Theory predicts that dewetting of a homogeneous liquid film from a solid surface may proceed via unstable surface waves on the liquid. This phenomenon, usually termed spinodal dewetting, has been sought after in many systems. Observations in liquid crystal and liquid metal films showed that, as expected, the emerging structures were similar to those found for spinodal decomposition in mixtures. Certain differences, however, could be attributed to peculiarities of the wetting forces in these two dissimilar systems, thereby demonstrating the role of nonlinearities inherent in the wetting forces.

The stability of thin liquid coatings plays a fundamental role in everyday life. We studied the stability conditions of thin (3 to 300 nm) liquid polymer films on various substrates. The key role is played by the effective interface potential φ of the system air/film/substrate, which determines the dewetting scenario in case the film is not stable. We describe in this study how to distinguish a spinodal dewetting scenario from heterogeneous and homogeneous dewetting by analysing the emerging structures of the film surface by e.g. Minkowski measures. We also include line tension studies of tiny droplets, showing that the long-range part of φ does affect the drop profile, but only very close to the three phase boundary line. The dynamic properties of the films are characterized via various experimental methods: the form of the dewetting front, for example, was recorded by scanning probe microscopy and gives insight into the boundary condition between the liquid and the substrate. We further report experiments probing the viscosity and the glass transition temperature of nm-thick films using e.g. ellipsometry. Here we find that even short-chained polymer melts exhibit a significant reduction of the glass transition temperature as the film thickness is reduced below 100 nm.

The propensity of liquid films to bead off poorly wettable substrates leads to a wide variety of liquid structures via mechanisms which are far from being fully understood. In particular, dewetting via unstable surface waves may be driven at least by dispersion forces, electrostatic forces, or by Marangoni-type transport. A hierarchy of dynamical instabilities finally transforms the initial homogeneous film into the final state, consisting of an ensemble of individual, isolated droplets. While these processes of self-organized structure formation are interesting in themselves, it may also be desirable to generate liquid structures in a more well-defined and predictable way. We have therefore investigated experimentally the behaviour of various liquids on substrates, the wettability of which has been laterally structured. The resulting artificial liquid objects display several remarkable properties, both statically and dynamically. Aside from potential applications as `liquid microchips', it is shown how fundamental quantities can be extracted from the shapes of the liquid surfaces, as determined by scanning force microscopy. The three-phase contact line tensions obtained in this way are in fair agreement with theoretical predictions and might help to resolve long-standing debates on the role of wetting forces on the nanometre scale.

A surface instability is reported in thin nematic films of 5CB and 8CB, occurring near the nematic-isotropic phase transition. Although this instability leads to patterns reminiscent of spinodal dewetting, we show that it is actually based on a nucleation mechanism. Its characteristic wavelength does not depend markedly on film thickness, but strongly on the heating rate. 64.70.Md,61.30.Pq Following several studies on the spreading behavior of liquid crystals (LC) from the nCB homologous series (4'-n-alkyl-4-cyanobiphenyl) [1][2][3][4][5][6], undulative instabilities have been observed in thin films of the LC 5AB 4 [7], and 5CB [8] near the nematic-isotropic (N-I) phase transition. In the case of 5CB, these results have led to some discussion whether a spinodal dewetting mechanism driven by van der Waals forces is at work, as proposed by Vandenbrouck et al. [8], or wether the instability is driven by a pseudo-Casimir force based on the director fluctuations in thin nematic films [9][10][11][12][13]. In the present paper, we show that neither is true for nCB thin films. Instead, the instability is caused by textures in the nematic film which largely determine the characteristic wavelength of the emerging pattern.5CB and 8CB were obtained from Merck KGaA (Darmstadt, Germany) and Frinton Laboratories Inc. (Vineland, NJ) respectively, and used without further purification. Silicon wafers (100-oriented, p-(Boron-) doped) with a native oxide layer of 2 nm provided by Wacker Chemitronics (Burghausen, Germany) were used as solid substrates. The wafers were cut to samples approximately 1 cm 2 in size and cleaned with a Snowjet T M (Tectra, Frankfurt/M, Germany), a cold CO 2 stream effectively removing particulate and organic contamination [14], followed by ultrasonication in ethanol, acetone, and hexane, subsequently. Immediately after this cleaning process, LC films were spincast onto the samples from hexane solutions. Variation of concentration and spinning rate allows to deposit films of variable thickness. The preparation procedure was performed in a class 100 clean room environment at room temperature. Therefore, the films were initially in the nematic (5CB) or smectic A (8CB) state, respectively. Film thicknesses were recorded with an ellipsometer (Optrel GbR, Berlin, Germany). The samples were placed on a heat stage (Linkam THMSG 600, temperature control better than 0.1• C) and observed in situ with a Zeiss Axiophot microscope equipped with a digital camera. Unless otherwise noted, no polarizers were used in the microscope setup.Observations at room temperature showed films of 5CB and 8CB to be stable for hours at thicknesses ranging from 50 nm to 200 nm. Upon heating, a surface undulation with a characteristic wavelength can be observed in both types of samples (see Figure 1) close to, but consistently below the N-I transition temperature (T N I ). The observed undulation does not lead to a complete dewetting of the LC film but rather disappears above T N I , such that the film becomes homogeneous again in the...