Electrophoretic investigations by laser light scattering

The electrophoresis of various colloidal suspensions has been investigated by laser light scattering. The electric field was applied to the solution by means of a narrow gap electrode configuration using a frequency modulated square wave electric field. The power spectrum of the intensity fluctuations of the scattered light was determined by direct sampling of the light intensity using an A/D converter and an on-line minicomputer. This experimental arrangement is capable of measuring very small Doppler shifts.     

Heterodyne detection of multiply scattered monochromatic light with a multipixel detector

A new technique is presented for measuring the spectral broadening of light that has been multiply scattered from scatterers in motion. In our method the scattered light is detected by a heterodyne receiver that uses a CCD as a multipixel detector. We obtain the frequency spectrum of the scattered light by sweeping the heterodyne local oscillator frequency. Our detection scheme combines a high optical etendue (product of the surface by the detection solid angle) with an optimal detection of the scattered photons (shot noise). Using this technique, we measure, in vivo, the frequency spectrum of the light scattered through the breast of a female volunteer.     

Heterodyne spectroscopy at very low frequencies

We have studied the spectrum of light scattered by capillary waves, thermally excited at the free surface of highly viscous liquids, by heterodyne spectroscopy techniques. We were able to measure spectral widths down to 0.2 Hz.     

Light scattering from colloids

This paper presents a review of light scattering results on static and dynamic properties of ordered colloidal suspensions of charged polystyrene particles and fractal colloidal aggregates. Our studies on static structure factor,S(Q), of ordered monodisperse colloidal suspensions and binary mixtures of particles with different particle diameters, measured by angle-resolved Rayleigh scattering will be discussed. This will include determination of bulk modulus using gravitational compression and observation of colloidal glass (inferred from splitting of the second peak inS(Q)). Dynamic light scattering, with real time analysis of scattered intensity fluctuations, is used to get information about Brownian dynamics of the particles. Recent advances in the field of light scattering from colloidal aggregates which show fractal geometry will also be discussed.     

Invited paperFourier-transform heterodyne spectroscopy of liquid and solid surfaces

Fourier-transform heterodyne spectroscopy is a simple but powerful technique to study narrow line shapes and small frequency shifts. We review the technique and its application to the study of hydrodynamic fluctuations. We introduce a simple scheme for obtaining the full spectrum of light scattered from liquid and solid surfaces. Using this scheme we obtained a spectral resolution of 200 Hz. Two applications of the technique, one involving the detection of counter-propagating capillary waves, the other measurement of the epitaxial crystal-growth speed of a crystalline silicon interface, are presented. Received: 21 August 1996     

Dynamic light scattering in localized coherence volumes

We introduce a novel light-scattering technique for investigating the dynamics of random media with a broad range of optical densities. By use of the spatial coherence properties of a single-mode optical fiber and the temporal coherence of a broadband source, the measurement volume is isolated at the end of the optical waveguide. Optical mixing between the fluctuating scattered light and the Fresnel-reflected field at the fiber-medium interface is analyzed directly in the frequency domain. The unique characteristics of this new technique are discussed in the context of simultaneous measurement of average scatterer size and concentration in dense colloidal suspensions.     

高散射媒质中的光程可分割动态光散射研究

为研究散射光强度随光子在散射媒质中散射光程的变化,基于单散射理论与扩散波光谱理论,采用了低相干动态光散射装置对不同粒径大小的聚苯乙烯悬浮液进行研究。将测量得到的背散射光光场强度谱的线宽与相应的理论计算结果相比较发现,在短光程区域,考虑容器壁附近拖曳效应的影响后,对于不同粒径的颗粒,光程为约5倍粒子平均自由程的区域可看成为单散射区域;对于光程大于225 m的区域可看成为扩散光波区域。实验结果表明低相干动态光散射法可实现高散射媒质从单散射区域到低次散射再到扩散区域的全光程的可分割的光场强度谱测量。     

Random surfaces that suppress single scattering

We present a method for numerically generating a one-dimensional random surface, defined by the equation x(3)=zeta(x(1)), that suppresses single-scattering processes in the scattering of light from the surface within a specified range of scattering angles. Rigorous numerical calculations of the scattering of light from surfaces generated by this approach show that the single-scattering contribution to the mean scattered intensity is indeed suppressed within that range of angles.     

