Self-organization of unconventional gradient concentric rings on precast PMMA films

Concentric polymethyl methacrylate (PMMA) rings with unconventional wavelength and amplitude gradients are formed both inside and outside a copper ring, which is placed on a precast PMMA film to control the evaporation behavior of a toluene droplet. The experimental results show that both the wavelength and amplitude of the PMMA concentric rings inside and outside the copper ring decrease as the PMMA rings approach the copper ring. For the PMMA rings with the same distance to the copper ring, the concentric rings outside the copper ring have larger wavelength and amplitude than those inside the copper ring. The characteristics of the concentric rings are dependent on the size of the copper ring and the film thickness. A featureless zone in the center of the copper ring is formed due to the evaporation-induced dewetting, and the size of which increases linearly with the increase of the copper ring size.