No abstract

Micrometer scale colloidal particles experiencing ∼kT scale interactions and suspended in a fluid are relevant to a broad spectrum of applications. Often, colloidal particles are anisotropic, either by design or by nature. Yet, there are few techniques by which ∼kT scale interactions of anisotropic particles can be measured. Herein, we present the initial development of scattering morphology resolved total internal reflection microscopy (SMR-TIRM). The hypothesis of this work is that the morphology of light scattered by an anisotropic particle from an evanescent wave is a sensitive function of particle orientation. This hypothesis was tested with experiments and simulations mapping the scattered light from colloidal ellipsoids at systemically varied orientations. Scattering morphologies were first fitted with a two-dimensional (2D) Gaussian surface. The fitted morphology was parameterized by the morphology’s orientation angle M ϕ and aspect ratio M AR. Data from both experiments and simulations show M ϕ to be a function of the particle azimuthal angle, while M AR was a sensitive function of the polar angle. This analysis shows that both azimuthal and polar angles of a colloidal ellipsoid could be resolved from scattering morphology as well or better than using bright-field microscopy. The integrated scattering intensity, which will be used for determining the separation distance, was also found to be a sensitive function of particle orientation. A procedure for interpreting these confounding effects was developed that in principle would uniquely determine the separation distance, the azimuthal angle, and the polar angle. Tracking these three quantities is necessary for calculating the potential energy landscape sampled by a colloidal ellipsoid.

Nanometer to micrometer scale colloidal particles are regularly found in applications in which surface forces dominate behavior. Consequently, a wide range of surface force measurement tools have been developed to probe interactions as a function of physiochemical properties. One tool, Total Internal Reflection Microscopy (TIRM), is an exceptionally sensitive probe of both conservative and non-conservative surface forces. A recent variant of TIRM called Scattering Morphology Resolved (SMR) TIRM utilizes the morphology of scattered light in concert with the integrated intensity to measure the position and orientation of a colloidal particle. Although the target of SMR-TIRM is the field of non-spherical "anisotropic" particles, spherical particles have been found to scatter evanescent waves with surprising morphology. Herein, we present experiments and simulations of the scattering morphology of a spherical particle. The morphology was probed as a function of particle size, incident beam polarization, and particle separation distance. We found that spherical particles scattered light with a noncircular morphology. Moreover, we found the morphology depended upon both the scaled particle size with respect to the incident beam wavelength and the incident beam polarization. Although the scattering morphology from the sphere was surprisingly complex, we did not find that these effects would alter the interpretation of scattering as a function of particle separation distance. This manuscript was submitted as part of the Lomonosov Readings 2020, Section: Mathematical models and methods in electromagnetics for particles simulations, characterization and synthesis.