(356c) Evaporation-Induced Self-Organized Surface Structures on Pre-Cast Polymer Thin Films

In this study, we fabricated well-ordered concentric rings with unconventional gradient on pre-cast PMMA films via evaporating a pure toluene droplet which was confined by a circular copper ring. Concentric rings were formed both inside and outside the copper ring due to the “stick-slip” motion of contact lines. Both the wavelength and the amplitude of the concentric rings inside and outside the copper ring decreases as the concentric rings approach to the copper ring. For the external rings, the wavelength and amplitude of the rings of larger diameters which are far away from the copper ring are larger than those of smaller diameters closer to the copper ring. This behavior is similar to the gradient rings fabricated by other methods.^{1, 2} On the other side, the internal rings exhibit the opposite trend: the rings of smaller diameters have larger wavelength and amplitude. The concentric rings display asymmetric characteristics with the reference to the copper ring. The external rings have larger wavelength and amplitude than the corresponding internal rings approximately with the same distance to the copper ring. The result suggests that positive curvature (for the external rings) tends to induce concentric rings of larger wavelength and amplitude than negative curvature (for the internal rings). The characteristics of the concentric rings can also be controlled by the thickness of the pre-cast films. Concentric rings of larger wavelength and amplitude can be formed on a thicker film when other experimental conditions remain the same. In addition, a featureless zone was formed in the center of the copper ring due to the evaporative dewetting. The size of the featureless zone is proportional to the size of copper ring. The “ring-on-film” template has the potential to fabricate regular concentric rings with controllable characteristics in a simple way.

References

(1) J. Xu, J. F. Xia, S. W. Hong, Z. Q. Lin, F. Qiu and Y. L. Yang, Self-assembly of gradient concentric rings via solvent evaporation from a capillary bridge, Physical Review Letters, 2006, 96.

(2) Z. Q. Lin and S. Granick, Patterns formed by droplet evaporation from a restricted geometry, Journal of the American Chemical Society, 2005, 127, 2816-2817.