Solution Thin Films Research Articles

Determination of the ice quantity by quantitative microscopic imaging of vitrification solutions.

Based on Boutron's semi-empirical crystallization theory, a quantitative microscopic imaging method was developed to determine the quantity of ice in thin films of vitrification solutions. A universal equation was obtained for various vitrification solutions. The new method was applied to determine the ice quantity and critical cooling rates (Vcc) for two new vitrification solutions. Results reveal that the new quantitative imaging method is reliable. This new method has several advantages including the fact that it is cheaper than differential scanning calorimetry (DSC) and that it can be used to study vitrification solutions with Vcc values extending beyond the maximum cooling rate of a typical DSC system. Also, this method should permit quantitative imaging of the volume fraction of ice in space and time in microscope samples.

Generalization of the Gibbs–Kelvin–Köhler and Ostwald–Freundlich equations for a liquid film on a soluble nanoparticle

A derivation of chemical equilibrium equations for a spherical thin film of solution around a soluble solid nanoparticle is presented. The equations obtained generalize the Gibbs-Kelvin-Kohler and Ostwald-Freundlich equations for a soluble particle immersed in the bulk phase. The generalized equations describe the dependence of the chemical potentials of a condensate and dissolved nanoparticle matter in the thin solution film, the condensate saturated pressure, and the solubility of the nanoparticle matter on the film thickness, and the nanoparticle size with account of the disjoining pressure of the liquid film.

Studies of the Polydiacetylene Bis-p-chlorocinnamate of 10,12-Docosadiyn-1,22-diol

The bis-p-chlorocinnamate of 10,12-docosadiyn-1,22-diol polymerized with 60Co gamma radiation to give a soluble polydiacetylene (PDA), PDA-ClCIN-22. We have studied PDA-ClCIN-22 in both crystallographic powder and solution coated films. Thin films of solution coated PDA-ClCIN-22 exhibit an absorption maximum of 544 nm, similar to the concentrated solution in chlorobenzene. Irradiation of these films with 254 nm UV light in air leads to loss of intensity in the visible spectrum. The thin films of PDA-ClCIN-22 are amorphous by X-ray powder diffraction. Irradiation of both PDA-ClCIN-22 powder and film with 254 nm light did not result in photochemical reactivity of the chlorocinnamate group as recorded by FTIR spectra. Differential scanning calorimetry (DSC) studies of PDA-ClCIN-22 reveal that presence of monomer and endothermic transitions near 116°C and 134°C on a first heating. On a second heating of a sample of PDA-ClCIN-22 taken to 150°C on first heating, the 116°C and 134°C endotherms are absent. These endotherms are also lost by monomer extraction with boiling cyclohexane. One possible source of the endotherms would be conformational transitions in the side chain.

Effect of Channel Geometry Variations on the Performance of a Constrained Microscale-Film Ammonia-Water Bubble Absorber

An experimental study of the absorption of ammonia vapor in a constrained thin film of ammonia-water solution is presented. A large aspect ratio microchannel with one of its walls formed of a porous material is used to constrain the thickness of the liquid film. Experiments are performed at a pressure of 2.5 bar absolute and 4 bar absolute and at a fixed weak solution inlet temperature. Weak solution flow rates are varied from 10 g/min to 30 g/min (corresponding to the weak solution Reynolds number, Re, from 15 to 45), inlet mass concentrations are varied from 0% to 15%, and gas flow rates are varied between 1 g/min and 3 g/min (corresponding to the vapor Re from 160 to 520). Six geometries, including three smooth-bottom-walled channels of differing depths and three channels with structured bottom walls, are considered. Results indicate that, for identical rates of vapor absorption, the overall heat transfer coefficient of the 400 μm absorber is in most cases significantly larger than that of other absorbers. For the 150 μm and 400 μm absorbers, a trade-off between the high overall heat and mass transfer coefficients is achieved for the highest vapor to solution flow rate ratio.

