Water front recession and the formation of various types of wrinkles in dried graphene oxide droplets

We systematically investigate the spontaneous formation of various types of wrinkle patterns in dried GO droplets using different drying conditions including temperature, GO concentration, and ionic concentration. A coffee-ring effect occurs in biphasic dispersions with low GO concentration, which is suppressed in high-concentration nematic phases. In dried nematic GO droplets, more than five different wrinkle patterns, composed of two types of wrinkle lines parallel and perpendicular to the direction of water recession, are spontaneously produced depending on the drying conditions. The wrinkle patterns are dependent on the recession behavior of the water front and interparticle friction, both of which influence the shape and density of the wrinkles. The receding water front flattens the GO sheets on a flat substrate from the edge to the center of the droplet, causing an anisotropic compressive force on the GO sheets that orders the wrinkle pattern. Interparticle friction, which is enhanced by increasing either the GO or the ionic concentration, increases the wrinkle density and disturbs the order of the wrinkle lines.