Instability Of Thin Films Research Articles

Self-organized composite morphologies and interface instability of immiscible alloy thin films during physical vapor deposition: Insights from phase-field simulations

In this paper, we carried out phase-field simulations for investigating self-organized composite morphologies formation and interface instability of two phase immiscible binary alloy thin-film growth during physical vapor deposition. Predicted results show that for lower the deposition rate the solid-gas interface keeps planar and the lateral composition modulation (LCM) configuration occurs. As the deposition rate increases, the self-organized composite morphology gradually transfers to vertical composition modulations (VCM) configuration and the random composition modulations (RCM) configuration with the decrease of the boundary layer of the incident vapor and the unsteady solid-gas interface. Results show that the nanoprecipitate concentration modulation (NCPM) depends strongly on the nominally phase fraction. Moreover, we found that the historical dependence of self-organized composite morphologies and solid-gas interface stability, i.e., the initial condition can strongly influence the microstructure of the thin film during physical vapor deposition. Finally, we provide an analysis method for interface instability based on fast Fourier transform (FFT), and the frequency characteristics of cellular and bump structures are identified.

Bounce and contact mode regimes for drop impact on smooth surfaces: The influence of gas kinetics and electrostatics

When liquid drops impact on solid surfaces, an air layer forms in between the drop and the surface, acting as a cushion to mitigate the impact. In this work, we focus on delineating the bounce and contact mode regimes of impacting drops on smooth surfaces, specifically discerning whether drops rebound from the air layer or make contact with the solid surfaces, and pinpointing the precise contact modes between the drop and solid surfaces by resolving the gas film evolution and rupture. Our simulation model incorporates gas kinetics and electrostatics effects, both of which have been validated by experiments documented in the literature or theoretical models regarding thin film instabilities. We undertake a comprehensive review and categorization of the contact modes and elucidate how they change under different conditions of impact velocities, ambient pressures, and electric field intensities. We also provide some perspectives on the regime map for the lubricated surfaces, which contains an unresolved issue that the critical Weber number for bouncing-wetting transition is significantly reduced compared to the solid smooth surfaces like mica. These insights have noteworthy practical implications offering guidance for a wide range of scenarios, from normal-pressure environments to low-pressure conditions at high altitudes, encompassing high electric field conditions such as nanogenerators as well as low electric field conditions resembling glass surfaces with static electricity.

Hierarchical Surface Instability in Polymer Films.

This study demonstrates hierarchical instabilities in thin films. The hierarchical instabilities display three morphological characteristics: (1) windmill-like patterns at the macroscale, (2) Bénard cells and striations at the microscale, and (3) holes at the mesoscale. Such hierarchical instabilities occurred when spin coating was performed on high-volatile solutions under a high relative humidity (RH) but were suppressed when spin coating was performed on low-volatile solutions regardless of the RH. The high-volatile solutions comprise poly(4-vinylpyridine) (P4VP) in methanol or ethanol. The low-volatility solutions comprise P4VP in propanol or butanol. P4VP molecular weights, P4VP concentrations, spin rates, and film thicknesses are not vital factors in forming hierarchical instability in spin-coated P4VP films. Instead, the formation of hierarchical instabilities depends on the RH and solvent types. Namely, the hierarchical instabilities are driven by Bénard-Marangoni convection, water vapor condensation, and disturbance of spin-up and spin-off stages during spin coating of highly volatile solutions under high RH. Mechanisms of hierarchical instabilities are interpreted in detail.

Thermal Instability of Gold Thin Films

The disintegration of a continuous metallic thin film leads to the formation of isolated islands, which can be used for the preparation of plasmonic structures. The transformation mechanism is driven by a thermally accelerated diffusion that leads to the minimalization of surface free energy in the system. In this paper, we report the results of our study on the disintegration of gold thin film and the formation of nanoislands on silicon substrates, both pure and with native silicon dioxide film. To study the processes leading to the formation of gold nanostructures and to investigate the effect of the oxide layer on silicon diffusion, metallic film with a thickness of 3 nm was deposited by molecular beam epitaxy (MBE) technique on both pure and oxidized silicon substrates. Transformation of the thin film was observed by low-energy electron microscopy (LEEM) and a scanning electron microscope (SEM), while the nanostructures formed were observed by atomic force microscope (AFM) method. Structural investigations were performed by low-energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS) methods. Our experiments confirmed a strong correlation between the formation of nanoislands and the presence of native oxide on silicon substrates.

