Cooperativity of multiple kinesin-1 motors mechanically coupled through a shared load

Recent experiments using single-molecule techniques have characterized the mechanical properties of single kinesin molecules in vitro at a range of loads and ATP concentrations. These experiments have shown that kinesin moves processively along microtubules by alternately advancing each of its motor domains in a hand-over-hand fashion, using Brownian motion and the energy from ATP hydrolysis. We have extended the theoretical analysis of kinesin through a mechanistic model that is capable of describing transient and steady-state behavior. Transient dynamics are needed to describe the effect of external perturbations (e.g. interactions with other kinesin molecules). Quantitative metrics are tailored to characterize the synchronization of nonlinear, nonsmooth systems such as kinesin. These metrics are employed to analyze the simulation results and to quantify the effect of the cargo linker stiffness, the load, and the difference in intrinsic velocity on the synchronization of two coupled motor proteins. Herein, the mechanistic model and the new analysis techniques are demonstrated for the case of two coupled kinesin motors.     

Acoustic inspection of bond strength of steel-reinforced mortar after exposure to elevated temperatures

In order to evaluate the bond strength between the reinforcement and concrete after fire damage, a combination of acoustic through-transmission and pull-out tests were used. Previous studies have shown a 25% decrease in the ultrasonic pulse velocity at 90% of the maximum load at room temperature. The specimens were kept in the oven at an elevated temperature for 1, 2, or 3 h. They were then removed and cooled to room temperature. Inspection was conducted using a high-power ultrasonic pulse velocity system while a pull-out load was applied. The correlation between preheated temperature, acoustic wave velocity, and the applied load was analyzed. Initial results show that bond strength and pulse velocity decreased substantially as the temperature or the heating time increased.     

The effect of load on guided wave propagation

The motivation for this work is the development of load measurement techniques based on the velocity of propagating guided waves in structural members such as cable and rail. A finite element technique for modelling the dispersion characteristics of guided waves in a waveguide of arbitrary cross section subjected to axial load is presented. The results from the FE model are compared to results obtained from a simple Euler–Bernoulli beam model. A dimensionless measure of the sensitivity of phase and group velocity to load is defined as the fractional change in velocity divided by the applied strain. At frequency waveguide-characteristic-dimension products (fd) of greater than around 1 for phase velocity and 5 for group velocity the sensitivity to strain levels likely to be encountered in engineering materials is strain independent (indicating that the change in velocity is proportional to strain) and decreases with increasing frequency. In this fd range, phase velocity increases under tensile loading and group velocity decreases. For waveguides with simple cross sections, such as plates and circular rods, it is shown that the Euler–Bernoulli beam model provides acceptable results over the majority of the fd range where there is measurable sensitivity to load. However, for waveguides with more complex cross sections such as rail, the Euler–Bernoulli beam model is less satisfactory. In particular, it does not predict the subtleties of the sensitivity of certain modes at high frequencies, nor any sensitivity for the torsional fundamental mode.     

Force generation by polymerizing microtubules

Polymerization ratchets formed by the assembly of actin filaments and microtubules are possibly the simplest realizations of biological thermal ratchets. A variety of experimental evidence exists that significant forces are generated by these processes, but quantitative studies lag far behind similar studies for molecular motors such as kinesin and myosin. Here we present a discussion of the theory of polymerization ratchets as well as experimental techniques used in our laboratory for the study of forces generated by single growing microtubules. Data obtained with these techniques provide us with valuable information that may eventually allow us to distinguish between different models for the growth of microtubules. Received: 15 January 2002 / Accepted: 11 February 2002 / Published online: 22 April 2002     

Transition radiation of elastic waves at the interface of two elastic half-planes

Radiation of elastic waves is studied that is emitted by a point load that crosses the interface of two elastic half-planes. It is assumed that the load has a constant magnitude, moves along a straight line normal to the interface, and has a constant speed that is smaller than the minimum shear wave speed in the half-planes. In this case the mechanism of excitation of elastic waves is conventionally referred to as transition radiation. The adopted model allows to obtain an analytical expression for the elastic field excited by the load in the frequency-wavenumber domain. Using this expression, the energy of transition radiation is derived in a closed form. It is shown that transition radiation of the body waves occurs at any non-zero velocity of the load. Additionally, transition radiation of interface waves may occur provided that parameters of the half-planes allow existence of Stoneley waves. A parametric analysis of the directivity diagram of radiated body waves is accomplished focusing on dependence of the diagram on the load speed, load direction, and parameters of the half-planes. Using parameters that allow radiation of interface waves, the energy of this radiation is compared to that of the body waves. It is shown that the energy of the interface waves is greater unless the load velocity is close to the lowest body wave velocity.     

