Showing papers by "Eindhoven University of Technology published in 2000"

Effects of mechanical forces on maintenance and adaptation of form in trabecular bone

Abstract: The architecture of trabecular bone, the porous bone found in the spine and at articulating joints, provides the requirements for optimal load transfer, by pairing suitable strength and stiffness to minimal weight according to rules of mathematical design1,2,3,4,5,6. But, as it is unlikely that the architecture is fully pre-programmed in the genes7, how are the bone cells informed about these rules, which so obviously dictate architecture? A relationship exists between bone architecture and mechanical usage8—while strenuous exercise increases bone mass9, disuse, as in microgravity and inactivity, reduces it10. Bone resorption cells (osteoclasts) and bone formation cells (osteoblasts) normally balance bone mass in a coupled homeostatic process of remodelling, which renews some 25% of trabecular bone volume per year. Here we present a computational model of the metabolic process in bone that confirms that cell coupling is governed by feedback from mechanical load transfer11,12,13.This model can explain the emergence and maintenance of trabecular architecture as an optimal mechanical structure, as well as its adaptation to alternative external loads.

A characterisation of logistics networks for product recovery

Abstract: Recovery of used products is receiving much attention recently due to growing environmental concern. Efficient implementation requires appropriate logistics structures to be set up for the arising goods flow from users to producers. We investigate the design of such logistics networks. As a basis for our analysis we review recent case studies on logistics network design for product recovery in different industries. We identify general characteristics of product recovery networks and compare them with traditional logistics structures. Moreover, we derive a classification scheme for different types of recovery networks.

Modelling of premixed laminar flames using flamelet-generated manifolds

Abstract: In order to reduce the computational cost of flame simulations, several methods have been developed during the last decades, which simplify the description of the reaction kinetics. Most of these methods are based on partial-equilibrium and steady-state assumptions, assuming that most chemical processes have a much smaller time scale than the flow time scale. These assumptions, however, give poor approximations in the ‘colder’ regions of a flame, where transport processes are also important. The method presented here, can be considered as a combination of two approaches to simplify flame calculations, i.e. a flamelet and a manifold approach. The method, to which we will refer as the Flamelet-Generated Manifold (FGM) method, shares the idea with flamelet approaches that a multi-dimensional flame may be considered as an ensemble of one-dimensional flames. The implementation, however, is typical for manifold methods: a low-dimensional manifold in composition space is constructed, and the thermo-chemical vari...

Selecting Cryptographic Key Sizes.

Abstract: In this article we offer guidelines for the determination of key sizes for symmetric cryptosystems, RSA, and discrete logarithm-based cryptosystems both over finite fields and over groups of elliptic curves over prime fields. Our recommendations are based on a set of explicitly formulated parameter settings, combined with existing data points about the cryptosystems.

Supramolecular Polymer Materials: Chain Extension of Telechelic Polymers Using a Reactive Hydrogen-Bonding Synthon**

Abstract: Functionalizing the termini of low-molecular-weight telechelic polymers with strongly associating hydrogen bonding units (see Figure) results in a new set of supramolecular materials, as reported here. These materials possess the unique combination of polymer-like properties at room temperature and monomer-like properties at elevated temperatures.

Helical self-assembled polymers from cooperative stacking of hydrogen-bonded pairs

Abstract: The double helix of DNA epitomizes this molecule's ability to self-assemble in aqueous solutions into a complex chiral structure using hydrogen bonding and hydrophobic interactions. Non-covalently interacting molecules in organic solvents are used to design systems that similarly form controlled architectures. Peripheral chiral centres in assemblies and chiral side chains attached to a polymer backbone, have been shown to induce chirality at the supramolecular level, and highly ordered structures stable in water are also known. However, it remains difficult to rationally exploit non-covalent interactions for the formation of chiral assemblies that are stable in water, where solvent molecules can compete effectively for hydrogen bonds. Here we describe a general strategy for the design of functionalized monomer units and their association in either water or alkanes into non-covalently linked polymeric structures with controlled helicity and chain length. The monomers consist of bifunctionalized ureidotriazine units connected by a spacer and carrying solubilizing chains at the periphery. This design allows for dimerization through self-complementary quadruple hydrogen bonding between the units and solvophobically induced stacking of the dimers into columnar polymeric architectures, whose structure and helicity can be adjusted by tuning the nature of the solubilizing side chains.