Breakup of thin liquid films with viscous interfaces

Thin liquid films are ubiquitous in nature and have many practical applications. From biological films to the curtain coating process, thin films are present in both large and small scales. Despite their importance, understanding the stability of these films remains a significant challenge due to the fluid–fluid interface that is free to deform, affected by interfacial tension and complex rheological behavior. Instabilities in thin films are often caused by van der Waals attractions, which can lead to the rupture of the layer. To investigate the rupture dynamics, numerical methods are commonly used, such as asymptotic derivations of the lubrication theory or interface tracking methods. In this paper, we present a computational study of the breakup dynamics of a stationary thin liquid sheet bounded by a passive gas with a viscous interface, using the arbitrary Lagrangian–Eulerian method and the Boussinesq–Scriven constitutive law to model the rheological behavior. Our results demonstrate that the stability of thin liquid films is influenced by both surface rheology and disjoining effects and that the viscous character of the interface can delay sheet breakup, leading to more stable films.

Experiments on Marangoni spreading – evidence of a new type of interfacial instability

Marangoni spreading on thin films is widely observed in nature and applied in industry. It has serious implications for airway drug delivery, especially in surfactant displacement therapy. This paper reports the results of experimental investigations of a surfactant-laden droplet spreading on films made of more viscous Newtonian fluids as well as on films made of viscoelastic fluids. The experiments used particle seeding, the transmission-speckle method and particle tracking velocimetry (PTV) to determine the deformation of the film–droplet interface and to measure velocity fields. Radially aligned patterns were observed on Newtonian films. Similar patterns, but with much smaller wavenumber, were observed on viscoelastic films in combination with rapid azimuthal variations of the film thickness. The Saffman–Taylor instability at the film–droplet interface explains the formation of patterns on a more viscous Newtonian film, and their onset requires exceeding the critical capillary number. The pattern formation on viscoelastic films is correlated with an instability at the film–droplet–air contact line when the liquid is expelled radially by the spreading droplet. PTV revealed azimuthal variations of the velocity field in the vicinity of the contact line. The observed contact line instability is different from previously reported fingering instabilities of Newtonian thin films. A simple scaling law accounting for the Marangoni-stress-induced elastic shear deformation is proposed to describe the flow field in the patterns formed in the viscoelastic films.

Exploiting the natural instability in thin and flexible dielectric solid films for sensing and photonic applications – INVITED

Flexible and stretchable photonics are emerging fields aiming to develop novel applications where the devices need to conform to uneven surfaces or whenever lightness and reduced thickness are major requirements. However, owing to the relatively small refractive index of transparent soft matter, these materials are not well adapted for light management at visible and near-infrared frequencies. Here we demonstrate simple, low cost and efficient protocols for fabricating Si1−xGex-based, sub-micrometric dielectric antennas with ensuing hybrid integration into different plastic supports. The dielectric antennas are realized exploiting the natural instability of thin solid films to form regular patterns of monocrystalline atomically smooth silicon and germanium nanostructures. Efficient protocols for encapsulating them into flexible and transparent, organic supports are investigated and validated. We benchmark the optical quality of the antennas with light scattering measurements, demonstrating the control of the islands structural colour and the onset of sharp Mie modes after encapsulation.

Instability and rupture of surfactant-laden bilayer thin liquid films.

The stability of surfactant-laden bilayer thin films, where the top layer is subject to van der Waals driven breakup, is of particular relevance to applications where one thin liquid layer is spread on another, such as film-forming firefighting foams and multilayer coatings. Although there has been much prior modeling work on the stability of thin liquid bilayers, additional physical effects and assumptions were incorporated in those studies, making it difficult to isolate the influence of surfactant on the rupture of the top layer. The present work addresses this issue through application of the lubrication approximation to derive a coupled system of nonlinear evolution equations describing the perturbations to the liquid-liquid and liquid-air interfaces and the surfactant interfacial concentrations. The surfactant is assumed to be insoluble and can be present at each interface. Linear stability analysis suggests, and nonlinear simulations confirm, that by using surfactant that adsorbs to both interfaces, the rupture time can be increased by an order of magnitude relative to the surfactant-free case. However, we find it crucial to have the right amount of surfactant to generate strongly stabilizing Marangoni stresses without reducing the interfacial tension too much. Nonlinear simulations and linear stability analysis provide insight into the mechanisms of the delayed rupture and show how the direction and strength of the Marangoni stresses strongly depend on the viscosity ratio of the layers. These results can help guide the choice and design of surfactants to achieve more effective firefighting foams and more stable liquid coatings.