Dynamic response of a pre-stressed cylindrical shell to a moving load

The work presented in this paper is concerned with the response of a pre-stressed, finite, thin circular cylindrical shell under a moving local load with a constant velocity. An analysis is carried out by a dynamic method, and the solutions which are bounded even at the critical velocity are obtained. The effects of the initial stresses on the dynamic responses of the displacement and the stresses are examined in connection with the velocity of the load.     

Steady state response of an infinite beam on a viscoelastic foundation with moving distributed mass and load

Compared with the moving concentrated load model, it is more realistic and proper to use the moving distributed mass and load model to simulate the dynamics of a train moving along a railway track. In the problem of a moving concentrated load, there is only one critical velocity, which divides the load moving velocity into two categories: subcritical and supercritical. The locus of a concentrated load demarcates the space into two parts: the waves in these two domains are called the front and rear waves,respectively. In comparison, in the problem of moving distributed mass and load, there are two critical velocities, which results in three categories of the distributed mass moving velocity. Due to the presence of the distributed mass and load, the space is divided into three domains, in which three different waves exist. Much richer and different variation patterns of wave shapes arise in the problem of the moving distributed mass and load. The mechanisms responsible for these variation patterns are systematically studied. A semi-analytical solution to the steady-state is also obtained, which recovers that of the classical problem of a moving concentrated load when the length of the distributed mass and load approaches zero.     

受力腓肠肌剪切波速度的超声检测

肌肉组织受力时的弹性特征变化,是其功能特性、健康状态的重要评估参量.该文利用在离体肌肉组织表面施加低频振动引起组织发生形变,在组织内部产生剪切波的方法,经过快速超声成像,通过剪切波图像估计剪切波的传播速度.研究结果表明顺肌纤维方向传播的剪切波速度快于垂直肌纤维方向传播的剪切波速度,剪切波的传播速度随受力的增加而变大,且...     

Dry sliding wear behavior of Al-SiO2 composites

Abstract

Al-base composites with different amount of silica (5, 10, 15 and 20 wt.%) were developed using powder metallurgy route and compacts were sintered at 550 °C for 2 h. XRD analysis of all compositions was conducted for phases and amount of the second phase present. Morphology of the composites shows quite uniform distribution of the SiO₂ particles but at higher percentage of SiO₂ particles the clustering starts. Mechanical properties such as uniaxial compressive strength (UCS) and hardness were evaluated and it is seen that among all compositions, composite with 10 wt.% SiO₂ has maximum UCS and hardness. Wear behavior of all composites was studied with sliding distance, applied loads, sliding velocity and composition. All composites show a linear increase in cumulative wear with distance and load. Wear rate with load increases continuously for all compositions, however, composite with 10 wt.% SiO₂ revealed minimum wear rate with distance, sliding velocity and loads. Wear rate with sliding velocity increases sharply after attaining minima at 3 m/s sliding velocity. SEM analysis of wear tracks is in agreement with wear results. Al-10 wt.%SiO₂ also shows minimum wear coefficient values for all loads, however, wear coefficient decreases with load for all compositions.     

