The coherent scattering function in the reptation model: analysis beyond asymptotic limits

We calculate the coherent dynamical scattering function S_c(q,t;N) of a flexible chain of length N, diffusing through an ordered background of topological obstacles. As an instructive generalization, we also calculate the scattering function S_c(q,t;M,N) for the central piece of length M < N of the chain. Using the full reptation model, we treat global creep, tube length fluctuations, and internal relaxation within a consistent and unified approach. Our theory concentrates on the universal aspects of reptational motion, and our results in all details show excellent agreement with our simulations of the Evans-Edwards model, provided we allow for a phenomenological prefactor which accounts for non-universal effects of the micro-structure of the Monte Carlo chain, present for short times. Previous approaches to the coherent structure function can be analyzed as special limits of our theory. First, the effects of internal relaxation can be isolated by studying the limit $N \to \infty$, M fixed. The results do not support the model of a `Rouse chain in a tube'. We trace this back to the non-equilibrium initial conditions of the latter model. Second, in the limit of long chains $(M = N \to \infty)$ and times large compared to the internal relaxation time $(t/N^2 \to \infty)$, our theory reproduces the results of the primitive chain model. This limiting form applies only to extremely long chains, and for chain lengths accessible in practice, effects of, e.g., tube length fluctuations are not negligible.Comment: 35 pages revtex style, 9 figures, submitted on January 5, 2002, references updated. Phys. Rev. E, to appea

The coherent scattering function of the reptation model: simulations compared to theory

We present results of Monte Carlo simulations measuring the coherent structure function of a chain moving through an ordered lattice of fixed topological obstacles. Our computer experiments use chains up to 320 beads and cover a large range of wave vectors and a time range exceeding the reptation time. -- We compare our results (i) to the predictions of the primitive chain model, (ii) to an approximate form resulting from Rouse motion in a coiled tube, and (iii) to our recent evaluation of the full reptation model. (i) The primitive chain model can fit the data for times t \gt 20 T_2, where T_2 is the Rouse time of the chain. Besides some phenomenological amplitude factor this fit involves the reptation time T_3 as a second fit parameter. For the chain lengths measured, the asymptotic behavior T_3 ~ N^3 is not attained. (ii) The model of Rouse motion in a tube, which we have criticized before on theoretical grounds, is shown to fail also on the purely phenomenological level. (iii) Our evaluation of the full reptation model yields an excellent fit to the data for both total chains and internal pieces and for all wave vectors and all times, provided specific micro-structure effects of the MC-dynamics are negligible. Such micro-structure effects show up for wave vectors of the order of the inverse segment size. For the dynamics of the total chain our data analysis based on the full reptation model shows the importance of tube length fluctuations. Universal (Rouse-type) internal relaxation is unimportant. It can be observed only in the form of the diffusive motion of a short central subchain in the tube. -- Finally we present a fit formula which in a large range of wave vectors and chain lengths reproduces the numerical results of our theory for the scattering from the total chain.Comment: 26 pages, 12 figure

DTN7: An Open-Source Disruption-tolerant Networking Implementation of Bundle Protocol 7

In disruption-tolerant networking (DTN), data is transmitted in a store-carry-forward fashion from network node to network node. In this paper, we present an open source DTN implementation, called DTN7, of the recently released Bundle Protocol Version 7 (draft version 13). DTN7 is written in Go and provides features like memory safety and concurrent execution. With its modular design and interchangeable components, DTN7 facilitates DTN research and application development. Furthermore, we present results of a comparative experimental evaluation of DTN7 and other DTN systems including Serval, IBR-DTN, and Forban. Our results indicate that DTN7 is a flexible and efficient open-source multi-platform implementation of the most recent Bundle Protocol Version 7.Comment: 18th International Conference on Ad Hoc Networks and Wireless (AdHoc-Now 2019

Smart Street Lights and Mobile Citizen Apps for Resilient Communication in a Digital City

Currently, nearly four billion people live in urban areas. Since this trend is increasing, natural disasters or terrorist attacks in such areas affect an increasing number of people. While information and communication technology is crucial for the operation of urban infrastructures and the well-being of its inhabitants, current technology is quite vulnerable to disruptions of various kinds. In future smart cities, a more resilient urban infrastructure is imperative to handle the increasing number of hazardous situations. We present a novel resilient communication approach based on smart street lights as part of the public infrastructure. It supports people in their everyday life and adapts its functionality to the challenges of emergency situations. Our approach relies on various environmental sensors and in-situ processing for automatic situation assessment, and a range of communication mechanisms (e.g., public WiFi hotspot functionality and mesh networking) for maintaining a communication network. Furthermore, resilience is not only achieved based on infrastructure deployed by a digital city's municipality, but also based on integrating citizens through software that runs on their mobile devices (e.g., smartphones and tablets). Web-based zero-installation and platform-agnostic apps can switch to device-to-device communication to continue benefiting people even during a disaster situation. Our approach, featuring a covert channel for professional responders and the zero-installation app, is evaluated through a prototype implementation based on a commercially available street light.Comment: 2019 IEEE Global Humanitarian Technology Conference (GHTC

Short Time Behavior in De Gennes' Reptation Model

To establish a standard for the distinction of reptation from other modes of polymer diffusion, we analytically and numerically study the displacement of the central bead of a chain diffusing through an ordered obstacle array for times $t < O(N^2)$. Our theory and simulations agree quantitatively and show that the second moment approaches the $t^{1/4}$ often viewed as signature of reptation only after a very long transient and only for long chains (N > 100). Our analytically solvable model furthermore predicts a very short transient for the fourth moment. This is verified by computer experiment.Comment: 4 pages, revtex, 4 ps file

