Formation and interaction of membrane tubes

We show that the formation of membrane tubes (or membrane tethers), which is a crucial step in many biological processes, is highly nontrivial and involves first-order shape transitions. The force exerted by an emerging tube is a nonmonotonic function of its length. We point out that tubes attract each other, which eventually leads to their coalescence. We also show that detached tubes behave like semiflexible filaments with a rather short persistence length. We suggest that these properties play an important role in the formation and structure of tubular organelles.     

Compressing a rigid filament: Buckling and cyclization

We study elastic properties of rigid filaments modeled as stiff chains shorter than their persistence length. By rigid filaments we mean that fluctuations around the optimal filament shape are weak and that low-order expansions (quadratic or quartic) in the deviation from the optimal shape are sufficient to describe them. Our main interest lies in the profiles of force vs. projected filament length, closure probability and weakly buckled states. Results may be relevant to experiments on self-assembled biological (microtubules, actin filaments) and synthetic (organo-gelators) filaments, carbon nanotubes and polymers grafted with strongly repelling side chains, some of which are discussed here.     

The propagation dynamics of ultraviolet light filament with Rayleigh scattering in air

In this paper we present for the first time the effects of Rayleigh scattering on the long distance propagation of ultraviolet （UV） light filament in air based on the stationary analysis. The simulation results show that the effects of Rayleigh scattering on the propagation of UV laser filaments may not be ignored. These influences are slightly dependent on the laser wavelength. We also compare the UV filament propagations at different input powers in the presence and the absence of the Rayleigh scattering and discuss the mechanisms of power loss and beam defocusing. In the absence of Rayleigh scattering, the filament propagation is determined by the oscillating behaviour of the beam size. In the presence of the scattering, the propagation lengths of filament are close to each other at different initial powers and determined by the Rayleigh scattering.     

Optically directed assembly of continuous mesoscale filaments

We demonstrate irreversible continuous filament formation when a weak laser focus is positioned near the edge of an evaporating colloidal droplet containing carbon and gold nanoparticles. Optical trapping, hydrothermal, and chemical interactions lead to controlled colloidal synthesis of stable, irreversible mesoscale filaments of arbitrary shape and size. Mechanisms for this optically directed assembly are discussed with fluid dynamics, molecular dynamics, and lattice kinetic Monte Carlo calculations.     

Dynamics of cell membranes and the underlying cytoskeletons observed by noninterferometric widefield optical profilometry and fluorescence microscopy

Combining the noninterferometric wide-field optical profilometry technique with fluorescence microscopy, we observe the membrane activities of a living cell as well as the structures of its cytoskeletons. The membrane ripples of a lamellipodium are related to similar structures of the underlying actin filaments. However, we find the ripples appear prior to and disappear later than the corresponding actin filament structures, which supports the elastic Brownian ratchet model of cell motility. In addition, we measure the three-dimensional movement of a fibronectin-coated latex bead on the membrane. The bead motion is determined by the movement and branching of the actin molecules on the filament, as well as by the displacement of the filament itself.     

Filament instability and rotational tissue anisotropy: A numerical study using detailed cardiac models

The role of cardiac tissue anisotropy in the breakup of vortex filaments is studied using two detailed cardiac models. In the Beeler-Reuter model, modified to produce stable spiral waves in two dimensions, we find that anisotropy can destabilize a vortex filament in a parallelepipedal slab of tissue. The mechanisms of the instability are similar to the ones reported in previous work on a simplified cardiac model by Fenton and Karma [Chaos 8, 20 (1998)]. In the Luo-Rudy model, also modified to produce stable spiral waves in two dimensions, we find that anisotropy does not destabilize filaments. A possible explanation for this model-dependent behavior based on spiral tip trajectories is offered. (c) 2001 American Institute of Physics.     

Unstable membrane outgrowth induced by filament tip spreading

We analyse the dynamics of a network of regularly spaced parallel filaments, growing perpendicularly against an elongating membrane which limits the diffusion of monomers in the half-space where filaments grow. When the membrane bulges, some filaments spread, so that less monomers are absorbed at the bulge periphery. It results in both a local increase of the free monomer concentration and an enhancement of the filament polymerisation rate, leading to an original growth instability. This one, stabilised here at short wavelengths by filament tip diffusion along the membrane and the drawback force exerted by a subjacent network of crosslinking molecules, may play a noticeable part in the initiation of neuritic and lamellipodial structures.     

