Flattened and wrinkled encapsulated droplets: Shape-morphing induced by gravity and evaporation

TL;DR

This study investigates how evaporation and gravity induce shape changes in encapsulated protein droplets, revealing different morphologies in pendant and sessile droplets and providing a model to predict these transformations.

Contribution

It introduces a gravito-elasto-capillary model explaining shape morphing in evaporating droplets with elastic films, emphasizing gravity's role even at small scales.

Findings

Elastic films form during evaporation at critical concentrations.

Sessile droplets crumple near the apex, pendant droplets wrinkle near the contact line.

Gravity influences droplet morphology regardless of size.

Abstract

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.