Optical Study of Interactions of Glass Beads ER Particles

The amount of ER effect is determined by the difference of dielectric constants of ER particles base liquid.The intensity of ER fluids can be characterized by the interaction of two particles. A double optical tweezers system suitable to study particle interaction is used to measure the particle aggregation time, and it is found that the particle aggregation time is proportional to the square of the electric field. This is the first time to directly measure the interaction of the electric dipoles of ER particles. A method is developed to use high speed CCD in measuring diffusing-wave spectroscopy (DWS), and, for the first time, the auto-correlation functions of nonegordic system of particle structure are measured to study the ER mechanism. Structure response time and force response time are obtained for glass beads ER fluid, and the time variation of characteristic decay times of system correlation functions under different electric fields is also measured. Diffusing coefficients under different fields imply that the interaction is proportional to the square of fields.     

Transient response of a Brownian particle with general damping

We study the transient response of a Brownian particle with general damping in a system of metastable potential well. The escape rate is evaluated as a function of time after an infinite wall is removed from the potential barrier. It takes a relaxation time for the rate to reach its limit value and this rate relaxation time differs from the relaxation time of the majority of the probability around the bottom of the potential well. The rate relaxation time is found to depend on the temperature as well as the damping constant. It involves the diffusion time and the instanton time, in general agreement with recent studies of the overdamped case by Bier et al. [Phys. Rev. E 59, 6422 (1999)].     

射流携带床反应器液体停留时间分布及模拟

对射流携带床液体停留时间分布进行了研究,考察了液体流量、气体流量和气体动量对液体停留时间分布的影响。结果表明,增加液体流量使停留时间分布密度曲线变得高而窄,且平均停留时间变短;气体流量增大使得停留时间出峰略有提前,气体流量大于4L/min时,继续增大气体流量对液体停留时间分布影响较小;当液体流量小于60L/h时,气体动量对液体停留时间的影响较明显,主要表现气体动量越大液体平均停留时间越长。基于实验结果分析及实验中观测的现象,将射流携带床内液体流动结构分为中心区和壁面区进行研究,建立了描述射流携带床内液体停留时间分布的数学模型,模型模拟结果和实验数据吻合良好。     

电吸附除氯过程的电化学阻抗谱及动力学研究

借助三电极体系, 基于电化学交流阻抗谱图, 提出了一种对已吸附Cl^-的活性炭再次吸附一个Cl^-弛豫时间的测定方法, 根据弛豫时间确定速率控制步骤 . 研究了阳极电势、 预处理时间和预处理浓度对电化学过程的影响, 基于得到的电化学交流阻抗谱图上的参数, 求出不同条件下电吸附Cl^-的弛豫时间及覆盖度. 结果表明, 不同条件下得到的复数平面图均由一个容抗弧和一个感抗弧构成, 分别代表Cl^-在阳极上发生电荷转移的过程和 Cl^-在活性炭电极上的吸附过程 . 增加阳极极化可有效缩短弛豫时间, 阳极极化时, 弛豫时间为2.0×10^-5 s; 增加预处理时间, 弛豫时间逐渐增加, 预处理时间为180 min时, 弛豫时间增加到4.9×10^-5 s; 预处理浓度对弛豫时间的影响可忽略. 弛豫时间分析结果表明, Cl^-吸附速率比扩散速率小, 吸附是电吸附过程的速率控制步骤. 电极表面的覆盖度较低, 仅有10^-4...     

Phase Resolved Method of Photoacoustic Spectrum on Amino Acid L-Tryptophan Compounds

Abstract

A discussion on using the phase shifts of the photoacoustic signal of a monophase sample for resolving its overlapping absorption peaks is presented. It is experimentally shown that the method is simple and useful. And the time delay which includes the nonradiative relaxation time, the thermal-diffusion time and the time of generating the acoustic signal is calculated. This is demonstrated using L-Tryptophan compounds.     

