Fabrication of asymmetric-gradient-concentric ring patterns via evaporation of droplets of PMMA solution at different substrate temperatures

Gradient-concentric ring patterns were constructed via the evaporation of droplets of PMMA-toluene solution at different substrate temperatures, using a copper ring to control the evaporative behavior of the droplets and the motion of the contact line. The wavelength and amplitude of the concentric rings were found to decrease with the decrease of the distance to the copper ring. Curvature-induced asymmetry of the surface patterns with respect to the copper ring was observed. The geometric characteristics of the concentric rings were dependent on the solution concentration and the substrate temperature. Higher substrate temperature led to the formation of concentric rings of larger wavelengths and smaller ratio of the amplitude to the wavelength for the inner concentric rings. Microscale structures were observed in the trenches between two adjacent polymer ridges, which changed with the wavelength of the concentric rings and the substrate temperature.