共查询到20条相似文献，搜索用时 15 毫秒
1.
We have developed a coaxial flow focusing geometry that can be fabricated using soft lithography in poly(dimethylsiloxane) (PDMS). Like coaxial flow focusing in glass capillary microfluidics, our geometry can form double emulsions in channels with uniform wettability and of a size much smaller than the channel dimensions. However, In contrast to glass capillary coaxial flow focusing, our geometry can be fabricated using lithographic techniques, allowing it to be integrated as the drop making unit in parallel drop maker arrays. Our geometry enables scalable formation of emulsions down 7 μm in diameter, in large channels that are robust against fouling and clogging. 相似文献
2.
The usual use of fractals involves self-similar geometrical objects to fill a space, where the self-similar iterations may continue ad infinitum. This is the first paper to propose the use of self-similar mechanical objects that fill an alloted space, while achieving an invariance property as the self-similar iterations continue (e.g. invariant strength). Moreover, for compressive loads, this paper shows how to achieve minimal mass and invariant strength from self-similar structures. The topology optimization procedure uses self-similar iteration until minimal mass is achieved, and this problem is completely solved, with global optimal solutions given in closed form. The optimal topology remains independent of the magnitude of the load. Mass is minimized subject to yield and/or buckling constraints. Formulas are also given to optimize the complexity of the structure, and the optimal complexity turns out to be finite. That is, a continuum is never the optimal structural for a compressive load under any constraints on the physical dimension (diameter). After each additional self-similar iteration, the number of bars and strings increase, but, for a certain choice of unit topology shown, the total mass of bars and strings decreases. For certain structures, the string mass monotonically increases with iteration, while the bar mass monotonically reduces, leading to minimal total mass in a finite number of iterations, and hence a finite optimal complexity for the structure. The number of iterations required to achieve minimal mass is given explicitly in closed form by a formula relating the chosen unit geometry and the material properties. It runs out that the optimal structures produced by our theory fall in the category of structures we call tensegrity. Hence our self-similar algorithms can generate tensegrity fractals. 相似文献
3.
Foams have many useful applications that arise from the structure and size distribution of the bubbles within them. Microfluidics allows for the rapid formation of uniform bubbles, where bubble size and volume fraction are functions of the input gas pressure, liquid flow rate, and device geometry. After formation, the microchannel confines the bubbles and determines the resulting foam structure. Bubbly structures can vary from a single row ("dripping"), to multiple rows ("alternating"), to densely packed bubbles ("bamboo" and dry foams). We show that each configuration arises in a distinct region of the operating space defined by bubble volume and volume fraction. We describe the boundaries between these regions using geometric arguments and show that the boundaries are functions of the channel aspect ratio. We compare these geometric arguments with foam structures observed in experiments using flow-focusing, T-junction, and co-flow designs to generate stable nitrogen bubbles in aqueous surfactant solution and stable droplets in oil containing dissolved surfactant. The outcome of this work is a set of design parameters that can be used to achieve desired foam structures as a function of device geometry and experimental control parameters. 相似文献
4.
According to the mathematical classification of topological band structures, there exist a number of fascinating topological states in dimensions larger than three with exotic boundary phenomena and interesting topological responses. While these topological states are not accessible in condensed matter systems, recent works have shown that synthetic systems, such as photonic crystals or electric circuits, can realize higher-dimensional band structures. Here, we argue that, because of its symmetry properties, the 4D spinless topological insulator is particularly well suited for implementation in these synthetic systems. We explicitly construct a 2D electric circuit lattice, whose resonance frequency spectrum simulates the 4D spinless topological insulator. We perform detailed numerical calculations of the circuit lattice and show that the resonance frequency spectrum exhibits pairs of 3D Weyl boundary states, a hallmark of the nontrivial topology. These pairs of 3D Weyl states with the same chirality are protected by classical time-reversal symmetry that squares to +1, which is inherent in the proposed circuit lattice. We also discuss how the simulated 4D topological band structure can be observed in experiments. 相似文献
5.
