Investigation and Analysis of Scattered Light from Colloidal Spheroids with Scattering Morphology Resolved Total Internal Reflection Microscopy (SMR-TIRM)

Microscale colloidal particles are regularly used in industrial products, such as lotions, paints, and drug delivery systems, in which the performance and associated properties of the product are profoundly impacted by the surface forces operating between particles. Among many surface forces measurement tools, Total Internal Reflection Microscopy (TIRM) has high energy and spatial sensitivity for the measurement of conservative and non-conservative surface forces. Recently, our lab started to develop a variant of TIRM named Scattering Morphology Resolved (SMR) TIRM, which utilizes scattering morphology (rather than integrated intensity alone) for the measurement of anisotropic colloidal particle orientation and position. Herein, we summarize the results of the scattering morphology from both isotropic and anisotropic colloidal particles. Specifically, the morphology was investigated as a function of particle size, incident beam polarization, and particle separation distance. We found that particles scattered evanescent waves with morphologies deviated from a circle, even for isotropic particles. The morphology aspect ratio depended upon the scaled particle size with respect to the incident beam wavelength (a/λ) and polarization. Specifically, the results from experiments and complimentary light scattering simulations suggest when increasing a/λ from 0.1 to 0.5, the aspect ratio of scattering morphology (M_AR) stays ~1.0 for p-polarization, but decreases from ~1.3 to ~1.0 for s-polarization. However, increasing a/λ from 0.5 to 23.7 resulted in a rise of M_AR, from ~1.0 to ~1.3 for both s- and p- polarization. Further, simulations confirmed the integrated scattering intensity would exponentially decay with increasing separation distance regardless of the value of M_AR. The work summarized herein supplements our existing understanding of scattering morphology dependence on particle size, shape, and beam polarization.