(458d) Measuring the Interactions of Anisotropic Particles Using Total Internal Reflection Microscopy

Controlled self-assembly in colloidal systems has benefited from a set of surface force measurement techniques optimised for systems of spherical particles and other symmetric geometries. With growing interest in the assembly of anisotropic particles (e.g. carbon nanotubes settling at an interface, or the alignment of rod-like zinc oxide crystals during solar cell fabrication), there is a strong incentive to develop surface force measurement techniques optimised for systems exhibiting either geometric or compositional anisotropy. One promising approach is the extension of total internal reflection microscopy (TIRM), an established direct force measurement technique capable of providing three-dimensional particle position with nanometre resolution at sub-millisecond timescales. Conventional TIRM is used to measure the hydrodynamics and order-k_BT interaction potentials of spherical micro or nanoparticles levitated above a surface, and is based on observing the inelastic scattering of evanescent fields; for spheres, this scattering is well-described by generalised Lorentz-Mie theory. An extension of TIRM for anisotropic particles requires a description of evanescent scattering to include up to three additional spatial dimensions (i.e. pitch, tilt and yaw) to account for non-degenerate rotational degrees of freedom. Bridging this gap in complexity has required the construction of finite-element models to solve Maxwellâ??s field equations directly, enabling the quantification of scattering in systems of particles with arbitrary geometry and composition. The insights gained from this modeling work have guided the design and assembly of an experimental apparatus that extends the utility of TIRM to observe systems of diffusing anisotropic particles while retaining the advantages of high spatiotemporal resolution. We will discuss our development of anisotropic TIRM, describe the way in which evanescent scattering models have been used to enable and complement experimental analysis, and present some innovations in particle sizing related to this new method.