Concurrent particle diffusion and sedimentation measurements using two-dimensional tracking in a vertical sample arrangement

Abstract

This letter reports a method for simultaneous tracking of Brownian motion and superimposed sedimentation movement of multiple micro- and nanoparticles in liquid. Simple two-dimensional particle tracking can be employed because the thin liquid sample film is arranged vertically and viewed from the side with a dark field video microscopy setup. Therefore, both diffusion and sedimentation can be used for particle size calculation, allowing analyses over a wide range of sizes and mass densities. To validate the method, size distributions of reference particles with known density and diameters ranging from 100 nm to 6 μm were determined. Brownian motion for size calculation is useful for sufficiently small particles, whereas sedimentation can only be applied if there is significant settling motion superimposed on Brownian motion (which requires large diameters and/or densities). Within a certain range, both principles are suitable for size measurements. As a consequence, this method can be used to determine the size and density of unknown particles in a single measurement step, provided that they exhibit both sedimentation and diffusive motion.