A laplace transform technique for calculating diffusion time lags

A simple formula for diffusional time lags is derived for systems governed by linear equations with source terms. This formula is used to calculate the time lag for systems in which diffusion, dissolution and reversible or irreversible reactions occur. It is also shown that time lags can be calculated for nondiffusional systems. The application of time lags to pharmacokinetics is discussed in particular     

剪切均匀性对流动诱导等规聚丙烯结晶的影响

固定应变和最终应变速率,采用瞬时和缓慢2种电机加速方式对样品施加剪切,研究了流场加载模式对样品流变和结晶行为的影响.实验结果显示缓慢加速能够消除剪切过程中流场的非均匀性,使样品取向度增加,提高流场对聚合物熔体的作用效果.同时,流动诱导结晶对于加速时间有依赖性.对于速率为17.7 s-1的剪切,加速时间为1 s时,熔体流动均匀且流动诱导的晶体取向最强,短加速时间(0.5 s)和长加速时间(1.5 s)样品的流动诱导结晶效果都弱于加速时间为1 s的样品.但是,对于不同剪切速率,其对应的最优加速时间不同.对于流动诱导结晶来说,加速时间应当作为一个重要参数来考虑,其背后的真实物理含义还需要进一步研究来说明.     

Rupture of thin stagnant films on a solid surface due to random thermal and mechanical perturbations

A generalized formalism for the rupture of a nondraining thin film on a solid support due to imposed random thermal and mechanical perturbations, modeled as a Gaussian white noise, is presented. The evolution of amplitude of perturbation is described by a stochastic differential equation. The average film rupture time is the average time for the amplitude of perturbation to equal to the film thickness and is calculated by employing a first passage time analysis for different amplitudes of imposed perturbations, wavenumbers, film thickness, van der Waals and electrostatic interactions and surface tensions. The results indicate the existence of an optimum wavenumber at which the rupture time is minimum. A critical film thickness is identified based on the sign of the disjoining pressure gradient, below which the film is unstable in that the rupture time is very small. The calculated values of rupture time as well as the optimum wavenumber in the present analysis agree well with the results of linear stability analysis for immobile as well as completely mobile gas-liquid film interfaces. For stable films, the rupture time is found to increase dramatically with film thickness near the critical film thickness. As expected, the average rupture time was found to be higher for smaller amplitudes of imposed perturbations, larger surface potentials, larger surface tensions and smaller Hamaker constants.     

Delayed yielding of polycarbonate under constant load

The time-dependent yielding of glassy polycarbonate subjected to constant tensile loads has been studied. Application of a constant stress of a magnitude between the yield stress and the stress required for propagation of a neck in constant strain tests results in inhomogeneous yielding after a well-defined time lag. This delay time increases with decreasing stress and temperature. The critical stress for slowly cooled material is greater than that for quenched material in which the delay time is divided in two regions. The delay time is regarded as the time required for the initiation of inhomogeneous yielding at either edge of the specimen and growth over a certain distance across the specimen. Geometrical observations revealed that the inhomogeneous yielding is shear yielding which is initiated due to stress inhomogeneities caused by mechanical imperfections at the edge of the specimen. The Eyring treatment of delayed yielding can describe fairly well the stress and temperature dependence of the delay time.     

Monte Carlo simulation based on dynamic disorder model in organic semiconductors: from coherent to incoherent transport

The dynamic disorder model for charge carrier transport in organic semiconductors has been extensively studied in recent years. Although it is successful on determining the value of bandlike mobility in the organic crystalline materials, the incoherent hopping, the typical transport characteristic in amorphous molecular semiconductors, cannot be described. In this work, the decoherence process is taken into account via a phenomenological parameter, say, decoherence time, and the projective and Monte Carlo method are applied for this model to determine the waiting time and thus the diffusion coefficient. It is obtained that the type of transport is changed from coherent to incoherent with a sufficiently short decoherence time, which indicates the essential role of decoherence time in determining the type of transport in organics. We have also discussed the spatial extent of carriers for different decoherence time, and the transition from delocalization (carrier resides in about 10 molecules) to localization is observed. Based on the experimental results of spatial extent, we estimate that the decoherence time in pentacene has the order of 1 ps. Furthermore, the dependence of diffusion coefficient on decoherence time is also investigated, and corresponding experiments are discussed.     

