We compare the dispersion relations for spinodally dewetting thin liquid films for increasing magnitude of interfacial slip length in the lubrication limit. While the shape of the dispersion relation, in particular the position of the maximum, are equal for no-slip up to moderate slip lengths, the position of the maximum shifts to much larger wavelengths for large slip lengths. Here, we discuss the implications of this fact for recently developed methods to assess the disjoining pressure in spinodally unstable thin films by measuring the shape of the roughness power spectrum. For PS films on OTS covered Si wafers (with slip length b approx. 1 mum) we predict a 20% shift of the position of the maximum of the power spectrum which should be detectable in experiments.
Numerical simulations of spontaneous dewetting of non-slipping, variable viscosity unstable thin liquid films on homogeneous substrates reveal the existence of sub-spinodal lengthscales through formation of satellite holes, a marker of nucleated dewetting and/or heterogeneous substrates, in the late stages of dewetting if the liquid viscosity decreases continually with decreasing film thickness. These films also show established signatures of slipping films such as faster rupture and flatter morphologies in the early stages even without invoking any slippage.
The stability of thin liquid films on a surface depends on the excess free energy of the system involving various short-range and long-range interactions. In an unstable condition, thin liquid films may dewet into multiple small-sized droplets via spinodal, homogeneous, or heterogeneous nucleation process. However, if the total excess free energy of the system can be manipulated using an external stimulus, one can control the stability of thin liquid films on demand. Here, we study the reversible dewetting process of thin lubricating films underneath aqueous drops on slippery surfaces using an…
We study dewetting of thin polymer films with built-in topographical fluctuations produced by rubbing the film surface with a rayon cloth. By varying the density of imposed surface defects, we unambiguously distinguish spinodal dewetting, which dominates in liquid films thinner than a characteristic thickness = 13.3 nm, from heterogeneous nucleation in the thicker films. Invariance of this characteristic thickness upon more than a decade change in the defect density makes kinetic effect an unseemly origin. A crossover of the spinodal line provides a consistent picture. This interpretation, how…
Slippage of Newtonian liquids in the presence of a solid substrate is a newly found phenomenon the origin of which is still under debate. In this paper, we present a new analysis method to extract the slip length. Enhancing the slip of liquids is an important issue for microfluidic devices that demand for high throughput at low pumping power. We study the velocity of short-chained liquid polystyrene (PS) films dewetting from non-wettable solid substrates. We show how the dynamics of dewetting is influenced by slippage and we compare the results of two types of substrates that give rise to diff…
Dewetting of thin polystyrene films deposited onto silicone wafers at temperatures close to the glass transition exhibits unusual dynamics and front morphologies. Here, we present a new theoretical approach of these phenomena taking into account both the viscoelastic properties of the film and the non-zero velocity of the film at the interface with the substrate (due to slippage). We then show how these two ingredients lead to : (a) A very asymmetric shape of the rim as the film dewetts, (b) A decrease of the dewetting velocity with time like $t^{-{1/2}}$ for times shorter than the reptation t…
Experiments on dewetting thin polymer films confirm the theoretical prediction that thermal noise can strongly influence characteristic time-scales of fluid flow and cause coarsening of typical length scales. Comparing the experiments with deterministic simulations, we show that the Navier-Stokes equation has to be extended by a conserved bulk noise term to accomplish the observed spectrum of capillary waves. Due to thermal fluctuations the spectrum changes from an exponential to a power law decay for large wavevectors. Also the time evolution of the typical wavevector of unstable perturbation…
The stability of thin liquid coatings is of fundamental interest in every- day 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 article 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.