“阳”加速器磁驱动平面飞片实验和速度计算校验

脉冲功率驱动源作为磁驱动加载的重要手段,通过调整其电路参数可调节负载电流波形,实现对样品无冲击准等熵加载。在"阳"加速器上,开展了一系列轴对称结构和带状结构构型的磁驱动平面飞片发射实验,电极材料采用不锈钢和LY-12铝。实验中测量了进入负载的电流历史和电极后自由面速度历史,并通过时序控制将两者时间关联起来。本文以测量到的电流历史数据为基础,引入负载电流分布系数,并结合已知的LY-12铝的状态方程数据,计算电极后自由面速度历史和飞片速度历史。通过实验测量自由面速度历史校验负载各个位置的电流分布系数。另外,基于装置参数和实验数据确定了考虑负载电感变化的装置等效电路模型,形成了计算样品压力加载历史和电极后自由面速度历史估算程序。此外,初步分析不同厚度电极的自由面速度历史,获取了电极材料的准等熵加载波剖面信息,观察到一系列准等熵加载下材料动力学性能引起的物理现象。     

Stochastic Model of Microtubule Dynamics

We introduce a continuous time stochastic process on strings made of two types of particle, whose dynamics mimics that of microtubules in a living cell. The long term behaviour of the system is described in terms of the velocity v of the string end. We show that v is an analytic function of its parameters and study its monotonicity properties. We give a complete characterisation of the phase diagram of the model and derive several criteria of the growth (\(v>0\)) and the shrinking (\(v<0\)) regimes of the dynamics.     

Abrasive wear on the atomic scale

A scanning force microscope in ultrahigh vacuum has been used to realize and detect atomic-scale abrasion on KBr(001). The continuous time evolution of the lateral force under scratching reveals that the wear mechanism is due to the removal and the rearrangement of single ion pairs. The debris is reorganized in regular terraces with the same periodicity and orientation as the unscratched surface, as in local epitaxial growth. The applied load has a strong influence on the abrasive process, whereas the scan velocity is less relevant.     

Modeling polymerization of microtubules: A semi-classical nonlinear field theory approach

In this paper, for the first time, a three-dimensional treatment of microtubules’ polymerization is presented. Starting from fundamental biochemical reactions during microtubule’s assembly and disassembly processes, we systematically derive a nonlinear system of equations that determines the dynamics of microtubules in three dimensions. We found that the dynamics of a microtubule is mathematically expressed via a cubic-quintic nonlinear Schrödinger (NLS) equation. We show that in 3D a vortex filament, a generic solution of the NLS equation, exhibits linear growth/shrinkage in time as well as temporal fluctuations about some mean value which is qualitatively similar to the dynamic instability of microtubules. By solving equations numerically, we have found spatio-temporal patterns consistent with experimental observations.     

Effect of interfacial bonding on impact properties of chopped glass fiber polymer nanocomposites

This paper aims to presents the investigations made on the effect of impact response of chopped glass fiber–epoxy nanocomposite laminates subjected to low velocity impact using instrumented falling weight impact tests. The laminates were prepared using six layers of chopped strand mat with density of 610 gsm with epoxy resin and nanoclay content varied from 1, 3, and 5 wt%, by hand lay-up method. The nanoclay was dispersed into the epoxy by high shear mixing process in order to obtained uniform distribution of individual nanoclay. Laminates were impacted at constant mass with different impact energies. During these low velocity impact tests, the maximum load, absorbed energy, and deflection at peak load were recorded. It was observed that by dispersion of nanoclay as reinforcement, there was significant improvement in load carrying capacity and energy absorption. When a small amount of nanoclay (1 wt%) was added into the epoxy, the maximum load was enhanced by 20%. The presence of stiffer nanoclay significantly reduced the surface cracks propagation and controlled delamination area. Scanning electron microscopy was performed to characterize the damage progression.     

高生长速度条件下的“层片↔棒状”共晶转变机理研究

低速生长条件下, 共晶“层片↔棒状”转变只由两相的体积分数控制. 高速情况下, 这种转变有时亦发生, 其转变机理不清楚. 本文应用竞争生长准则, 结合高速生长条件下层片共晶和棒状共晶生长模型研究了生长速度引起的“层片↔棒状”转变机理. 结果显示: 体积分数在临界值附近很小的范围内, 生长速度和溶质配分系数的增大可引起“棒状→ 层片”共晶转变; 而当体积分数远离临界值时, 转变不发生. 生长速度名义上引起“层片↔棒状”共晶转变实际上是由生长速度变化引起的体积分数的变化导致的. 关键词： “层片↔棒状”共晶转变 竞争生长 生长速度 体积分数     

