Destabilising effect of long-range forces in thin liquid films on wettablesubstrates

Using optical microscopy, we followed the evolution of thin and initially s mooth liquid polymer films on wettable solid substrates for different surro unding media. For thicknesses between 30 and 110 nm, we observed an instabi lity of the film, eventually leading to the formation of nanodroplets on a remaining wetting layer. Exchanging the surrounding medium allowed to rever se the process and to re-establish a smooth film. Our results are in accord ance with theoretical considerations and computer simulations. Weak but lon g-ranged dispersion forces may have drastic consequences concerning the mor phology of thin liquid films even if short-range interactions force the fil m to stay on the substrate.