayed flooding of the surface and b) correlation length has to surpass a critical value for dropwise condensation (DWC) to be sustained in hierarchical structured surfaces, when the droplets are growing in a partial Cassie state. Following this rationale, droplets are categorized in three different energy and wetting states (Wenzel droplets, Cassie droplets of low kinetic energy and high energy jumping droplets), depending on the correlation length of the topography. Heat transfer experiments demonstrate an increase of 126% in the heat transfer coefficient (HTC) of surfaces exhibiting the maximum correlation length when compared to the flat hydrophobic surface.     

Thin Film Condensation on Nanostructured Surfaces

Water vapor condensation is a ubiquitous process in nature and industry. Over the past century, methods achieving dropwise condensation using a thin (<1 µm) hydrophobic “promoter” layer have been developed, which increases the condensation heat transfer by ten times compared to filmwise condensation. Unfortunately, implementations of dropwise condensation have been limited due to poor durability of the promoter coatings. Here, thin‐film condensation which utilizes a promoter layer not as a condensation surface, but rather to confine the condensate within a porous biphilic nanostructure, nickel inverse opals (NIO) with a thin (<20 nm) hydrophobic top layer of decomposed polyimide is developed. Filmwise condensation confined to thicknesses <10 µm is demonstrated. To test the stability of thin‐film condensation, condensation experiments are performed to show that at higher supersaturations droplets coalescing on top of the hydrophobic layer are absorbed into the superhydrophilic layer through coalescence‐induced transitions. Through detailed thermal‐hydrodynamic modeling, it is shown that thin‐film condensation has the potential to achieve heat transfer coefficients approaching ≈100 kW m^?2 while avoiding durability issues by significantly reducing nucleation on the hydrophobic surface. The work presented here develops an approach to potentially ensure durable and high‐performance condensation comparable to dropwise condensation.     

Tunable Anisotropic Wettability of Rice Leaf‐Like Wavy Surfaces

Rice leaves can directionally shed water droplets along the longitudinal direction of the leaf. Inspired by the hierarchical structures of rice leaf surfaces, synthetic rice leaf‐like wavy surfaces are fabricated that display a tunable anisotropic wettability by using electrostatic layer‐by‐layer assembly on anisotropic microwrinkled substrates. The nanoscale roughness of the rice leaf‐like surfaces is controlled to yield tunable anisotropic wettability and hydrophobic properties that transitioned between the anisotropic/pinned, anisotropic/rollable, and isotropic/rollable water droplet behavior states. These remarkable changes result from discontinuities in the three‐phase (solid–liquid–gas) contact line due to the presence of air trapped beneath the liquid, which is controlled by the surface roughness of the hierarchical nanostructures. The mechanism underlying the directional water‐rolling properties of the rice leaf‐like surfaces provides insight into the development of a range of innovative applications that require control over directional flow.     

A Multiscale Strategy for Constructing Finned Microporous Superhydrophobic Surface for Highly-Efficient Condensation Heat Transfer Enhancement Enabled by Lifespan Regulation of Droplets

Spontaneous droplet jumping on micro-/nano-structured superhydrophobic surfaces has been exploited as an efficient means for enhancing steam condensation heat transfer. However, the good performance of such surfaces quickly decays with raising the degree of subcooling, due to the mismatch between the characteristic length scales and droplet sizes when they grow up. Herein, a novel strategy for multiscale droplet regulation is proposed by combining sub-millimeter fin structure with a hierarchical microporous superhydrophobic surface. A superior condensation heat transfer performance is attained on such hierarchical superhydrophobic finned tube (F-SHB), in comparison to the baseline case of superhydrophobic non-finned (SHB) tube under well-controlled test conditions. Although the droplet jumping is not as vigorous as that on the SHB tube, the finned geometry of the F-SHB tube leads to a condensation heat transfer enhancement even under high degrees of subcooling up to 36 K, because of the accelerated departure of large droplets by imposing Laplace force gradient in the presence of V-shaped sub-millimeter fins. This multiscale enhancement strategy is shown to enable a cascading regulation over the entire lifespan of condensate droplets. The fabrication of F-SHB tubes is facile and easy to be scaled up, showing great potential in practical steam condensation applications.     

A Rubberlike Stretchable Fibrous Membrane with Anti‐Wettability and Gas Breathability

The fabrication of a stable, anti‐wetting surface is a very challenging issue in surface chemistry. In general, superhydrophobicity highly depends on the surface structure. Moreover, mechanical deformation of the surface structure can produce dramatic changes in the surface wetting state, and in some cases, may even result in a complete loss of the surface's unique wettability. However, the study of stable surfaces under mechanical deformation conditions has been limited to flexible surfaces or small strain. Here, a mechanically stable superhydrophobic membrane is presented, which possesses high stretchability and gas breathability. The membrane, which consists of an elastic polyurethane fibrous matrix coated with polyaniline hairy nanostructures and polytetrafluoroethylene, exhibites excellent superhydrophobic properties under ≥300% strain. The breathability and wettability of the membrane is examined by examining various static and dynamic wetting parameters. The robust membrane maintaines its anti‐wettability (water contact angle ≈160°, hysteresis ≈10°) for 1000 stretching cycles. It is also determined that the stretchable and superhydrophobic surface suppresses the fragmentation and rebound of impact droplets, compared with rigid superhydrophobic surfaces. Finally, underwater gas sensing is demonstrated as a novel application.     

