Using dynamic light scattering to characterize mixed phase single particles levitated in a quasi-electrostatic balance

We use Dynamic Light Scattering ( DLS) to characterize non- spherical, micrometre- sized, single aerosol particles levitated in an electrodynamic or in a quasi- electrostatic balance. These are either solid salt particles efforesced from an aqueous salt solution droplet upon drying, or mixed phase aerosol particles, i. e. aqueous solution droplets containing a single solid salt inclusion. We show that the shortest decay of the temporal intensity autocorrelation function measured in the far field scattering pattern can be quantitatively analyzed. We treat the scattering pattern as if arising from an equivalent sized Mie sphere, and we attribute the temporal intensity fluctuations to rotational Brownian motion of the whole particle. This analysis allows sizing of non- spherical particles. We have indications that the long tails of the autocorrelation functions are due to deviations of the scattering pattern from that of a Mie sphere, leading to spikes in the temporal evolution of the intensity because of the rotational Brownian motion. We also show that the diffusional motion of an inclusion within the aqueous solution of a host droplet is masked by rotational Brownian motion, prohibiting even a qualitative analysis.