Critical Film Thickness Research Articles

Universal scaling of polygonal desiccation crack patterns

Polygonal desiccation crack patterns are commonly observed in natural systems. Despite their quotidian nature, it is unclear whether similar crack patterns which span orders of magnitude in length scales share the same underlying physics. In thin films, the characteristic length of polygonal cracks is known to monotonically increase with the film thickness; however, existing theories that consider the mechanical, thermodynamic, hydrodynamic, and statistical properties of cracking often lead to contradictory predictions. Here we experimentally investigate polygonal cracks in drying suspensions of micron-sized particles by varying film thickness, boundary adhesion, packing fraction, and solvent. Although polygonal cracks were observed in most systems above a critical film thickness, in cornstarch-water mixtures, multiscale crack patterns were observed due to two distinct desiccation mechanisms. Large-scale, primary polygons initially form due to capillary-induced film shrinkage, whereas small-scale, secondary polygons appear later due to the deswelling of the hygroscopic particles. In addition, we find that the characteristic area of the polygonal cracks, A_{p}, obeys a universal power law, A_{p}=αh^{4/3}, where h is the film thickness. By quantitatively linking α with the material properties during crack formation, we provide a robust framework for understanding multiscale polygonal crack patterns from microscopic to geologic scales.

Effect of sessile binary droplet evaporation at different ambient temperatures on interfacial profile near the triple line

Heat and mass transfer near the triple line of an evaporating droplet contains complex physics, while the mechanisms of evaporation influenced by the ambient temperature remain unclear. This paper describes an experimental study of interfacial profile near the triple line during evaporation of pure water and binary mixtures including dilute aqueous ethanol and n-butanol solutions at different ambient temperatures. The difference of interfacial profile between single and multi-component sessile evaporative droplets were experimentally demonstrated. The result shows that the thin film evaporation of the binary droplets is greatly suppressed when the ambient temperature drops. The thin film thickness reduces with the time under various ambient temperature conditions which are below 25 °C. The thickness difference at between 0 s and 40 s decreases in the order of water, ethanol solution and n-butanol solution. For water droplet, the critical thin film thickness at which the evaporation mode changes decreases with increasing ambient temperature, which is opposite to the binary droplets. Meanwhile, the rate of change of thin film thickness increases with increasing ambient temperature for water or ethanol solution, while it decreases with increasing temperature for n-butanol solution, which is attributed to the contact line energy barrier that decreases with increasing ambient temperature.

Spontaneous wrinkle emergence in nascent eumelanin thin films.

Self-patterning processes originated by physical stimuli have been extensively documented in thin films, whereas spontaneous wrinkling phenomena due to chemical transformation processes are, to the best of our knowledge, unprecedented. Herein we report a case of spontaneous polymerization-driven surface nano-patterning (∼500 nm) that develops in smooth thin solid films of 5,6-dihydroxyindole (DHI), a major precursor of eumelanin polymers, over a time scale of 30 to 60 days in air at room temperature. The phenomenon can be observed only above a critical film thickness of ∼250 nm and it is affected by exposure to ammonia vapors causing acceleration of the oxidation process. The thickness-dependent onset of wrinkling can be attributed to non-homogeneous rates of oxidation through the film causing slow swelling/expansion of the inner layers followed by fast stiffening and cross-linking in the outer layer exposed to higher oxygen levels.

Effects of wettability on explosive boiling of nanoscale liquid films: Whether the classical nucleation theory fails or not?

Abstract Two main reasons including practical and scientific significance continuously motivate the studies of explosive boiling of nanoscale liquid films. Whether the nanoscale explosive boiling agrees with classical nucleation theory is still an open issue. In this work, we study the effects of surface wettability on the explosive boiling of nanoscale liquid films using molecular dynamics simulations. A critical film thickness is proposed to address the debate of whether the classical nucleation theory fails in nanoscale boiling. The explosive boiling modes and dynamics are summarized with the phase diagram based on various simulation cases, considering the effects of surface wettability, film thickness, and surface superheat. When the films thickness exceeds the critical thickness, hydrophobic surfaces are more favorable for explosive boiling, which still agrees with the classical nucleation theory. However, a much higher superheat is required to trigger the explosive boiling of a thin liquid film when its thickness is less than the critical thickness on hydrophobic surfaces, which consists with previous molecular dynamics simulations. Furthermore, we also find that the explosive boiling is trigged faster for films on hydrophilic surfaces than those on hydrophobic surfaces with the same superheat owing to the lower Kapitza thermal resistance. With such heat transfer superiority, hydrophilic surfaces can heat liquid films faster to explosion. The deliveries of this work and the concept of the critical thickness might help to understand the still fledgling field of nanoscale boiling phase change and its relevant mechanisms.

