Instability of Thin Liquid Films by Density Variations: A New Mechanism that Mimics Spinodal Dewetting (original) (raw)
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Instability and Pattern Formation in Thin Liquid Films on Chemically Heterogeneous Substrates
Langmuir, 2000
The surface instability, dynamics, morphology, and spontaneous dewetting of a thin liquid film on chemically heterogeneous substrates are studied on the basis of 3D nonlinear simulations. A new mechanism of dewetting in the presence of heterogeneity is proposed where the instability is engendered by the gradient of intermolecular interactions that lead to a microscale wettability contrast. The time scale of instability, which can be several orders smaller than the spinodal dewetting time scale on homogeneous surfaces, varies inversely with the potential difference induced by the heterogeneity. Heterogeneity can even destabilize spinodally stable films, reduce the time of rupture substantially for thicker films, and decrease the dependence of rupture time on the film thickness. The presence of heterogeneity produces complex and locally ordered morphological features that are not predicted by the spinodal dewetting, for example "ripples" and "castle-moat" structures, radially symmetric structures, and a lack of undulations before the birth of a hole. The precise morphological pattern selection depends on the size of the heterogeneity, the potential difference caused by the heterogeneity, the film thickness, and also the spinodal characteristics of the substrate. The resulting morphologies can be understood on the basis of simple arguments that consider interplay among these factors.
Instability and morphology of thin liquid films on chemically heterogeneous substrates
Physical Review Letters, 2000
A new mechanism for the surface instability and dewetting of thin films on chemically heterogeneous substrates is identified and simulated. The time scale for instability varies inversely with the potential difference due to the heterogeneity. Heterogeneities can even destabilize spinodally stable films, reduce the time of rupture substantially for thicker films, and produce complex and locally ordered morphological features (e.g., ripples and castle-moat structures, lack of undulations before hole formation) that are not predicted by the spinodal mechanism. PACS numbers: 68.15. + e, 47.20.Ma, 47.54. + r, Ultrathin fluid films (e.g., coatings) on solid substrates have attracted widespread interest in recent years [6]. The current theoretical understanding of thin film stability, dynamics, and morphology is limited to films on chemically homogeneous substrates. Such films are rendered unstable by a spinodal mechanism whenever the excess intermolecular interaction energy per unit area (DG) shows positive curvature with respect to the film thickness, h, e.g., when the effective Hamaker constant, A s is positive for the long-range van der Waals interaction. Deliberately tailored chemically heterogeneous substrates are increasingly being used to engineer desired nano-and micropatterns in thin films . Also, experiments even on the substrates that were thought to be relatively homogeneous show remarkable departures from the theory of spinodal dewetting [6]. These observations have repeatedly suggested the involvement of heterogeneous sites, e.g., dust, trapped microcavities, chemical contamination, variation of oxide layer thickness on silicon, variable chain adsorption, etc., all of which generate localized patches of surface properties or potential different from the surrounding substrate, as shown in . This Letter proposes and simulates a new mechanism of thin film instability and its evolution on chemically heterogeneous substrates. The general dynamical and morphological features of the instability are then clearly contrasted with the spinodal dewetting. One of our main results is that the heterogeneities, even on length scales substantially smaller than the spinodal scales can greatly accelerate the growth of surface instabilities and can even engender the rupture of spinodally stable films.
Stability and Dewetting of Thin Liquid Films
Series in Soft Condensed Matter, 2008
The stability of thin liquid coatings is of fundamental interest in everyday life. Homogeneous and non-volatile liquid coatings may dewet either by heterogeneous nucleation, thermal nucleation, or spinodal dewetting. Wetting and dewetting is explained on a fundamental level, including a discussion of relevant interactions. The chapter will also address the various dewetting scenarios and explain how the effective interface potential governs the behavior obtained for various stratified substrates and film thicknesses.
The Journal of chemical physics, 1999
Various stages of evolution of the surface instability and pattern formation are investigated for unstable thin ͑Ͻ100 nm͒ fluid films subjected to the long-range van der Waals repulsion and a shorter range attraction. The complete three-dimensional morphology is resolved based on numerical solutions of the nonlinear 2D thin film equation. In the first phase of evolution, initial random nonhomogeneities are quickly reorganized into a small amplitude undulating structure consisting of long ''hills'' and ''valleys.'' Different types of patterns are formed thereafter, depending on the initial mean thickness vis-à-vis location of the minimum in the intermolecular force curve. Dewetting of relatively thick films occurs by circular isolated holes which grow and coalesce to form a large-scale structure with intervening pools and ridges of the liquid, which eventually decay into increasingly circular droplets. In thinner films, the shallow depressions merge and the long ridges of the bicontinuous structure mature, fragment, and directly transform into increasingly circular droplets, which continue to grow by ripening and merger. The characteristics of a pattern, its pathway of evolution, and the morphology at the onset of dewetting thus depend crucially on the form of the intermolecular potential in an extended neighborhood of the initial thickness. The linear and 1D nonlinear analyses used hitherto fail completely in prediction of morphological patterns, but can predict their length scales rather well.