Thermocapillary Patterning of Highly Uniform Microarrays by Resonant Wavelength Excitation

For decades, researchers have been exploring pattern-forming instabilities in free surface thin films in the hope of developing alternative lithographic techniques for applications only requiring resolution limits in the submicron range. Previous studies have shown how the pitch and shape of elements in an array can be varied by adjusting the magnitude of surface forces and growth time prior to solidification in situ. Since the formations emerge naturally from an initial flat molten film, the final arrays exhibit ultrasmooth interfaces and are therefore ideally suited to beam-shaping applications such as thin-film micro-optics. Progress in this field has stalled, however, due to the very nature of the formation process. Even when great care is taken to ensure that initial films are defect-free, final arrays still exhibit unacceptable variability in pitch, shape, and height due to ubiquitous sources of noise responsible for instability and growth. In this work, we focus on a thermocapillary instability in slender molten films exposed to a very large thermal gradient. We begin with a discussion and demonstration of why this instability inextricably leads to highly disordered arrays even if initialized by a film with very small amplitude surface roughness. We then demonstrate how spatially periodic modulation of the thermal field, implemented in three different ways, can induce synchronous growth of highly uniform periodic arrays despite noisy initial conditions. Results based on linear and weakly nonlinear stability analysis, Bloch wave analysis, and direct numerical simulation of the interface equation reveal how resonant wavelength excitations occurring between the modulation and instability driving fields are responsible for such rapid and coherent growth. An additional benefit is that the modulation field can be selected to yield an array pitch much smaller than in unmodulated systems.

Effect of the odd viscosity on Faraday wave instability

Faraday waves arise in fluid systems with free surfaces subject to vertical oscillations of sufficient strength due to parametric resonance. The odd viscosity is a peculiar part of the viscosity stress tensor that does not result in dissipation and is allowed when parity symmetry is broken spontaneously or due to external magnetic fields or rotations. The effect of the odd viscosity on the classic Faraday instability of thin liquid films in infinite horizontal plates is investigated by utilizing both linear Floquet theory and nonlinear lubrication theory based on the weighted residual model. This work derives the nonlinear evolution equations about the flow rate and free surface height, and linear stability analysis is performed to achieve the damped Mathieu equation. The results show that the neutral stability curves derived from the Mathieu equation agree well with those obtained from the linear Floquet analysis, especially for lower viscosity ratios μ. The nonlinear numerical results simulated by the method of lines indicate interesting results where the odd viscosity gives rise to a “sliding” of the wave configuration parallel to the wall, and the interface wave then translates into a traveling wave.

Evolution of Thin-Film Wrinkle Patterns on a Soft Substrate: Direct Simulations and the Effects of the Deformation History.

Surface wrinkling instability in thin films attached to a compliant substrate is a well-recognized form of deformation under mechanical loading. The influence of the loading history on the formation of instability patterns has not been studied. In this work, the effects of the deformation history involving different loading sequences were investigated via comprehensive large-scale finite element simulations. We employed a recently developed embedded imperfection technique which is capable of direct numerical predictions of the surface instability patterns and eliminates the need for re-defining the imperfection after each analysis step. Attention was devoted to both uniaxial compression and biaxial compression. We show that, after the formation of wrinkles, the surface patterns could still be eliminated upon complete unloading of the elastic film–substrate structure. The loading path, however, played an important role in the temporal development of wrinkle configurations. With the same final biaxial state, different deformation histories could lead to different surface patterns. The finding brings about possibilities for creating variants of wrinkle morphologies controlled by the actual deformation path. This study also offers a mechanistic rationale for prior experimental observations.

