Introduction to Systems Approach

Main aspects of systems theory are outlined. Emphasis is on the interface of between time and systems - natural or artificial

Precise time and time interval (PTTI), an overview

Present applications of precise time and frequency (T/F) technology can be grouped as follows: (1) Communications systems which require T/F for time division multiplexing and for using spread spectrum techniques. (2) Navigation systems which need T/F for position fixing using a timed signal. (3) Scientific-Metrological applications which use T/F as the most precisely reproducible standard of measurement. (4) Astronomical-Space applications which cover a variety of the most demanding applications such as pulsar research, Very Long Baseline Interferometry (VLBI) and laser/radar ranging. In particular, pulsar time-of-arrival measurements require submicrosecond precision over a period of one-half year referred to an extraterrestrial inertial system, and constitute the most stringent requirements for uniform timekeeping to date

Semiflexible polymers under external fields confined to two dimensions

The non-equilibrium structural and dynamical properties of semiflexible polymers confined to two dimensions are investigated by molecular dynamics simulations. Three different scenarios are considered: The force-extension relation of tethered polymers, the relaxation of an initially stretched semiflexible polymer, and semiflexible polymers under shear flow. We find quantitative agreement with theoretical predictions for the force-extension relation and the time dependence of the entropically contracting polymer. The semiflexible polymers under shear flow exhibit significant conformational changes at large shear rates, where less stiff polymers are extended by the flow, whereas rather stiff polymers are contracted. In addition, the polymers are aligned by the flow, thereby the two-dimensional semiflexible polymers behave similarly to flexible polymers in three dimensions. The tumbling times display a power-law dependence at high shear rate rates with an exponent comparable to the one of flexible polymers in three-dimensional systems.Comment: Accepted for publication in J. Chem. Phy

Negative association in uniform forests and connected graphs

We consider three probability measures on subsets of edges of a given finite graph $G$, namely those which govern, respectively, a uniform forest, a uniform spanning tree, and a uniform connected subgraph. A conjecture concerning the negative association of two edges is reviewed for a uniform forest, and a related conjecture is posed for a uniform connected subgraph. The former conjecture is verified numerically for all graphs $G$ having eight or fewer vertices, or having nine vertices and no more than eighteen edges, using a certain computer algorithm which is summarised in this paper. Negative association is known already to be valid for a uniform spanning tree. The three cases of uniform forest, uniform spanning tree, and uniform connected subgraph are special cases of a more general conjecture arising from the random-cluster model of statistical mechanics.Comment: With minor correction

Conformations, hydrodynamic interactions, and instabilities of sedimenting semiflexible filaments

The conformations and dynamics of semiflexible filaments subject to a homogeneous external (gravitational) field, e.g., in a centrifuge, are studied numerically and analytically. The competition between hydrodynamic drag and bending elasticity generates new shapes and dynamical features. We show that the shape of a semiflexible filament undergoes instabilities as the external field increases. We identify two transitions that correspond to the excitation of higher bending modes. In particular, for strong fields the filament stabilizes in a non-planar shape, resulting in a sideways drift or in helical trajectories. For two interacting filaments, we find the same transitions, with the important consequence that the new non-planar shapes have an effective hydrodynamic repulsion, in contrast to the planar shapes which attract themselves even when their osculating planes are rotated with respect to each other. For the case of planar filaments, we show analytically and numerically that the relative velocity is not necessarily due to a different drag of the individual filaments, but to the hydrodynamic interactions induced by their shape asymmetry.Comment: 9 pages, 7 figures in Soft Matter (2015

Migration of semiflexible polymers in microcapillary flow

The non-equilibrium structural and dynamical properties of a semiflexible polymer confined in a cylindrical microchannel and exposed to a Poiseuille flow is studied by mesoscale hydrodynamic simulations. For a polymer with a length half of its persistence length, large variations in orientation and conformations are found as a function of radial distance and flow strength. In particular, the polymer exhibits U-shaped conformations near the channel center. Hydrodynamic interactions lead to strong cross-streamline migration. Outward migration is governed by the polymer orientation and the corresponding anisotropy in its diffusivity. Strong tumbling motion is observed, with a tumbling time which exhibits the same dependence on Peclet number as a polymer in shear flow.Comment: 6 pages, 7 figures, accepted by EP

External gates and transport in biased bilayer graphene

We formulate a theory of transport in graphene bilayers in the weak momentum scattering regime in such a way as to take into account contributions to the electrical conductivity to leading and next-to-leading order in the scattering potential. The response of bilayers to an electric field cannot be regarded as a sum of terms due to individual layers. Rather, interlayer tunneling and coherence between positive- and negative-energy states give the main contributions to the conductivity. At low energies, the dominant effect of scattering on transport comes from scattering within each energy band, yet a simple picture encapsulating the role of collisions in a set of scattering times is not applicable. Coherence between positive- and negative-energy states gives, as in monolayers, a term in the conductivity which depends on the order of limits. The application of an external gate, which introduces a gap between positive- and negative-energy states, does not affect transport. Nevertheless the solution to the kinetic equation in the presence of such a gate is very revealing for transport in both bilayers and monolayers.Comment: 6 pages, accepted for publication in Physical Review

A preliminary discussion of gravitational physics experiments for the Spacelab era

An overview of past, present, and proposed future experiments in gravitational physics is given. These experiments are concerned with the measurement of relativistic gravity effects to test theories of gravitation. Certain experiments which could be performed on shuttle and Spacelab missions and the potential of Spacelab for gravitation physics research are discussed