Instability Of Thin Films Research Articles

Patterns on a roll: a method of continuous feed nanoprinting

Exploiting elastic instability in thin films has proven a robust method for creating complex patterns and structures across a wide range of lengthscales. Even the simplest of systems, an elastic membrane with a lattice of pores, under mechanical strain, generates complex patterns featuring long-range orientational order. When we promote this system to a curved surface, in particular, a cylindrical membrane, a novel set of features, patterns and broken symmetries appears. The newfound periodicity of the cylinder allows for a novel continuous method for nanoprinting.

Ferroelectric Instability and Dimensionality in Bi-Layered Perovskites and Thin Films

The dielectric and thermal properties of Bi (bismuth)-layered perovskite SrBi2Ta2O9(SBT) are discussed in comparison with ferroelectric thin BaTiO3films. Although these two perovskites exhibit quite a different nature, the dielectric properties of BaTiO3thin film are similar to those in bulk SBT. The dielectric properties and pseudo-two-dimensional structure between SBT and thin film suggest that the bulk layered ferroelectric SBT is a good model of ultra-thin ferroelectric film with two perovskite layers, free from any misfit lattice strain with substrate and surface charge at the interface with electrodes. Based on the mechanism of ferroelectric phase transition of SBT, it seems plausible that the ferroelectric interaction is still prominent but shows a crossover from ferroelectric to antiferroelectric interaction in perovskite ultra-thin films along the tetragonal axis.

Electrohydrodynamic instabilities of polymer thin films: Filler effect

The influence of various nanoparticles with different dimension, density, dielectric constant, and surface property on electrohydrodynamic (EHD) instabilities of polymer/nanoparticle nanocomposite thin films was examined as a function of nanoparticle concentration. Transmission electron microscopy (TEM) images of polystyrene (PS)/nanoparticles (NPs) thin films demonstrated that all the nanoparticles were uniformly distributed in polymer matrix and the homogeneous dispersions of nanoparticles were not affected by thermal annealing above glass transition temperature. Optical microscopy (OM) observations indicated that thin films of polystyrene containing silica (SiO 2), gold (Au), cadmium selenide (CdSe), and titania (TiO 2) nanoparticles showed electrohydrodynamic instability patterns similar to those seen in pure polystyrene, up to 3 vol% nanoparticles. The presence of nanoparticles changed the dielectric constant of the thin films, which led to systematic variations in the wavelengths of the surface instabilities, which were consistent with calculated values. Cross-sectional transmission electron microscopy (TEM) images showed that migration or aggregation of the nanoparticles occurred only for silica contrary to other nanoparticles. This work points to a simple route to reduce the scale of final well-ordered columnar structures.

Thermophysics of the Interfacial Region and Its Impact on Instability and Rupture of Thin Free Liquid Films

Merging of adjacent bubbles is a particularly important process in nucleate boiling heat transfer, bubbly two-phase flow in small tubes, and the mechanisms that dictate the Leidenfrost transition. The merging process is ultimately associated with the rupture of the liquid film separating two bubbles, and the stability of the liquid film dictates whether coalescence is likely to occur. This paper examines two methods of exploring how the molecular-level features of the thin film and its interfaces affect the stability of the film and its subsequent impact on bubble merging. One method is an extended molecular capillarity model that has been developed for a free liquid film bounded by two liquid–vapor interfacial regions. This model predicts that as film thickness diminishes, a point is reached at which the mean centerline density in the film is between the spinodal limits for the fluid, implying that the film core lacks intrinsic stability. This suggests that loss of intrinsic stability may be the mechanism for film rupture in some cases. The second method examined here is use of molecular dynamics simulations to examine film structure and stability. Simulations using a Lennard–Jones interaction potential for non-polar fluids and the SPC/E potential for water and salt water are examined. Results of these explorations indicate that for water–NaCl solution films, surface tension increases with increasing NaCl concentration. These predictions are shown to be consistent with measured surface-tension data for such solutions. The results also indicate that for pure water and NaCl solutions, below a threshold thickness, the surface tension decreases with film thickness. Wave interface stability theory implies that this tends to make the film less stable. The implications of the results of these investigations for bubble merging in two-phase flow and boiling processes are also examined.

