Pattern Formation In Thin Films Research Articles

Recent progress on crack pattern formation in thin films.

Fascinating pattern formation by quasi-static crack growth in thin films has received increasing interest in both interdisciplinary science and engineering applications. The paper mainly reviews recent experimental and theoretical progress on the morphogenesis and propagation of various quasi-static crack patterns in thin films. Several key factors due to changes in loading types and substrate confinement for choosing crack paths toward different patterns are summarized. Moreover, the effect of crack propagation coupled to other competing or coexisting stress-relaxation processes in thin films, such as interface debonding/delamination and buckling instability, on the formation and transition of crack patterns is discussed. Discussions on the sources and changes in the driving force that determine crack pattern evolution may provide guidelines for the reliability and failure mechanism of thin film structures by cracking and for controllable fabrication of various crack patterns in thin films.

Light-assisted self-organization and pattern formation in thin films of azobenzene-containing polyurea

In this work, ordered patterns having nanoscale features and resolutions in the range of 5–75 μm are produced from thin films of azo-polyurea (azo-PU) using photoinduced dewetting. Azo-PU is synthesized through polyaddition reactions, and reversible isomerization between cis – trans states is demonstrated by exposure to light of wavelengths 365 and 450 nm. This photochemical response is exploited to produce patterns by selective exposure using photomasks followed by subsequent annealing to induce polymer flow and dewetting. Marangoni flow is found to be a dominant mechanism that controls morphological evolution of patterns. The patterns can be switched between pillar-like and hole-like morphologies by stimulating the forward or backward isomerization reaction, and pattern resolution can be controlled using photomasks of different feature sizes. Preferential adsorption of proteins on these patterns is proposed as a potential application.

Bi-continuous pattern formation in thin films via solid-state interfacial dealloying studied by multimodal characterization

Solid-state interfacial dealloying creates bi-continuous nanostructured thin films with 3D interconnected morphology, revealed by multimodal characterization, offering potential applications in catalysis, biomedical sensing and energy storage.

Pattern formation in thin films of polymer solutions: Theory and simulations.

Self-assembly has been recognized as an efficient tool for generating a wide range of functional, chemically, or physically textured surfaces for applications in small scale devices. In this work, we investigate the stability of thin films of polymer solutions. For low concentrations of polymer in the solution, long length scale dewetting patterns are obtained with wavelength approximately few microns. Whereas, for concentrations above a critical value, bimodal dispersion curves are obtained with the dominant wavelength being up to two orders smaller than the usual dewetting length scale. We further show that the short wavelength corresponds to the phase separation in the film resulting in uniformly distributed high and low concentration regions. Interestingly, due to the solvent entropy, at very high concentration values of polymer, a re-entrant behaviour is observed with the dominant length scale now again corresponding to the dewetting wavelength. Thus, we show that the binary films of polymer solutions provide additional control parameters that can be utilized for generating functional textured surfaces for various applications.

Adhesion-induced instabilities and pattern formation in thin films of elastomers and gels

A hydrostatically stressed soft elastic film circumvents the imposed constraint by undergoing a morphological instability, the wavelength of which is dictated by the minimization of the surface and the elastic strain energies of the film. While for a single film, the wavelength is entirely dependent on its thickness, a co-operative energy minimization dictates that the wavelength depends on both the elastic moduli and thicknesses of two contacting films. The wavelength can also depend on the material properties of a film if its surface tension has a pronounced effect in comparison to its elasticity. When such a confined film is subjected to a continually increasing normal displacement, the morphological patterns evolve into cracks, which, in turn, govern the adhesive fracture behavior of the interface. While, in general, the thickness provides the relevant length scale underlying the well-known Griffith-Kendall criterion of debonding of a rigid disc from a confined film, it is modified non-trivially by the elasto-capillary number for an ultra-soft film. Depending upon the degree of confinement and the spatial distribution of external stress, various analogs of the canonical instability patterns in liquid systems can also be reproduced with thin confined elastic films.