Thickness dependence of the flux-flow velocity and the vortex instability in nanocrystalline γ-Mo2N thin films

The influence of the thickness on the vortex instability of nanocrystalline γ-Mo2N thin films is analyzed. The samples were grown on Si (100) using reactive sputtering. The quasiparticle relaxation time for films with thickness between 7 and 26 nm is analyzed in the framework of Larkin–Ovchinnikov instability by performing current-voltage curves. Considering self-heating effects due to finite heat removal from the substrate, we determine a fast quasiparticle relaxation time τ ≈ 50 ps for all the samples at low temperatures. On the other hand, close to Tc, the vortex velocity becomes magnetic field-independent, and τ increases from ≈ 40 to 110 ps as the films are made thicker. The results are discussed considering the disorder's contribution and the bridges' geometry on the vortex instability.

Instability and rupture of ultrathin freestanding viscoelastic solid films.

We analyze the instability of viscoelastic solid freestanding thin films under the influence of van der Waals forces using linear stability analysis and nonlinear simulations. Linear stability analysis shows that the zero-frequency elastic modulus governs the onset of instability and stabilizes the film beyond a critical value analogous to thin supported viscoelastic solid films. However, for freestanding solid films, the critical shear modulus is found to be independent of surface tension, unlike that of thin supported viscoelastic solid films. It is further shown that freestanding viscoelastic solid films with higher moduli can be destabilized for a given film thickness and Hamaker constant compared to supported solid films. In contrast to thin viscoelastic liquid films where the growth rate is enhanced due to elastic effects but length scale is unaltered, freestanding films with solidlike viscoelasticity show a retarded growth rate and enhanced length scale. The presence of solidlike viscoelasticity has a stabilizing effect and affects all the key aspects of instability such as critical wave number, dominant wave number, and maximum growth rate. We numerically solve the set of coupled nonlinear evolution equationsfor film thickness and tangential displacement in order to elucidate the dynamics of film rupture. Our simulations show that, consistent with the linear stability predictions, an increase in the elastic modulus delays film rupture. The dynamics exhibits self-similarity in the vicinity of film rupture and the film thins as (t_{r}-t)^{3/4}, where t_{r} is the rupture time and t_{r}-t is the time remaining until film rupture. The scaling exponent 3/4 obtained for a thin freestanding viscoelastic solid film is significantly greater than the scaling exponent (1/3) for a thin freestanding viscous film.

Linear and nonlinear optical properties of dewetted SiGe islands

We propose to exploit the natural mechanical instability of thin solid films to form regular patterns of monocrystalline atomically smooth silicon and germanium nanostructures that cannot be realized with conventional methods. The solid-state dewetting dynamics is guided by pre-patterning the sample by a combination of electron-beam lithography and reactive-ion etching, obtaining precise control over number, size, shape, and relative position of the final Si1-xGex structures. Here we describe our progress in the spectroscopic investigation of individual dewetted Si1-xGex nanoislands: in the linear regime, bright Mie-type localized resonances are detected in the visible spectral range, with a spectral position that can be tuned by modifying the size of the nanoparticles. In the non-linear regime, instead, sizable third-harmonic generation is observed at the level of single islands. We believe that these results will be pivotal to a novel approach in spectral filtering, sensing and structural color with all-dielectric photonic devices.

Vertically aligned two-dimensional halide perovskites for reliably operable artificial synapses

Halide perovskites, fascinating memristive materials owing to mixed ionic-electronic conductivity, have been attracting great attention as artificial synapses recently. However, polycrystalline nature in thin film form and instability under ambient air hamper them to be implemented in demonstrating reliable neuromorphic devices. Here, we successfully fabricated vertically aligned 2D halide perovskite films (V-HPs) for active layers of artificial synapses, showing moisture stability for several months. Unlike random-oriented HPs, which exhibit negligible current hysteresis, the V-HPs possess multilevel analog memristive characteristics, programmable potentiation and depression with distinguished multi-states, long-short-term plasticity, paired-pulse facilitation, and even spike-timing-dependent plasticity. Furthermore, high classification accuracy is obtained with implementation in deep neural networks. These remarkable improvements are attributed to the vertically well-aligned lead iodide octahedra acting as the ion transport channel, confirmed by first-principles calculations. This study paves the way for understanding HPs nanophysics and demonstrating their potential utility in neuromorphic computing systems.

