Evolution of radius and light scattering properties of single drying microdroplets of colloidal suspension

- Electrodynamic levitation technique was used to study crystallization and aggregation processes of evaporating microdroplets of colloidal suspensions and deposition of final dry aggregates.
- Combination of interferometric and electrical weighting size measurement method allows accurate sizing of drying droplet/aggregate size evolution
- Character of micro objects formations from drying microdroplet of colloidal suspension can be analysed using droplet size evolution and elastically scattered light intensities.
- Scanning Electron Microscopy was used to observe wet droplets at the initial evaporation and final dry aggregates of SDS and SDS/SiO2 structures.

We report on observation of well-pronounced characteristic features of elastic light scattering of evaporating solution and suspension microdroplet of the anionic surfactant sodium dodecyl sulfate (SDS) and colloidal silica (SiO2) nanospheres in diethylene glycol (DEG) during SDS surface layer and structure formation (crystallization). For pure DEG/SDS solution droplet evaporation process, characteristic evaporation transitions manifested in the evolution of the droplet radius, a(t) for all the SDS concentrations (C = 20  mM, 40  mM and 100  mM) studied as well as well-pronounced intensity signals characterizing SDS soft gel-solid transitions for initial SDS concentrations, C  >  40  mM. In the case of microdroplets composed of DEG/SDS with controlled addition of colloidal silica, the intensity fluctuations were enhanced and had profiles dependent on the initial composition of the suspension. Exemplary wet droplets at the initial evaporation stages and final dry aggregates of SDS and SDS/SiO2 were deposited on a substrate and observed with Scanning Electron Microscopy (SEM). Features of the deposited structures correlate well with the elastic scattered light measurements characterizing the drying processes.