Deposit Formation from Evaporating Urea-Water Droplets on Substrates of Different Wettability

The influence of wall temperature and surface properties on the formation of solid urea deposits from an evaporating aqueous urea solution (UWS) is investigated. Special attention is paid to the dynamics of crystallization and the governing processes. The investigations aim to contribute to the fundamental understanding of the deposit formation process from UWS.Three different surface treatments and controlled heating of the wall allow variation of the wetting and evaporation behavior of single UWS droplets while crystallization is monitored in-situ using a high-speed camera. The deposit morphology is then captured by confocal microscopy which provides the basis for subsequent analysis via the power spectral density method (PSD). This allows the distribution of the height of different surface features into the respective lateral dimensions to be determined for each deposit individually, providing valuable information about the localized crystallization history.The results show significant impact of the varied parameters on the crystallization process as well as on the shape and morphology of the final deposit. The operating points are classified to regimes with regard to crystallization dynamics, based on the relative ratio of heat transfer, and hence evaporation, to internal mass transport in the liquid. The PSD analysis provides characteristic distributions following a smooth inverse power law which indicate self-affine surfaces. The respective scaling tendencies of the deposit aggregates as well as localized variations in aggregate size agree well with the proposed crystallization regimes. The latter are manifested as trends in an additionally conducted analysis of the surface fractal dimensions