Self-Organized Wrinkling in Thin Polymer Films under Solvent–Nonsolvent Solutions: Patterning Strategy for Microfluidic Applications

Self-organized wrinkling instabilities in thin polymer films have instigated a field of versatile surface patterning and have spurred several research efforts in developing micro- and nanopatterned templates for a wide range of applications. Here, we report for the first time a distinct class of wrinkles in a thin polymer (polystyrene, PS) film coated on a substrate under a mixture of organic solvent and aqueous nonsolvent. The solvent (dimethyl formamide, DMF) softens and swells the polymer and paves the way for wetting of the hydrophilic substrate (≥46 mJ/m2) by the solvent–nonsolvent (S-NS) mixture, leading to wrinkle formation. It is investigated that selective delamination-induced wrinkling is a generic phenomenon and takes place in various polymers as well as different combinations of solvent–nonsolvent mixtures. The surface energy of the substrate and the composition of the solvent–nonsolvent mixture play a critical role as wrinkling is not observed on substrates with lower surface energy (<46 mJ/m2). An isotropically distributed yet disordered self-organized wrinkle network of hollow buried channels is formed, and it is illustrated that these can be exploited to generate a mesh of microwires and harnessed to form highly directional patterns using electron beam lithography, which can turn the new leaf for nano- and microfluidic device fabrication platforms.

Interfacial instability of thin films in soft microfluidic configurations actuated by electro-osmotic flow

Interfacial instability of thin films in soft microfluidic configurations actuated by electro-osmotic flow

Mechanical buckling can pattern the light-diffracting cuticle of Hibiscus trionum.

Many species have cuticular striations that play a range of roles, from pollinator attraction to surface wettability. In Hibiscus trionum, the striations span multiple cells at the base of the petal to form a pattern that produces a type of iridescence. It is postulated, using theoretical models, that the pattern of striations could result from mechanical instabilities. By combining the application of mechanical stress with high-resolution imaging, we demonstrate that the cuticle buckles to create a striated pattern. Through mechanical modeling and cryo-SEM fractures, we show that the cuticle behaves like a bilayer system with a stiff film on a compliant substrate. The pattern of buckling aligns with the direction of the stress to create a larger-scale pattern. Our findings contribute to the understanding of the formation of tissue-wide patterns in living organisms.

Modulational instability in thin liquid film flowing down an inclined uniformly heated plate

The modulational instability properties regarding the evolution of interfacial disturbances of the flow of a thin liquid film down an inclined uniformly heated plate subject to thermal Marangoni (thermocapillary) effects are investigated under the framework of linear stability analysis. The investigation has been performed both analytically and numerically using a complex cubic Ginzburg–Landau equation without the driving term to provide comprehensive pictures of the influence of nonlinearity, dissipation, and dispersion on interfacial disturbance generation and evolution. It is shown that when the interplay between linear and nonlinear effects is relatively important, the disturbances evolve as a superposition of groups of traveling periodic waves with different amplitudes, and the interfacial disturbances evolve as smooth modulations. Furthermore, the dynamic modes of these disturbances become aperiodic. Conversely, when the evolution of instabilities is influenced by strong nonlinearity, the flow saturates, and different situations lead to different possible modulated wavy structures, caused by the interplay between nonlinear and linear dispersive and dissipative effects. Moreover, the appearance and the spatial and temporal evolution of the modulated disturbance profiles are influenced by both the amplitude of the disturbances and the linear dissipative term. Here, based on our investigation, two cases are highlighted. In the first case, which corresponds to very small amplitude of the disturbances, the dynamic modes of the disturbances evolve from periodic traveling waves to spatial and temporal modulated periodic solitary wave patterns. In the second case, by increasing the amplitude of the disturbances, the appearance of modulational modes is rapid, and therefore, we can observe the development of modulationally marginal-like stable patterns or spatial and temporal modulated patterns with nonuniform profiles.

Thin film instability driven dimple mode of air film failure during drop impact on smooth surfaces

Drop impact with a smooth surface can produce a dimple mode of contact due to a combined effect of a capillary wave and a thin film instability.