(637f) Differential Dynamic Microscopy of Weakly-Scattering and Polydisperse Suspensions of Colloids or Protein-Rich Liquid Clusters

Differential dynamic microscopy (DDM) was recently developed to quantify dynamics of submicron particles in solutions from fluctuations of intensity in optical micrographs. DDM is well-established for monodisperse particle populations but has not been applied to solutions containing weakly-scattering polydisperse particles, which are widely employed in industrial and technological applications. Here, we use brightfield DDM (b-DDM) to measure the dynamics of polydisperse weakly-scattering particles using two systems: (i) a bidisperse mixture of nanoparticles of radii 300 nm and 50 nm, and (ii) solutions of protein-rich liquid clusters, whose size ranges from tens to hundreds of nanometers and whose total volume fraction is less than 10^-5. We evaluate diffusion coefficients from the dependence of the diffusive relaxation time on the scattering wavevector using a polydisperse fitting function. The average diffusion coefficient measured using b-DDM is consistently lower than that obtained from dynamic light scattering (DLS) at a scattering angle of 90°. This apparent discrepancy is due to Mie scattering from the polydisperse mixtures, in which larger particles or clusters preferentially scatter more light in the forward direction. We establish that for weakly scattering populations an optimal thickness of the sample chamber exists, at which increasing noise compensates stronger b-DDM signals.