Single-step synthesis of wrinkled MoSe2 thin films

Controlled and simple synthesis strategy of two-dimensional (2D) nanomaterials is critical for designing the related heterostructures with different geometries, which influence strain, electronic structures, light-interactions, and surface reactions. Here, we report a single-step growth method for wrinkled MoSe2 thin films on flat SiO2/Si substrates by using high-vacuum co-evaporation. Above the critical film thickness of 15–20 nm, the MoSe2 films start to form wrinkled structure with lateral periodicity of 7–10 μm, which significantly reduces light reflectance in the visible range. We demonstrate formation of the wrinkled pattern on the selective area either by etching the substrate surface or by depositing gold films. In addition, we observe switching of the microscopic growth mode from a layered stacking to a vertically aligned growth, which results in 2D-3D hybrid microstructures. Our single-step synthesis approach might be generalized to obtain wrinkled 2D-3D hybrid nanomaterials and utilized for flexible electronic & optoelectronic devices, efficient solar cells, catalytic electrodes, and anti-counterfeiting applications.

The Influence of Polysaccharides on Film Stability and Bubble Attachment at the Talc Surface

The wetting characteristics and water film stability at the talc surface have been studied, particularly the effect of polysaccharides such as guar gum, starch, and dextrin. Talc is a gangue mineral in the flotation of base metal sulfide ores, precious metal sulfide ores, and platinum group metal (PGM) sulfide ores. Talc surfaces were investigated using surface analysis techniques including atomic force microscopy, high-speed video bubble attachment measurements, and wetting film stability measurements using a synchronized tri-wavelength reflection interferometry microscope (STRIM). In the presence of polysaccharides, there is a significant increase in bubble attachment time at the talc surface, but only a slight change in contact angle, which suggests that polysaccharide depression of talc is due primarily to the slow rate of bubble attachment and not due to a change in contact angle. The critical rupture thickness (hc) for a hydrophobic talc surface was found to be 56 nm, while the hydrophilic phlogopite surface of similar structure has an equilibrium film thickness (he) of 25 nm. At low polysaccharide concentrations, the wetting films formed on the talc surfaces were unstable, but at high concentrations the wetting films became stable with similar thickness values as the critical rupture thickness, and bubble attachment did not occur. However, it was found that the critical and equilibrium film thickness values do not change significantly with the polysaccharide type or concentration. The results from this research help us understand further details of film rupture and displacement during bubble attachment.

An in situ Study of the Kinetics of a Solid-Phase Reaction Activated by the Energy of Elastic Stresses Arisen upon the Formation of the Cu/As2Se3 Nanosized Film Structure

For the first time, the kinetics of a solid-phase chemical reaction activated by the energy of elastic stresses generated upon the formation of the Cu/As2Se3 nanosized film structure is investigated in situ. It is shown that the time at which a solid-phase chemical reaction starts, as well as the voltage of the Cu/As2Se3 heterolayer, significantly depends on the thickness of the As2Se3 film. At a critical thickness of the As2Se3 film, which is equal to 110 nm, a threshold value of the energy of elastic stresses is achieved. Relaxation of this energy over new defects (micropores and microcracks) generated in the film system leads to the activation of a solid-phase chemical reaction and an increase in its rate. A mechanism of the operation of a positive feedback between the chemical reaction in a solid phase and elastic stresses is proposed.