Side wall effects on the instability of thin gravity-driven films—From long-wave to short-wave instability

We study the effects of side walls on the primary instability of a gravity-driven thin liquid film flowing down a flat channel. The influences of different capillary boundary layer effects at the side walls on the instability of the free surface are resolved experimentally, by varying the crosswise side wall distance of the measurement positions between 5 mm and the channel center-line. The height of the capillary elevation and, thus, the resulting pretensioning of the free surface and the magnitude of a possible velocity overshoot have been adjusted by changing the contact angle between the liquid and the side wall. The influence of the contact angle on the stability of the flow, and especially its range, is remarkable. The difference of the neutral stability curves for the two investigated contact angles is up to 25% and remains significant even up to a side wall distance of 17 times the capillary length. Irrespective of the contact angle, the type of the free surface instability undergoes a transition from long-wave in the center of the channel to short-wave, as is well known for boundary layer flows, when the side wall distance is reduced. Furthermore, we have found that the presence of a velocity overshoot tends to destabilize the free surface.

Dry-spot nucleation in thin liquid films on chemically patterned surfaces

We systematically study the influence of chemical patterning on the instability of thin liquid films induced by chemical heterogeneities on a flat, horizontal, and partially wetting substrate. We consider common geometric shapes like wedges, circles, and stripes and determine the time required for nucleation of a dry-spot as a function of film thickness, contact angle, pattern dimensions, and geometry. Moreover, we characterized the resulting liquid distribution and identified conditions that avoid the formation of residual droplets on the less wettable regions, which is usually undesirable in technological applications.

Electro-hydrodynamic instability patterning of polymers

Electro-HydroDynamic Instability Patterning, EHDIP, is a novel micro-manufacturing process that makes use of the instability of viscous polymeric thin films when exposed to electrostatic fields. By using non uniform electrostatic fields, it is possible to shape the polymer into defined meso- and micro-scale structures which are subsequently cured to defined 2D and 3D microstructures. The relatively rapid process time, the one-step manufacturing approach, as well as the ability to produce hitherto unrealised topographies - such as continuous profile structures, makes EHDIP an attractive manufacturing process.

Production of highly aligned collagen lamellae by combining shear force and thin film confinement

Load-bearing tissues owe their mechanical strength to their highly anisotropic collagenous structure. To date attempts to engineer mechanically strong connective tissue have failed, mainly due to a lack of ability to reproduce the native collagen organization in constructs synthesized by cultured cells in vitro. The ability to influence the orientation of self-assembling collagen molecules to produce highly anisotropic structures has applications ranging from de novo engineering of complex tissues to the production of organized scaffolds for cell culture contact guidance. In this investigation we have used the simple technique of spin-coating to produce highly aligned arrays of collagen fibrils. By a simple modification of the method we have also successfully produced orthogonal collagen lamellae. Alternating collagen lamellae are frequently seen in load-bearing tissues such as cornea, annulus fibrosus, and cortical bone. Culturing of corneal fibroblasts on aligned collagen shows that the cells adopt the organization of fibrils. In this investigation we observed the reversal of fibrillar growth direction or “hook” formation similar to that seen previously in a microfluidic shear flow chamber. Although the results of this investigation clearly show that it is possible to produce small areas (1 cm 2) of collagen fibrils with enough alignment to guide fibroblasts, there is evidence that thin film instabilities are likely to be a significant barrier to producing organized collagen fibrils over larger areas. Successful application of this method to produce highly controlled and organized collagenous structures will require the development of techniques to control thin film instability and will be the subject of future work.

