Analyse des Weines

Ohne Zusammenfassung     

Avoiding unphysical kinetic traps in Monte Carlo simulations of strongly attractive particles

We introduce a "virtual-move" Monte Carlo algorithm for systems of pairwise-interacting particles. This algorithm facilitates the simulation of particles possessing attractions of short range and arbitrary strength and geometry, an important realization being self-assembling particles endowed with strong, short-ranged, and angularly specific ("patchy") attractions. Standard Monte Carlo techniques employ sequential updates of particles and can suffer from low acceptance rates when attractions are strong. In this event, collective motion can be strongly suppressed. Our algorithm avoids this problem by proposing simultaneous moves of collections (clusters) of particles according to gradients of interaction energies. One particle first executes a "virtual" trial move. We determine which of its neighbors move in a similar fashion by calculating individual bond energies before and after the proposed move. We iterate this procedure and update simultaneously the positions of all affected particles. Particles move according to an approximation of realistic dynamics without requiring the explicit computation of forces and without the step size restrictions required when integrating equations of motion. We employ a size- and shape-dependent damping of cluster movements, motivated by collective hydrodynamic effects neglected in simple implementations of Brownian dynamics. We discuss the virtual-move algorithm in the context of other Monte Carlo cluster-move schemes and demonstrate its utility by applying it to a model of biological self-assembly.     

Air stream-mediated vortex agitation of microlitre entities on a fluidic chip

The method presented in this paper uses air flux to induce spiral motion in small-scale (e.g., microlitre) fluid entities in an efficient, technologically convenient manner. The set-up entails a manifold that modulates the air flux and projects it onto a liquid-containing reservoir. The flow behaviour of the liquid phase has been visualized through the dissolution of rhodamine B crystallites and the motion of fluorescent microbeads. In addition, the method proved effective to restoring a suspension of spherical particles upon sedimentation, promoting displacement and capture of the beads within a microfluidic system.     

Insensitivity to salt of assembly of a rigid biopolymer aggrecan

Many polyelectrolytes, ranging from sulfonated polystyrene to DNA, exhibit a strong sensitivity of their phase behavior to salt concentration, especially to higher valence salts, which often lead to phase separation. We show that the stiff polyelectrolyte aggrecan exhibits a qualitatively different behavior. Specifically, the scattering properties of aggrecan solutions are exceptionally insensitive to the addition of calcium salt, conferring on aggrecan the role of an ion reservoir mediating calcium metabolism in cartilage and bone, and also providing osmotic resilience to compressive load.     

NMR measurements on cis-1,4-polybutadiene as a function of pressure

Measurements are reported of the nuclear spin—lattice relaxation time T₁ in cis-1,4-polybutadiene at room temperature up to a pressure of 3,500 bar. Up to 2,000 bar the relaxation curves are described by a single T₁, the pressure variation of which indicates, in the Arrhenius model, an activation volume of 14.5 cm³ mole^?1. Above 2,000 bar the effects of strain due to partial crystallization become evident, and multiple relaxation is observed. By measuring the compressibility of this material up to 10 kbar an estimate of the free volume is made, giving with P in kbar. This measurement is applied in turn to two models of the glassy state, that of Cohen and Turnbull¹ which yields an activation volume of 84 cm³ mole^?1, and that of Adam and Gibbs,² which leads to a critical configurational entropy of 7.7 k_B. In the liquid state the transverse nuclear relaxation, measured by the spin-echo technique, appears to be governed by the same process as T₁.     

Design considerations for table-top, laser-based VUV and X-ray free electron lasers

A recent breakthrough in laser-plasma accelerators, based upon ultrashort high-intensity lasers, demonstrated the generation of quasi-monoenergetic GeV-electrons. With future Petawatt lasers ultra-high beam currents of ∼100 kA in ∼10 fs can be expected, allowing for drastic reduction in the undulator length of free-electron-lasers (FELs). We present a discussion of the key aspects of a table-top FEL design, including energy loss and chirps induced by space-charge and wakefields. These effects become important for an optimized table-top FEL operation. A first proof-of-principle VUV case is considered as well as a table-top X-ray-FEL which may also open a brilliant light source for new methods in clinical diagnostics. PACS 41.60.Cr; 52.38.Kd     

Self-phase-matched high harmonic generation

Our theoretical analysis reveals that tunnel ionization significantly modifies the electric field of few-cycle laser pulses within a single oscillation period. This subcycle self-modulation is predicted to result in phase matching, making high harmonic generation in the x-ray regime possible for the first time. Such a radiation source opens novel possibilities in the investigation of matter with x-ray techniques, such as time resolved x-ray diffraction and absorption.