Preparation and characterization of (001)- and (110)-oriented 0.6FeTiO3⋅0.4Fe2O3 films for room temperature magnetic semiconductors

Thin films of ilmenite-hematite solid solution 0.6FeTiO3⋅0.4Fe2O3 were prepared on α-Al2O3 (001) and (110) single-crystalline substrates. The oxide phases formed in the thin films strongly depended on the oxygen partial pressure (PO2) during deposition. At PO2=1.3×10−3Pa, regardless of thesubstrate orientation, well-ordered 0.6FeTiO3⋅0.4Fe2O3 films with R3¯ symmetry were epitaxially formed. Large saturation magnetization at room temperature was observed in both (001)- and (110)-oriented films. The differences in the magnetization and electrical resistivity curves between the (001)- and (110)-oriented films indicated the anisotropic nature of 0.6FeTiO3⋅0.4Fe2O3.

Coupled laser molecular trapping, cluster assembly, and deposition fed by laser-induced Marangoni convection

A coupled mechanism for molecular aggregation in a thin water solution film by laser-tweezers is suggested based on (i) simulation of light intensity distribution and (ii) order of magnitude analysis of heat and mass transport induced by Marangoni convection. The analysis suggests that the laser induced temperature distribution develops within 1 ms and Marangoni convection flow commences within 0.01-1 s, which increases by 1-2 orders of magnitude the mass transfer of dissolved molecules into the laser focus where they are trapped and aggregate by attractive van der Waals forces. This mechanism, considered for the particular case of polymer assembly, suggests that it can also be successfully applied for assembling other types of clusters and molecular aggregates from solutions.

Organic Crystal Fibers Aligned into Oriented Bundles with Polarized Emission

We demonstrate a simple method for growing organic crystal fiber bundles as long as centimeters by controlling the shape and dimension of the evaporation channel of the chloroform solution of N,N'-bis(1-ethylpropyl)-3,4,9,10-perylenebis(dicarboximide) (EPPTC). The capillary effect induces a thin solution film (capillary film) on the wall of the evaporation channel, and fast evaporation of the solvent gives rise to a concentration gradient along the channel. Thus, the strong pi-stacking between the EPPTC molecules in the capillary film results in formation of crystal fibers. Nearly linearly polarized emission centered around 620 nm has been detected from these crystal fibers under optical excitation. Measurements of the electron diffraction pattern and optical microscopic properties show well-defined stacking of the molecules in the crystal fibers with excellent alignment.

Drying of Thin Films of Polymer Solutions Coated over Impermeable Substrates

The manufacturing of adhesive tapes, photographic films, magnetic media, and many other products involves the coating and drying of thin films of polymeric solutions or emulsions over an impermeable substrate. Wet thickness may vary from 1 micron to 500 microns and solid content from 3% to 50% or more. The drying process generally occurs in convective ovens, where air temperature, air flow rate, and solvent partial pressure may be adjusted. In many cases, it is the manufacturing step that controls the productivity; therefore, it is very important to optimize the oven conditions to minimize the oven residence time without causing defects on the coated film and keeping the residual solvent inside the final product within specification limits. A drying model and a simulation software based on it were developed in order to solve the transient mass and heat transfer equations that describe the drying of a polymer solution containing two different solvents. The resulting system of equations shows strong non-linearity due to the free surface and the dependence of diffusion coefficients on solvent concentration and film temperature. The integration in time was performed with a fully implicit method, and at each time step, Newton's method was used to solve the resulting non-linear system of algebraic equations. An example of using the drying simulator to optimize an industrial adhesive tape line is discussed. The gain in productivity achieved by the analysis was greater than 40%.

