(637g) Dark-Field Differential Dynamic Microscopy of Gold Nanoparticles

Differential dynamic microscopy (DDM) is an emerging characterization technique that measures the ensemble dynamics of colloidal and complex fluid motion using simple optical microscopy. By correlating the intensity fluctuations of a video micrograph into Fourier space, it is possible to obtain the dynamic structure factor in systems that would otherwise be difficult to measure using photon correlation spectroscopy. To date, DDM has successfully been applied to bright-field, fluorescence, confocal and phase-contrast micrographs to study diverse dynamic phenomena, including bacterial motility, colloidal aggregation and anisotropic particle motion. In this work, we show for the first time how DDM analysis can be extended to dark-field imaging, i.e. a linear space variant (LSV) imaging mode. Specifically, we present a correction to the image structure function obtained by DDM that accounts for scatterers with a non-homogeneous intensity distribution as they move within the imaging plane. To validate the analysis, we study the Brownian motion of gold nanoparticles, whose plasmonic structure allows for nanometer-scale particles to be imaged under dark-field illumination, in various simple and complex fluids. We find that the diffusion coefficients of Au nanoparticles extracted from dark-field differential dynamic microscopy agree well with those measured via multiple-particle tracking microrheology. These results demonstrate the potential for DDM analysis to be applied to linear space variant forms of microscopy, providing access to experimental systems unavailable to other imaging modes.