A geometric theory for scroll wave filaments in anisotropic excitable media

Scroll waves are an important example of self-organisation in excitable media. In cardiac tissue, scroll waves of electrical activity underlie lethal ventricular arrhythmias and fibrillation. They rotate around a topological line defect which has been termed the filament. Numerical investigation has shown that anisotropy can substantially affect the dynamics of scroll waves. It has recently been hypothesised that stationary scroll wave filaments in cardiac tissue describe geodesics in a space whose metric is the inverse diffusion tensor. Several computational studies have validated this hypothesis, but until now no quantitative theory has been provided to study the effects of anisotropy on scroll wave filaments. Here, we review in detail the recently developed covariant formalism for scroll wave dynamics in general anisotropy and derive the equations of motion of filaments. These equations are fully covariant under general spatial coordinate transformations and describe the motion of filaments in a curved space whose metric tensor is the inverse diffusion tensor. Our dynamic equations are valid for thin filaments and for general anisotropy and we show that stationary filaments obey the geodesic equation. We extend previous work by allowing spatial variations in the determinant of the diffusion tensor and the reaction parameters, leading to drift of the filament.     

Long-range spectrally and spatially resolved radiation from filaments in air

The distance-resolved spectral intensity distribution of the backscattered light from long filaments generated in air using ultra-short and intense laser pulses is presented. A clean fluorescence spectrum from N₂ molecules and ions, which is produced by the high peak intensity inside the plasma filament of the fundamental pulse, was clearly resolved from the backscattered supercontinuum. The supercontinuum generated by both the fundamental and the third-harmonic pulses developed progressively and became fully developed only at the end of the filamentation.     

Z箍缩靶用聚合物丝的弛豫特性

 聚苯乙烯（PS）,聚乙烯（PE）及其的氘代物是Z箍缩驱动惯性约束聚变（ICF）实验中的重要固体燃料容器材料,针对物理实验对其形状的特殊要求,利用高压毛细管流变仪及HAUL-OFF熔体拉伸测试单元进行熔融纺丝,制备出直径为30～100 μm的聚合物丝样品。通过对PS,PE以及氘代聚苯乙烯（DPS）丝的力学弛豫性质研究发现：在相同的恒定应力下,实验用PS丝的蠕变量明显小于PE丝,PS丝表现出更好的尺寸稳定性;当定伸长为1％时,PS丝的松弛率明显小于PE丝;DPS丝的蠕变及应力松弛行为与PS丝具有相同的变化趋势。     

Vortex dynamics in three-dimensional continuous myocardium with fiber rotation: Filament instability and fibrillation

Wave propagation in ventricular muscle is rendered highly anisotropic by the intramural rotation of the fiber. This rotational anisotropy is especially important because it can produce a twist of electrical vortices, which measures the rate of rotation (in degree/mm) of activation wavefronts in successive planes perpendicular to a line of phase singularity, or filament. This twist can then significantly alter the dynamics of the filament. This paper explores this dynamics via numerical simulation. After a review of the literature, we present modeling tools that include: (i) a simplified ionic model with three membrane currents that approximates well the restitution properties and spiral wave behavior of more complex ionic models of cardiac action potential (Beeler-Reuter and others), and (ii) a semi-implicit algorithm for the fast solution of monodomain cable equations with rotational anisotropy. We then discuss selected results of a simulation study of vortex dynamics in a parallelepipedal slab of ventricular muscle of varying wall thickness (S) and fiber rotation rate (theta(z)). The main finding is that rotational anisotropy generates a sufficiently large twist to destabilize a single transmural filament and cause a transition to a wave turbulent state characterized by a high density of chaotically moving filaments. This instability is manifested by the propagation of localized disturbances along the filament and has no previously known analog in isotropic excitable media. These disturbances correspond to highly twisted and distorted regions of filament, or "twistons," that create vortex rings when colliding with the natural boundaries of the ventricle. Moreover, when sufficiently twisted, these rings expand and create additional filaments by further colliding with boundaries. This instability mechanism is distinct from the commonly invoked patchy failure or wave breakup that is not observed here during the initial instability. For modified Beeler-Reuter-like kinetics with stable reentry in two dimensions, decay into turbulence occurs in the left ventricle in about one second above a critical wall thickness in the range of 4-6 mm that matches experiment. However this decay is suppressed by uniformly decreasing excitability. Specific experiments to test these results, and a method to characterize the filament density during fibrillation are discussed. Results are contrasted with other mechanisms of fibrillation and future prospects are summarized. (c)1998 American Institute of Physics.     