Instabilities in thin films on hyperelastic substrates by 3D finite elements

Spatial pattern formation in thin films on rubberlike compliant substrates is investigated based on a fully nonlinear 3D finite element model, associating nonlinear shell formulation for the film and finite strain hyperelasticity for the substrate. The model incorporates Asymptotic Numerical Method (ANM) as a robust path-following technique to predict a sequence of secondary bifurcations on their post-buckling evolution path. Automatic Differentiation (AD) is employed to improve the ease of the ANM implementation through an operator overloading, which allows one to introduce various potential energy functions of hyperelasticity in quite a simple way. Typical post-buckling patterns include sinusoidal and checkerboard, with possible spatial modulations, localizations and boundary effects. The proposed finite element procedure allows accurately describing these bifurcation portraits by taking into account various finite strain hyperelastic laws from the quantitative standpoint. The occurrence and evolution of 3D instability modes including fold-like patterns will be highlighted. The need of finite strain modeling is also discussed according to the stiffness ratio of Young’s modulus.

Effects of Substrate Constraint on Crack Pattern Formation in Thin Films of Colloidal Polystyrene Particles

Crack formation and the evolution of stress in drying films of colloidal particles were studied using optical microscopy and a modified cantilever deflection technique, respectively. Drying experiments were performed using polystyrene particles with diameters of 47 ± 10 nm, 100 ± 16 nm, and 274 ± 44 nm that were suspended in water. As the films dried, cracks with a well-defined spacing were observed to form. The crack spacing was found to be independent of the particle size used, but to increase with the film thickness. The characteristic crack spacing was found to vary between 20 and 300 μm for films with thickness values in the range 3-70 μm. Cantilever deflection measurements revealed that the stresses that develop in the film increase with decreasing film thickness (increasing surface-to-volume ratio). The latter observation was interpreted in terms of the effects of a substrate constraint which causes the build up of stresses in the films. This interpretation was confirmed by crack formation experiments that were performed on liquid mercury surfaces in which removal of the substrate constraint prevented crack formation. Experiments were also performed on compliant elastomer surfaces in which the level of constraint was varied by changing the substrate modulus. The cracking length scale was found to increase with decreasing substrate modulus. A simple theory was also developed to describe the substrate modulus dependence of the cracking length scale. These combined experiments and theory provide convincing evidence that substrate constraints are an important factor in driving crack formation in thin colloidal films.

Sol-Gel-derived ZnO Coating with Nanopatterns Fabricated by Nanoimprinting

The influence of an applied pressure on the relaxation of stress for sol–gel-derived zinc oxide (ZnO) thin film and the formation of nanopatterns during nanoimprinting has been carried out. Wrinkling pattern formation in thin films caused by the relaxation of stress is a phenomenon frequently encountered. In this study, a route for sol–gel-derived ZnO film imprinted with poly(dimethylsiloxane) mold was used to attain pattern transfer with excellent uniformity and smooth profile. The ZnO nanoscale rib patterns are obtained by altering the processing parameters, such as baking conditions, applied pressure, and content of solvent within the gel films. The surface roughness that caused by the relaxation of stress, and the wrinkles generated to relieve the stress can self-assemble into the contour of the mold during the imprinting process with an applied pressure. By altering the pretreatment parameters to render some residual solvent in the film can enhance the fluidity of the film, which in turn leads to a faster and deeper penetration of the mold into the film. In conclusion, the effect of processing conditions for eliminating wrinkling pattern of sol–gel-derived ZnO, and to prepare nanopatterns with good uniformity and smooth surface has been demonstrated.

Phase Separation Dynamics and Pattern Formation in Thin Films of a Liquid Crystalline Copolyester in Its Biphasic Region

We present the experimental results concerning a phase separation dynamics and relevant pattern formation in thin film samples (ca. 10 μm in thickness) of a liquid crystalline copolyester in its biphasic region. The copolyester separates into an isotropic phase and an anisotropic phase in the biphasic region due to its composition heterogeneity, though it is homopolymer. The entire phase-separation process was in situ monitored in real space by polarized light microscopy. The structural evolution that appeared in the digital images was further analyzed using the fast Fourier transform method. The process can be described by the following steps: (1) The formation of a percolated network consisting of phase-separated isotropic and anisotropic liquids. The network growth obeys a scaling law Λm(t) ∝ qm(t)-1 ∝ tα, where Λm(t) is the characteristic length of the domains, qm(t) is the characteristic wavenumber, and t is the time. A crossover of the exponent from 1/3 to 1/2 was observed. (2) The network breaks up at a critical value of Λm(t) = 50 μm to form some anisotropic fragments with irregular shapes, followed by the further shrinking and reshaping into anisotropic drops. (3) The diffusion−coalescence of the drops to form large merged drops and the reshaping of these merged drops followed by orientational ordering within the merged drops. We studied the temperature dependence of these individual processes and discussed the mechanisms and the scaling behavior of the domain growth in the first step.