Workflow Verification: Finding Control-Flow Errors Using Petri-Net-Based Techniques

Abstract: Workflow management systems facilitate the everyday operation of business processes by taking care of the logistic control of work. In contrast to traditional information systems, they attempt to support frequent changes of the workflows at hand. Therefore, the need for analysis methods to verify the correctness of workflows is becoming more prominent. In this chapter we present a method based on Petri nets. This analysis method exploits the structure of the Petri net to find potential errors in the design of the workflow. Moreover, the analysis method allows for the compositional verification of workflows.

Critically evaluated rate coefficients for free-radical polymerization, 2.. Propagation rate coefficients for methyl methacrylate

Abstract: Propagation rate coefficients, k(P), for free-radical polymerization of butyl acrylate (BA) previously reported by several groups are critically evaluated. All data were determined by the combination of pulsed-laser polymerization (PLP) and subsequent polymer analysis by size exclusion (SEC) chromatography. The PLP-SEC technique has been recommended as the method of choice for the determination of k(P) by the IUPAC Working Party on Modeling of Polymerization Kinetics and Processes. Application of the technique to acrylates has proven to be very difficult and, along with other experimental evidence, has led to the conclusion that acrylate chain-growth kinetics are complicated by intramolecular transfer (backbiting) events to form a mid-chain radical structure of lower reactivity. These mechanisms have a significant effect on acrylate polymerization rate even at low temperatures, and have limited the PLP-SEC determination of k(P) of chain-end radicals to low temperatures (<20 degreesC) using high pulse repetition rates. Nonetheless, the values for BA from six different laboratories, determined at ambient pressure in the temperature range of -65 to 20 degreesC mostly for bulk monomer with few data in solution, fulfill consistency criteria and show excellent agreement, and are therefore combined together into a benchmark data set. The data are fitted well by an Arrhenius relation resulting in a pre-exponential factor of 2.21 x 10(7) L (.) mol(-1) (.) s(-1) and an activation energy of 17.9 kJ (.) mol(-1). It must be emphasized that these PLP-determined k(P) values are for monomer addition to a chain-end radical and that, even at low temperatures, it is necessary to consider the presence of two radical structures that have very different reactivity. Studies for other alkyl acrylates do not provide sufficient results to construct benchmark data sets, but indicate that the family behavior previously documented for alkyl methacrylates also holds true within the alkyl acrylate family of monomers. [GRAPHICS] Arrhenius plot of propagation rate coefficients, k(P), for BA as measured by PLP-SEC.

Ultrafast magneto-optics in nickel: magnetism or optics?

Abstract: Several magnetic and optical processes contribute to the magneto-optical response of nickel thin films after excitation by a femtosecond laser pulse. We achieved a first complete identification by explicitly measuring the time-resolved Kerr ellipticity and rotation, as well as its temperature and magnetic field dependence in epitaxially grown (111) and (001) oriented Cu/Ni/Cu wedges. The first hundreds of femtoseconds the response is dominated by state filling effects. The true demagnetization takes approximately 0.5-1 ps. At the longer (sub-ns) time scales the spins are found to precess in their anisotropy field. Simple and transparent models are introduced to substantiate our interpretation.

Stability and Lifetime of Quadruply Hydrogen Bonded 2-Ureido-4[1H]-pyrimidinone Dimers

Abstract: 2-Ureido-4[1H]-pyrimidinones are known to dimerize via a strong quadruple hydrogen bond array. A detailed study of the dimerization constant and lifetime of the dimer is presented here. Excimer fluorescence of pyrene-labeled 2-ureido-4[1H]-pyrimidinone 1b was used to determine a dimerization constant Kdim of 6 × 107 M-1 in CHCl3, 1 × 107 M-1 in chloroform saturated with water, and 6 × 108 M-1 in toluene (all at 298 K). Under these conditions, the preexchange lifetime of the similar dimers of both 1d and 1e is 170 ms in CDCl3, 80 ms in wet CDCl3, and 1.7 s in toluene-d8, as determined by dynamic NMR spectroscopy. Association rate constants were calculated from the Kdim values and the preexchange lifetimes. The resulting values are significantly lower than the diffusion-controlled association rate constants calculated using the Stokes−Einstein and the Debeije equations. This difference is ascribed to a tautomeric equilibrium of the monomer between the dimerizing 4[1H]-pyrimidinone and nondimerizing 6[1H]-py...