Impact of a surface TiO2 atomic sheet on the electronic transport properties of LaAlO3/SrTiO3 heterointerfaces

Here, we report the impact of a surface TiO2 atomic sheet on the electronic transport properties of LaAlO3/SrTiO3 heterointerfaces. We demonstrate that when a LaAlO3 film is deposited on a reconstructed SrTiO3(001)-(13×13)-R33.7° substrate, the critical thickness of the LaAlO3 film for emergence of high-mobility electrons is two unit cells. In addition, we observe that as a result of surface passivation by the TiO2 atomic sheet, an electron density of 2.0 × 1014 cm−2 is maintained at the interface owing to the suppression of electron transfer to the surface. A combination of scanning transmission electron microscopy and photoemission spectroscopy reveals that the TiO2 sheet on LaAlO3 plays a key role in determining the conducting properties. This study demonstrates the importance of surface terminations to the electronic structures of buried interfaces.

Atomic layer deposited Al2O3 thin films as humidity barrier coatings for nanoparticle-based strain sensors

Al2O3 thin films deposited by atomic layer deposition on Pt nanoparticle-based strain sensors were studied as humidity barrier coatings for sensor protection. The effect of two deposition parameters—film thickness and growth temperature—is discussed in relation to an Al2O3 coating’s ability to isolate the nanoparticle surface and protect strain sensitivity from humidity variations. It is shown that transmission electron microscopy images cannot confirm the effective protection of a nanoparticle surface, thus x-ray photoelectron spectroscopy and electrical measurements have been employed. The existence of a critical thickness of the Al2O3 protective film above which resistance and gauge factor variations are suppressed during humidity change was observed at different deposition temperatures. This ability is linked with the existence of incorporated pinholes and intrinsic hydroxyl groups in the Al2O3 thin film, which are responsible for humidity transport through the oxide.

Dynamic Interaction between a Millimeter-Sized Bubble and Surface Microbubbles in Water.

The coalescence between microbubbles and millimeter-sized bubbles is an elementary process in various industrial applications such as froth flotation and wastewater treatment. Fundamental understanding of the coalescence behavior between two colliding bubbles requires knowledge of water drainage from the thin liquid film between the deformable air-water surfaces, a simple phenomenon with high complexity in physics because of the interplay of surface forces, hydrodynamic drainage, and surface rheology. In this work, we performed simultaneous measurements of the interaction force and spatial thin-film thickness during the collision between a millimeter-sized bubble (radius 1.2 mm) and surface microbubbles (radii between 30 and 700 μm) using our recently developed dynamic force apparatus. The interaction force during the collision agrees well with the prediction from the Stokes-Reynolds-Young-Laplace model with the tangentially immobile boundary condition at the air-liquid interface. However, the measured coalescence times for different bubble sizes are shorter than the model predictions, possibly caused by a rapid drainage behavior along with the loss of symmetry of the thin liquid film. In dozens of experimental runs, the bubbles coalesced at a critical film thickness of 25 ± 15 nm, which agrees reasonably well with the predicted rupture thickness using attractive van der Waals interaction force. These results suggest that the nonsymmetric drainage process, rather than the rupture thickness, contributes to the scattering of the experimental coalescence time between two fast-colliding air bubbles. Furthermore, our results suggest that smaller surface bubbles (30-100 μm) are more effective for the attachment onto a large bubble as the coalescence time decreases considerably when the microbubbles are smaller than 100 μm.

Mechanical properties of zinc-aluminum film on steel cable substrate in corrosion environment

This paper presents an experimental program aimed at finding out the effect of salt spray corrosion on the mechanical properties of the film in the zinc-aluminum film/304 stainless steel substrate system. At first, the salt spray tests were carried out on the film/substrate specimens to observe the corrosion phenomenon and obtain the porosity of the zinc-aluminum film. Then, the three-point bending tests were carried out on the specimens with different film thicknesses and different corrosion time by a universal testing device equipped with an acoustic emission system (AE) to obtain the effective elastic modulus of the corroded film. After that, the crack-initiation threshold of the film was accurately detected to further obtain the fracture toughness of the corroded film by AE system. Also, tensile tests were used to obtain the critical film thicknesses and the relationship between crack spacing and the film thickness of the corroded film under different strains. The test results illustrated that the effective elastic modulus and fracture toughness of zinc-aluminum film decrease with the increase of corrosion time; the porosity of the film decreases with the increase of the film thickness; the decline rate of the effective elastic modulus and fracture toughness of the corroded film can be decreased by slightly increasing the film thickness; the critical thickness of the film decreases with the increase of tensile strain and the corroded film is easier to crack than the non-corroded film due to the decrease of critical cracking strain.

