(90b) Protein Depositions from Drying Droplets
The fluid flow driven by evaporation of a sessile droplet is surprisingly rich, and highly relevant to important process ranging from the manufacture of DNA microarrays to ink-jet printing. Here we use a drying water droplet to deposit nanoparticles and proteins, such as BSA and rhodamine, onto substrates, including the affect of buffer salts, such as sodium bicarbonate, on the deposition patterns. We use fluorescently stained proteins to image the patterns in a fluorescence microscope, and video microscopy to follow the dynamics of the pattern formation. We find a wide range of phenomena, including Marangoni instabilities, wave-like crystallization fronts, and jerky contact line motion. We also find a wide variety of patterns, including concentric circles, dendritic patterns, bizarre squares, and partially filled patterns. These patterns are sensitive to protein content and concentration, buffer content and concentration, drying conditions, and substrate treatments. These phenomena are largely still mysterious, although they can be partially explained by contact line self-pinning, and supersaturation of salts for which nucleation is suddenly initiated. Possibly applications are discussed.