Eur. Phys. J. E 12, 383–387 (2003) DOI: 10.1140/epje/e2004-00006-7 T HE EUROPEAN P HYSICAL JOURNAL E Unstable thin films: new questions H. Kaya a and B. J´ erˆome Complex Fluids Group, Departement of Chemical Engineering, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands Received 1st August 2003 Published online 20 January 2004 –c  EDP Sciences, Societ`a Italiana di Fisica, Springer-Verlag 2004 Abstract. We consider recent developments in the theory and modelling of unstable thin films, pointing out some recently suggested alternative routes and methods to account for phenomena such as nucleations and density fluctuations. PACS. 68.15.+e Liquid thin films – 47.20.Ma Interfacial instability 1 Introduction Thin films on solid substrates have over the last four decades been a topic of intense scientific research, both theoretically and experimentally. From the point of view of industrial applications, the study of thin films is im- portant for coatings, with which the surface characteris- tics of a material can be modified without affecting the bulk properties. On a more advanced level, which have give thin film research a new boost, the pattern formation taking place during the dewetting films can be of great im- portance in nanotechnology, with the possibility of design- ing patterned surfaces for specific applications. Before this can be realized, however, it is necessary to understand the processes governing the dynamics and equilibrium statics of a thin film on a fundamental level. At a macroscopic level, the description of the wetting behaviour of a liq- uid droplet deposited on a solid substrate is provided by Young’s law [1], which relates the observable contact an- gle to the surface tensions between the solid substrate, the liquid droplet, and the surrounding vapour phase. A thin film appears a small system is terms of size, and hence one should expect it to represent a simplified system with fewer degrees of freedom than its bulky coun- terpart, thus facilitating the study of physical processes taking place within the film. This is, however, not the case. Instead, the reduced dimensions render a material in a thin film state sensitive to a number of influencing factors that are negligible in the bulk state. This finite- size effect is indeed a typical feature of small systems [2]. Contaminations like dust particles, for instance, may be of no importance in a bulk liquid, but if the same liquid is in a thin film state, the impurities cause visible and often undesirable changes in the film structure when they act as sites for heterogeneous nucleation. For the experimen- talist working on thin films, such effects call for meticu- a e-mail: kaya@science.uva.nl lous preparation and careful investigation. Another small- system feature is the sensitivity to fluctuations in for in- stance temperature and density. The length-scale of these fluctuations can be comparable to those of the governing interactions, thus causing dramatic effects. The walls con- fining the thin film can also be considered as defects, since they disrupt the continuum of the liquid film. More im- portant, however, are the surface interactions with ranges comparable to the thickness of the film. The stability of a liquid thin film deposited on a solid substrate can in the simplest case be directly inferred from the sign of the net film-substrate interaction. On the other hand, small per- turbations of these interactions may change their sign in certain ranges of film thickness, thus causing a complete opposite behaviour. The causes of these changes will be discussed in Section 2. The treatment and modelling of the forces and inter- actions acting upon a thin film are still on a highly simpli- fied stage, and thus they represent an ongoing challenge for theoretical work, and for the experiments which the theories seek to explain. The detailed balance between bulk and surface forces makes numerical simulations of thin film evolution a highly complicated matter, and even small adjustments may lead to new complications. This will be addressed in Section 3. The nucleation vs. spin- odal dewetting problem is the topic of Section 4, where the possibilities of applying classical nucleation theory to dewetting phenomena is mentioned. 2 Interactions 2.1 Fundamental interactions The shape of the interface potential as a function of film thickness contains crucial information about the stability of a thin film [3–5]. From the shape of the potential of