Nucleation of stripe domains in thin ferromagnetic films

For almost 50 years, the formation of stripe domains in magnetic thin films has been considered as an abrupt magnetization phase transition driven by an interplay of the intrinsic perpendicular anisotropy and the shape anisotropy. This allows the determination of a critical film thickness for a given value of the intrinsic perpendicular anisotropy. The present experimental and theoretical analysis shows that micromagnetic structures such as domain walls and their junctions considerably soften a magnetic thin film. The consequence is a significant reduction of the critical film thickness with respect to the originally derived value.

Observation of a mixed regime for an impinging droplet on a sessile droplet

This article reports experimental results of the presence of a mixed regime where rebound or coalescence occurs when a liquid droplet with low impact velocity impinges on a static sessile droplet, deposited on a flat aluminum surface. In particular, the present study suggests possible reasons why this mixed occurs with nearly the same probabilities with the use of the sequential images captured by two high-speed cameras from the 767 experimental cases. It is also demonstrated that the mixed regime is substantially affected by static droplet volumes, Weber numbers, and offset distances between impinging and static droplets. In the mixed regime, rebound may occur because a relatively large air-film maintains to prevent film drainage for coalescence. The lubrication theory is utilized to estimate film thickness between two droplets and is compared to the critical film thickness. The upper limit of the Weber numbers in the mixed regime decreases with an increase in the offset distance, owing to a rotational droplet motion causing air-film elongation. The contact time during rebound slightly increases with the static sessile droplet volume. It is observed that the measured contact times increases with a decrease in impact velocity, showing cushioning effect by air-film. On the other hand, a coalescence is observed to intermittently in the mixed regime because the air-film layer existing at the interface has an irregular shape by oscillation of an impinging droplet.

Enhanced microwave absorption property of Reduced Graphene Oxide (RGO)–Strontium Hexaferrite (SF)/Poly (Vinylidene) Fluoride (PVDF)

Enormous growth in the use of electronic gadgets presents a challenge to develop efficient electromagnetic interference (EMI) shielding materials specifically using composites with properties such as light weight and high shielding due to absorption. In the present work we have synthesized a composite material with three components; Reduced Graphene Oxide (RGO) and Strontium Hexaferrite (SrFe12O19) (SF) bound together by polymer Poly (Vinylidene) Fluoride (PVDF). Composite RGO/SF (RGOSF) was prepared by facile one port chemical reduction method. Surface anchoring of ~500 nm of magnetic particles over the graphene sheet was confirmed by Field Emission Scanning Electron Microscopy (FESEM) and Transmission Electron Microscopy (TEM). Crystal structure of SF particles was analyzed before and after composite formation with RGO. Effective crystallite size was estimated from both XRD and Raman Spectroscopy which showed similar trend moving from SF to RGOSF to RGOSFPVDF. Polymer composite films of RGOSFPVDF were prepared by hot pressing. Both real and imaginary part of magnetic and dielectric parameters were studied. It was observed that enhancement of interfacial polarization and anisotropic heat loss in PVDF matrix was generated by RGOSF which ultimately created more scattering center and helped in absorption of electromagnetic radiation. Critical film thickness of 3 mm was capable to shield up to 33 dB EM energy of which >99.9% was due to absorption.

Multi-scale simulation of droplet–droplet interaction and coalescence

In this work, droplet–droplet interaction is modelled using a multi-scale approach which couples multiphase flow simulation using a volume of fluid method to a surface thin film model operating on the sub-grid scale. The volume of fluid model is based on a multiple marker method with a smoothed surface tension calculation, and a thin film model is derived to simulate film drainage using a Reynolds equation approach. A novel method of coupling the two allows for the prediction of coalescence or rebound of colliding droplets essentially from first principles, relying only on a critical film thickness parameter. The model is implemented using the open source tool set OpenFOAM and tested against experimental results of colliding hydrocarbon droplets from the literature. It is found to produce accurate interface deformation results for the duration of the collision, and to consistently predict the outcome of the collision process.

