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

The desiccation of bio-fluid 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 20oC and 40oC, on the desiccation deposits of foetal 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 20oC, to cell-like for substrate temperatures between 25oC and 40oC. Calculation of the thermal and solutal Bénard-Marangoni numbers shows that, whilst thermal effects are negligible when drying takes place at 20oC, for higher substrate temperatures (25oC-40oC), 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 towards the end of drying, maintaining circulation within the cells and leading to crystallisation of salts in the formed cells. Thus, we believe that the cell-like structures result from 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 with temperature and the area occupied by each cell decreases. The average distance between cells decreases linearly with substrate temperature.