(552b) Texturing Freezing Droplets

Ice templating provides a means of generating textures with a well-defined topography. Recent applications involve the freezing of water droplets, with or without colloids, on flat or textured surfaces. An interesting feature of water droplets freezing on a substrate is the formation of a pointy tip of constant angle, regardless of the substrate temperature, surface energy, or droplet volume. Here, by adding polymer to water, we demonstrate how to manipulate and even prevent the formation of such icy tip. We find that the sharpness of the tip decreases with increasing polymer concentration until completely disappearing above a critical concentration. Our experiments reveal that, upon freezing, phase separation occurs, resulting in the freezing ice-front pushing the polymer towards the edges of the droplet and subsequently forming a thin polymer-layer around the surface of the droplet. As a result, the extent of tip formation is determined by the competition of the liquid-water-to-ice expansion and the elasticity of the thin polymer membrane. Moreover, the volume expansion due to ice crystallization stretches the polymer membrane, inducing the formation of wrinkles, subsequently texturing the frozen droplet. Together our results provide new ways to control the shape of frozen droplets for ice templating or microstructure fabrication, with applications in tissue engineering, separation membranes, and soft robotics.