Showing papers by "Beihang University published in 2010"

Directional water collection on wetted spider silk

Abstract: Many biological surfaces in both the plant and animal kingdom possess unusual structural features at the micro- and nanometre-scale that control their interaction with water and hence wettability. An intriguing example is provided by desert beetles, which use micrometre-sized patterns of hydrophobic and hydrophilic regions on their backs to capture water from humid air. As anyone who has admired spider webs adorned with dew drops will appreciate, spider silk is also capable of efficiently collecting water from air. Here we show that the water-collecting ability of the capture silk of the cribellate spider Uloborus walckenaerius is the result of a unique fibre structure that forms after wetting, with the 'wet-rebuilt' fibres characterized by periodic spindle-knots made of random nanofibrils and separated by joints made of aligned nanofibrils. These structural features result in a surface energy gradient between the spindle-knots and the joints and also in a difference in Laplace pressure, with both factors acting together to achieve continuous condensation and directional collection of water drops around spindle-knots. Submillimetre-sized liquid drops have been driven by surface energy gradients or a difference in Laplace pressure, but until now neither force on its own has been used to overcome the larger hysteresis effects that make the movement of micrometre-sized drops more difficult. By tapping into both driving forces, spider silk achieves this task. Inspired by this finding, we designed artificial fibres that mimic the structural features of silk and exhibit its directional water-collecting ability.

Local Derivative Pattern Versus Local Binary Pattern: Face Recognition With High-Order Local Pattern Descriptor

Abstract: This paper proposes a novel high-order local pattern descriptor, local derivative pattern (LDP), for face recognition. LDP is a general framework to encode directional pattern features based on local derivative variations. The nth-order LDP is proposed to encode the (n-1)th -order local derivative direction variations, which can capture more detailed information than the first-order local pattern used in local binary pattern (LBP). Different from LBP encoding the relationship between the central point and its neighbors, the LDP templates extract high-order local information by encoding various distinctive spatial relationships contained in a given local region. Both gray-level images and Gabor feature images are used to evaluate the comparative performances of LDP and LBP. Extensive experimental results on FERET, CAS-PEAL, CMU-PIE, Extended Yale B, and FRGC databases show that the high-order LDP consistently performs much better than LBP for both face identification and face verification under various conditions.

Recent developments in bio-inspired special wettability

Abstract: Nature is a school for scientists and engineers. After four and a half billion years of stringent evolution, some creatures in nature exhibit fascinating surface wettability. Biomimetics, mimicking nature for engineering solutions, provides a model for the development of functional surfaces with special wettability. Recently, bio-inspired special wetting surfaces have attracted wide scientific attention for both fundamental research and practical applications, which has become an increasingly hot research topic. This Critical Review summarizes the recent work in bio-inspired special wettability, with a focus on lotus leaf inspired self-cleaning surfaces, plants and insects inspired anisotropic superhydrophobic surfaces, mosquito eyes inspired superhydrophobic antifogging coatings, insects inspired superhydrophobic antireflection coatings, rose petals and gecko feet inspired high adhesive superhydrophobic surfaces, bio-inspired water collecting surfaces, and superlyophobic surfaces, with particular focus on the last two years. The research prospects and directions of this rapidly developing field are also briefly addressed (159 references).

Two-Dimensional Graphene Bridges Enhanced Photoinduced Charge Transport in Dye-Sensitized Solar Cells

Abstract: As a novel two-dimensional (2D) material, graphene shows great benefits in electric and material science. Compared to 1D nanomaterials, it may show more excellent properties. Here, we introduced graphene as 2D bridges into the nanocrystalline electrodes of dye-sensitized solar cells, which brought a faster electron transport and a lower recombination, together with a higher light scattering. On the basis of these advantages, the short-circuit current density was increased by 45% without sacrificing the open-circuit voltage, and the total conversion efficiency was 6.97%, which was increased by 39%, comparing with the nanocrystalline titanium dioxide photoanode, and it was also much better than the 1D nanomaterial composite electrode.