Growth of Au/Ag nanowires in thin surfactant solution films: An electron microscopy study

The surfactant templated gold–silver nanowire growth process in a thin solution film was probed by cryo-transmission electron microscopy (cryo-TEM). The increasing surfactant concentration upon film drying appears to induce phase transformations in the film and form a liquid crystalline template for the nanowires growth. High-resolution transmission electron microscopy (HRTEM) and electron holography revealed that the nanowires were polycrystalline with some preferred crystallite orientations and had a roughly cylindrical cross-section. Further improvement of the technique may lead to highly ordered metal nanowire arrays within the surfactant matrix similar to the closely related mesoporous materials.

AFM surface force measurements conducted with silica in C nTACl solutions: Effect of chain length on hydrophobic force

Surface forces were measured using an AFM with silica surfaces immersed in C n TACl ( n = 12–18) solutions in the absence of added salt. The results showed long-range attractive forces that cannot be explained by the DLVO theory. The long-range attractions increased with increasing surfactant concentration, reaching a maximum at the point of charge neutralization (p.c.n.) and then decreased. The long-range forces decayed exponentially, with the decay lengths increasing from 3 to 32 nm as the chain length of the surfactants increased from C-12 to C-18. The measured forces can be fitted to the charged-patch model of Miklavic et al. [S.J. Miklavic, D.Y.C. Chan, L.R. White, T.W. Healy, J. Phys. Chem. 98 (1994) 9022–9032] by assuming patch sizes that are much larger than the values reported in the literature. It was found that the decay length decreases linearly with the effective concentration of the CH 2/CH 3 groups of the C n TACl homologues raised to the power of −1/2, which is in line with the Eriksson et al.'s hydrophobic force model derived using a mean-field approach. It appears, therefore, that the long-range attractions observed in the present work are hydrophobic forces originating from changes in water structure across the thin surfactant solution film between the silica surfaces. It is conceivable that hydrocarbon chains in solution disrupt the surface-induced water structure and cause a decrease in hydrophobic force. This observation may also provide an explanation for the very long-range forces observed with silylated, LB-deposited, and thiol-coated surfaces.

Liquid Templating for Nanoparticle Organization into Complex Patterns

Dewetting of thin films of charged polymer solutions produces complex patterns that can be applied to direct nanoparticle organization on solid substrates. The morphology produced by dewetting can be controlled by the solution properties, temperature, and substrate wetting. In this work, new results on this liquid-template self-assembly system are presented, with special emphasis on producing large arrays of organized nanoparticles. On a hydrophilic substrate with complete wetting, the patterns include polygonal networks and parallel-track arrays that extend over several hundreds of microns. These large structures are formed under well-controlled drying conditions and characterized by scanning electron microscopy, which is better suited for the examination of large as well as small areas than atomic force microscopy. On partial wetting substrates, new patterns are observed, including a complex set of parallel curved bands with variable particle number densities.

Coating of a thin layer of NaBH4 solution for mercury vapor generation coupled to atomic fluorescence spectrometry

Sodium tetrahydroborate (NaBH4) was, for the first time, coated onto PTFE (polytetrafluoroethylene), FEP (fluorinated ethylene propylene), Excelon (microline) and PEEK (polytetrafluoroethylene) surfaces and used for mercury vapor generation-atomic fluorescence spectrometric determination. A lab-on-valve (LOV) incorporating a miniature membrane phase separation chamber was employed to facilitate the operation. A 600–1200 nm thin film of NaBH4 solution was coated onto the interior surface of tubes by sweeping through 30 μl of NaBH4 solution with an argon stream. Mercury cold vapor was generated as sample solution flowing through the pre-coated tubing carried by argon. The film coating was renewed for each operating run. The vapor was either immediately excited in the outlet of the separator in the LOV and detected by directing the fluorescence to a photomultiplier with optical fibers, or directed to a commercial AFS-920 atomic fluorescence spectrometer. NaBH4 coating not only minimized fluorescence quenching by concomitant hydrogen encountered in conventional AFS, but the coating process also effectively eliminated mercury memory effects. In addition, the coating substantially simplified the operation of vapor generation using a single syringe pump for fluidic delivery. When using PTFE tubing as a coating medium, limits of detection of 0.06 and 0.005 μg l−1 were derived by monitoring with a LOV-AFS system and an AFS-920 spectrometer, respectively, with a sample volume of 500 μl. A sampling frequency of 60 h−1 along with RSDs of 2.9% and 2.0% at 2.0 μg l−1 were achieved. Mercury contents in certified reference materials and a local coastal seawater were measured.

