Evanescent Wave Scattering from Colloidal Ellipsoids

Anisotropic colloidal particles are regularly found in applications ranging from health to energy. These particles, typically with non-uniform shape or surface chemistry, interact with boundaries in unique ways, offering pathways to complex assemblies and active systems. Our lab is focused on developing experimental, computational, and theoretical techniques for the measurement and interpretation of the dynamics of these particles in confined geometries. This talk will summarize our development of Scattering Morphology Resolved Total Internal Reflection Microscopy (SMR-TIRM), which is an ultra-microscopy technique suitable for the direct measurement of the dynamics of an anisotropic particle very near a boundary. The hypothesis driving SMR-TIRM is the morphology of light scattered from a colloidal ellipsoid can be used to resolve the orientation and position of a particle. Namely, the morphology angle (Mφ) and aspect ratio (MAR) of a 2D Gaussian fit of the scattered light are reporters of the azimuthal and polar orientation of a spheroid very near a boundary interacting with an evanescent field. Experimental data from both ensembles and individual colloidal ellipsoids supported this hypothesis, with independent theory and computation further supporting this hypothesis and agreeing with experimental data. Scattering is predicted using the T-matrix method and rotation addition theorem for spherical vector wave functions, and the scattered light's image is calculated with the Debye diffraction integral. The experimental and computation results facilitated development of an analytical expression for scattering that relates the measured quantities to the orientation of the particle. Related examples of how this technique can be used for non-model systems, such as red blood cells and oil filled capsules, will also be described.