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

Abstract

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

The Strength of Long-Range Forces across Thin Liquid Films

“…The stability of thin films and the search for the possible origins of instabilities have become topics of much recent interest (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13). A major question in this context is if van der Waals forces can provoke an instability which eventually results in a complete destruction of the film.…”

“…Thus, we used silicone oils, which have very low surface energies. Our experimental approach is based on bi-modal polydimethylsiloxane (PDMS) brushes on silicon wafers covered with a thin PDMS film of variable thickness (13). Such films were stable if they were kept in air.…”

Control of Spatial Periodicities of Dewetting Structures of Oleo-Liquid Films

“…ting structure have been extensively studied (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16). Here, we have investigated the dewetting of oleo-liquid film containing octyl p-methoxycinnamate and octylsilyl titanium dioxide particle having average diameter of 35 nm on glass surface.…”

“…The dewetting of thin polymer film on solid substrate by treating with water has been investigated (12)(13)(14)(15)(16). However, adjusting the viscosity of polymer and the interfacial tension between polymer and water to appropriate value for controlling the characteristic length of the dewetting pattern is relatively difficult.…”

Multidimensional Generation of Combinatorial Organic Arrays by Selective Wetting Inscription

“…In fact, long-range vdW forces were found to propagate over a few tens of nanometers in a film that experiences vdW force-driven dewetting on a wettable layer. [16] In light of the above idea, a hydrophobic fluoropolymer, EGC-1700, was examined for use as a CL with a wetting transition using xylene, which dissolves light-emitting polymers (LEPs) for element patterns. In a modified version of the intermolecular theory, [17] the contact angle can be described as…”