The dewetting behavior of a liquid film from a liquid substrate has been studied as a function of the substrate viscosity, using two highly viscous polymers as a model system. The dewetting velocity exhibits a minimum as a function of substrate viscosity. This behavior results from the competition of (1) the mass transport in the substrate, and (2) the retarded deformation of the liquid-liquid interface. Atomic force microscopy is used to image both the liquid-air interfaces and the buried liquid-liquid interface. The shape of the latter changes significantly with increasing substrate viscosity.The dewetting of thin liquid films from a flat surface is a common phenomenon with crucial impact on various technological processes. In recent years, the stability of thin liquid films has received considerable scientific interest as well, and a basic understanding of spreading and dewetting has evolved both for simple liquids and complex fluids [1][2][3][4][5][6][7][8][9][10][11][12]. Thin polymer films with high molecular weights have proven to be ideal model systems in these studies owing to their low vapor pressure and due to the fact that their high viscosity facilitates the study of the dynamic behavior. While most studies so far have dealt with thin liquid films on a solid substrate, the more complex situation of a liquid dewetting from a liquid substrate has received much less attention [12][13][14][15][16]. Brochard-Wyart, Martin, and Redon [12] recently presented a detailed theoretical study suggesting that liquid-liquid dewetting should exhibit a variety of different regimes depending mainly on the relative viscosities of the two liquids, the thicknesses of the respective liquid layers, and the surface and interfacial tensions involved. Martin, Buguin, and Brochard-Wyart [13] showed that the dewetting velocity depends on the viscosity of the substrate and decreases with increasing substrate viscosity. This apparently intuitive result points to the importance of the particular shape of the liquid-liquid interface around the growing hole and suggests that, in contrast to a solid substrate, the rim extends into the underlying liquid. However, so far direct verification of this model has not been possible, as the necessary three-dimensional imaging of the liquid-liquid interface during the dewetting process is a tedious experimental task. As will be shown below, the situation can be even more involved as the amount to which the rim penetrates into the substrate is in itself determined by the relative mobilities in the two liquids.In this Letter, we present a systematic study of the dewetting process at a polymer/polymer interface for varying molecular weight of the polymeric substrate. It is found that the dewetting velocity initially decreases with increasing substrate molecular weight; however, at large substrate viscosities, the opposite trend is observed. We present a novel technique to image the shapes of both the growing hole and the interface between the two polymers at any stage of the dewetting process. Th...

We have studied the dynamics of dewetting at the interface between polystyrene (PS) and poly(methyl methacrylate) (PMMA) as a function of the molecular weights of the two materials. Optical microscopy and atomic force microscopy have been used to follow the dewetting process and to image both the surface and interfacial structure in the vicinity of the growing holes. We determined the scaling behavior of the dewetting velocity as a function of the molecular weight of the dewetting species (PS). Different regimes are found depending on the PMMA molecular weight. For large PMMA molecular weights the dewetting velocity scales inversely with the PS viscosity, while for low PMMA molecular weights, the dewetting velocity is almost independent of the PS viscosity. The experimental data are in quantitative agreement with recent theoretical predictions.