A Phase-Field – Finite Element Model for Instabilities in Multilayer Thin Films

ABSTRACTCahn-Hilliard type of phase-field (PF) model coupled with elasticity equations is used to study the instabilities in multilayer thin films. The governing equations of the solid state phase transformation include a 4th order partial differential equation representing the evolution of the conserved PF variable (concentration) coupled to 2nd order partial differential equations representing the mechanical equilibrium. A mixed order Galerkin finite element (FE) model is used including C0 interpolation functions for the displacement, and C1 interpolation functions for the concentration. It is shown that quadratic convergence, expected for conforming elements, is achieved from this coupled mixed-order FE model.Using the PF – FE model, first, we studied the effect of compositional strain on the PF interface thickness and the results of simulations are compared with the analytical solutions of an infinite thin film diffusion couple with a flat interface.Morphological instabilities in binary multilayer thin films are investigated. The alloys with and without intermediate phase are considered, as well as the cases with stable and metastable intermediate phase. Maps of transformations in multilayer systems are carried out considering the effects of initial thickness of layers, compositional strain, and growth of a stable/unstable intermediate phase on the instability of the multilayer thin films. It is shown that at some cases phase transformation, intermediate phase nucleation and growth, or deformation of layers due to high compositional strain can lead to the coarsening of the layers which can result in different mechanical and materials behaviors of the original designed multilayer.

Spontaneous instability of soft thin films on curved substrates due to van der Waals interaction

The linear bifurcation theory is used to investigate the stability of soft thin films bonded to curved substrates. It is found that such a film can spontaneously lose its stability due to van der Waals or electrostatic interaction when its thickness reduces to the order of microns or nanometers. We first present the generic method for analyzing the surface stability of a thin film interacting with the substrate and then discuss several important geometric configurations with either a positive or negative mean curvature. The critical conditions for the onset of spontaneous instability in these representative examples are established analytically. Besides the surface energy and Poisson's ratio of the thin film, the curvature of the substrate is demonstrated to have a significant influence on the wrinkling behavior of the film. The results suggest that one may fabricate nanopatterns or enhance the surface stability of soft thin films on curved solid surfaces by modulating the mechanical properties of the films and/or such geometrical properties as film thickness and substrate curvature. This study can also help to understand various phenomena associated with surface instability.

Electrohydrodynamic instabilities in thin liquid trilayer films

Experiments by Dickey et al. [Langmuir 22, 4315 (2006)] and Leach et al. [Chaos 15, 047506 (2005)] show that novel pillar shapes can be generated from electrohydrodynamic instabilities at the interfaces of thin polymer/polymer/air trilayer films. In this paper, we use linear stability analysis to investigate the effect of free charge and ac electric fields on the stability of trilayer systems. Our work is also motivated by our recent theoretical study [S. A. Roberts and S. Kumar, J. Fluid Mech. 631, 255 (2009)] which demonstrates how ac electric fields can be used to increase control over the pillar formation process in thin liquid bilayer films. For perfect dielectric films, the effect of an ac electric field can be understood by considering an equivalent dc field. Leaky dielectric films yield pillar configurations that are drastically different from perfect dielectric films, and ac fields can be used to control the location of free charge within the trilayer system. This can alter the pillar instability modes and generate smaller diameter pillars when conductivities are mismatched. The results presented here may be of interest for the creation of complex topographical patterns on polymer coatings and in microelectronics.

Thin film lubrication dynamics of a binary mixture: Example of an oscillatory instability

We study thin film instabilities in liquid films with deformable surface using the lubrication theory. An externally applied vertical temperature gradient may give cause to an instability (Marangoni instability) of the flat motionless film. Contrary to the earlier work where mostly pure fluids were discussed, the focus of the present paper lays on instabilities in mixtures of two completely miscible liquids. We show that the normally found monotonic long-wave instability may turn into an oscillatory one if the two components have a different surface tension and if the Soret coefficient establishes a stabilizing vertical concentration gradient. A systematic derivation of the basic equations in long-wave approximation is given. The character of instabilities is studied using linear stability analysis. Finally, a real system consisting of a water-isopropanol mixture is discussed in some detail.

