Geometrically Templated Dynamic Wrinkling from Suspended Poly(vinyl alcohol) Soap Films

Abstract

Wrinkling is commonly observed as mechanical instability when a stiff thin film bound on a compliant thick substrate undergoes in-plane compression exceeding a threshold. Despite significant efforts to create a broad range of surface patterns via wrinkling, little has been studied about a dynamic and transient wrinkling process, where a suspended polymer thin film undergoes liquid-to-solid phase transitions. Here, a spontaneous wrinkling process is reported, when drying poly(vinyl alcohol) (PVA) soap films suspended on 3D printed wireframes with near zero or negative Gaussian curvatures. As water evaporates, a thickness gradient across the sample is developed, leading to non-uniform drying rates, and a concentration gradient between the inner and outer sides (exposed to air) of the suspended PVA soap film induces a differential osmotic pressure. Together, these effects contribute to an in-plane compressive stress, leading to the formation of surface wrinkles, whose growth is guided by the geometry of the frame. Importantly, the wrinkles evolve dynamically: the wavelength and number of the wrinkles can be tuned by altering the concentration of the PVA aqueous solutions, the initial mass, the relative humidity of the drying environment; the patterns of the resulting wrinkles can be programmed by the geometry of the wireframe.