Understanding of Internal Clustering Validation Measures

Abstract: Clustering validation has long been recognized as one of the vital issues essential to the success of clustering applications. In general, clustering validation can be categorized into two classes, external clustering validation and internal clustering validation. In this paper, we focus on internal clustering validation and present a detailed study of 11 widely used internal clustering validation measures for crisp clustering. From five conventional aspects of clustering, we investigate their validation properties. Experiment results show that S\_Dbw is the only internal validation measure which performs well in all five aspects, while other measures have certain limitations in different application scenarios.

Reversible Image Watermarking Using Interpolation Technique

Abstract: Watermarking embeds information into a digital signal like audio, image, or video. Reversible image watermarking can restore the original image without any distortion after the hidden data is extracted. In this paper, we present a novel reversible watermarking scheme using an interpolation technique, which can embed a large amount of covert data into images with imperceptible modification. Different from previous watermarking schemes, we utilize the interpolation-error, the difference between interpolation value and corresponding pixel value, to embed bit ?1? or ?0? by expanding it additively or leaving it unchanged. Due to the slight modification of pixels, high image quality is preserved. Experimental results also demonstrate that the proposed scheme can provide greater payload capacity and higher image fidelity compared with other state-of-the-art schemes.

Bioinspired super-antiwetting interfaces with special liquid-solid adhesion.

Abstract: Super-antiwetting interfaces, such as superhydrophobic and superamphiphobic surfaces in air and superoleophobic interfaces in water, with special liquid-solid adhesion have recently attracted worldwide attention. Through tuning surface microstructures and compositions to achieve certain solid/liquid contact modes, we can effectively control the liquid-solid adhesion in a super-antiwetting state. In this Account, we review our recent progress in the design and fabrication of these bioinspired super-antiwetting interfaces with special liquid-solid adhesion. Low-adhesion superhydrophobic surfaces are biologically inspired, typically by the lotus leaf. Wettability investigated at micro- and nanoscale reveals that the low adhesion of the lotus surface originates from the composite contact mode, a microdroplet bridging several contacts, within the hierarchical structures. Recently high-adhesion superhydrophobic surfaces have also attracted research attention. These surfaces are inspired by the surfaces of gecko feet and rose petals. Accordingly, we propose two biomimetic approaches for the fabrication of high-adhesion superhydrophobic surfaces. First, to mimic a sticky gecko's foot, we designed structures with nanoscale pores that could trap air isolated from the atmosphere. In this case, the negative pressure induced by the volume change of sealed air as the droplet is pulled away from surface can produce a normal adhesive force. Second, we constructed microstructures with size and topography similar to that of a rose petal. The resulting materials hold air gaps in their nanoscale folds, controlling the superhydrophobicity in a Wenzel state on the microscale. Furthermore, we can tune the liquid-solid adhesion on the same superhydrophobic surface by dynamically controlling the orientations of microstructures without altering the surface composition. The superhydrophobic wings of the butterfly (Morpho aega) show directional adhesion: a droplet easily rolls off the surface of wings along one direction but is pinned tightly against rolling in the opposite direction. Through coordinating the stimuli-responsive materials and appropriate surface-geometry structures, we developed materials with reversible transitions between a low-adhesive rolling state and a high-adhesive pinning state for water droplets on the superhydrophobic surfaces, which were controlled by temperature and magnetic and electric fields. In addition to the experiments done in air, we also demonstrated bioinspired superoleophobic water/solid interfaces with special adhesion to underwater oil droplets and platelets. In these experiments, the high content of water trapped in the micro- and nanostructures played a key role in reducing the adhesion of the oil droplets and platelets. These findings will offer innovative insights into the design of novel antibioadhesion materials.