Scroll waves in spherical shell geometries

The evolution of scroll waves in excitable media with spherical shell geometries is studied as a function of shell thickness and outer radius. The motion of scroll wave filaments that are the locii of phaseless points in the medium and organize the wave pattern is investigated. When the inner radius is sufficiently large the filaments remain attached to both the inner and outer surfaces. The minimum size of the sphere that supports spiral waves and the maximum number of spiral waves that can be sustained on a sphere of given size are determined for both regular and random initial distributions. When the inner radius is too small to support spiral waves the filaments detach from the inner surface and form a curved filament connecting the two spiral tips in the surface. In certain parameter domains the filament is an arc of a circle that shrinks with constant shape. For parameter values close to the meandering border, the filament grows and collisions with the sphere walls lead to turbulent filament dynamics. (c) 2001 American Institute of Physics.     

The observation of field-aligned current during type-III ELM in EAST

Measurements of current-carrying filaments associated with Type-III edge localized mode (ELM) have recently been made in the Experimental Advanced Superconducting Tokamak by direct probing of the edge plasma using an advanced, fast-moving electrostatic and magnetic probe system. Contrary to expectations, no current filaments were detected near the separatrix. However, a clear signature of current filament has been observed in the far scrape-off layer (SOL) where the difference of the voltage between the divertor plates connecting a filament is sufficiently large, thus strongly suggesting that the current-carrying filaments form in the SOL, rather than being ejected from the plasma inside the separatrix. These findings provide, for the first time, information on the formation and sustainment of current filaments during type-III ELMy H-modes.     

How kinesins walk,assemble and transport: A birds-eye-view of some unresolved questions

Eukaryotic cells contain an intricate network of microtubule filaments inside. It provides the mechanical support for maintaining cell shape as well as a railway for intracellular traffic. A special class of ATP hydrolyzing enzymes bind microtubule inside the cells and ‘walk’ along the filament. Kinesins constitute a subset of these so called ‘motor’ proteins. These are a diverse set of proteins capable of converting the chemical energy of ATP hydrolysis to mechanical force and move from one end of the cell to the other carrying a variety of different cargoes. Although the composition, structure and their force generating mechanism is understood in considerable detail, several questions regarding the mechanism of kinesin mediated transport remained unanswered. Here, in this review, I have provided a brief overview of kinesin structure and functions in different intracellular transports and highlighted some of the key unresolved issues.     

Atomistic origin of urbach tails in amorphous silicon

Exponential band edges have been observed in a variety of materials, both crystalline and amorphous. In this Letter, we infer the structural origins of these tails in amorphous and defective crystalline Si by direct calculation with current ab initio methods. We find that exponential tails appear in relaxed models of diamond silicon with suitable extended defects that emerge from relaxing point defects. In amorphous silicon (a-Si), we find that structural filaments of short bonds and long bonds exist in the network, and that the tail states near the extreme edges of both band tails are also filamentary, with much localization on the structural filaments. We connect the existence of both filament systems to structural relaxation in the presence of defects and of topological disorder.     

Chirp control of femtosecond laser-filamentation-induced snow formation in a cloud chamber

Filamentation-induced water condensation and snow formation are investigated using laser pulses with different chirps and pulse widths. Chirped pulses result in the laser filamentation with different spatial lengths and intensities, which has a great impact on airflow motion and snow formation. The experiments show that snow formation mainly relates to the filament intensity distribution. Negative chirped pulses produce a greater amount of snow because of higher intensity inside the filaments as compared with the positive chirped pulses.     

Interference effects in the conical emission of a femtosecond filament in fused silica

It is shown both experimentally and theoretically that interference effects play the key role in the formation of frequency-angular spectrum of the filament conical emission. For the first time, we investigated experimentally the transformation of the conical emission frequency-angular spectrum with an increase in the filament length inside fused silica. We discovered the appearance of fine structure of the conical emission rings produced by lengthy filament. It is shown that the conical emission frequency-angular spectrum is produced by interference of coherent radiation from one or several moving point sources in the filament. The shape of the conical emission spectrum depends on the medium material dispersion, the spectrum structure is determined by length and relative location of filament emitting regions.     

Covariant stringlike dynamics of scroll wave filaments in anisotropic cardiac tissue

It has been hypothesized that stationary scroll wave filaments in cardiac tissue describe a geodesic in a curved space whose metric is the inverse diffusion tensor. Several numerical studies support this hypothesis, but no analytical proof has been provided yet for general anisotropy. In this Letter, we derive dynamic equations for the filament in the case of general anisotropy. These equations are covariant under general spatial coordinate transformations and describe the motion of a stringlike object in a curved space whose metric tensor is the inverse diffusion tensor. Therefore the behavior of scroll wave filaments in excitable media with anisotropy is similar to the one of cosmic strings in a curved universe. Our dynamic equations are valid for thin filaments and for general anisotropy. We show that stationary filaments obey the geodesic equation.