Sizing multimodal suspensions using differential dynamic microscopy*

Particle sizing is vital in both fundamental and applied colloid science. Differential Dynamic Microscopy (DDM) combines real space imaging with analysis in Fourier space to extract the same quantity as dynamic light scattering (DLS), the intermediate scattering function, at considerably lower wavevectors. But, it is based on different physics. Fitting of the ISF yields diffusion coefficients and hence particle size.

However, the limits of DDM in sizing have not been investigated in depth. In particular most studies have focused on monodisperse systems. Using well-characterised multimodal suspensions we study the ability of DDM to size small particles in the presence of large ones. Under both brightfield and phase contrast conditions the DDM signal scales with the sixth power of particle radius (the same as DLS). Nevertheless, by selecting different regions of the real-space images to analyse with a DDM algorithm we were able to isolate the signal from small particles. This allows DDM to extract the size of small particles in the presence of a higher concentration of larger species than DLS.