  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.