Physical interpretation of mean local accumulation time of morphogen gradient formation

The paper deals with a reaction-diffusion problem that arises in developmental biology when describing the formation of the concentration profiles of signaling molecules, called morphogens, which control gene expression and, hence, cell differentiation. The mean local accumulation time, which is the mean time required to reach the steady state at a fixed point of a patterned tissue, is an important characteristic of the formation process. We show that this time is a sum of two times, the conditional mean first-passage time from the source to the observation point and the mean local accumulation time in the situation when the source is localized at the observation point.     

Photon counting statistics for blinking CdSe-ZnS quantum dots: a Lévy walk process

We analyze photon statistics of blinking CdSe-ZnS nanocrystals interacting with a continuous wave laser field, showing that the process is described by a ballistic Lévy walk. In particular, we show that Mandel's Q parameter, describing the fluctuations of the photon counts, is increasing with time even in the limit of long time. This behavior is in agreement with the theory of Silbey and co-workers (Jung et al. Chem. Phys. 2002, 284, 181), and in contrast to all existing examples where Q approaches a constant, independent of time in the long time limit. We then analyze the distribution of the time averaged intensities, showing that they exhibit a nonergodic behavior, namely, the time averages remain random even in the limit of a long measurement time. In particular, the distribution of occupation times in the on-state compares favorably to a theory of weak ergodicity breaking of blinking nanocrystals. We show how our data analysis yields information on the amplitudes of power-law decaying on and off time distributions, information not available using standard data analysis of on and off time histograms. Photon statistics reveals fluctuations in the intensity of the bright state indicating that it is composed of several states. Photon statistics exhibits a Lévy walk behavior also when an ensemble of 100 dots is investigated, indicating that the strange kinetics can be observed already at the level of small ensembles.     

响应面法优化巴旦木壳对废水Pb、Cu和Cd的吸附及同时测定

研究废弃巴旦木壳对模拟废水中Pb、Cu和Cd的去除率。在单因素试验的基础上,采用响应面法对吸附剂投加量、吸附时间和pH值3因素进行优化。实验结果表明,最佳吸附条件为Pb：为吸附剂投加量0.4g、吸附时间49.38min、pH值为9.96;Cu：吸附剂投加量0.4g、吸附时间49.91min、pH值为10.13;Cd：吸附剂投加量0.4g、吸附时间49.83min、pH值为10.42;在此条件下,Pb、Cu和Cd的去除率分别为87.42%、73.49%和85.11%。采用偏最小二乘法(PLS)对Pb、Cu和Cd模拟混合试样吸附后的溶液进行同时测定,计算得出吸附剂对Pb、Cu和Cd的去除率分别为83.2%、66.0%和83.3%。     

A low cost spectrum multiscaling analyzer based on personal computer for studying the decay properties of short-lived nuclides

A low cost spectrum multiscaling analyzer system based on an IBM PC is described. Interrupt service routines were used to handle both the digital data converted by ADC and record the dead time profile. A dead time counter was provided to handle the dead time problem by counting the busy time of amplifier and ADC, which is important in correcting the decay rate of the short-lived nuclides during the counting interval.     