Annie Ruimi Yueming Liang Robert M. McMeeking 《Journal of The Franklin Institute》2012,349(7):2294-2312
The performance of a microscale aluminum nitride piezoelectric resonator in the shape of a trampoline is analyzed using three-dimensional finite element simulations. The air-suspended resonator is supported by beams and is designed to respond to longitudinal through-thickness vibrations. The device is targeted to operate at UHF frequencies (3 GHz) suitable for wireless filtering applications. Energy loss due to material damping is accounted for in the model. Other sources of damping are considered. We analyze if and how the material thickness, number of beams and beam length affect the resonator performance. This is intended to provide useful information at the design stages and eliminate the high costs associated with manufacturing a filter with poor performance. Performance is evaluated by means of the electromechanical coupling coefficient (K2) and the quality factor (Q) calculated from the electrical impedance frequency response plots. The results indicate that (i) K2 is insensitive to geometry (K2～6.5%), (ii) Q increases linearly with the AlN thickness attaining Q～1900 for a 1.7 μm thick resonator and (iii) a trampoline resonator with three beams has a better performance capability than the resonator with four or eight beams with a figure of merit K2Q～120 and resonating at a higher frequency value than its counterparts resonators, peaking at 3.21 GHz. The performance figures agree well with those predicted by a one dimensional theory. The value of K2 also agrees well with test data but that of Q is higher than the one recorded in the lab. 相似文献
6.
钢梁柱节点域在达到屈服强度之后,还可以承受相当的剪力。其屈服后抗剪强度的提高取决于节点域周边柱翼缘、横向加劲肋对板域的约束程度。柱翼缘以及横向加劲肋的厚度宽度的变化都会对节点域的受力性能发生影响。本文利用大型通用有限元软件ANSYS8.0,主要针对在单调加载作用下,讨论以上四个参数的变化对节点域各受力性能产生的影响。 相似文献
7.
William E. McGrath 《Information processing & management》1978,14(1):17-28
In a university library, variability in circulation of books by subject area is partly but directly dependent on the academic program—e.g. if the anthropology department enrolls more students than the physics department, then more anthropology books should circulate. However, if the number of students enrolled in each subject is the same, the number of books circulated in one subject may still be larger than in the other. Therefore, other sources accounting for variability must be sought. One possible source is in the nature of the subjects themselves. For example, subjects (as represented by academic disciplines or departments) are often called hard or soft, pure or applied, life or nonlife. The purpose of this paper was to determine the relationship between these characteristics and the number of books charged out of an academic library by students. Three major hypotheses were formulated:(1) the softer a subject, the more books charged on that subject; the harder the subject, the fewer the books charged;(2) the purer the subject, the more books charged; the more applied the subject, the fewer the books charged;(3) The more a subject can be characterized as life oriented, the greater the difference in number of books charged between that subject and those characterized as nonlife oriented. A scale value for each characteristic in each of 60 academic subjects was determined by a survey of faculty of the University of Southwestern Louisiana. Correlation and multiple regression were employed to assess the proportion of circulation variance accounted for by each of the three characteristics. Other variables—masters, upper and lower level enrollments, credit hours being taught, number of books already in the library, and level of degree offered—were tested as controls. Hypothesis (1) was weakly supported under the condition that shelflist and masters enrollments were held constant (3% of variance). Support was somewhat firmer (10% variance) when the dependent variable was defined as proportion of shelflist circulated. Hard/soft was also significant (15% of variance) when pure/applied was held constant, but shelflist and enrollments were not. Hypothesis (2) was modestly supported under the condition that other variables were not held constant. Otherwise pure/applied was not significant. Together, pure/applied and hard/soft, in that order, and when no other variables were controlled accounted for 26% of the variance. Hypothesis (3) was not supported under any of the conditions tested. Results have implications in sociological understanding of the relationship between disciplines and library use, in formulating a rationale for library collection building, and in administrative, budget allocations to subjects. 相似文献
8.
This study predicts the impact of technological, organizational and environmental (TOE) determinants on e-maintenance technology readiness in manufacturing firms. Survey responses of 308 managers from a wide spectrum of manufacturing firms have been validated and analyzed by means of structural equation modelling. The findings indicate that dimensions of e-maintenance technology readiness in manufacturing firms are mainly influenced by technological and organizational determinants involving technological infrastructure and competence, expected benefits and challenges of e-maintenance, and firm size and ownership. Surprisingly, there is no significant effect of competitive pressures on e-maintenance readiness. This study offers managers and vendors a frame of reference to analyze firm's situation before initiating new innovations. In case of e-maintenance technology, adoption strategies should be built around fostering level of employees’ technological knowledge and skills, technology infrastructure as well as sustaining potential benefits and encountering potential challenges associated with e-maintenance technology. This paper is one of the early studies that predict dimensions of technology readiness index (TRI) through the determinants of technology–organization–environment (TOE) framework. Also, it is among the first attempts to link prominent technology adoption models to e-maintenance technology as a novel form of enterprise innovations. 相似文献
9.