弱熔体对流对定向凝固中棒状共晶生长的影响

利用渐近方法求出在弱对流熔体中定向凝固棒状共晶生长的浓度场的渐近解,研究了弱熔体对流对定向凝固中棒状共晶生长的影响.结果表明,弱熔体对流对定向凝固中棒状共晶生长有显著的作用;平均界面过冷度不仅与棒状共晶的棒间距、生长速度有关,还与流动强度有关;当生长速度一定时,随着流动强度增大,棒状共晶的平均界面过冷度减小.利用最小过冷原则,获得棒间距与生长速度和流动强度的关系.结果表明,当生长速度比较小时,随着流动强度增大,棒状共晶的棒间距增大;当生长速度比较大时,随着流动强度增大,棒状共晶的棒间距变化减弱;棒状共晶的生长速度越小,流动对棒状共晶生长的影响越大.利用本文的解析结果计算在对流条件下Al-Cu共晶的棒间距,结果显示随着转速增大或径向距离增大,共晶的间距增大,这与Junze等的实验结果相符合.     

A general procedure for the dynamic analysis of finite and infinite beams on piece-wise homogeneous foundation under moving loads

Transversal vibrations induced by a load moving at a constant speed along a finite or an infinite beam resting on a piece-wise homogeneous visco-elastic foundation are studied. Special attention is paid to the amplification of the vibrations which arise as the point load traverses a foundation discontinuity. The governing equations of the problem are solved by the normal-mode analysis. The solution is expressed in the form of an infinite sum of orthogonal natural modes multiplied by the generalized displacements. The natural frequencies are obtained numerically exploiting the concept of the global dynamic stiffness matrix. This ensures that the frequencies obtained are accurate. The methodology is neither restricted by load velocity nor damping and is simple to use, though obtaining the numerical expression of the results is not straightforward. A general procedure for numerical implementation is presented and verified. There is no restriction for finite structures, however, for infinite structures, validity of the results is restricted to a “region of interest” of finite length. To illustrate the methodology, the probability of exceeding an admissible upward displacement is determined when the load travels at a certain velocity according to the normal distribution. In this problem, the given structure has an intermediate part of adaptable foundation stiffness, which is optimized in a parametric way, enabling to draw important conclusions about the optimum intermediate stiffness. The results obtained have direct application on the analysis of railway track vibrations induced by high-speed trains crossing regions with significantly different foundation stiffness.     

Absolute stability of the solidification interface in a laser resolidified Zn--2wt.%Cu hypoperitectic alloy

This paper reports on laser surface remelting experiments performed on a Zn--2wt.%}Cu hypoperitectic alloy by employing a 5kW CW CO₂ laser at scanning velocities between 6 and 1207mm/s. The growth velocities of the microstructures in the laser molten pool were accurately measured. The planar interface structure caused by the high velocity absolute stability was achieved at a growth velocity of 210~mm/s. An implicit expression of the critical solidification velocity for the cellular--planar transition was carried out by nonlinear stability analyses of the planar interface. The results showed a better agreement with the measured critical velocity than that predicted by M--S theory. Cell-free structures were observed throughout the whole molten pool at a scanning velocity of 652~mm/s and the calculated minimum temperature gradient in this molten pool was very close to the critical temperature gradient for high gradient absolute stability (HGAS) of the \eta phase. This indicates that HGAS was successfully achieved in the present experiments.     

Influence of boundary structure and near neighbor crystallographic orientation on the dynamic damage evolution during shock loading

The role of crystallographic orientation on damage evolution in ductile metals during shock loading has been investigated. By utilizing large-grained copper specimens, it has been shown that the development of intragranular damage, in the form of void growth and coalescence, is influenced by the grain orientation with respect to the applied load. Additionally, strain incompatibility and the inability to promote transmission or activation of secondary dislocation slip across a grain boundary, are proposed as the likely cause for intergranular failure. Finally, the free surface velocity profiles of each grain, specifically the decay of the oscillations after the pull-back, correlated well with the amount of damage measured within the respective grain.