Electric field induced instability and pattern formation in thin liquid films

Electrostatic field induced instability, morphology, and patterning of a thin liquid film confined between two electrodes with an air gap are studied on the basis of nonlinear 3D simulations, both for spatially homogeneous and heterogeneous fields. In addition to the spinodal flow resulting from the variation of field because of local thickness changes, a heterogeneous imposed field also moves the liquid from the regions of low field to high field, thus allowing a more precise control of pattern. Hexagonal packing of liquid columns is observed for a spatially homogeneous electric field, which is in accord with the e-field experiments on thin polymer films (Schaffer et al. Nature 2000, 403, 874). For a large liquid volume fraction in the gap, varphi > or = 0.75, the coalescence of columns causes a phase inversion, leading to the formation of air columns or cylindrical holes trapped in the liquid matrix (air-in-liquid dispersion). Locally ordered aligned patterns are formed by imposing a spatial variation of the electrostatic field by using a topographically patterned electrode. For example, multiple rows/lines of liquid columns are formed near the edge of a step-like heterogeneity of the electrode and annular rings of ordered columns or concentric ripples are formed around a heterogeneous circular patch. Simulations predict that the electrode pattern is replicated in the film only when the pattern periodicity, L(p), exceeds the instability length scale on the basis of the minimum interelectrode separation distance, L(p) > or = lambda(m)-d(min). Thus, the formation of secondary structures can be suppressed by employing an electrode with deep grooves and stronger field gradients, which produces almost ideal templating. The number density of the electric field induced patterns can be altered by tuning the mean film thickness (or the volume fraction of liquid in the gap), periodicity and depth (amplitude) of the grooves on the top electrode, and the applied voltage. The implications are in electrostatic lithography, pattern replication in soft materials, and the design and interpretation of thin film experiments involving electric fields.     

Pattern formation in PMMA film induced by electric field

Micro‐sized patterns were created on thin poly(methyl methacrylate) (PMMA) films by the effect of external field, perpendicular to the film surface. The PMMA film, prepared by spin‐coating onto Si wafer, was heated to the fluid temperature (275 °C) and a linear pattern was created by the effect of electric field produced by a strip electrode. In another experiment, a round pattern was created as a result of local laser heating of the PMMA film under homogenous electrical field. The created patterns were analyzed by optical microscopy and profile meter. The dependence of the form and size of the created patterns on the intensity of the electric field, exposure time, and initial film thickness was examined. Wave guiding property of a linear pattern, produced by the above technique, was examined in a simple experiment. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1131–1135, 2009     

An experimental test of the fractal model for drainage of foam films

Drainage of foam films with different radii (50-150 μm), stabilized by hexathylene glycol dodecyl ether C(12)E(6) and in a presence of 0.024 M NaCl, were analyzed in the light of a recent dynamic fractal classification of [1]. The latter accounts for the effect of film surface corrugations developed during the film drainage. For simplicity, the film surface mobility is neglected since the presence of surfactants reduces dramatically the film surface velocity. The magnitude of surface non-homogeneities, caused by the film drainage, is accounted via a dynamic fractal dimension parameter α being spanned between zero and two. Depending on the α-value the film drains by different kinetic laws. For example, if the thin film is planar α=2 and it drains according to the Reynolds law; if α=1 the film contains an axisymmetric dimple causing faster drainage; if α=1/2 the film exhibits number of asymmetric dimples and the film drains even faster; finally if α=0 the film contains spatially uncorrelated domains causing the fastest possible drainage. The present analysis of experimental data suggests that the parameter α is inversely proportional to the film radius R and it is independent of the type and concentration of surfactants. A semi-empirical model for α is proposed, thus completing the generic dynamic fractal classification.     

Droplet size effects on film drainage between droplet and substrate

When a droplet approaches a solid surface, the thin liquid film between the droplet and the surface drains until an instability forms and then ruptures. In this study, we utilize microfluidics to investigate the effects of film thickness on the time to film rupture for water droplets in a flowing continuous phase of silicone oil deposited on solid poly(dimethylsiloxane) (PDMS) surfaces. The water droplets ranged in size from millimeters to micrometers, resulting in estimated values of the film thickness at rupture ranging from 600 nm down to 6 nm. The Stefan-Reynolds equation is used to model film drainage beneath both millimeter- and micrometer-scale droplets. For millimeter-scale droplets, the experimental and analytical film rupture times agree well, whereas large differences are observed for micrometer-scale droplets. We speculate that the differences in the micrometer-scale data result from the increases in the local thin film viscosity due to confinement-induced molecular structure changes in the silicone oil. A modified Stefan-Reynolds equation is used to account for the increased thin film viscosity of the micrometer-scale droplet drainage case.     