Biotechnology and the European public

Abstract: The latest European sample survey of public perceptions of biotechnology reveals widespread opposition to genetically modified (GM) food in much of Europe, but public attitudes to medical and environmental applications remain positive.

Advanced Workflow Patterns

Abstract: Conventional workflow functionality like task sequencing, split parallelism, join synchronization and iteration have proven effective for business process automation and have widespread support in current workflow products. However, newer requirements for workflows are encountered in practice, opening grave uncertainties about the extensions for current languages. Different concepts, although outwardly appearing to be more or less the same, are based on different paradigms, have fundamentally different semantics and different levels of applicability – more specialized for modeling or more generalized for workflow engine posit. By way of developmental insight of new requirements, we define workflow patterns which are described imperatively but independently of current workflow languages. These patterns provide the basis for an in-depth comparison of 12 workflow management systems. As such, the work reported in this paper can be seen as the academic response to evaluations made by prestigious consulting companies. Typically, these evaluations hardly consider the workflow modeling language and routing capabilities and focus more on the purely technical and commercial aspects.

Third‐party logistics in Europe – five years later

Abstract: Discusses the results of a survey among shippers in several European countries involved in outsourcing their transport, warehousing and other logistics activities. The survey, which is a joint initiative of the universities of Cranfield, Eindhoven, Linkoping and Rotterdam (Erasmus) and of ManDat GmbH, is a sequel to a similar survey carried out five years ago and has led to a renewed insight into the characteristics of logistics partnerships, the key success factors for making partnerships work, the concerns that shippers have about outsourcing and the benefits they expect to receive from it. The survey has led to two major observations: first, that the scope and level of sophistication of the partnerships has increased over the last five years, be it very gradually, and second, that the perception of outsourcing by shippers has not changed over the last five years.

Measuring the bullwhip effect in the supply chain

Abstract: Increased demand variability in supply chains (the bullwhip effect) has been discussed in the literature. The practical measurement of this effect, however, entails some problems that have not received much attention in the literature and that have to do with the aggregation of data, incompleteness of data, the isolation of demand data for defined supply chains that are part of a greater supply web. This paper discusses these conceptual measurement problems and discusses experiences in dealing with some of these problems in an industrial project. Also presents empirical results of measurements of the bullwhip effect in two supply chains.

Using equity theory to examine the difference between burnout and depression

Abstract: This study among a sample of 154 Dutch teachers examines the discriminant validity of burnout and depression, as measured by the Maslach Burnout Inventory (MBI) and the Center for Epidemiologic Studies Depression Scale (CES-D), respectively. Confirmatory factor analyses show that burnout can be statistically discriminated from depression. Results corroborate the three-factor structure of the MBI and partly confirm the four-factor structure of the CES-D. Furthermore, results of structural equation modeling analyses show that a lack of reciprocity in the relationship with one's partner predicts depression (and not burnout), and that a lack of reciprocity in the relationship with students predicts burnout (and only indirectly depression). These findings are consistent with equity theory, and confirm the central hypothesis that burnout is work-related, whereas depression is context-free.

Coronary pressure measurement to assess the hemodynamic significance of serial stenoses within one coronary-artery : validation in humans

Abstract: Background—When several stenoses are present within 1 coronary artery, the hemodynamic significance of each stenosis is influenced by the presence of the other(s), and the calculation of coronary and fractional flow reserve (CFR and FFR) for each individual stenosis is confounded. Recently, we developed and experimentally validated a method to determine the true FFR of each stenosis as it would be after the removal of the other stenosis; the true FFR can be reliably predicted by coronary pressures measured before treatment at specific locations within the coronary artery using equations accounting for stenosis interaction. The aim of the present study was to test the validity of these equations in humans. Methods and Results—In this study of 32 patients with 2 serial stenoses in 1 coronary artery, relevant pressures were measured before the intervention, after the treatment of 1 stenosis, and after the treatment of both stenoses. The true FFR of each stenosis (FFRtrue) was directly measured after the elim...