介绍了BEPCⅡ(北京正负电子对撞机二期工程)对撞点束流轨道反馈系统的设计.由于BEPCⅡ将采用双环、多束团、交叉角对撞,并且多种原因会使束流处于非最佳对撞状态甚至无法对撞,因此一套基于束-束偏转技术的对撞点束流反馈系统的建立,以使得对撞机保持在最佳对撞状态是非常必要的.此外,为了同时测量对撞点处正、负电子束的位置,两套带有8电极的特殊束流位置探测器(BPM)将安装在对撞点两侧.8电极BPM的工作原理以及采用有限元法进行模拟计算的结果在文章中也进行了介绍. 相似文献
10.
介绍了BEPCII(北京正负电子对撞机二期工程)对撞点束流轨道反馈系统的设计.由于BEPCII将采用双环、多束团、交叉角对撞,并且多种原因会使束流处于非最佳对撞状态甚至无法对撞,因此一套基于束—束偏转技术的对撞点束流反馈系统的建立,以使得对撞机保持在最佳对撞状态是非常必要的.此外,为了同时测量对撞点处正、负电子束的位置,两套带有8电极的特殊束流位置探测器(BPM)将安装在对撞点两侧.8电极BPM的工作原理以及采用有限元法进行模拟计算的结果在文章中也进行了介绍 相似文献
11.
A novel actuation method of transporting droplets by using electrical charging of droplet in a dielectric fluid 总被引：1，自引：0，他引：1
We evaluate the feasibility of manipulating droplets in two dimensions by exploiting Coulombic forces acting on conductive droplets immersed in a dielectric fluid. When a droplet suspended in an immiscible fluid is located near an electrode under a dc voltage, the droplet can be charged by direct contact, by charge transfer along an electrically conducting path, or by both mechanisms. This process is called electrical charging of droplet (ECOD). This charged droplet may then be transported rapidly by exploiting Coulombic forces. We experimentally demonstrate electrical actuation of a charged droplet by applying voltage sequences. A charged droplet is two dimensionally actuated by following the direction of the electrical field signal. The droplet does not contact the surface of the microfluidic chip when it moves. This characteristic is very advantageous because treatments of the substrate surfaces of microfluidic chip become simpler. In order to test the feasibility of using ECOD in a droplet-based microreactor, electrocoalescence of two oppositely charged droplets is also studied. When two droplets approach each other due to Coulombic attraction, a liquid bridge is formed between them. We postulate that if the applied electric field is weaker than a certain critical level, the two droplets coalesce instantaneously when the charges are exchanged and redistributed through this liquid bridge. 相似文献
12.
Herbert J. Carlin Pier Paolo Civalleri Jeffrey C. Hantgan 《Journal of The Franklin Institute》1981,311(4):209-230
A general procedure is outlined for obtaining single or coupled transmission line models to represent the propagation of surface wave modes in conductively unshielded dielectric waveguides. The procedure uses a homogeneous electrically or magnetically walled waveguide having the same dimensions as the dielectric of the surface waveguide, to produce a set of orthogonal eigenfunctions. These eigenfunctions are projected upon Maxwell's equations resulting in a system of transmission lines coupled together through a wave immittance, which represents the ratio of a longitudinal and a transverse field component at the dielectric-air interface. Examples are given for various modes of the dielectric slab and the dielectric rod, in particular the HE1n modes for the latter. The transmission line models derived for these examples consist of a single trasmission line found directly by projection or reduced from a coupled transmission line model by port elimination, or of two transmission lines coupled together. All circuit models derived preserve the basic properties of surface waves (e.g. no solution below cut-off), and any of the single line models can be solved to give explicit approximate algebraic formulae for the propagation constant as a function of frequency. Numerical results show that the dispersion curves calculated from the models versus exact values are generally excellent over the entire frequency spectrum. 相似文献
13.
