Drop Spreading and Confinement in Swelling-Driven Folding of Thin Films

Abstract

Surface instabilities are a versatile method for generating three-dimensional (3D) surface microstructure. When an elastomeric film weakly bonded to a substrate is swollen with solvent, buckle delamination and subsequent sliding of the film on the substrate lead to the formation of tall, self-contacting, and permanent folds. This paper explores the mechanics of fold development when such folding is induced by placing a drop on the surface of the film. We show that capillary effects can induce a strong coupling between folding and drop spreading: as folds develop, they wick the solvent toward the periphery of the drop, further propagating radially aligned folds. Accordingly, a solvent drop spreads far more on films that are weakly adhered to the substrate. As drop size reduces and folding becomes increasingly confined, debonding propagates along the perimeter of the wetted region, thus leading to corral-shaped fold patterns. On the other hand, as drop size increases and confinement effects weaken, isotropically oriented folds appear at a spacing that reduces as swelling increases. The spacing between the folds and the size of the corrals are both determined by the extent to which a single fold relieves compressive stress in its vicinity by sliding. We develop a model for folding which explicitly accounts for the fact that folds must initiate with near-zero volume under the buckle. The model shows that folds can appear even at very low swelling if there are large pre-existing debonded regions at the film–substrate interface.

Pattern Formation in Evaporative Drying of a Polymer Solution Droplet over a Soft Swellable Substrate

“…The key emphasis of this work is to understand the effect of an interacting swellable substrate − during evaporative drying of a dilute PMMA solution droplet with different C i . On a non-interacting rigid substrate such as a silicon wafer or glass, evaporation of a polymer solution droplet is generally associated with the pinning of the contact line leading to deposition of most of the polymer along the pinned droplet periphery, followed by a late-stage stick-slip retraction of the de-pinned contact line.…”

Controllable Buckle Delaminations in Polymer-Supported Periodic Gradient Films by Mechanical Compression

“…The instability modes of the wrinkle and buckle-delamination can be frequently observed in experiments. Intuitively, the wrinkle forms in the case of soft substrate and/or strong interfacial adhesion, − while buckle-delamination appears in the case of rigid substrate and/or weak adhesion. − These two instability modes have been extensively investigated in the past decades. Based on the continuum elasticity theory, the critical strain for initiation of wrinkle is expressed as − ε normalw = 1 4 true( 3 E̅ normals E̅ normalf true) 2 / 3 where E̅ normalf = E f / false( 1 − ν normalf 2 false) , E̅ normals = E s / false( 1 − ν normals 2 false) , E is the elastic modulus, ν is Poisson’s ratio, and the subscripts f and s denote the film and substrate, respectively.…”

“…In the case of rigid substrate and weak interfacial adhesion, the film tends to locally detach from its substrate at some positions, leading to the formation of buckle-delaminations. , The delaminations are originated from the stress localization due to film impurities or interfacial weaknesses. , They are usually spontaneously formed in a homogeneous film–substrate system, and their initiation positions and geometrical parameters are hardly precisely controlled experimentally. On the other hand, the buckle-delaminations possess higher aspect ratios and thus can endure larger deformations compared with the wrinkles and other advanced modes.…”

Strategy for Patterning Titania Dendrites by Gas–Solution Interaction at Droplet Surfaces

“…Recent works have focused on the investigation of colloidal droplet evaporation on a solid substrate and on the resulting formation of ordered patterns. , Evaporation-induced crystallization is observed with decreasing volume of a single or multicomponent solution droplet on the surface of a solid substrate. , The effect of surfactant on evaporation in a droplet is being studied . The presence of a temperature gradient in a drop contributes to the occurrence of complicated, even fractal-like, deposit patterns . The intermittent movement of the contact line during the droplet evaporation can lead to the appearance of single or multiple coffee rings .…”

“…6 The presence of a temperature gradient in a drop contributes to the occurrence of complicated, even fractal-like, deposit patterns. 7 The intermittent movement of the contact line during the droplet evaporation can lead to the appearance of single or multiple coffee rings. 8 In this work, 9 the diphenylalanine self-assembled microtubes have been grown via evaporation-driven crystallization, making use of the Marangoni flow in the drying droplets.…”