Linear scaling computation of the Fock matrix. VII. Parallel computation of the Coulomb matrix

We present parallelization of a quantum-chemical tree-code for linear scaling computation of the Coulomb matrix. Equal time partition is used to load balance computation of the Coulomb matrix. Equal time partition is a measurement based algorithm for domain decomposition that exploits small variation of the density between self-consistent-field cycles to achieve load balance. Efficiency of the equal time partition is illustrated by several tests involving both finite and periodic systems. It is found that equal time partition is able to deliver 91%-98% efficiency with 128 processors in the most time consuming part of the Coulomb matrix calculation. The current parallel quantum chemical tree code is able to deliver 63%-81% overall efficiency on 128 processors with fine grained parallelism (less than two heavy atoms per processor).     

全二维气相色谱第二维死时间的测定

建立了两种恒压模式下全二维气相色谱第二维死时间的测定方法。一种方法是利用不同压力下的相对保留时间差规律,计算非同步调制的全二维气相色谱第二维的保留时间,再利用正构烷烃同系物的保留规律线性拟合计算第二维的死时间;测定的第二维的死时间与温度的线性相关系数大于0.997。另一种方法是在已知化合物保留因子和温度关系的条件下,在一次程序升温中测定此化合物的3个以上不同流出温度条件下的表观保留时间,再根据该表观保留时间计算出死时间与温度的关系。实验结果表明,两种方法对死时间测定的偏差小于0.05 s。这两种方法适合于各种类型的全二维气相色谱,无论其调制方式是同步还是非同步。     

Light-scattering photography of polymers near the glass transition

Laser light scattered from a block of polystyrene is observed to have a pattern of bright and dark regions called speckle. We observe that the speckle pattern has a lifetime consistent with that of the average relaxation lifetime 〈τ〉 of the intensity fluctuations of the medium as determined by photon correlation spectroscopy. In order to study the pattern of the scattered light, a series of photographs was taken at a variety of exposure times. For each series, the value of the average relaxation lifetime is defined by the temperature and pressure of the polystyrene sample. When the value of 〈τ〉 is short relative to the exposure time, the photograph displays only a random pattern of exposed grains. This is due to the large number of fluctuations that have occurred during the exposure time with random phases relative to one another. As the average relaxation time is increased at a constant exposure time, the speckle pattern appears when the value of 〈τ〉 becomes comparable to the exposure time. The phenomenon of laser speckle allows the time scale of the slowly relaxing fluctuations near the glass transition to be visualized. A digitized series of such pictures could be analyzed to obtain the average relaxation time for the fluctuations, just as in normal photon correlation spectroscopy where the intensity of one coherence area is measured as a function of time.     

Luminescence kinetics and association equilibria of lanthanide ions in aqueous solutions

Luminescence life time measurement can be used under certain conditions to determine the thermodynamic constants of complex formation between the luminescent (central ion) and ligand. The basic equations correlating the life time and the equilibria constants were derived for two cases: the time for establishing the thermodynamic equilibrium is much shorter than the life time of the excited state of the central ion; and the time for establishing the equilibrium and the life time are of the same order of magnitude.     

Nanosensors lost in space. A random walk study of single molecule detection with single-nanopore sensors

Nanopores by providing single molecule detection and manipulation are lately in the forefront of life science and nanotechnology research. While single nanopore sensors can detect the residence of even one molecule or nanoparticle within the nanopore, the analytical significance of this process is often misunderstood. A fundamental problem of nanosensors is that their sensing zone is generally infinitesimal with respect of the probed sample volume. Consequently, the probability to have in extremely diluted solutions target molecules or nanoparticles encountering the nanosensor is low. Thus, eventhough the sensor by itself has single molecule detection capability the average time frame in which this occurs is by far not irrelevant for the analysis. In this paper we report on random walk simulations to determine the average time (encounter time) needed by a single molecule to encounter a single nanopore sensor. By assigning the simulation environment with real space and time values a semi-empirical equation for expressing the average encounter time in purely diffusive systems is provided. We also show that random walk simulations can be adapted to evaluate the encounter time in the presence of an external force field acting on the target molecule. As practically relevant application the case of electrophoretically driving DNA strands towards the nanopore sensor is presented and a semi-empirical equation for the encounter time is provided.