Trajectory tracking control design with command-filtered compensation for a quadrotor

Abstract: The design of a flight control system capable of not only stabilising attitude but also tracking a trajectory accurately for an under-actuated quadrotor aircraft is quite challenging. This study constructs the relationship between the attitude and linear acceleration of a small quadrotor unmanned aircraft and proposes a trajectory tracking control design algorithm, based on the relationship, using a new command-filtered backstepping technique to stabilise the attitude and a linear tracking differentiator to eliminate the classical inner/outer-loop structure. Finally, the validity and the effectiveness of this algorithm are demonstrated by various numerical simulations under different conditions.

New parametrization for the nuclear covariant energy density functional with a point-coupling interaction

Abstract: A new parametrization PC-PK1 for the nuclear covariant energy density functional with nonlinear point-coupling interaction is proposed by fitting to observables of 60 selected spherical nuclei, including the binding energies, charge radii, and empirical pairing gaps. The success of PC-PK1 is illustrated in the description of infinite nuclear matter and finite nuclei including the ground-state and low-lying excited states. In particular, PC-PK1 provides a good description for the isospin dependence of binding energy along either the isotopic or the isotonic chain, which makes it reliable for application in exotic nuclei. The predictive power of PC-PK1 is also illustrated for the nuclear low-lying excitation states in a five-dimensional collective Hamiltonian in which the parameters are determined by constrained calculations for triaxial shapes.

Consensus of a Class of Second-Order Multi-Agent Systems With Time-Delay and Jointly-Connected Topologies

Abstract: This technical note investigates consensus problems of a class of second-order continuous-time multi-agent systems with time-delay and jointly-connected topologies. We first introduce a neighbor-based linear protocol with time-delay. Then we derive a sufficient condition in terms of linear matrix inequalities (LMIs) for average consensus of the system. Furthermore, we discuss the case where the time-delay affects only the information that is being transmitted and show that consensus can be reached with arbitrary bounded time-delay. Finally, simulation results are provided to demonstrate the effectiveness of our theoretical results.

Equivalence of digital image correlation criteria for pattern matching.

Abstract: In digital image correlation (DIC), to obtain the displacements of each point of interest, a correlation criterion must be predefined to evaluate the similarity between the reference subset and the target subset. The correlation criterion is of fundamental importance in DIC, and various correlation criteria have been designed and used in literature. However, little research has been carried out to investigate their relations. In this paper, we first provide a comprehensive overview of various correlation criteria used in DIC. Then we focus on three robust and most widely used correlation criteria, i.e., a zero-mean normalized cross-correlation (ZNCC) criterion, a zero-mean normalized sum of squared difference (ZNSSD) criterion, and a parametric sum of squared difference (PSSD(ab)) criterion with two additional unknown parameters, since they are insensitive to the scale and offset changes of the target subset intensity and have been highly recommended for practical use in literature. The three correlation criteria are analyzed to establish their transversal relationships, and the theoretical analyses clearly indicate that the three correlation criteria are actually equivalent, which elegantly unifies these correlation criteria for pattern matching. Finally, the equivalence of these correlation criteria is further validated by numerical simulation and actual experiment.

Composite disturbance-observer-based control and H∞ control for complex continuous models

Abstract: A novel type of control scheme combining the disturbance-observer-based control (DOBC) with H∞ control is proposed for a class of complex continuous models with disturbances. The disturbances are supposed to include two parts. One part in the input channel is generated by an exogenous system with uncertainty, which can represent the harmonic signals with modeling perturbations. The other part is supposed to have the bounded H2-norm. Parametric uncertainties exist both in concerned plant and in exogenous subsystem. The disturbance observers based on regional pole placement and D-stability theory are designed and integrated with conventional H∞ control laws. The new composite DOBC and H∞ control scheme is applied to complex continuous models for the case with known and unknown nonlinearity, respectively. Then the first type of disturbances can be estimated and rejected, and the second type can be attenuated; simultaneously, the desired dynamic performances can be guaranteed. Simulations for a flight control system are given to demonstrate the effectiveness of the results and compare the proposed results with the previous schemes. Copyright © 2009 John Wiley & Sons, Ltd.