Roberto De A. Martins 《Journal of The Franklin Institute》1981,311(5):331-337
A paper by Macagno (3) in this Journal is discussed. The origin of the concept of physical dimensions is traced back to ideas previously used in analytic geometry. Descartes' use of the word “dimension” in the study of physical magnitudes is shown to have properties completely different from Fourier's dimensions, being therefore unimportant to the evolution of dimensional analysis. It is also shown that the principle of homogeneity was used in the derivation of physical equations sixty years prior to the publication of Fourier's work, and that the latter was aware of this paper, that may be considered the earliest publication on dimensional analysis. 相似文献
14.
1/4波长的垂直GP天线广泛用于中波广播以及移动无线电通讯等领域。GP天线即地网天线英文全称为Ground Plane antenna可知地网是GP天线的组成部分,地网的性质会对GP天线产生非常重要的影响。本文借助仿真软件HFSS分析了地网的大小形状材料对GP天线阻抗,共振频率,增益,方向图等天线主要参数的影响。这些结果将为GP天线在有限地平面中的设计提供理论支持。 相似文献
15.
Using femtosecond laser to fabricate highly precise interior three-dimensional microstructures in polymeric flow chip 总被引：1，自引：0，他引：1
This paper reports using femtosecond laser marker to fabricate the three-dimensional interior microstructures in one closed flow channel of plastic substrate. Strip-like slots in the dimensions of 800 μm×400 μm×65 μm were ablated with pulse Ti:sapphire laser at 800 nm (pulse duration of ～120 fs with 1 kHz repetition rate) on acrylic slide. After ablation, defocused beams were used to finish the surface of microstructures. Having finally polished with sonication, the laser fabricated structures are highly precise with the arithmetic roughness of 1.5 and 4.5 nm. Fabricating such highly precise microstructures cannot be accomplished with nanosecond laser marking or other mechanical drilling methods. In addition, since laser ablation can directly engrave interior microstructures in one closed chip, glue smearing problems to damage molded microstructures possibly to occur during the chip sealing procedures can be avoided too. 相似文献
16.
《Journal of The Franklin Institute》2023,360(3):1706-1732
One of the drawbacks of the controllability theory for nonlinear systems is that most existing controllability criteria are not algebraically verifiable, which makes them difficult to apply especially if the system dimension is high. Thus, it is a significant task to seek algebraically verifiable controllability criteria for nonlinear systems. In this paper, we study controllability of discrete-time inhomogeneous bilinear systems. In the classical results on controllability of such systems, a necessary condition is that the linear part has to be controllable. However, we will show that this condition is in fact not necessary for controllability. Specifically, we first define the spectrum for discrete-time inhomogeneous bilinear systems and reveal that the spectrum is a fundamental property which is very useful in investigating the controllability problems. We then present controllability criteria for the systems with real spectrum, which are algebraically verifiable. Furthermore, we also provide algorithms for the controllable systems to compute the exact or approximated control inputs to achieve the transition between any given pair of states. The presented controllability criteria and algorithms work for the systems in any finite dimension and are easy to implement. More importantly, through our controllability criteria, we reveal that controllability of the linear part is not necessary for discrete-time inhomogeneous bilinear systems to be controllable. Examples are given to illustrate the presented algebraic controllability criteria. 相似文献
17.
18.