We investigate the effect of short chains on slip of highly entangled polystyrenes (PS) during thin film dewetting from non-wetting fluorinated surfaces. Binary and ternary mixtures were prepared from monodisperse PS with weight average molecular weights $5 < M_\textrm{w} < 490$ kg/mol. Flow dynamics and rim morphology of dewetting holes were captured using optical and atomic force microscopy. Slip properties are assessed in the framework of hydrodynamic models describing the rim height profile of dewetting holes. We show that short chains with $M_\textrm{w}$ below the polymer critical molecul…
This study reveals the influence of the surface energy and solid/liquid boundary condition on the breakup mechanism of dewetting ultra-thin polymer films. Using silane self-assembled monolayers, SiO$_2$ substrates are rendered hydrophobic and provide a strong slip rather than a no-slip solid/liquid boundary condition. On undergoing these changes, the thin-film breakup morphology changes dramatically -- from a spinodal mechanism to a breakup which is governed by nucleation and growth. The experiments reveal a dependence of the hole density on film thickness and temperature. The combination of l…
Hydrodynamic slip of Newtonian liquids is a new phenomenon, the origin of which is not yet clarified. There are various direct and indirect techniques to measure slippage. Here we describe a method to characterize the influence of slippage on the shape of rims surrounding growing holes in thin polymer films. Atomic force microscopy is used to study the shape of the rim; by analyzing its profile and applying an appropriate lubrication model we are able to determine the slip length for polystyrene films. In the experiments we study polymer films below the entanglement length that dewet from hydr…
Ultrathin polymer films on non-wettable substrates display dynamic features which have been attributed to either viscoelastic or slip effects. Here we show that in the weak and strong slip regime effects of viscoelastic relaxation are either absent or not distinguishable from slip effects. Strong-slip modifies the fastest unstable mode in a rupturing thin film, which questions the standard approach to reconstruct the effective interface potential from dewetting experiments.
Stability of thin lubricating fluid coated slippery surfaces depends on the surface energy of the underlying solid surface. High energy solid surfaces, coated with thin lubricating oil, lead to the dewetting of the oil films upon depositing aqueous drops on it. The total surface energy, which is due to the long range and short range interactions, also predict the instability of thin lubricating films under the given condition. In this article, we present experimental study of dewetting of thin lubricating oil films sandwiched between hydrophilic solid surface and aqueous drops. Fluorescence im…
We have studied the breakup and subsequent fluid flow in very thin films of partially wetting liquid on solid substrates, using molecular dynamics simulations. The liquid is made of short chain molecules interacting with Lennard-Jones interactions, and the solid is modeled as a clean crystal lattice whose atoms have thermal oscillations. Films below a critical thickness are found to exhibit a spontaneous spinodal-like instability leading to dry patches, as predicted theoretically and observed in some experiments. Liquid withdrawing from a dry patch collects in a moving rim whose fluid dynamics…
The influence of wall slip on the instability of a non-wetting liquid film placed on a solid substrate is analyzed in the limit of negligible inertia. In particular, we focus on the stability properties of the film, comparing the performance of the three lubrication models available in the literature, namely the weak, intermediate and strong slip models, with the Stokes equations. Since none of the aforementioned leading-order lubrication models is shown to be able to predict the growth rate of perturbations for the whole range of slipping lengths, we develop a parabolic model able to accurate…
The dewetting dynamics of ultrathin polymer films, e.g. in the model system of polystyrene on a polydimethylsiloxane-covered substrate, exhibits interesting behavior like a fast decay of the dewetting velocity and a maximum in the width of the built-up rim in the course of time. These features have been recently ascribed to the relaxation of residual stresses in the film that stem from the nonequilibrium preparation of the samples. Recent experiments by Coppee et al. on PS with low molecular weight, where such stresses could not be evidenced, showed however similar behavior. By scaling argumen…
Classical hydrodynamic models predict that infinite work is required to move a three-phase contact line, defined here as the line where a liquid/vapor interface intersects a solid surface. Assuming a slip boundary condition, in which the liquid slides against the solid, such an unphysical prediction is avoided. In this article, we present the results of experiments in which a contact line moves and where slip is a dominating and controllable factor. Spherical cap shaped polystyrene microdroplets, with non-equilibrium contact angle, are placed on solid self-assembled monolayer coatings from whi…
The instability, dynamics and morphological transitions of patterns in thin liquid films on periodic striped surfaces (consisting of alternating less and more wettable stripes) are investigated based on 3-D nonlinear simulations that account for the inter-site hydrodynamic and surface-energetic interactions. The film breakup is suppressed on some potentially destabilizing nonwettable sites when their spacing is below a characteristic lengthscale of the instability, the upper bound for which is close to the spinodal lengthscale. The thin film pattern replicates the substrate surface energy patt…
