Temperature-dependent Pattern Formation in Drying Aqueous Droplets of Lysozyme

Drying colloidal droplets have a wide range of applications from medical diagnostics to coatings for industries. This paper explores the effects of the substrate temperature (ranging from $25$ to $55 ^{\circ}$C) and various initial concentrations ($\phi$) of $1$ to $20$ wt% of lysozyme in an aqueous solution on its drying and final dried film state using bright-field optical microscopy. The $\phi$ is divided into three regimes, ultra-concentrated ($20$ $<$ $\phi$ $\leq$ $17$ wt%), concentrated ($17$ $<$ $\phi$ $\leq$ $9$ wt%) and diluted ($9$ $<$ $\phi$ $\leq$ $1$ wt%). Increasing $\phi$ in these regimes finds that this movement in the later non-linear region slows down as the front carries and deposits protein molecules until the supply in solution is exhausted. In the ultra-concentrated regime, the fluid front moves linearly throughout the drying process. The deposition of protein onto the surface by the fluid front creates the "coffee-ring" and increases with increasing $\phi$. A dimple is observed in a central mound-structure, which grows with increasing $\phi$. As the temperature increases the drying rate increases, reducing the time for convective flow and the deposition of material at the fluid front. Interestingly, at (T, $\phi$) = ($55 ^{\circ}$C, $20$ wt%), the droplet forms the thickest film with suppressed ring formation. The dimple diminishes in the ultra-concentrated regime, whereas it changes to an expanded spot in some samples of the diluted and concentrated regimes with elevated temperatures. Both initial concentration and substrate temperature lead to surface tension and temperature gradients across the droplet, affecting the morphological patterns. This study provides insights into protein-protein and protein-substrate interactions driven by temperature and concentration for biomedical and biosensing applications.