Influence of the surface pretilt angle on spatially periodic deformations in nematic layers

The deformations induced by electric field in twisted or untwisted flexoelectric nematic layers can be homogeneous (i.e. one-dimensional) or spatially periodic (i.e. two-dimensional). The periodic deformations are undesirable from an applicative point of view since they destroy the homogeneous appearance of the area of an excited pixel of a display. They are particularly favoured when the nematic material possesses flexoelectric properties. In order to check whether the unwanted periodic deformations can be eliminated by means of suitable surface pretilt angle, the small deformations arising just above the corresponding threshold voltage were investigated numerically. The nematic materials exhibiting both weak and strong flexoelectricity were taken into account. The surface pretilt angles ranging from 0° to 30° were adopted. It was shown that the periodic patterns, arising in the case of planar surface alignment, disappear if sufficiently large surface pretilt angle is applied.     

Heterogeneity in styrene-butadiene latex films

Low-Tg styrene-butadiene (SB) latex films were investigated by noncontact atomic force microscopy and scanning electric potential microscopy, revealing a number of different morphologies and electric potential patterns across films cast from the same SB latex dispersions under the same conditions. Surface leveling and charge dispersion throughout the films are, thus, restrained even at temperatures above Tg and the minimum film-formation temperature. An unprecedented electric pattern is observed, in which the particle cores are more positive than the contacting particle outer layers. Different packing patterns, including cubic and hexagonal arrays, coexist in neighboring areas. Zonal centrifugation of the SB latex in sucrose density gradient shows that particles cover a broad range of densities. Thus, film surface heterogeneity is at least partly due to particle heterogeneity. Fractal dimensions of topographic profiles are lower than those of the electric potential profiles, showing that charge mobility is much more restrained than polymer chain motion at the film surface and that it imposes a limit to the charged chain-ends motion.     

Effect of mass transfer on the film drainage between colliding drops

The influence of mass transfer on the drainage behaviour of the thin liquid film between two drops immersed in another liquid colliding at constant approach velocity has been studied experimentally. The liquid-liquid system used is glycerol in silicone oil. The transferred solute is acetone and the volume concentration difference across the interface ranges from 1 to 5%. The film thickness evolution has been measured using a laser interferometry technique. The direction of mass transfer (from the drops towards the film phase and inversely) has been investigated and the results compared to the case with no mass transfer. When the solute transfers from the drops towards the continuous phase, the drainage rate is significantly higher than in the case with no mass transfer. This result is interpreted as a consequence of the mass transfer induced surface mobility in the film region (the so-called Marangoni effect) due to localized surface tension differences. This effect has been demonstrated by the visualization of the flow patterns in the drops and in the film phase (using a particle tracer technique). In this case, the slope of the film height as a function of time seems to be independent of the approach velocity condition imposed on the drop and appears to be controlled by the interfacial tension gradient. In the opposite case, when the solute transfers from the continuous phase towards the drops, the film drainage rate is lowered with respect to the case of no mass transfer, goes to zero or even changes its sign depending on the mass transfer intensity. The results also show that in the range of solute concentration studied, the effect of mass transfer on the film drainage process takes place at large distances compared to the scales at which lubrication theory is valid.     

Theoretical study of the adsorption and dissociation of oxygen on Pt(111) in the presence of homogeneous electric fields

The effect of homogeneous electric fields on the adsorption energies of atomic and molecular oxygen and the dissociation activation energy of molecular oxygen on Pt(111) were studied by density functional theory (DFT). Positive electric fields, corresponding to positively charged surfaces, reduce the adsorption energies of the oxygen species on Pt(111), whereas negative fields increase the adsorption energies. The magnitude of the energy change for a given field is primarily determined by the static surface dipole moment induced by adsorption. On 10-atom Pt(111) clusters, the adsorption energy of atomic oxygen decreased by ca. 0.25 eV in the presence of a 0.51 V/A (0.01 au) electric field. This energy change, however, is heavily dependent on the number of atoms in the Pt(111) cluster, as the static dipole moment decreases with cluster size. Similar calculations with periodic slab models revealed a change in energy smaller by roughly an order of magnitude relative to the 10-atom cluster results. Calculations with adsorbed molecular oxygen and its transition state for dissociation showed similar behavior. Additionally, substrate relaxation in periodic slab models lowers the static dipole moment and, therefore, the effect of electric field on binding energy. The results presented in this paper indicate that the electrostatic effect of electric fields at fuel cell cathodes may be sufficiently large to influence the oxygen reduction reaction kinetics by increasing the activation energy for dissociation.     