Enhanced instability in thin liquid films by improved compatibility
Physical Review Letters, 2000
We investigated experimentally the morphological evolution of thin polydimethylsiloxane films sandwiched between a silicon wafer and different bounding liquids with interfacial tensions varying by 2 orders of magnitude. It is shown that increasing the compatibility between film and bounding liquid by adding a few surfactant molecules results in a faster instability of shorter characteristic wavelength. Inversely, based on the characteristic parameters describing the instability we determined extremely small interfacial tensions with a remarkable accuracy. 68.15. + e, Thin liquid films sandwiched between a solid substrate and a bounding fluid are highly relevant in many scientific and technological processes such as adsorption, flotation, lubrication, and coatings. Attractive long-range interactions across the film may cause these films to become unstable and break up to form dry patches on the underlying substrate . The interfacial tension (g fb ) at the filmbounding fluid interface is a key parameter in the spatial and temporal evolution of this instability. Qualitatively, g fb plays a purely stabilizing role before the appearance of dry patches or holes as it resists the deformation of the film surface. The higher g fb is, the more difficult it is to rupture the film. This situation before the breakup of the film has to be contrasted to the subsequent dewetting. There, the interplay of three interfacial tensions (substrate-bounding fluid, substrate-film fluid, and film-bounding fluid), which determine the contact angle at the three phase contact line , affects the morphological evolution of the film, e.g., the growth of dry patches. In addition, the dewetting velocity is proportional to g fb . Quantitatively, linear theory as well as a dimensional analysis of the nonlinear governing equations predicts a linear dependence of length and time scales of the instability on the interfacial tension . These theoretical results have never been verified or disproved experimentally. It has to be emphasized that varying g fb by changing the bounding liquid affects, at the same time, the strength of the long-range interactions across the film . Here, we avoided such a correlation by simply adding a few surfactant molecules to the bounding liquid. These surfactant molecules adsorbed at the interface between the film and the bounding fluid and thereby reduced g fb without significantly changing the long-range interactions.
2000
Dynamics, stability, morphology, and dewetting of a thin film (< 100 nm) under the influence of a long-range van der Waals attraction combined with a short-range repulsion are studied based on numerical solutions of the nonlinear two-dimensional (2-D) thin film equation. Area and connectivity measures are used to analyze the morphology and the distinct pathways of evolution of the surface instability. The initial disturbance resolves into an undulating structure of uneven'hills and valleys'.
Spinodal dewetting of thin liquid films at soft interfaces
The European Physical Journal E, 2000
We study theoretically the behavior of nanoscopic liquid films L (thickness e) intercalated between a solid S and a rubber R (elastic modulus µ). Thickness modulations involve a healing length ζR, which results from a competition between elastic and disjoining pressure. With van der Waals interactions, ζR = e 4 /(a 2 h0), where a is a molecular size and h0 the rubber capillary length (h0 = γLR/µ, γLR = L/R interfacial tension). If the Hamaker constant of the intercalated liquid is negative, the film dewets by amplification of peristaltic fluctuations ("spinodal dewetting"). The typical size of the S/R contacts is predicted to scale like ζ R for films of thicknesses e > √ h0a. The rise time τ of the fastest mode, predicted to scale like τ ≈ e 9 , should be very sensitive to the film thickness.
Spinodal instability and pattern formation in thin liquid films confined between two plates
Journal of Colloid and Interface Science, 2006
The instability, morphology and pattern formation engendered by the van der Waals force in a thin liquid film of thickness h confined between two closely placed solid surfaces (at distance d > h) are investigated based on nonlinear 3D simulations. The initial and the final stages of dewetting and pattern formation are found to be crucially dependent on the volumetric (thickness) ratio of air and liquid and its deviation from the location of the maximum of the spinodal parameter versus volumetric ratio curve. On a low energy surface, relatively thinner films and wider air gaps favor initial dewetting of the lower plate by the formation of holes, whereas thicker films with thinner air gaps initially evolve by the formation of columns/bridges that join the upper plate. In the later stage of evolution, the initial holes in thinner films evolve into columns/drops, while a rapid coalescence of columns in the thicker films eventually causes formation of holes. Thus, a phase inversion, either from liquid-in-air to air-in-liquid dispersion or vice versa, occurs during the final stages of evolution. A thin film confined between two high-energy solid surfaces forms columns (bridges) only when its mean thickness, h 0 , is greater than a critical thickness (h c) or the air gap is smaller than a critical distance. The patterns can be aligned by using a topographically patterned confining surface. Conditions on pattern periodicity, amplitude, and the volumetric ratio of air and liquid in the gap are explored for the formation of various types of ordered patterns including annular rings of columns, concentric ripples, parallel channels and a rectangular array of complex features. The results are of significance in soft lithographies such as LISA, soft stamping and capillary force lithography.
Physical Review Letters, 2013
A thermodynamically consistent gradient dynamics model for the evolution of thin layers of liquid mixtures, solutions and suspensions on solid substrates is presented which is based on a film height-and mean concentration-dependent free energy functional. It is able to describe a large variety of structuring processes including coupled dewetting and decomposition processes. As an example, the model is employed to investigate the dewetting of thin films of liquid mixtures and suspensions under the influence of effective long-range van der Waals forces that depend on solute concentration. The occurring fluxes are discussed and it is shown that spinodal dewetting may be triggered through the coupling of film height and concentration fluctuations. Fully nonlinear calculations provide the time evolution and resulting steady film height and concentration profiles.