利用散射光增强弱吸收固体混合物中主要光吸收物质的光声光谱特征

在传统的透射和反射光谱中, 散射光通常是误差的主要来源之一, 然而对光声光谱, 散射光有可能成为促进光谱测量的积极因素. 本文研究了弱吸收固体混合物–-奥氮平药片及其粉末的光声光谱. 为了排除光声池吸收散射光所产生背景光声信号的干扰, 取试样与其空白物的差值谱进行碳黑归一化, 得到了只与试样自身性质相关的归一化光声光谱.实验发现通过将药片碾成粉末, 可以使奥氮平药片的主成分奥氮平原料药的光声光谱特征凸显出来. 分析表明光散射效应是这一现象产生的主要原因. 传统光谱技术的障碍–-散射光却能促进光声光谱测量, 这显示出光声技术在光谱测量领域的独特优势. 上述实验提供了一种初步快速鉴别弱吸收固体混合物中少量光吸收物质的新方法, 这一方法有望应用于固体药物、矿物和土壤分析等领域. 关键词： 光声光谱 散射光 弱吸收固体混合物 奥氮平     

浓悬浮液中纳米SiO_2团聚体的渗透率

本文基于悬浮液中渗透性颗粒的短时扩散动力学理论,采用低相干光纤动态光散射方法,测量了相同粒径的纳米SiO_2团聚体在不同体积分数时的扩散系数,利用扩散系数随渗透率的变化关系得到纳米SiO_2团聚体的渗透率.结果表明:恒温条件下,具有一定渗透率的团聚体颗粒扩散得比硬球颗粒快.实验测量得到的团聚体渗透率与采用photoshop CS6对团聚体SEM图像进行处理计算得到的渗透率符合较好.     

基于空间外差的拉曼信号处理方法研究

空间外差拉曼光谱技术是近年兴起的一种超光谱探测技术,它既具有拉曼光谱测量的非接触、快速、简单、可重复、无需样品准备等特点,更具有空间外差的超光谱分辨率、高通量、无运动部件优点,能在被测物质特征波长中心范围探测,实现微弱拉曼光信号的直接测量。由于被测信号微弱、光学元件加工精度、器件封装及仪器安装误差等原因,会导致空间外差拉曼光谱仪(SHRS)接收到的干涉图存在光强分布不均匀、干涉条纹倾斜或扭曲等现象,从而使普通光谱恢复方法得到光谱信号准确度下降或无法识别。根据SHRS探测到的干涉图所存在的误差特点,将二维傅里叶变换应用于SHRS干涉图的光谱复原,提出基于二维频域谱重采样的最强直线方向光谱提取方法,以实现SHRS光谱的获取。提取过程为：将采集的目标干涉图进行二维傅里叶变换,获得二维频谱图,通过使用相同实验系统采集的单波长或多波长光源的二维频谱信号特征峰的位置信息,拟合获取光信息强度最大方向直线方程。然后根据该直线与目标二维频谱图各列交点坐标位置,确定重采样贡献像元及权重。对拟合直线方程经过的所有列像元进行重采样,得到最终的光谱信号。将该方法应用于自行搭建实验系统采集的三叶草干涉图数据,同时与其他方法光谱复原结果进行对比。结果表明：与一维行平均光谱法比较,该方法获取的光谱在探测波段中心区域信号强度更加明显,同时消除了探测器热噪声的影响;与二维频谱中心行直接提取法比较,该方法是该方法的改进版本,两者结果比较接近。但是由于考虑到干涉条纹y分量的影响,沿二维频谱信号最强方向重采样得到的最终光谱,其主峰半峰宽更窄、边频噪声强度更小,且随着y分量的增加,光谱复原效果及优势将越发明显。该方法是空间外差拉曼光谱技术数据处理的一种有益补充与尝试。     

A fibre-optic sensor for flocculation detection by electric field light scattering

A laser-based apparatus is described that is able to differentiate between the isolated and flocculated state of colloidal particles in concentrated suspensions. It is based on the principle that the intensity of the scattered light changes when anisodiametric particles are made to rotate and align in an electric field. The system is demonstrated by measurements on kaolinite clay suspensions and can be readily adapted to monitor and control suspensions deflocculation in ‘on-line’ industrial processes.     