Labyrinthine instability in thin liquid films

When a thin liquid film on a solid surface has a thickness corresponding to a particularpart the spinodal region of the disjoining pressure versus thickness isotherm, the filmbreaks down. One of the patterns that emerges on the breakdown has been referred to aswavy instability. It is compared here to the labyrinthine instability seen in magnetic films.The system is modeled following the procedure used in magnetic systems, and the patternof wavy instability is broken down into a curved thick–thin film in equilibrium with a flatthin–thin film of constant thickness. Minimization of free energy leads to expressions forvarious length scales that characterize the system. Comparisons with publishedexperimental results on nematic liquid crystals for a number of very different features aresatisfactory. They include film thicknesses in the bulk at equilibrium where thecapillary pressure is not zero, and is determined as a part of the solution, as wellas film thicknesses in the ledge where the capillary pressure is zero. Stabilityanalysis shows that the system is unstable in both directions with some qualifiers. Amodel is proposed in the form of a tiled structure to explain the labyrinthineform.

Substrate Heterogeneity Induced Instability and Slip in Polymer Thin Films: Dewetting on Silanized Surfaces with Variable Grafting Density

We employ dewetting behavior of thin polystyrene films to investigate the stability and slippage on the silanized Si wafers with different grafting density of silane brush. PS films are stable on our nonsilanized substrate, but an increase in the grafting density increases the contact angle from zero, causing the appearance and growth of holes. A clear signature of slippage (hole radius ∼ time2/3) is obtained at low grafting density and low contact angles, which changes to no-slip behavior (hole radius ∼ time) on a uniform high density brush. It is also found that the molecular weight of PS does not influence the slippage implying that the slippage observed is caused neither by the entanglement of polymer chains, nor by high nonwettability of the substrate, but by the substrate heterogeneity or physicochemical roughness imparted by the incomplete, low density brush with intervening adsorbed PS chains.

Interfacial slippage effect on the spontaneous instability of soft thin viscoelastic films

Based on the continuum mechanics and the bifurcation theory, a three-dimensional theoretical model for a soft thin viscoelastic film bonded to a rigid substrate is investigated. Considering the effect of interfacial slippage and the competition among van der Waals interaction potential energy, strain energy, and surface energy, a set of three-dimensional governing equations of the spontaneous instability is derived, and the analytical results of time-dependent critical conditions are obtained. Furthermore, the phase diagram of instability due to van der Waals interaction and the variation of the dimensionless characteristic wavenumber and the critical interaction stiffness with the critical time are discussed by considering the effect of interfacial slippage.

On the Breakup of Patterned Nanoscale Copper Rings into Droplets via Pulsed-Laser-Induced Dewetting: Competing Liquid-Phase Instability and Transport Mechanisms

Nanolithographically patterned copper rings were synthesized, and the self-assembly of the rings into ordered nanoparticle/nanodrop arrays was accomplished via nanosecond pulsed laser heating above the melt threshold. The resultant length scale was correlated to the transport and instability growths that occur during the liquid lifetime of the melted copper rings. For 13-nm-thick rings, a change in the nanoparticle spacing with the ring width is attributed to a transition from a Raleigh-Plateau instability to a thin film instability because of competition between the cumulative transport and instability timescales. To explore the competition between instability mechanisms further, we carried out experiments with 7-nm-thick rings. In agreement with the theoretical predictions, these rings break up in both the azimuthal and radial directions, confirming that a simple hydrodynamic model captures the main features of the processes leading to the breakup.