Transient electroosmotic flow induced by AC electric field in micro-channel with patchwise surface heterogeneities

This paper investigates two-dimensional, time-dependent electroosmotic flow driven by an AC electric field via patchwise surface heterogeneities distributed along the micro-channel walls. The time-dependent flow fields through the micro-channel are simulated for various patchwise heterogeneous surface patterns using the backwards-Euler time stepping numerical method. Different heterogeneous surface patterns are found to create significantly different electrokinetic transport phenomena. The transient behavior characteristics of the generated electroosmotic flow are then discussed in terms of the influence of the patchwise surface heterogeneities, the direction of the applied AC electric field, and the velocity of the bulk flow. It is shown that the presence of oppositely charged surface heterogeneities on the micro-channel walls results in the formation of localized flow circulations within the bulk flow. These circulation regions grow and decay periodically in phase with the applied periodic AC electric field intensity. The location and rotational direction of the induced circulations are determined by the directions of the bulk flow velocity and the applied electric field.     

Surface enhanced second harmonic generation from macrocycle, catenane, and rotaxane thin films: experiments and theory

Surface enhanced second harmonic generation (SE SHG) experiments on molecular structures, macrocycles, catenanes, and rotaxanes, deposited as monolayers and multilayers by vacuum sublimation on silver, are reported. The measurements show that the molecules form ordered thin films, where the highest degree of order is observed in the case of macrocycle monolayers and the lowest in the case of rotaxane multilayers. The second harmonic generation activity is interpreted in terms of electric field induced second harmonic (EFISH) generation where the electric field is created by the substrate silver atoms. The measured second order nonlinear optical susceptibility for a rotaxane thin film is compared with that obtained by considering only EFISH contribution to SHG intensity. The electric field on the surface of a silver layer is calculated by using the Delphi4 program for structures obtained with TINKER molecular mechanics/dynamics simulations. An excellent agreement is observed between the calculated and the measured SHG susceptibilities.     

Measurement of the viscosity of ferroelectric liquid crystals in free-standing films

A new implementation of the technique for the measurement of the Goldstone mode rotational viscosity in the S_c phase is presented. An electric field is applied parallel to the surface of a free-standing liquid crystal film. The optical transmission change of the film is recorded. The viscosity can be calculated from the reorientation time between optically separated positions of the director. A comparison with the viscosity values measured in thin cells is given. The values determined in cells are always higher than the results obtained from free-standing films. This indicates the great influence of the forces of interaction between the liquid crystal molecules and the surface layer in the cells.     

An Insoluble Surfactant Model for a Vertical Draining Free Film

A mathematical model is constructed to study the evolution of a vertically oriented thin liquid film draining under gravity when there is an insoluble surfactant with finite surface viscosity on its free surface. Lubrication theory for this free film results in three coupled nonlinear partial differential equations describing the free surface shape, the surface velocity, and the surfactant transport at leading order. We will show that in the limit of large surface viscosity, the evolution of the free surface is that obtained for the tangentially immobile case. For mobile films with small surface viscosity, transition from a mobile to an essentially immobile film is observed for large Marangoni effects. It is verified that increasing surface viscosity and the Marangoni effect retard drainage, thereby enhancing film stability. The theoretical results are compared with experiment; the purpose of both is to act as a model problem to evaluate the effectiveness of surfactants for potential use in foam-fabrication processes. Copyright 2000 Academic Press.     

Superficial wrinkles in stretched, drying gelatin films

When a thin film of initially hydrated gelatin is allowed to dry from the surface, superficial changes in the structure of the material affect the local mechanical properties of the drying region. If the film is simultaneously subjected to large strain deformation (above 20%), a periodic pattern of wrinkles appears on the surface of the gelatin along the length of the sample in the direction of the applied force. These wrinkles are uniformly distributed on the surface of the gelatin with a wavelength that is much smaller than the sample thickness, which changes with sample composition, aging time, and deformation rate. We investigate these patterns via in situ environmental scanning electron microscopy (ESEM) and provide a theory for their origin.     