We review recent experiments on dewetting thin films of evaporating colloidal nanoparticle suspensions (nanofluids) and discuss several theoretical approaches to describe the ongoing processes including coupled transport and phase changes. These approaches range from microscopic discrete stochastic theories to mesoscopic continuous deterministic descriptions. In particular, we describe (i) a microscopic kinetic Monte Carlo model, (ii) a dynamical density functional theory and (iii) a hydrodynamic thin film model. Models (i) and (ii) are employed to discuss the formation of polygonal networks, …
In many macroscopic dynamic wetting problems, it is assumed that the macroscopic interface is quasistatic, and the dissipation appears only in the region close to the contact line. When approaching the moving contact line, a microscopic mechanism is required to regularize the singularity of viscous dissipation. On the other hand, if the characteristic size of a fluidic system is reduced to a range comparable to the microscopic regularization length scale, the assumption that viscous effects are localized near the contact line is no longer justified. In the present work, such microscopic length…
In this study, we present a novel method to assess the slip length and the viscosity of thin films of highly viscous Newtonian liquids. We quantitatively analyse dewetting fronts of low molecular weight polystyrene melts on Octadecyl- (OTS) and Dodecyltrichlorosilane (DTS) polymer brushes. Using a thin film (lubrication) model derived in the limit of large slip lengths, we can extract slip length and viscosity. We study polymer films with thicknesses between 50 nm and 230 nm and various temperatures above the glass transition. We find slip lengths from 100 nm up to 1 micron on OTS and between …
A gradient dynamics model based on an extended interface Hamiltonian is presented that is able to describe the dynamics of structuring processes in thin films of liquid mixtures, solutions and suspensions on solid substrates including coupled dewetting and decomposition. After discussing known limiting cases 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 trigge…
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.
A transient 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.
Probing the fluid dynamics of thin films is an excellent tool to study the solid/liquid boundary condition. There is no need for external stimulation or pumping of the liquid due to the fact that the dewetting process, an internal mechanism, acts as a driving force for liquid flow. Viscous dissipation within the liquid and slippage balance interfacial forces. Thereby, friction at the solid/liquid interface plays a key role towards the flow dynamics of the liquid. Probing the temporal and spatial evolution of growing holes or retracting straight fronts gives, in combination with theoretical mod…
Alkylsilane self-assembled monolayers (SAMs) are often used as model substrates for their ease of preparation and hydrophobic properties. We have observed that these atomically smooth monolayers also provide a slip boundary condition for dewetting films composed of unentangled polymers. This slip length, an indirect measure of the friction between a given liquid and different solids, is switchable and can be increased [Fetzer et al., Phys. Rev. Lett. 2005; Bäumchen et al. J. Phys.: Cond. Matt. 2012] if the alkyl chain length is changed from 18 to 12 backbone carbons, for example. Typically, th…
The stability of nonvolatile thin liquid films and of sessile droplets is strongly affected by finite size effects. We analyze their stability within the framework of density functional theory using the sharp kink approximation, i.e., on the basis of an effective interface Hamiltonian. We show that finite size effects suppress spinodal dewetting of films because it is driven by a long-wavelength instability. Therefore nonvolatile films are stable if the substrate area is too small. Similarly, nonvolatile droplets connected to a wetting film become unstable if the substrate area is too large. T…
Thin liquid films on surfaces are part of our everyday life, they serve e.g. as coatings or lubricants. The stability of a thin layer is governed by interfacial forces, described by the effective interface potential, and has been subject of many studies in the last decades. In recent years, the dynamics of thin liquid films came into focus since results on the reduction of the glass transition temperature raised new questions on the behavior of especially polymeric liquids in confined geometries. The new focus was fired by theoretical models that proposed significant implication of the boundar…
In capillary-driven fluid dynamics, simple departures from equilibrium offer the chance to quantitatively model the resulting relaxations. These dynamics in turn provide insight on both practical and fundamental aspects of thin-film hydrodynamics. In this work, we describe a model trilayer dewetting experiment elucidating the effect of solid, no-slip confining boundaries on the bursting of a liquid film in a viscous environment. This experiment was inspired by an industrial polymer processing technique, multilayer coextrusion, in which thousands of alternating layers are stacked atop one anoth…
We propose a novel approach to the numerical simulation of thin film flows, based on the lattice Boltzmann method. We outline the basic features of the method, show in which limits the expected thin film equations are recovered and perform validation tests. The numerical scheme is applied to the viscous Rayleigh-Taylor instability of a thin film and to the spreading of a sessile drop towards its equilibrium contact angle configuration. We show that the Cox-Voinov law is satisfied, and that the effect of a tunable slip length on the substrate is correctly captured. We address, then, the problem…
