Altman Lauren E, Quddus Rushna, Cheong Fook Chiong, Grier David G
Department of Physics and Center for Soft Matter Research, New York University, New York, NY 10003, USA.
Department of Chemistry, New York University, New York, NY 10003, USA.
Soft Matter. 2021 Mar 18;17(10):2695-2703. doi: 10.1039/d0sm02262d.
An in-line hologram of a colloidal sphere can be analyzed with the Lorenz-Mie theory of light scattering to measure the sphere's three-dimensional position with nanometer-scale precision while also measuring its diameter and refractive index with part-per-thousand precision. Applying the same technique to aspherical or inhomogeneous particles yields measurements of the position, diameter and refractive index of an effective sphere that represents an average over the particle's geometry and composition. This effective-sphere interpretation has been applied successfully to porous, dimpled and coated spheres, as well as to fractal clusters of nanoparticles, all of whose inhomogeneities appear on length scales smaller than the wavelength of light. Here, we combine numerical and experimental studies to investigate effective-sphere characterization of symmetric dimers of micrometer-scale spheres, a class of aspherical objects that appear commonly in real-world dispersions. Our studies demonstrate that the effective-sphere interpretation usefully distinguishes small colloidal clusters in holographic characterization studies of monodisperse colloidal spheres. The effective-sphere estimate for a dimer's axial position closely follows the ground truth for its center of mass. Trends in the effective-sphere diameter and refractive index, furthermore, can be used to measure a dimer's three-dimensional orientation. When applied to colloidal dimers transported in a Poiseuille flow, the estimated orientation distribution is consistent with expectations for Brownian particles undergoing Jeffery orbits.
可以用洛伦兹 - 米氏光散射理论分析胶体球的在线全息图，以纳米级精度测量球体的三维位置，同时还能以千分之一的精度测量其直径和折射率。将相同技术应用于非球形或不均匀颗粒，可得到一个有效球体的位置、直径和折射率的测量值，该有效球体代表了颗粒几何形状和组成的平均值。这种有效球体解释已成功应用于多孔、有凹坑和有涂层的球体，以及纳米颗粒的分形簇，所有这些物体的不均匀性都出现在小于光波长的长度尺度上。在这里，我们结合数值和实验研究，来研究微米级球体对称二聚体的有效球体表征，这是一类在实际分散体系中常见的非球形物体。我们的研究表明，在单分散胶体球的全息表征研究中，有效球体解释有助于区分小胶体簇。二聚体轴向位置的有效球体估计值与其质心的实际情况非常接近。此外，有效球体直径和折射率的趋势可用于测量二聚体的三维取向。当应用于在泊肃叶流中传输的胶体二聚体时，估计的取向分布与经历杰弗里轨道的布朗粒子的预期一致。