Flow and instability of thin films on a cylinder and sphere

We investigate the dynamics of thin films driven by gravity on the outer surface of a cylinder and sphere. The surface is rigid, stationary and the axis of the cylinder is horizontal. An instantaneous release of a constant volume of fluid at the top of the cylinder or sphere results initially in a two-dimensional or axisymmetric current respectively. The resultant flow of a thin film of fluid is described using lubrication theory when gravity and viscous forces govern the dynamics. We show that the thickness of the flow remains uniform in space and decreases in time liket−1/2near the top of both the cylinder and the sphere. Analytic solutions for the extent of the flow agree well with our experiments until the advancing front splits into a series of rivulets. We discuss scalings of the flow at the onset of the instability as a function of the Bond number, which characterizes the relative importance of gravity and surface tension. The experiments, conducted within an intermediate range of Bond numbers, suggest that the advancing front becomes unstable after it has propagated a critical distance, which depends primarily and monotonically on the volume of fluid and not on the viscosity of fluid. Releasing a sufficiently large volume of fluid ensures that rivulets do not develop on either a cylinder or sphere.

A Unified Theory of Instabilities in Viscoelastic Thin Films: From Wetting to Confined Films, From Viscous to Elastic Films, and From Short to Long Waves

A general unified theory of field (van der Waals, electric, etc.)-induced surface instabilities in thin viscoelastic films that accounts for a destabilizing field and stabilizing effects of elastic strain and surface energy is presented. The present theory seamlessly covers the instability and its different regimes in films ranging from elastic to viscous, from adhesive (confined) to wetting (free surface), and from short- to long-wave instabilities. The critical conditions for the onset of instability are found to be strongly dependent on elastic properties such as the shear modulus of the film, but the dominant wavelength is strikingly independent of the film rheology. Different regimes based on a nondimensional parameter (gamma/mu h) are uncovered, where gamma is the surface energy, mu is the elastic shear modulus, and h is the film thickness. A short-wave, elasticlike response with wavelength lambda approximately = 2.96 h is obtained for gamma/mu h < 0.1, whereas long waves that depend nonlinearly on the field strength and surface energy are obtained for gamma/mu h > 1. Owing to their small critical thickness, wetting films destabilized by intermolecular forces always display long-wave instability regardless of their viscoelasticity. Furthermore, our numerical simulations based on energy minimization for unstable wetting elastic films show the formation of islands for ultrathin films and a morphological phase transition to holes embedded in the film for relatively thicker films. Unlike viscous films, however, unstable elastic films do not display a unique dominant wavelength but a bimodal distribution of wavelengths.

Electric field induced dewetting and pattern formation in thin conducting polymer film

Electric field induced instability and two-dimensional sub-micropattern formation in thin films of polyaniline (PANi) (conducting polymer (CP)) at different levels of doping with camphor sulfonic acid (CSA) are studied experimentally. Optical microscopic and AFM analysis reveal that the application of electric field across the thin PANi-EB (emeraldine base) film–air interface between two electrodes causes instability and formation of uniformly dispersed sub-micron column/drop like elevations surrounded by annular depression. The number density and height of such pattern increase with voltage increment. Number density analysis reveals that below a threshold voltage, CP (at its base state) behaves like a dielectric polymer. On the contrary, a 50% doped PANi-ES (emeraldine salt) thin film–air interface forms stripe like on-plane pattern instead of columnar patterns while subjected to electric field. Performance study reveals faster response of columnar morphology of PANi-EB while exposed to HCl vapor compared to that of thin films of PANi-EB. The significance is in design of chemical vapor sensor of low diffusional resistance utilizing high surface to volume ratio.

Morphological instability of ferromagnetic thin films

We investigate the morphological instability of ferromagnetic planar thin films using linear perturbation theory. We find that, for a uniformly magnetized ferromagnetic planar film with a nonzero component of the magnetization along the normal to the planar film, the planar surface is unstable with respect to perturbations having a wave number smaller than a critical value. In addition, the critical wave number changes with the direction of the surface perturbations, with the perturbations perpendicular to the in-plane component of the magnetization having the largest critical wave number. Such anisotropy could be important in controlling the evolution of the morphology of the film and nanoscale-island formation.