Dispersion forces are present everywhere. Their importance, however, is largely neglected because directly at a surface or at an interface they are mostly weak compared with specific interaction of short range. Here, we show that these forces are nonetheless extremely relevant and may have drastic consequences on the stability of thin films. We demonstrate that a force (per unit area) of <1 Pa is capable of “destroying” 100 nm (!) thick films, even if they are “glued” to the substrate by end-grafted polymers. We present the temporal evolution of different morphologies of unstable thin liquid polymer films caused by destabilizing intermolecular forces.

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We present experimental results on the friction force exerted by a network of poly(dimethylsiloxane) (PDMS) moving on a solid substrate coated with the same polymer. Two different coatings are compared: a surface, densely grafted with short chains (which can be seen as a model impenetrable surface) onto which a well-defined number of long chains (connectors) can be gradually added and adsorbed PDMS layers. Increasing the sliding velocity between 10 -5 and 10 -1 m/s suggests a transition between "liquidlike" and "solidlike" frictional behavior. Increasing the molecular weight of adsorbed or grafted chains shifts this transition to higher sliding velocities. Increasing systematically the areal density (Σ) of connectors yields two opposite trends: (i) At our lowest velocities, an increase of Σ results in higher friction. This increase of friction is due to the pull-out process of the grafted chains from the network. (ii) At higher velocities, the same connectors lead to a reduction of friction (lubrication effect!) with respect to a surface without connectors.

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Using optical microscopy, we followed the evolution of thin and initially smooth liquid polymer films on wettable solid substrates for different surrounding media. For thicknesses between 30 and 110 nm, we observed an instability of the film, eventually leading to the formation of nanodroplets on a remaining wetting layer. Exchanging the surrounding medium allowed to reverse the process and to re-establish a smooth film. Our results are in accordance with theoretical considerations and computer simulations. Weak but long-ranged dispersion forces may have drastic consequences concerning the morphology of thin liquid films even if short-range interactions force the film to stay on the substrate.The stability of thin films is essential in many applications like in coatings (paints) or in microelectronic devices (insulating layers). Due to the large surface-to-volume ratio interfacial properties become increasingly important. Especially if the film thickness gets much thinner than one micron or approaches molecular dimensions, intermolecular forces start to govern the system [1-5]. Short-range interaction, corresponding to forces acting mainly on contact between molecules, determine to a large extent interfacial tensions, wettability (identified by the spreading of a liquid on the substrate) or adhesion. It has been argued [4,5] that they may be insufficient to compete with effects arising from long-range interactions, even if these forces are weak. At a distance of 100 nm these forces (per unit area) are already at least 4-5 orders of magnitude weaker than atmospheric pressure. Short-and long-range forces may vary independently and may even have different signs as they originate from different types of interactions [5]. It is well known from colloid science [6,7] that two particles, named 1 and 2, immersed in a fluid, designated as 3, may either attract or repel each other, even over distances (h) of many tens of nanometers. Such long-range interaction may be due to ubiquitous apolar

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Fluorescence correlation experiments were performed on rhodamine 6G in PDMS spin-coated films on glass surfaces. With polarised excitation, ensemble bleaching of the dye and single molecule intensity fluctuations were observed. From the statistics of single molecule intensity data taken at different positions in the film, correlation functions were calculated. Two modes of motion with exponential decay shapes and correlation times of tau(c) = 0.15 s and tau(c) = 0.7 s could be detected. Potential origins of intensity fluctuations are lateral diffusion, rotational diffusion or intramolecular fluctuations of dyes involving spectral diffusion or photoinduced processes. From the experimental results, lateral diffusion can be ruled out as a motional mode. Single molecule fluctuations are assigned to changes of the molecular configuration of the dyes, which are rigidly bound to the glass. To assess the environmental influence on such molecular motions, the bulk viscosity of the PDMS was varied over two orders of magnitude, leading to changes of tau(c) of the slow mode by a factor of four. This result proves the sensitivity of the single molecule fluctuations to the molecular scale dynamics of the surrounding polymer matrix and makes the correlation time a measure of the local environment of dye probes.

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