CdS Quantum Dots-Sensitized TiO2 Nanorod Array on Transparent Conductive Glass Photoelectrodes

Abstract: An oriented single-crystalline TiO2 nanorod or wire array on transparent conductive substrates would be the most desirable nanostructure in preparing photoelectrochemical solar cells because of its efficient charge separation and transport properties as well as superior light harvesting efficiency. In this study, a TiO2 nanorod array film grown directly on transparent conductive glass (FTO) was prepared by a simple hydrothermal method. The formation of CdS quantum dots (QDs) on the vertically aligned TiO2 nanorods photoelectrode was carried out by chemical bath deposition. The as-prepared materials were characterized by scanning electron microscopy, transmission electron microscopy (TEM), high-resolution TEM, and X-ray diffraction. The results indicate that CdS QDs with a diameter smaller than 10 nm are uniformly covered on the surface of the single-crystalline TiO2 nanorods. Under AM 1.5 G illumination, the photoelectrode was found with a photocurrent intensity of 5.778 mA/cm2 at a potential of 0 V versu...

Ceria Hollow Nanospheres Produced by a Template-Free Microwave-Assisted Hydrothermal Method for Heavy Metal Ion Removal and Catalysis

Abstract: Ceria hollow nanospheres composed of CeO2 nanocrystals were synthesized via a template-free and microwave-assisted aqueous hydrothermal method. This is a low-cost and environmentally benign method. The chemicals used are all environmentally benign materials (cerium nitrate, urea, and water). An Ostwald ripening mechanism coupled with a self-templated, self-assembly process, in which amorphous solid spheres are converted to crystalline nanocrystals and the latter self-assemble into hollow structures, was proposed for the formation of the hollow structures. The products were characterized by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, high-resolution TEM, energy-dispersive X-ray analysis, X-ray photoelectron spectroscopy, and N2 adsorption−desorption methods. These ceria hollow nanospheres show an excellent adsorption capacity for heavy metal ions, for example, 22.4 mg g−1 for As(V) and 15.4 mg g−1 for Cr(VI). These values are significantly higher than reported ...

A Comprehensive Review of the Development of Adaptive Cruise Control Systems

Abstract: It has been 15 years since the first generation of adaptive cruise control (ACC)-equipped vehicles was available on the market and 7 years since the ISO standard for the first generation of ACC systems was produced. Since the next generation of ACC systems and more advanced driver-assistant systems are at the verge of complete introduction and deployment, it is necessary to summarise the development and research achievements of the first generation of ACC systems in order to provide more useful experiential guidance for the new deployment. From multidimensional perspectives, this paper looks into the related development and research achievements to objectively and comprehensively introduce an ACC system to researchers, automakers, governments and consumers. It attempts to simply explain what an ACC system is and how it operates from a systematic perspective. Then, it clearly draws a broad historical picture of ACC development by splitting the entire history into three different phases. Finally, the most significant research findings-related ACC systems have been reviewed and summarised from the human, traffic and social perspectives respectively.

Direction Controlled Driving of Tiny Water Drops on Bioinspired Artificial Spider Silks