Mechanical properties of natural-fibre-mat-reinforced thermoplastics based on flax fibres and polypropylene.

Abstract: Thermoplastic composites based on flax fibres and a polypropylene (PP) matrix were manufactured using (i) a film-stacking method based on random fibre mats and (ii) a paper making process based on chopped fibres. The influence of fibre length and fibre content on stiffness, strength and impact strength of these so-called natural-fibre-mat-reinforced thermoplastics (NMTs) is reported and compared with data for glass-mat-reinforced thermoplastics (GMTs), including the influence of the use of maleic-anhydride grafted PP for improved interfacial adhesion. In addition some preliminary data on the influence of fibre diameter on composite stiffness and strength is reported. The data is compared with the existing micro-mechanical models for strength and stiffness. A good agreement was found between theory and experiment in case of stiffness whereas in the case of strength the experimental values fall well below the theoretical predictions. Results indicated that NMTs are of interest for low-cost engineering applications and can compete with commercial GMTs when a high stiffness per unit weight is desirable. Results also indicated that future research towards significant improvements in tensile and impact strength of these types of composites should focus on the optimisation of fibre strength rather than interfacial bond strength.

Circularly polarized electroluminescence from liquid-crystalline chiral polyfluorenes

Abstract: Circularly polarized electroluminescence (CPEL) has potential application in optical data storage and processing. It was recently discovered that CPEL could be generated from chiral-substituted, conjugated polymers. These authors have synthesized a range of polyfluorenes (e.g., see Figure) that form liquid crystals and emit bluish light with a significant (200 times greater than in the first report) circularly polarized component

Environmental Durability of Flax Fibres and their Composites based on Polypropylene Matrix

Abstract: The environmental degradation behaviour of flax fibres and their polymer composites are explored. New upgraded Duralin flax fibres, which have been treated by a novel treatment process for improved moisture and rot sensitivity were studied. Environmental studies showed that these upgraded Duralin flax fibres absorb less moisture than untreated Green flax fibres, whereas the mechanical properties of the treated fibres were retained, if not improved. The effect of this novel flax fibre treatment on the environmental behaviour of natural-fibre-mat-reinforced thermoplastics (NMTs) is investigated by monitoring the moisture absorption and swelling, and measuring the residual mechanical properties of the flax/polypropylene composites at different moisture levels. The moisture absorption and swelling of the upgraded flax fibre composites is approximately 30% lower than that of composites based on Green flax fibres.

Using Behavioral Realism to Estimate Presence: A Study of the Utility of Postural Responses to Motion Stimuli

Abstract: We recently reported that direct subjective ratings of the sense of presence are potentially unstable and can be biased by previous judgments of the same stimuli (Freeman et al., 1999). Objective measures of the behavioral realism elicited by a display offer an alternative to subjective ratings. Behavioral measures and presence are linked by the premise that, when observers experience a mediated environment (VE or broadcast) that makes them feel present, they will respond to stimuli within the environment as they would to stimuli in the real world. The experiment presented here measured postural responses to a video sequence filmed from the hood of a car traversing a rally track, using stereoscopic and monoscopic presentation. Results demonstrated a positive effect of stereoscopic presentation on the magnitude of postural responses elicited. Posttest subjective ratings of presence, vection, and involvement were also higher for stereoscopically presented stimuli. The postural and subjective measures were not significantly correlated, indicating that nonproprioceptive postural responses are unlikely to provide accurate estimates of presence. Such postural responses may prove useful for the evaluation of displays for specific applications and in the corroboration of group subjective ratings of presence, but cannot be taken in place of subjective ratings.

High‐Quality Alkyl Monolayers on Silicon Surfaces

Abstract: Covalent attachment of functionalized monolayers onto silicon surfaces (see Figure for examples) is presented here as a strategy for surface modification. The preparation and structure of both unfunctionalized and functionalized alkyl-based monolayers are described, as are potential applications, for example, in the surface passivation of Si solar cells and for photopatterning of silicon surfaces.