Deposit formation from evaporating urea-water droplets on substrates of different wettability

During the evaporation of urea water solution (UWS), the wall temperature and surface properties influence the dynamics of deposit formation by affecting the internal mass transport. These effects are expected to be reflected in the resulting deposit morphology and allow different deposit regimes to be distinguished.

The temperature of metallic substrates is varied for three different surface treatments to analyze the wetting, evaporation behavior and the crystallization process of single UWS droplets in situ using a high-speed camera. The deposit morphology is captured by confocal microscopy and analyzed via the power spectral density method (PSD). PSD is used to extract the height of different surface features for each deposit, providing valuable information about the local crystallization history.

A significant influence of the surface properties on the crystallization process as well as on the morphology of the final deposit is found. The influence of wettability is described by the resulting internal mass transport, which determine the urea distribution. PSD analysis quantified distinct trends in the scaling tendencies of the deposit aggregates under different wall conditions. The local crystal growth history extracted by PSD agrees well with proposed crystallization mechanisms, which is further supported by high-speed and SEM imaging.