Abstract: www.MaterialsViews.com C O M M Direction Controlled Driving of Tiny Water Drops on Bioinspired Artifi cial Spider Silks U N IC A By Hao Bai , Xuelin Tian , Yongmei Zheng ,* Jie Ju , Yong Zhao , and Lei Jiang * IO N Directional driving of liquid drops is of signifi cant interest in many applications, such as microfl uidic devices, [ 1–6 ] fog harvesting, [ 7 ] fi ltration, [ 8 ] and condensers. [ 9 ] For this purpose, great progress has been made in driving drops larger than hundreds of micrometers [ 9–23 ] by introducing chemical, [ 1 , 10 , 12–15 , 23 ] thermal, [ 16–19 ] or shape [ 20–22 ] gradients on surfaces. However, driving micrometer-sized drops is much more diffi cult because they encounter a larger contact angle (CA) hysteresis effect. [ 7 , 24 ] In nature, the wetted silk of cribellate spider offers new insights into solving this problem by combining different gradients together. [ 7 ] Here, inspired by the spider silk, we fabricated a series of artifi cial spider silks with spindle-knots in which the chemical compositions and surface nanostructures were subtly designed. Our investigations demonstrated that tiny water drops (tens of picoliters) could be driven with controllable direction (“toward” or “away from” the knot) by optimizing the cooperation of curvature, chemical, and roughness gradients on the fi ber surfaces. The study will pave the way for designing smart materials and devices to drive tiny water drops in a controllable manner. When a nylon fi ber was immersed into polymer solution and drawn out horizontally, a string of polymer drops, which became spindle-knots after being dried, formed on the fi ber due to the Rayleigh instability [ 25 ] of the polymer solution. The surface energy of the spindle-knots was tailored by choosing different polymers including poly(vinyl acetate) (PVAc), poly(methyl methacrylate) (PMMA), polystyrene (PS), and poly(vinylidene fl uoride) (PVDF), which have intrinsic water contact angles of 56.7 ° , 68.4 ° , 93.3 ° , and 92.7 ° , respectively (see Supporting Information, Figure S1). On the other hand, the surface roughness (porous nanostructures) of the spindle-knots was also designed through phase separation

RegionKNN: A Scalable Hybrid Collaborative Filtering Algorithm for Personalized Web Service Recommendation

Abstract: Several approaches to web service selection and recommendation via collaborative filtering have been studied, but seldom have these studies considered the difference between web service recommendation and product recommendation used in e-commerce sites. In this paper, we present RegionKNN, a novel hybrid collaborative filtering algorithm that is designed for large scale web service recommendation. Different from other approaches, this method employs the characteristics of QoS by building an efficient region model. Based on this model, web service recommendations will be generated quickly by using modified memory-based collaborative filtering algorithm. Experimental results demonstrate that apart from being highly scalable, RegionKNN provides considerable improvement on the recommendation accuracy by comparing with other well-known collaborative filtering algorithms.

Nanowire-in-microtube structured core/shell fibers via multifluidic coaxial electrospinning

Abstract: A multifluidic coaxial electrospinning approach is reported here to fabricate core/shell ultrathin fibers with a novel nanowire-in-microtube structure from more optional fluid pairs than routine coaxial electrospinning. The advantage of this approach lies in the fact that it introduces an extra middle fluid between the core and shell fluids of traditional coaxial electrospinning, which can work as an effective spacer to decrease the interaction of the other two fluids. Under the protection of a proper middle fluid, more fluid pairs, even mutually miscible fluids, can be operated to generate "sandwich"-structured ultrathin fibers with a sharp boundary between the core and shell materials. It thereby largely extends the scope of optional materials. Selectively removing the middle layer of the as-prepared fibers results in an interesting nanowire-in-microtube structure. Either homogeneous or heterogeneous fibers with well-tailored sandwich structures have been successfully fabricated. This method is an important extension of traditional co-electrospinning that affords a more universal avenue to preparing core/shell fibers; moreover, the special hollow cavity structure may introduce some extra properties into the conventional core/shell structure, which may find potential applications such as optical applications, microelectronics, and others.

Convergence analysis of the Jacobi spectral-collocation methods for Volterra integral equations with a weakly singular kernel

Abstract: In this paper, a Jacobi-collocation spectral method is developed for Volterra integral equations of second kind with a weakly singular kernel. We use some function transformation and variable transformations to change the equation into a new Volterra integral equation deflned on the standard interval (i1;1), so that the solution of the new equation possesses better regularity and the Jacobi orthogonal polynomial theory can be applied conveniently. In order to obtain high order accuracy for the approx- imation, the integral term in the resulting equation is approximated by using Jacobi spectral quadrature rules. The convergence analysis of this novel method is based on the Lebesgue constants corresponding to the Lagrange interpolation polynomials, poly- nomials approximation theory for orthogonal polynomials and the operator theory. The spectral rate of convergence for the proposed method is established in the L 1 -norm and weighted L 2 -norm. Numerical results are presented to demonstrate the efiectiveness of the proposed method.

