(411g) Novel Extensions of TIRM Measurements Based on Finite-Element Analysis of Evanescent Field Scattering: Particle Sizing, Multiple Scattering and Non-Spherical Particles

Total internal reflection microscopy (TIRM) is a direct force measurement technique relying on evanescent field scattering to provide order k_BT interaction potentials for particles levitated above a plate. The theoretical basis of TIRM is well established for measuring surface forces in systems of isolated spherical particles, with far-field scattering adequately described by generalized Lorenz Mie theory (GLMT). Extending TIRM into confinement, such as micro-fluidic channels or measurements involving multiple particles, may not always be adequately described by single particles scattering evanescent filed in isolation, thus a more quantitative description of the scattering can provide additional insight into experimental design.  We use GMLT or finite element modeling (FEM) to explore methods to extend TIRM to either new applications or getting more information from existing methods.

Conventional TIRM experiments are primarily concerned with total scattered field intensity, disregarding field morphology. One often discarded morphological feature is the edge-localised scattering exhibited by particles immersed in an evanescent field, i.e. the scattering hot spot; in particular for a microsphere viewed through a low numerical aperture (NA) objective, an Airy disc pattern is formed with a centroid corresponding almost exactly to the projected edge of the particle. We demonstrate how this phenomenon can be exploited to furnish an accurate measure of particle size using a set of monochrome CCD/CMOS images and verify this computationally using GLMT coupled with a vectorial diffraction model to account for the NA of our objective lens.

Using high NA objectives, the scattered field is seen to radiate forward in the direction of incident field propagation with sustained intensity spanning several particle radii.  This far-field observation suggests a substantial spatial envelope surrounding a source particle within which the scattered near-field may be intense enough for significant interaction with adjacent particles or confinement structures (e.g. microfluidic channels, pillars or patterned surfaces). The probability of near-field coupling therefore arises in several scenarios of experimental interest. In order to quantify these effects, we have implemented FEM to solve Maxwell’s equations directly for arbitrary scattering bodies and incident fields. This model has been validated against far-field GLMT results for the base-case of an isolated microsphere, and is now being used for simulating near-field interactions to facilitate the design and interpretation of TIRM experiments. In addition we have used this model to explore the scattering behavior of non-spherical particles in TIRM measurements.