Transition from Dendritic to Cell-like Crystalline Structures in Drying Droplets of Fetal Bovine Serum under the Influence of Temperature

The desiccation of biofluid droplets leads to the formation of complex deposits which are morphologically affected by the environmental conditions, such as temperature. In this work, we examine the effect of substrate temperatures between 20 and 40 degrees C on the desiccation deposits of fetal bovine serum (FBS) droplets. The final dried deposits consist of different zones: a peripheral protein ring, a zone of protein structures, a protein gel, and a central crystalline zone. We focus on the crystalline zone showing that its morphological and topographical characteristics vary with substrate temperature. The area of the crystalline zone is found to shrink with increasing substrate temperature. Additionally, the morphology of the crystalline structures changes from dendritic at 20 degrees C to cell-like for substrate temperatures between 25 and 40 degrees C. Calculation of the thermal and solutal Benard-Marangoni numbers shows that while thermal effects are negligible when drying takes place at 20 degrees C, for higher substrate temperatures (25-40 degrees C), both thermal and solutal convective effects manifest within the drying drops. Thermal effects dominate earlier in the evaporation process leading, we believe, to the development of instabilities and, in turn, to the formation of convective cells in the drying drops. Solutal effects, on the other hand, are dominant toward the end of drying, maintaining circulation within the cells and leading to crystallization of salts in the formed cells. The cell-like structures are considered to form because of the interplay between thermal and solutal convection during drying. Dendritic growth is associated with a thicker fluid layer in the crystalline zone compared to cell-like growth with thinner layers. For cell-like structures, we show that the number of cells increases and the area occupied by each cell decreases with temperature. The average distance between cells decreases linearly with substrate temperature.

Automated summary

This study examines the effect of substrate temperature on the desiccation deposits of fetal bovine serum droplets. The morphology and characteristics of the crystalline zone vary with substrate temperature, with the area of the crystalline zone shrinking and the morphology changing from dendritic to cell-like. Thermal and solutal convective effects are observed in the drying drops, with thermal effects dominating earlier in the evaporation process and solutal effects becoming dominant toward the end of drying. The interplay between thermal and solutal convection leads to the formation of cell-like structures.