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Introductory Remarks

The production of coating layers through deposition and convective drying of colloidal droplets on a solid surface is a procedure that is utilized in a great number of technical applications. Hereby, the solids first form individual structures, driven by the internal flows of each droplet [1]. Then, in the course of the application of more droplets, these structures subsequently merge into a connected coating layer. This incremental layer formation process, that can be seen for example during spray fluidized bed coating, showed a strong dependence of the resulting layers on the drying conditions in previous experiments [2].

In the course of this project, the incremental layer build-up from silicon dioxide nanoparticles was mimicked within a laboratory scale drying chamber. This was done by applying droplets of various sizes from the micro- to nanoliter range on a flat glass surface. After application the sessile droplets were dried under varying conditions up to technical and the resulting layer structures were analyzed in a droplet-by-droplet manner. The porosity as well as layer and droplet residue structure were measured incrementally during layer build-up by means of white-light interferometry and X-ray micro-computed tomography, in search for specific correlations between the various drying parameters and layer porosity or morphology.

Through the conducted experiments, the single deposit coffee-ring structures described by Deegan et al. [3] were identified to also be a recurring and determining phenomenon in the layer build-up process of nanoparticles. The deposits emerging at the start are made up of several ring-like formations stacked over one another or positioned next to another. In the further course of the incremental process, these structures merge via formation of solid bridges in between. This eventually results in a grown coating layer with a uniform morphology. Regarding layer porosity, it could be observed that a faster drying process, i. e. higher drying airflow velocity or temperature, results in higher porosity values. Additionally, an increase of porosity alongside an increase of deposited droplets was detected. This was determined to be due to the frequent inclusion of cavities within the resulting layer.

[1] R. Bhardwaj, X. Fang, D. Attinger (2009): Pattern formation during the evaporation of a colloidal nanoliter drop: A numerical and experimental study. New Journal of Physics 11, 075020.

[2] C. Rieck, T. Hoffmann, A. Bück, M. Peglow, E. Tsotsas (2015): Influence of drying conditions on layer porosity in fluidized bed spray granulation. Powder Technology 272, 120–131.

[3] R. D. Deegan, O. Bakajin, T. F. Dupont, G. Huber, S. R. Nagel, T. A. Witten (2000): Contact line deposits in an evaporating drop. Physical Review E: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics 62, 756–765.