Branched and Dendritic Polymer Architectures: Functional Nanomaterials for Therapeutic Delivery

Barriers to therapeutic transport in biological systems can prevent accumulation of drugs at the intended site, thus limiting the therapeutic effect against various diseases. Advances in synthetic chemistry techniques have recently increased the accessibility of complex polymer architectures for drug delivery systems, including branched polymer architectures. This article first outlines drug delivery concepts, and then defines and illustrates all forms of branched polymers including highly branched polymers, hyperbranched polymers, dendrimers, and branched–linear hybrid polymers. Many new types of branched and dendritic polymers continue to be reported; however, there is often confusion about how to accurately describe these complex polymer architectures, particularly in the interdisciplinary field of nanomedicine where not all researchers have in‐depth polymer chemistry backgrounds. In this context, the present review describes and compares different branched polymer architectures and their application in therapeutic delivery in a simple and easy‐to‐understand way, with the aim of appealing to a multidisciplinary audience.     

Hierarchical Superhydrophobic Surfaces Fabricated by Dual‐Scale Electron‐Beam‐Lithography with Well‐Ordered Secondary Nanostructures

Recent studies on superhydrophobic surfaces have revealed the important roles of structural hierarchy in the overall properties of these surfaces. Here, a novel, versatile, and efficient technique is introduced for fabricating macroscopic hierarchical superhydrophobic surfaces with both well‐defined primary microstructures and well‐ordered secondary nanostructures using electron‐beam lithography. With this technique, the engineering capability of controlling the size, shape, and distribution of the secondary‐features is demonstrated, which allows a systematic and quantitative study of the individual effects of these parameters. Superhydrophobic surfaces produced by this new technique exhibit two distinctive wetting behaviors, high and low adhesion. The structural characteristics and structure‐property relations of each of those two regimes are discussed.     

无线异构网络的垂直切换判决算法

传统上的水平切换处理方法已经应用在同一接入技术的网络中（如无线蜂窝网络）。随着多种异构无线网络的出现,下一代网络必须支持垂直切换技术以保证用户从一个网络切换到内一个网络时仍然保持连接,垂直切换作为多网融合的基础,受到了学术界和工业界的广泛关注。对使用层次分析法进行垂直切换技术的工作原理及应用环境进行了分析,提出同时使用多种切换技术以解决不同环境下的切换问题。     

InP基连续面型方形微透镜设计与制作技术研究

建立了非球面连续面型方形口径微透镜性能仿真模型,利用ZEMAX软件对10°视场、50μm周期、填充因子接近100%的磷化铟微透镜面型参数进行了优化设计。开发了衍射调制动态曝光与感应耦合等离子体刻蚀系统误差补偿方法制作出的磷化铟微透镜,其表面粗糙度小于2 nm,面型数据与设计值相比误差小于1/4工作波长,达到完善成像水平。     

Bioinspired Assembly of Hierarchical Light‐Harvesting Architectures for Improved Photophosphorylation

Molecular assembly offers a bottom‐up way to construct biomimetic architectures with unique structures and properties. Although artificial photophosphorylation systems have long been developed, their microstructures have yet to achieve the sophisticated order and hierarchy of natural organisms. Herein, by utilizing principles in the natural plant leaves, it is shown that a biomimetic system with hierarchically ordered and compartmentalized structures, combining photosystem II (PSII) and adenosine triphosphate (ATP) synthase, can be obtained through template‐directed layer‐by‐layer assembly. Under light illumination, PSII in such a highly ordered light‐harvesting array, splits water to produce protons and electrons. Furthermore, a remarkable proton gradient is created across the covering ATP synthase‐reconstituted lipid membrane. As a consequence, highly efficient photophosphorylation is achieved. Outstandingly, the rate of ATP production in this hierarchical light‐harvesting architecture is enhanced 14 times, compared to that in the nature. This study paves a new way to assemble bioinspired systems with enhanced solar‐to‐chemical energy conversion efficiency.     