Analysis of the Stress Relieving Process in a Semiconductor Heterosystem with a (013) Interface

A theoretical analysis of the process of introducing misfit dislocations into a semiconductor heterostructure with a (013) interface is performed by assuming conditions of quasi-equilibrium process. The mechanism of generation is established for those misfit dislocations, which do not meet the requirement of minimum critical film thickness. The calculations are performed on the basis of the force balance model and allow for the shear stress field in the film and the type of the screw dislocation component.

Role of the different defects, their population and distribution in the LaAlO3/SrTiO3 heterostructure's behavior

Hetero-interfaces between epitaxial LaAlO3 films and SrTiO3 substrates can exhibit an insulator-metal transition at a critical film thickness above which a quasi-two-dimensional electron gas forms. This work aims to elucidate the significant role the defects play in determining the sources of non-mobile and mobile carriers, the critical thickness, and the dipolar field screening. A model is built based on a comprehensive investigation of the origin of charge carriers and the advanced analysis of structural factors that affect the electronic properties of these hetero-epitaxial interfaces.

Superstable Ultrathin Water Film Confined in a Hydrophilized Carbon Nanotube.

Fluids confined in a nanoscale space behave differently than in the bulk due to strong interactions between fluid molecules and solid atoms. Here, we observed water confined inside "open" hydrophilized carbon nanotubes (CNT), with diameter of tens of nanometers, using transmission electron microscopy (TEM). A 1-7 nm water film adhering to most of the inner wall surface was observed and remained stable in the high vacuum (order of 10-5 Pa) of the TEM. The superstability of this film was attributed to a combination of curvature, nanoroughness, and confinement resulting in a lower vapor pressure for water and hence inhibiting its vaporization. Occasional, suspended ultrathin water film with thickness of 3-20 nm were found and remained stable inside the CNT. This film thickness is 1 order of magnitude smaller than the critical film thickness (about 40 nm) reported by the Derjaguin-Landau-Verwey-Overbeek theory and previous experimental investigations. The stability of the suspended ultrathin water film is attributed to the additional molecular interactions due to the extended water meniscus, which balances the rest of the disjoining pressures.

Thickness Effect on (La0.26Bi0.74)2Ti4O11 Thin-Film Composition and Electrical Properties**Supported by the Basic Research Project of Shanghai Science and Technology Innovation Action under Grant No 17JC1400300, the National Key Basic Research Program of China under Grant No 2014CB921004, the National Natural Science Foundation of China under Grant No 61674044, and the Program of Shanghai Subject Chief Scientist under Grant No 17XD1400800.

Highly oriented (00l) (La0.26Bi0.74)2Ti4O11 thin films are deposited on (100) SrTiO3 substrates using the pulsed laser deposition technique. The grains form a texture of bar-like arrays along SrTiO3 ⟨110⟩ directions for the film thickness above 350 nm, in contrast to spherical grains for the reduced film thickness below 220 nm. X-ray diffraction patterns show that the highly ordered bar-like grains are the ensemble of two lattice-matched monoclinic (La,Bi)4Ti3O12 and TiO2 components above a critical film thickness. Otherwise, the phase decomposes into the random mixture of Bi2Ti2O7 and Bi4Ti3O4 spherical grains in thinner films. The critical thickness can increase up to 440 nm as the films are deposited on LaNiO3-buffered SrTiO3 substrates. The electrical measurements show the dielectric enhancement of the multi-components, and comprehensive charge injection into interfacial traps between (La,Bi)4Ti3O12 and TiO2 components occurs under the application of a threshold voltage for the realization of high-charge storage.

Thin Film Evolution Equation for a Strained Anisotropic Solid Film on a Deformable Isotropic Substrate

We consider a continuum model for the evolution of an epitaxially-strained dislocation-free anisotropic thin solid film on isotropic deformable substrate in the absence of vapor deposition. By using a thin film approximation we derived a nonlinear evolution equation. We examined the nonlinear evolution equation and found that there is a critical film thickness below which every film thickness is stable and a critical wave number above which every film thickness is stable.