Oxidation of thin films of zirconium and Zr-Ti solid solutions on single-crystal silicon

We have studied the oxidation of thin films of zirconium and Zr-Ti (1.2–2.03 at % Ti) solid solutions produced on single-crystal silicon wafers by magnetron sputtering. The films have been oxidized in flowing oxygen at temperatures from 473 to 673 K in a resistance furnace. The phase composition of the resultant oxide layers has been determined. Our results indicate that the oxidation kinetics of thin-film zirconium and Zr-based solid solutions are well represented by a linear-parabolic rate law. The composition, structure, and optical constants of the oxide layers depend on the heat-treatment conditions and the titanium content of the films.

Near-Field Optical Characterization of Sub-Micrometer Particle Arrays of Organic Pigment

ABSTRACT Sub-micrometer sized particles of organic dye, formed as precipitates on a glass surface when the organic solvent evaporated from a thin film of specimen solution at a precisely controlled evaporation rate, showed a typical one- or two-dimensional registration. We performed a near-field optical study of a single (or a few) particle(s) within these self-assembled particle arrays. Suppressing the background fluorescence that originated from the stray light in the clad region of probe fiber at the coupling point of the incident laser, we could manage to observe the near-field fluorescence from the separated particle. Further to overcome the near-field fluorescence decay in a minute or so, which was presumably due to photobleaching of surface-rich dye particles, we prepared them embedded in a transparent polymer film which is expected to protect the dye from oxygen.

Forced and Natural Convective Drying of Trehalose/Water Thin Films: Implication in the Desiccation Preservation of Mammalian Cells

Trehalose is believed to offer desiccation protection to mammalian cells by forming stable glassy matrices. The goal of the current study was to explore the desiccation kinetics of thin films of trehalose-water solution under forced and natural convective conditions and to investigate the thermophysical state of mammalian cells at the bottom of the thin film. We developed a finite difference model based on the mass and energy conservation equations coupled to the water transport model from the cells. The boundary conditions were obtained from correlations or experimental measurements and the Gordon-Taylor equation was used to predict the glass transition temperature at every location. Results indicated that there are three distinct regimes for drying for both forced and natural convection, characterized by the slope of the moisture content plot as a function of time. Our results also indicate that the surface of the solution reached the glassy state in less than 10 min for the Reynolds (forced) numbers explored and approximately 30 min for some Rayleigh (natural convective) numbers; however, significant water was trapped at this instant. Larger drying force hastened quicker glass formation but trapped more water. The numerical model was capable of predicting the drying kinetics for the dilute region accurately, but deviated while predicting the other regimes. Based on these experimental validations of the model, the osmotic response of different cells located at the bottom of the solution with orders of magnitude difference in their membrane permeability (Lp) was predicted. The results suggested that extracellular glass formed around cells at the bottom of a trehalose-water solution by the propagation of glass into the solution; however it takes more than an order of magnitude time (approximately 7 min to >100 min for forced convective drying) to remove sufficient water to form glass around cells from the time when the first surface glass is formed. This is attributed to low diffusivity of water through the glass. In addition, the water transport from the glassy matrix could be either diffusion or Lp limited. For diffusion-limited transport, lowering the film thickness at the beginning of drying by half almost lowers the drying time by an order of magnitude. In summary, the optimal design of convective desiccation protocols requires accounting for the size of the cell, their membrane permeability (Lp) and the starting thickness of the solution.

