  Pattern formation in drying protein droplets continues to attract
considerable research attention because it can be linked to specific
protein-protein interactions. An extensive study of the drying evolution and
the final crack patterns are presented, highlighting the concentration
dependence (from 1 to 13 wt%) on two globular proteins, lysozyme (Lys) and
bovine serum albumin (BSA), in de-ionized water. The drying evolution starts
with a constant contact radius mode and shifts to a mixed mode where both fluid
front and contact angle changes. The contact angle monotonically decreases,
whereas, the fluid front exhibits two regimes: an initial linear regime and a
later nonlinear regime. Unlike the linear regime, the non-linear regime is
faster for Lys droplets. This results in the formation of a "mound"-like
structure in the central region. A new feature, a "dimple" is observed in this
mound which is found to be dependent on the initial concentration. The
different crack morphology of BSA and Lys depends strongly on the initial state
of the solution and can be interpreted using a simple mechanical model. In
fact, when dried under uniform conditions (surface, humidity, temperature,
droplet diameter, etc.), the evolution and the final pattern displays as a
fingerprint of the initial state.