应用低相干动态光散射法测量悬浮液中的颗粒粒径分布

利用低相干动态光散射法测量不同浓度悬浮液中的颗粒粒径分布。由低相干光源和迈克尔逊干涉仪,结合传统的动态光散射技术组成低相干动态光散射装置,通过检测单次背散射光的能谱分布,利用CONTIN算法得到不同浓度悬浮液中颗粒的粒径分布。结果表明,对于1%～10%体积浓度范围内的单分散悬浮液羊品测量得到的颗粒粒径精确度在4%以内。     

Multispeckle diffusing wave spectroscopy of colloidal particles suspended in a random packing of glass spheres

We use a multispeckle diffusing wave spectroscopy (MSDWS) method to study the ensemble-averaged dynamics of the fluctuating speckle pattern when illuminating colloidal particles suspended in a static and opaque porous medium with a coherent light source. Experiments were performed with Brownian latex particles in a random packing of glass spheres. The mixing of the light scattered by the moving colloidal particles and the porous matrix gives rise to a plateau value of the intensity autocorrelation function in the long-waiting-time limit. From the plateau in the correlation function, we can determine the fraction of light scattered from moving particles and estimate the photon mean free path in the colloidal solution. The method opens up promising possibilities to probe the static fraction in semisolid materials.     

Small-angle light scattering from polymer-dispersed liquid-crystal films

A method is developed for modeling and computing the angular distribution of light scattered forward from a single-layer polymer-dispersed liquid-crystal (PDLC) film. The method is based on effective-medium approximation, anomalous diffraction approximation, and far-field single-scattering approximation. The angular distribution of forward-scattered light is analyzed for PDLC films with droplet size larger than the optical wavelength. The method can be used to study field-and temperature-induced phase transitions in LC droplets with cylindrical symmetry by measuring polarized scattered light intensity.     

Note on transport processes in dense colloidal suspensions

A new approach to transport processes in dense charged as well as neutral colloidal suspensions is presented. It is based on a far-reaching analogy between dense colloidal suspensions and dense hard-sphere fluids, implying, in turn, an analogy with atomic liquids. As a result, new expressions valid for a number of colloidal transport coefficients are predicted.     

Laser light scattering and correlation techniques for characterization of colloidal suspensions

Particle motion in colloidal suspensions causes Doppler shifts in scattered laser light. For pure diffusion, the resulting spectral broadening may be measured by intensity interferometry and photon correlation techniques. Measured line widths or decay times yield particle size information. Electrophoretic mobilities may be determined, if an electric field is applied to the suspension. However, resulting line shifts can be extremely small and real-time optical phase processing may be required to resolve it. Finally, information about particle interactions may be obtained by studying angular cross sections or by photon correlation measurements on a small number of particles, i.e. using a very small scattering volume.Dedicated to Professor Harry Thomas on the occasion of his 60th birthday     

Doppler spectrum-based polar nephelometer

The particle scattering asymmetry parameter of an ensemble of submicrometer particles, as well as the complete scattering phase function (angular distribution), is determined by measuring the Doppler spectrum of the scattered light. No calibration is required. The monitor incorporates a 532?nm laser with long coherence length, an integrating sphere to collect the scattered light, and a heterodyne (interferometric) detection scheme. Measurements of monodisperse polystyrene latex spheres and polydisperse ammonium sulfate particles indicate that the measured value of the asymmetry parameter is generally within ±0.03 of that calculated using Mie theory.     

电解质对浓悬浮液中胶体颗粒扩散特性的影响

本文利用相位调制光纤低相干动态光散射装置, 研究了不同物质量浓度下电解质 (NaCl及 BaCl₂) 对浓悬浮液中聚苯乙烯胶体颗粒扩散特性的影响. 实验结果表明, 当电解质浓度低于0.01 mol/L 时, 恒温条件下浓悬浮液中聚苯乙烯胶体颗粒扩散系数随电解质离子浓度以及离子化合价的增大而增大, 实验测量得到的扩散系数与Stern模型所得到的扩散系数符合较好. 关键词： 测量 电解质 浓悬浮液 低相干动态光散射     

A generalized Milne solution for the correlation effects of multiple light scattering with polarization

The Wiener-Hopf method is used to obtain a generalized Milne solution for scalar and electromagnetic fields with single-scattering anisotropy. For the scalar field, the solution found for the time correlation function and the interference component of backscattering is in good agreement with available experimental data. A solution of the Milne problem is constructed for the electromagnetic field. Given anisotropy, the generalized Milne equation is solved in the P₁ approximation for the quantity that describes the degree of depolarization of the scattered light. It is shown that the depolarization of the scattered light can change sign at large anisotropies.