Viscosity and Lubricity of Aqueous NaCl Solution Confined between Mica Surfaces Studied by Shear Resonance Measurement

We report the viscoelasticity of the thin film of aqueous NaCl solution confined between mica surfaces measured by shear resonance apparatus. The observed shear resonance curves at separations less than ca. 2 nm indicated that the solution exhibits the high lubrication effects under some loads. The effective viscosity (0.1-10 Pa s) obtained for the separations less than 1 nm from a mechanical model was 2-4 orders of magnitude larger than the bulk value. Our study employing a novel shear measurement provided a comprehensive picture for the dynamics of confined water thinner than a few nanometers.

Analysis of Molecular Orientation within Self-Organized Dye Particles with Near-Field and Polarized Evanescent-Field Excited Fluorescence

Typical one- or two-dimensional registration was observed in organic dye particles of micrometer and submicrometer size that were formed as precipitates on an ozone-treated hydrophilic glass surface when the organic solvent evaporated from a thin film of specimen solution at a precisely controlled evaporation rate. We successfully identified near-field-excited near-field fluorescence from single particles within these self-organized particle arrays. However, some of the particles, especially most of the small particles with diameters around 2 µm or less, did not show fluorescence under near-field observation. In contrast, far-field fluorescence, when excited by a polarized evanescent field, was observed, with its intensity depending on the excitation polarization, indicating that molecules' transition moment within dye particles orient parallel to the substrate surface. These two observations suggest that the near-field at the tip of the probe was polarized parallel to the probe axis. Another observation, that molecules seemed to orient in a similar direction among neighboring particles, suggests that the dewetting process contributed to the alignment of the molecular direction among adjacent particles, which further proves that the present specimen was formed by a self-organizing mechanism.

Formation of Gold−Silver Nanowires in Thin Surfactant Solution Films

Thin, long gold/silver nanowires were grown on substrates in thin surfactant solution films. This growth process occurred exclusively in thinning aqueous films as the water evaporated, and elongated surfactant template structures were formed. The nanowire growth depended on the presence of a relatively high concentration of silver ions (typical Ag:Au mole ratio of 1:1). Tuning the pH value to about 5 in the growth solution was crucial for the nanowire growth. Further development of this process may lead to a simple wet chemical technique for the fabrication of relatively uniform arrays of metal nanowires on surfaces.

Effect of phase transitions on the exciton absorption spectrum of RbxK1−x Ag4I5 superionic conductors

The absorption spectrum of thin films of solid solutions of RbxK1−xAg4I5 superionic conductors is investigated in the energy range 3–6 eV at temperatures from 90 to 290 K upon heating and cooling. It is established that the temperature dependences of the spectral position Em and the half-width Γ of the long-wavelength exciton band are determined by the exciton-phonon interaction and the generation of Frenkel defects at phase transitions to the superionic state. Fluctuations of the composition of solid solutions do not affect the behavior of the dependences Em(x) and Γ(x) at 90 K, which is indicative of the localization of excitons in the AgI sublattice of these compounds. A difference in the dependences Tc1(x) and Tc2(x) (the γ → β and β → α phase transitions, respectively) is revealed: the curve Tc1(x) has a minimum at x ≈ 0.5, whereas the curve Tc2(x) shows a weak maximum.

Improving slide-based assays by stirring: Application of liquid-on-liquid mixing to immunofluorescence staining of polytene chromosomes

A new method for stirring thin liquid films has been developed and demonstrated to increase the sensitivity of immunofluorescence staining of polytene chromosomes. This liquid-on-liquid mixing (LOLM) technique uses a stirrer fluid, immiscible with the thin film, to transmit shear at the liquid–liquid interface. Here, we stir mineral oil layered over an aqueous thin film of antibody solution, which stains transcription apparatuses on chromosomes previously fixed to a glass slide. The quality of staining was assessed at varying antibody concentrations and incubation or stirring times. Our data indicate that the LOLM technique overcomes the diffusion barrier associated with traditional slide-based biological assays.