Fine-grained channel access in wireless LAN

Abstract: Modern communication technologies are steadily advancing the physical layer (PHY) data rate in wireless LANs, from hundreds of Mbps in current 802.11n to over Gbps in the near future. As PHY data rates increase, however, the overhead of media access control (MAC) progressively degrades data throughput efficiency. This trend reflects a fundamental aspect of the current MAC protocol, which allocates the channel as a single resource at a time.This paper argues that, in a high data rate WLAN, the channel should be divided into separate subchannels whose width is commensurate with PHY data rate and typical frame size. Multiple stations can then contend for and use subchannels simultaneously according to their traffic demands, thereby increasing overall efficiency. We introduce FICA, a fine-grained channel access method that embodies this approach to media access using two novel techniques. First, it proposes a new PHY architecture based on OFDM that retains orthogonality among subchannels while relying solely on the coordination mechanisms in existing WLAN, carrier-sensing and broadcasting. Second, FICA employs a frequency-domain contention method that uses physical layer RTS/CTS signaling and frequency domain backoff to efficiently coordinate subchannel access. We have implemented FICA, both MAC and PHY layers, using a software radio platform, and our experiments demonstrate the feasibility of the FICA design. Further, our simulation results suggest FICA can improve the efficiency ratio of WLANs by up to 400% compared to existing 802.11.

Switchable adhesion on liquid/solid interfaces

Abstract: Construction and application of surfaces with switchable liquid-solid adhesion have generated worldwide interest during the past a few years. These surfaces are of great importance not only for fundamental research but for various practical applications in smart and fluid-controllable devices. This Feature Article reviews several techniques that have been developed to switch the adhesion on liquid/solid interfaces, including tuning the surface chemical composition, tailoring the surface morphology, and applying external stimuli. Particular attention is paid to superhydrophobic surfaces with reversible switching between low- and high-adhesion to water droplets in response to external stimuli. The dynamic behavior of water droplets on such surfaces can be controlled ranging from rolling to pinning state, while maintaining superhydrophobic states. In addition, smart adhesion in oil/water/solid system and platelet/water/solid system are also discussed, which is of importants for application in designing novel anti-bioadhesion materials.

Investigating behaviours of hydrogen in a tungsten grain boundary by first principles: from dissolution and diffusion to a trapping mechanism

Abstract: We have investigated the dissolution, segregation and diffusion of hydrogen (H) in a tungsten (W) grain boundary (GB) using a first-principles method in order to understand the GB trapping mechanism of H. Optimal charge density plays an essential role in such a GB trapping mechanism. Dissolution and segregation of H are directly associated with the optimal charge density, which can be reflected by the H solution and segregation energy sequence for the different interstitial sites. To occupy the optimal-charge-density site, H can be easily trapped by the W GB with the solution and segregation energy of −0.23 eV and −1.11 eV, respectively. Kinetically, such a trapping is easier to realize due to the much lower diffusion barrier of 0.13–0.16 eV from the bulk to the GB in comparison with the segregation energy, suggesting that it is quite difficult for the trapped H to escape out of the GB. However, the GB can hold no more than 2 H atoms because the isosurface of optimal charge density almost disappears with the second H atom in, leading to the conclusion that H2 molecule and thus H bubble cannot form in the W GB. Taking into account the lower vacancy formation energy in the GB as compared with the bulk, we propose that the experimentally observed H bubble formation in the W GB should be via a vacancy trapping mechanism.