A Versatile Approach towards Multifunctional Robust Microcapsules with Tunable,Restorable, and Solvent‐Proof Superhydrophobicity for Self‐Healing and Self‐Cleaning Coatings

Numerous microencapsulation techniques have been developed to encase various chemicals, for which specific processing parameters are required to address the widely differing features of the encapsulated materials. Microencapsulation of reactive agents is a powerful technique that has been extensively applied to self‐healing materials. However, the poor solvent compatibility and insufficient thermal stability of microcapsules continue to pose challenges for long‐term storage, processing, and service in practical applications. Here, an easily modifiable and highly versatile method is reported for preparing various chemicals filled poly(urea‐formaldehyde) microcapsules that exhibit superior tightness against solvents and heat and that possess widely tunable, repetitiously self‐restorable, and solvent‐proof superhydrophobicity. In addition, the low‐cost fabrication of biomimetic multifunctional smart coatings is demonstrated for self‐healing anticorrosion and self‐cleaning antifouling applications by directly dispersing the superhydrophobic microcapsules into and onto a polymer matrix. The methodology presented in this study should inspire the development of multifunctional intelligent materials for applications in related fields.     

Superhydrophobic “Pump”: Continuous and Spontaneous Antigravity Water Delivery

Antigravity transportation of water, which is often observed in nature, is becoming a vital demand for advanced devices and new technology. Many studies have been devoted to the motion of a single droplet on a horizontal or inclined substrate under specific assistance. However, the self‐propelled water motion, especially continuous antigravity water delivery, still remains a considerable challenge. Here, a novel self‐ascending phenomenon driven only by the surface energy release of water droplets is found, and a superhydrophobic mesh to pump water up to a height of centimeter scale is designed. An integrated antigravity transportation system is also demonstrated to continuously and spontaneously pump water droplets without additional driving forces. The present novel finding and integrated devices should serve as a source of inspiration for the design of advanced materials and for the development of new technology with exciting applications in microfluidics, microdetectors, and intelligent systems.     

Paper‐Based Surfaces with Extreme Wettabilities for Novel,Open‐Channel Microfluidic Devices

In this work, a facile methodology is discussed, involving fluoro‐silanization followed by oxygen plasma etching, for the fabrication of surfaces with extreme wettabilities, i.e., surfaces that display all four possible combinations of wettabilities with water and different oils: hydrophobic–oleophilic, hydrophilic–oleophobic, omniphobic, and omniphilic. Open‐channel, paper‐based microfluidic devices fabricated using these surfaces with extreme wettabilities allow for the localization, manipulation, and transport of virtually all high‐ and low‐surface tension liquids. This in turn expands the utility of paper‐based microfluidic devices to a range of applications never before considered. These include, as demonstrated here, continuous oil–water separation, liquid–liquid extraction, open‐channel microfluidic emulsification, microparticle fabrication, and precise measurement of mixtures' composition. Finally, the biocompatibility of the developed microfluidic devices and their utility for cell patterning are demonstrated.     

分层异构网络无线资源管理技术探讨

随着通信技术的不断发展,用户随时随地获得高质量无线业务体验的需求造成网络的分层异构覆盖特征越来越明显.对分层异构网络中的无线资源管理关键技术进行了探讨,利用无线资源管理技术可以极大地提高分层异构无线网络环境下的传输速率并节约能量.     

各向异性腐蚀表面粗糙度的"应力模型"

微电子机械系统(MEMS)关键的体加工工艺——各向异性腐蚀的样品表面会出现一定程度的不平面貌，针对这一现象提出了“应力模型”，用来解释腐蚀后表面粗糙度出现的物理机制，对表面激活能、腐蚀液激活能等物理概念进行了分析，提出了腐蚀表面粗糙度的计算公式，解释了各向异性腐蚀与表面形貌相关的实验现象。     

On Performance of Node Placement Approaches for Hierarchical Heterogeneous Sensor Networks

This paper considers a two-tier hierarchical heterogeneous wireless sensor network using the concept of clustering. The network has two type of nodes: regular sensor nodes (litenodes or LN) with limited communications, storage, energy, and computation power; and high-end sophisticated nodes (SNs), or clusterheads, with significantly additional resources. The litenodes communicate their data to the SNs and the SNs forward all collected data to a central gateway node called the base station (BS). Our network architecture allows the LNs to reach a SN via multiple hops through other LNs. We investigate the problem of optimally placing a minimum number of sophisticated nodes to handle the traffic generated by the lite nodes, while ensuring that the SNs form a connected network using their wireless links. This placement problem is formulated and solved as multi-constraint optimization problem using well known approaches: Binary Integer Linear Programming (BILP) approach, Greedy approach (GREEDY) and Genetic Algorithm (GA) approach. It was found through simulations that BILP performed best for regular grid topologies, while GA performed better for random LN deployment. Furthermore, the effects of various parameters on the solution are also presented. The paper also proposes a HYBRID approach that uses the solutions provided by GREEDY and/or BILP as the initial solution to the GA. Using HYBRID, results comparable to original GA could be obtained in only 11.46% of the time required for the original GA. Part of the research was supported by a grant from Air Force Office of Scientific Research (AFOSR) grant No. FA9550-06-1-0103.