Sizing of single evaporating droplet with Near-Forward Elastic Scattering Spectroscopy

- An optical setup and numerical models to analyse spectral properties of single evaporating microdroplet were developed.
- Advantages of the electrodynamic trapping and the broadband spectral analysis were exploited.
- Radius evolution of evaporating droplets of pure liquids and suspensions of nanoparticles with maximal accuracy of 25  nm was evaluated.
- Newly developed Near-Forward Elastic Scattering Spectroscopy was compared with other sizing techniques.

We have developed an optical setup and related numerical models to study evolution of single evaporating micro-droplets by analysis of their spectral properties. Our approach combines the advantages of the electrodynamic trapping with the broadband spectral analysis with the supercontinuum laser illumination. The elastically scattered light within the spectral range of 500-900  nm is observed by a spectrometer placed at the near-forward scattering angles between 4.3 ° and 16.2 ° and compared with the numerically generated lookup table of the broadband Mie scattering. Our solution has been successfully applied to infer the size evolution of the evaporating droplets of pure liquids (diethylene and ethylene glycol) and suspensions of nanoparticles (silica and gold nanoparticles in diethylene glycol), with maximal accuracy of  ± 25 nm. The obtained results have been compared with the previously developed sizing techniques: (i) based on the analysis of the Mie scattering images - the Mie Scattering Lookup Table Method and (ii) the droplet weighting. Our approach provides possibility to handle levitating objects with much larger size range (radius from 0.5  µm to 30  µm) than with the use of optical tweezers (typically radius below 8  µm) and analyse them with much wider spectral range than with commonly used LED sources.