  We report surprising morphological changes of suspension droplets (containing
class II hydrophobin protein HFBI from Trichoderma reesei and water) as they
evaporate with a contact line pinned on a rigid solid substrate. Both pendant
and sessile droplets display the formation of an encapsulating elastic film as
the bulk concentration of solute reaches a critical value during evaporation,
but the morphology of the droplet varies significantly: for sessile droplets,
the elastic film ultimately crumples in a nearly flattened area close to the
apex while in pendant droplets, circumferential wrinkling occurs close to the
contact line. These different morphologies are understood through a
gravito-elasto-capillary model that predicts the droplet morphology and the
onset of shape changes, as well as showing that the influence of the direction
of gravity remains crucial even for very small droplets (where the effect of
gravity can normally be neglected). The results pave the way to control droplet
shape in several engineering and biomedical applications.
