Dynamic Reversible Evolution of Wrinkles on Floating Polymer Films under Magnetic Control

In this paper, we present a simple and versatile method to dynamically and reversibly tailor surface wrinkles on a floating polymer film by combining a magnetic droplet and neodymium magnet. The magnetic force from the attraction of the neodymium magnet to the magnetic droplet is the main reason for surface instabilities of floating polymer films, which can induce radial stress in the radial direction, and further, compressive stress in the circumferential direction. This compressive stress can trigger not only floating film wrinkling but also a wrinkle-fold transition. Surface morphologies on the floating polymer film have been systematically studied, by varying the distance between the magnetic droplet and neodymium magnet, polymer film thickness, and magnetic droplet volume. With the decrease in the distance between a magnetic droplet and a neodymium magnet, the decrease in polymer film thickness, and the increase in the magnetic droplet volume, the wrinkle numbers increase and even a wrinkle-fold transition happens. Additionally, the coupling effect of multiple magnetic droplets on the floating film has also been used to achieve novel surface wrinkle patterns, which greatly widens the applications of surface wrinkling.