ReactiFi: Reactive Programming of Wi-Fi Firmware on Mobile Devices

Network programmability will be required to handle future increased network traffic and constantly changing application needs. However, there is currently no way of using a high-level, easy to use programming language to program Wi-Fi firmware. This impedes rapid prototyping and deployment of novel network services/applications and hinders continuous performance optimization in Wi-Fi networks, since expert knowledge is required for both the used hardware platforms and the Wi-Fi domain. In this paper, we present ReactiFi, a high-level reactive programming language to program Wi-Fi chips on mobile consumer devices. ReactiFi enables programmers to implement extensions of PHY, MAC, and IP layer mechanisms without requiring expert knowledge of Wi-Fi chips, allowing for novel applications and network protocols. ReactiFi programs are executed directly on the Wi-Fi chip, improving performance and power consumption compared to execution on the main CPU. ReactiFi is conceptually similar to functional reactive languages, but is dedicated to the domain-specific needs of Wi-Fi firmware. First, it handles low-level platform-specific details without interfering with the core functionality of Wi-Fi chips. Second, it supports static reasoning about memory usage of applications, which is important for typically memory-constrained Wi-Fi chips. Third, it limits dynamic changes of dependencies between computations to dynamic branching, in order to enable static reasoning about the order of computations. We evaluate ReactiFi empirically in two real-world case studies. Our results show that throughput, latency, and power consumption are significantly improved when executing applications on the Wi-Fi chip rather than in the operating system kernel or in user space. Moreover, we show that the high-level programming abstractions of ReactiFi have no performance overhead compared to manually written C code

Protein C Inhibitor—A Novel Antimicrobial Agent

Protein C inhibitor (PCI) is a heparin-binding serine proteinase inhibitor belonging to the family of serpin proteins. Here we describe that PCI exerts broad antimicrobial activity against bacterial pathogens. This ability is mediated by the interaction of PCI with lipid membranes, which subsequently leads to their permeabilization. As shown by negative staining electron microscopy, treatment of Escherichia coli or Streptococcus pyogenes bacteria with PCI triggers membrane disruption followed by the efflux of bacterial cytosolic contents and bacterial killing. The antimicrobial activity of PCI is located to the heparin-binding site of the protein and a peptide spanning this region was found to mimic the antimicrobial activity of PCI, without causing lysis or membrane destruction of eukaryotic cells. Finally, we show that platelets can assemble PCI on their surface upon activation. As platelets are recruited to the site of a bacterial infection, these results may explain our finding that PCI levels are increased in tissue biopsies from patients suffering from necrotizing fasciitis caused by S. pyogenes. Taken together, our data describe a new function for PCI in innate immunity

Spectral dimension of fluid membranes

The spectral dimension ds of polymerized and fluid self-avoiding vesicles are investigated by Monte Carlo methods. For both cases we obtained ds=2, which indicates that these surfaces belong to the same class of "microcanonical" surfaces

Spontaneous Translocation of a Polymer across a Curved Membrane

©1995 The American Physical Society. The electronic version of this article is the complete one and can be found online at: http://link.aps.org/doi/10.1103/PhysRevLett.74.2142DOI: 10.1103/PhysRevLett.74.2142The translocation of a hydrophobic polymer across a curved bilayer membrane has been studied using Monte Carlo methods. It is found that for curved membranes the polymer crosses spontaneously and almost irreversibly from the side of lower curvature to the side of higher curvature. This phenomenon can be understood based upon the curvature-induced difference of lipid fluctuations between the two halves of the bilayer. The difference of fluctuations drives the polymer across the bilayer in order to maximize the conformational entropy of the polymer

Correlations among interpenetrating polymer coils : the probing of a fractal

We use renormalization group and ε-expansion to calculate the density distribution in a polymer coil of polymerisation index N2, interpenetrating a coil of polymerisation index N1. The distance of the coils is controlled by fixing the center of mass distance or by fixing the center of mass of coil N2 with respect to the central segment of coil N1. The effective free energies of the two-coil system in both situations are universal functions which tend to some finite limit as N1 →∞, N 2/N1 fixed. This is in sharp contrast to the prediction of mean field models which lead to hard sphere behaviour. Besides free energies we calculate the radius of gyration and the density distribution of coil N2. For N2 ⪡ N1 all our results show a pronounced dependence on the position of the probe coil N2 relative to coil N1, thus proving the strong inhomogeneity of the density of coil N1 characteristic for a fractal object. We carry through off-lattice Monte Carlo simulations which nicely confirm our analytical results.Le groupe de renormalisation et le développement en ε sont utilisés pour calculer la distribution de densité d'une chaîne polymère d'indice de polymérisation N2 entremêlée à une chaîne d'indice de polymérisation N1. La distance entre chaînes est contrôlée en fixant la distance des centres de gravité ou en fixant la position du centre de gravité de la chaîne N2 par rapport au segment central de la chaîne N 1. Les énergies libres du système tendent dans les deux cas vers des fonctions universelles finies lorsque N1→ ∞, N2/N1 fixé, contrairement à la prédiction de modèles de champs moyens qui conduisent à un comportement de type « sphère dure ». Les rayons de giration et la distribution de densité de la chaîne N 2 sont également calculés. Lorsque N2 ⪡ N 1 les résultats dépendent fortement de la position relative de la chaîne « sonde » N2 par rapport à la chaîne N 1. Ceci met en évidence d'importantes inhomogénéités dans la densité de la chaîne N1 qui caractérisent un objet fractal. Des simulations numériques de Monte Carlo confirment nos résultats analytiques