Particle Dynamics in Crystallizing Colloidal Suspensions

这项研究将率先使用一种新的光散射技术--扩散波光谱(DWS)来研究胶体体系中结构形成的动力学。在应用于胶体悬浮液的DWS中，光在离开系统之前从许多胶体颗粒上散射出去。散射光由PM管测量。对于每个特定的光子路径，路径(&gt；ri(T))中粒子的位移引起散射强度的变化。通过测量作为时间函数的散射强度，可以获得散射光的自相关G1(T)。假设(1)沿光子路径的散射的净相位变化是高斯随机变量，(2)光子路径中粒子的位移矢量(&gt；ri(T))和散射矢量qi不相关，以及(3)传播方向在长度L*上变得不相关。在这些假设下，单光子路径的关联函数的贡献可以用均方位移&GT来表示；R2，L*和路径长度S。为了得到S长度的光程对总散射强度的贡献，首先通过求解特定样品几何形状的扩散方程得到时间t和位置r的光的能量密度(单位体积的光子数U(t，r))，并从这个函数得到加权因子。输运平均自由程是通过对不相互作用的粒子测量自相关并从理论上估计均方位移的校准过程来获得的。这项技术将用于探索无序和有序系统中离子强度和浓度对短时粒子动力学的影响。

This study will pioneer the use of a newly emerging light scattering technique, diffusive wave spectroscopy (DWS), to the study of the dynamics of structure formation in colloidal systems. In DWS applied to colloidal suspensions, light scatters off of many colloid particles before leaving the system. The scattered light is measured by a PM tube. For each particular photon path, the displacement of the particles in the path (>ri(t)) gives rise to a change in the scattering intensity. By measuring the scattering intensity as a function of time, the autocorrelation g1(t) of the scattered light can be obtained. It is assumed that (1) the net phase change for the scatterings along a photon path is a Gaussian random variable, (2) the displacement vector of a particle in a photon path (>ri(t)) and the scattering vector qi are uncorrelated, and (3) the direction of propogation becomes uncorrelated over a length l*. With these assumptions, the contribution to the correlation function for one photon path can be written in terms of the mean square displacement >r2, l* and the path length s. To obtain the contribution of path lengths of length s to the total scattering intensity, the energy density of the light at time t and position r (the photons per unit volume U(t,r)) is first obtained by solving a diffusion equation for the particular sample geometry, and from this function the weighting factor can be obtained. The transport mean free path is obtained by a calibration procedure in which an autocorrelation is measured on particles which are not interacting and the mean square displacement can be estimated from theory. The technique will be used to probe the influence of ionic stregth and concentration on short time particle dynamics in disordered and ordered systems.