Formation of ordered mesoscopic patterns in polymer cast films by dewetting O. Karthaus, L. Gra ˚sjo ¨, N. Maruyama, M. Shimomura* Research Institute for Electronic Science, Hokkaido University, Kita 12, Nishi 6, Kita-ku, N12W6 Sapporo, Japan Abstract Ordered arrays of polymer aggregates can be prepared by a simple process: rinsing of smooth hydrophilic surfaces with a dilute solution of a hydrophobic polymer. The patterns consist of submicron-size polymer aggregates which can form superlattices over several tens of micrometers, thus self-assembling several hundred aggregates in an orderly fashion. Atomic force microscopy shows that the structures consist of isolated ‘polymer islands’, separated by the bare substrate. The ordering of the aggregates can be explained by the formation of regular, dissipative structures caused by convection during the evaporation of the solvent, followed by dewetting of the hydrophobic polymer solution from the smooth hydrophilic surface.  1998 Elsevier Science S.A. All rights reserved Keywords: Dissipative structure; Dewetting; Hydrophobic polymer; Mesoscopic pattern; Nanosize dot 1. Introduction Dewetting is a spatiotemporal process in which a thin liquid film contracts and forms droplets on a surface. Mostly, dewetting is an unwanted process. For coatings, lubrication or adhesion, thin homogeneous films are desir- able, thus a lot of attention is focused on the wetting and dewetting properties of polymer thin films. Dewetting has been intensively studied [1–4] and described theoretically [5,6] in the case of polymer melts on flat substrates. In no case has the formation of a regular pattern in dewetted films been observed. Recently we found the formation of an ordered polymer patterns upon rinsing a mica surface with a dilute solution of polystyrene, but we were unable to pro- vide a conclusive evidence for the formation mechanism [7]. In this paper we show that the pattern formation is not limited to polystyrene on mica, but can be extended to other hydrophobic polymers and to other hydrophilic substrates. The size of the aggregates can be controlled by using two different preparation techniques, rinsing and casting, respectively. 2. Experimental Polystyrene (M w 9800 g/mol; M w /M n  1.05, and M w 45 000 g/mol; M w /M n undetermined), poly(vinylstearate) (M w 90 000 g/mol), and poly(hexylthiophene) (M w 87 000 g/mol) were purchased from Aldrich; benzene was Uvasol Grade from Merck, Germany. All glassware was thoroughly cleaned with detergent solution (dcn 90, Decon Labora- tories, UK) followed by immersion in alkaline water/etha- nol (1:1) with sonication. Mica was cleaved immediately before use. Silicon wafers (Si(111) or (100)) (Nilaco, Japan) and glass substrates (Matsunami, Japan) were used as purchased. Samples were prepared by two methods. (1) Rinsing a freshly cleaved mica surface with a dilute benzene solution of polystyrene. The mica was held at an angle of 60–70° during the rinsing process. (2) Evaporation of a solution droplet on a horizontal substrate, e.g. silicon or slide glass. Optical micrographs were taken in the reflection mode (Olympus BH-2) and recorded with a CCD camera (Sony DXC 755). Atomic force microscopy (AFM) images were taken in the AC mode, in which a vibrating tip is scanned over the surface (Olympus NV 2500 aqac). NIH Image, Version 1.61, was used as the software for fast Four- ier transformation (FFT). Thin Solid Films 327–329 (1998) 829–832 0040-6090/98/$ - see front matter  1998 Elsevier Science S.A. All rights reserved PII S0040-6090(98)00771-8 * Corresponding author. Tel. +81 11 7063666; fax: +81 11 7064974; e-mail: okart@imdes.hokudai.ac.jp