In this paper, a detailed numerical and experimental investigation into the optimisation of hydrodynamic micro-trapping arrays for high-throughput capture of single polystyrene (PS) microparticles and three different types of live cells at trapping times of 30 min or less is described. Four different trap geometries (triangular, square, conical, and elliptical) were investigated within three different device generations, in which device architecture, channel geometry, inter-trap spacing, trap size, and trap density were varied. Numerical simulation confirmed that (1) the calculated device dimensions permitted partitioned flow between the main channel and the trap channel, and further, preferential flow through the trap channel in the absence of any obstruction; (2) different trap shapes, all having the same dimensional parameters in terms of depth, trapping channel lengths and widths, main channel lengths and widths, produce contrasting streamline plots and that the interaction of the fluid with the different geometries can produce areas of stagnated flow or distorted field lines; and (3) that once trapped, any motion of the trapped particle or cell or a shift in its configuration within the trap can result in significant increases in pressures on the cell surface and variations in the shear stress distribution across the cell’s surface. Numerical outcomes were then validated experimentally in terms of the impact of these variations in device design elements on the percent occupancy of the trapping array (with one or more particles or cells) within these targeted short timeframes. Limitations on obtaining high trap occupancies in the devices were shown to be primarily a result of particle aggregation, channel clogging and the trap aperture size. These limitations could be overcome somewhat by optimisation of these device design elements and other operational variables, such as the average carrier fluid velocity. For example, for the 20 μm polystyrene microparticles, the number of filled traps increased from 32% to 42% during 5–10 min experiments in devices with smaller apertures. Similarly, a 40%–60% reduction in trapping channel size resulted in an increase in the amount of filled traps, from 0% to almost 90% in 10 min, for the human bone marrow derived mesenchymal stem cells, and 15%–85% in 15 min for the human embryonic stem cells. Last, a reduction of the average carrier fluid velocity by 50% resulted in an increase from 80% to 92% occupancy of single algae cells in traps. Interestingly, changes in the physical properties of the species being trapped also had a substantial impact, as regardless of the trap shape, higher percent occupancies were observed with cells compared to single PS microparticles in the same device, even though they are of approximately the same size. This investigation showed that in microfluidic single cell capture arrays, the trap shape that maximizes cell viability is not necessarily the most efficient for high-speed single cell capture. However, high-speed trapping configurations for delicate mammalian cells are possible but must be optimised for each cell type and designed principally in accordance with the trap size to cell size ratio. 相似文献
19.
Oliva Saldanha Martha E. Brennich Manfred Burghammer Harald Herrmann Sarah K?ster 《Biomicrofluidics》2016,10(2)
The structural organization of metazoan cells and their shape are established through the coordinated interaction of a composite network consisting of three individual filament systems, collectively termed the cytoskeleton. Specifically, microtubules and actin filaments, which assemble from monomeric globular proteins, provide polar structures that serve motor proteins as tracks. In contrast, intermediate filaments (IFs) assemble from highly charged, extended coiled coils in a hierarchical assembly mechanism of lateral and longitudinal interaction steps into non-polar structures. IF proteins are expressed in a distinctly tissue-specific way and thereby serve to generate the precise plasticity of the respective cells and tissues. Accordingly, in the cell, numerous parameters such as pH and salt concentration are adjusted such that the generation of functional networks is ensured. Here, we transfer the problem for the mesenchymal IF protein vimentin to an in vitro setting and combine small angle x-ray scattering with microfluidics and finite element method simulations. Our approach is adapted to resolve the early assembly steps, which take place in the sub-second to second range. In particular, we reveal the influence of ion species and concentrations on the assembly. By tuning the flow rates and thus concentration profiles, we find a minimal critical salt concentration for the initiation of the assembly. Furthermore, our analysis of the surface sensitive Porod regime in the x-ray data reveals that the formation of first assembly intermediates, so-called unit length filaments, is not a one-step reaction but consists of distinct consecutive lateral association steps followed by radial compaction as well as smoothening of the surface of the full-width filament. 相似文献
20.
Chiara Manneschi Paola Fanzio Tapio Ala-Nissila Elena Angeli Luca Repetto Giuseppe Firpo Ugo Valbusa 《Biomicrofluidics》2014,8(6)
There is currently a growing interest in control of stretching of DNA inside nanoconfined regions due to the possibility to analyze and manipulate single biomolecules for applications such as DNA mapping and barcoding, which are based on stretching the DNA in a linear fashion. In the present work, we couple Finite Element Methods and Monte Carlo simulations in order to study the conformation of DNA molecules confined in nanofluidic channels with neutral and charged walls. We find that the electrostatic forces become more and more important when lowering the ionic strength of the solution. The influence of the nanochannel cross section geometry is also studied by evaluating the DNA elongation in square, rectangular, and triangular channels. We demonstrate that coupling electrostatically interacting walls with a triangular geometry is an efficient way to stretch DNA molecules at the scale of hundreds of nanometers. The paper reports experimental observations of λ-DNA molecules in poly(dimethylsiloxane) nanochannels filled with solutions of different ionic strength. The results are in good agreement with the theoretical predictions, confirming the crucial role of the electrostatic repulsion of the constraining walls on the molecule stretching. 相似文献