Arbeitsgruppe Prof. Seemann

The stability of thin liquid films with nanoscale thickness is of fundamental interest in nano scale design and optimization of surface structure and coating in various applications. On such small scales the competition of macroscopic and intermolecular forces may lead to a complex dynamics of wetting or dewetting. A full understanding of the dynamics assist us in controlling and predicting the behavior of those systems.

For that purpose, the spinodal dewetting of nanometric thin liquid polystyrene (PS) films on liquid polymethyl-methacrylate (PMMA) substrates are studied. The instability of PS films is driven by intermolecular (van der Waals) forces. The initial stage of dewetting consists of amplification of thermal fluctuations. According to theoretical predictions, the PS/air and the PS/PMMA interfaces shall evolve in a coupled way and we expect correlated pattern on both the free and the buried interface. Hence, when the amplitudes of the corrugations reached the film thickness, the two interfaces meet in some points and the PS-air interface ruptures leading to the formation of holes. The holes will grow meeting the neighbor holes eventually forming a set of PS droplets on PMMA.

The effect of various parameters on the dynamics of dewetting is monitored step by step using Atomic Force Microscopy and Ellipsometry and the experimental results are compared with theoretical predictions provided by our collaborators (B. Wagner, D. Peschka, L. Schmeller, WIAS). Interestingly, the roughness of the initially prepared films seems to have a strong impact on both the spinodal rupture time and the evolving spinodal wavelength.

Fig.1:Atomic force microscopy scans of the PS-air interface (left) and the corresponding buried PS-PMMA interface (right) that is imaged after removing the PS layer with a selective solvent. Initial film thickness of PS(62 kg/mol) is 6 nm, while that of PMMA(10 kg/mol) is 111 nm. Sample was annealed for 4 min at 124 °C. The amplitude of the corrugation of the PS-air interface is clearly larger than that of the PS-PMMA interface. The corrugation of both interfaces are antiphase, corresponding positions are indicated with a)-d).

External collaborations: Barbara Wagner, Dirk Peschka, Leonie Schmeller (WIAS, Berlin)

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Structure Formation in Thin Liquid-Liquid Films