Holographic characterization and tracking of colloidal dimers in the effective-sphere approximation

TL;DR

This study uses holographic techniques combined with numerical and experimental methods to characterize colloidal dimers, demonstrating effective-sphere approximation can accurately determine their position, orientation, and properties in flow conditions.

Contribution

It extends effective-sphere holographic characterization to symmetric colloidal dimers, enabling accurate measurement of their position and orientation.

Findings

Effective-sphere estimates closely match the dimer's center of mass.

Trends in diameter and refractive index reveal 3D orientation.

Orientation distribution in flow aligns with Brownian motion models.

Abstract

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 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 identifies dimers 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.