Single molecule magnets: from thin films to nano-patterns

Single molecule magnets (SMM) are a class of molecules exhibiting magnetic properties similar to those observed in conventional bulk magnets, but of molecular origin. SMMs have been proposed as potential candidates for several technological applications that require highly controlled thin films and patterns. Here we present an overview of the most important approaches for thin film growth and micro(nano)-patterning of SMM, giving special attention to Mn(12) based molecules. We present both conventional approaches to thin film growth (Langmuir-Blodgett, chemical approach, dip and dry, laser evaporation), patterning (micro-contact printing, deposition on patterned surface, moulding of homogeneous films) and new methods specifically developed for SMM (lithographically controlled wetting, lithographically controlled de-mixing).     

Film drainage and interfacial instabilities in polymeric systems with diffuse interfaces

We report an experimental investigation on the effect of mutual diffusion in polymeric systems on film drainage between two captive drops. The main objective is to study the influence of diffuse interfaces on film drainage. This is done by using material combinations with different interfacial properties and interferometric visualization of the film between two interacting drops. For highly diffusive systems film drainage is observed to be, in contrast to immiscible systems, non-axisymmetric and unstable immediately after the film formation (at a few micrometers film thickness). Depending on whether the total thickness of the diffusion layers in the film is smaller or larger than the thickness of the film, Marangoni convection is found to enhance or delay film drainage. Enhanced film drainage is determined to be in order of 100 times faster than predicted by the current models, while delayed film drainage is observed after a drainage period where experimental and predicted results (assuming, a partially mobile interface) are in close agreement.     

一种新型的铕络合物/TiO2发光薄膜

研究了Eu(DBM)3(DiBut-bpy)/TiO2/AA 单分子膜的表面压－分子面积(π－A)曲线和稳定性,单层膜中的TiO2纳米粒子是经TBT（tetrabutyloxyltitanium ,钛酸四丁酯）二维溶胶-凝胶法得到的.TiO2纳米粒子和铕络合物都被成功地转移到固体基片上，得到了具有良好的机械和热稳定性的新型发光薄膜.小角度X射线衍射结果证明这种复合膜具有层状有序的周期性结构.讨论了TiO2基质对复合膜发光机理的影响.     

Wetting films on chemically patterned surfaces

The behavior of thin wetting films on chemically patterned surfaces was investigated. The patterning was performed by means of imprinting of micro-grid on methylated glass surface with UV-light (λ=184.8 nm). Thus imprinted image of the grid contained hydrophilic cells and hydrophobic bars on the glass surface. For this aim three different patterns of grids were utilized with small, medium and large size of cells. The experiment showed that the drainage of the wetting aqueous films was not affected by the type of surface patterning. However, after film rupturing in the cases of small and medium cells of the patterned grid the liquid from the wetting film underwent fast self-organization in form of regularly ordered droplets covering completely the cells of the grid. The droplets reduced significantly their size upon time due to evaporation. In the cases of the largest cell grid, a wet spot on the place of the imprinted grid was formed after film rupturing. This wet spot disassembled slowly in time. In addition, formation of a periodical zigzag three-phase contact line (TPCL) was observed. This is a first study from the planned series of studies on this topic.     

Molecular and Supramolecular Deformations and Disclinations in a Liquid Crystalline Copolyether Thin Films under an Electrostatic Field

An alignment study of a liquid crystalline copolyether TPP‐7/11(5/5) thin films has been carried out in a 10 kV·cm^–1 electrostatic field parallel to the thin film surface normal. This copolyether possesses a negative dielectric anisotropy. The chain molecules are homogeneously aligned in the electric field and they form two‐dimensionally ordered lamellae in a tilted columnar phase when the samples were cooled to room temperature. It is observed that the chain molecules are splayed to form bent lamellae and the chain direction is perpendicular to the tangential direction of the lamellar surfaces. These lamellae thus become replicas of the chain orientation. Due to the flexoelectric effect and density fluctuation on the thin film free surface, disclinations having topological strength s = 1, c = π/4 and defect walls form. These s = 1 disclinations possesses both left‐ and right‐handednesses. Discussion of the defect formations have been attempted.