Modeling the elastic deformation of polymer crusts formed by sessile droplet evaporation

Abstract

Evaporating droplets of polymer or colloid solution may produce a glassy crust at the liquidvapour interface, which subsequently deforms as an elastic shell. For sessile droplets, the known radial outward flow of solvent is expected to generate crusts that are thicker near the pinned contact line than the apex. Here we investigate, by non-linear quasi-static simulation and scaling analysis, the deformation mode and stability properties of elastic caps with a non-uniform thickness profile. By suitably scaling the mean thickness and the contact angle between crust and substrate, we find data collapse onto a master curve for both buckling pressure and deformation mode, thus allowing us to predict when the deformed shape is a dimple, mexican hat, and so on. This master curve is parameterised by a dimensionless measure of the non-uniformity of the shell. We also speculate on how overlapping time-scales for gelation and deformation may alter our findings.

Droplets of Polymers

“…The final drop shape displays, in the latter case, a circular fold that can only be seen on profilometry measurements (Figure 22.4C). Axisymmetric deformation mode and stability properties of elastic caps were investigated by nonlinear quasi-static simulation and scaling analysis by Head (2006). The corresponding thickness value is 400 μm in rough agreement with optical observations.…”

Skin layer stratification in drying droplets of dairy colloids

“…Such enhanced robustness could depend not exclusively on colloid properties, but also on skin structural characteristics. In a previous work, for example, Head suggests that dry shells with homogeneous thickness do not undergo buckling instabilities [53]. In this light, the outcomes of Sadek et al [27] on the drying of WPI droplets experimentally corroborate this theory, since samples exhibited an almost constant thickness from the base to the apex height for different protein concentrations.…”

Mechanical Pinning of Liquids through Inelastic Wetting Ridge Formation on Thermally Stripped Acrylic Polymers

“…The same behavior was also observed after the surface energy of the polymer was modified with 50 Å of platinum to enhance their optical properties for imaging. The observed mechanism differs from that responsible for the formation of regular patterns from evaporating polymers solutions − and colloidal suspensions. − In these systems, an outward liquid flux forms to compensate the maximum rate of evaporation near the contact line. This transports the dissolved or suspended materials into the border region where it concentrates and then precipitates or aggregates forming an elevated ridge.…”