Concentration polarization at Langmuir monolayer deposition: the role of indifferent electrolytes

Under dynamic conditions of the charged Langmuir monolayer deposition onto a substrate surface, ion concentration and electric potential profiles are induced in the subphase around the three-phase contact line. Such local changes in the subphase influence the deposition process, particularly the monolayer adhesion work and the maximum deposition rate. If indifferent electrolytes (not interacting chemically with interfacial groups) are present in the solutions, they can affect electric potential distributions and therefore the monolayer charge and the deposition process as a whole. With increasing deposition rate, the indifferent electrolyte counterions replace gradually the potential-determining counterions in a close vicinity to the contact line. This leads to increasing monolayer ionization and increasing electrostatic repulsion between the monolayer and substrate. When the deposition rate approaches the critical one, the charge of the monolayer increases dramatically and the stationary monolayer deposition becomes impossible. Such a significant increase of the monolayer charge is not observed in the absence of indifferent electrolytes.     

Influence of fluid flow on the deposition of soluble surfactants through receding contact lines of volatile solvents

Soluble surfactants are often deposited from volatile solvents through moving contact lines. In this study, we demonstrate that altering the flow field near such a contact line fundamentally changes the deposited surfactant structure. At slow contact line speeds, the substrate emerges dry. A densely packed, tilted monolayer of surfactant is deposited along the solid-vapor interface from the rolling fluid motion at the contact line. At faster speeds, the substrate emerges with an evaporating thin film entrained on its surface. Surfactant is confined in the film in a constantly increasing concentration environment. Monodisperse crystalline islands nucleate and grow on the surface with sizes and shapes controlled by varying the deposition conditions. These results contrast with disordered deposits that result from evaporation at a pinned contact line. Our results suggest that dip-coating with control of dipping speed and evaporation rate may provide better control of deposition through contact lines of evaporating solvents.     

Formation of a polymer particle monolayer by continuous self-assembly from a colloidal solution

The preparation of two-dimensional monolayers of polymer particles over a large area was demonstrated via a facile solution process. Polymer microspheres were continuously self-assembled into a close-packed monolayer from a colloidal solution confined between two plates such that the top plate was carefully dragged at a constant velocity in the direction opposite that of the monolayer growth. In situ direct observation of the particle movement during the coating process confirmed that particle transport was directed toward the contact line of the solution meniscus by evaporation-induced convective flow. Sliding of the top plate apparently effectively counterbalanced the convective flow to provide the particles with a contact line for growth of a monolayer particle array. The influence of particle concentration, sliding speed of the top plate, and surface wettability of the bottom substrate were investigated and optimized. Monolayer particle arrays were successfully demonstrated as a template for the preparation of ZnO films with ordered hollow hemispherical structures. This approach is applicable to the fabrication of ordered structures of monodispersed particles composed of various materials over large areas.     

Surfactant-assisted spreading of a liquid drop on a smooth solid surface

Axisymmetric spreading of a liquid drop covered with an insoluble surfactant monolayer on a smooth solid substrate is numerically investigated. As the drop spreads, the adsorbed surfactant molecules are constantly redistributed along the air-liquid interface by convection and diffusion, leading to nonuniformities in surface tension along the interface. The resulting Marangoni stresses affect the spreading rate by altering the surface flow and the drop shape. In addition, surfactant accumulation in the vicinity of the moving contact line affects the spreading rate by altering the balance of line forces. Two different models for the constitutive relation at the moving contact line are used, in conjunction with a surface equation of state based on the Frumkin adsorption framework, to probe the surfactant influence. The coupled evolution equations for the drop shape and monolayer concentration profile are integrated using a pseudospectral method to determine the rate of surfactant-assisted spreading over a wide range of the dimensionless parameters governing the spreading process. The insoluble monolayer enhances spreading through two mechanisms; a reduction in the equilibrium contact angle, and an increase in the magnitude of the radial pressure gradient within the drop due to the formation of positive surface curvature near the moving contact line. Both mechanisms are driven by the accumulation of surfactant at the contact line due to surface convection. Although the Marangoni stresses induced at the air-liquid interface reduce the rate of spreading during the initial stages of spreading, their retarding effect is overwhelmed by the favorable effects of the aforementioned mechanisms to lead to an overall enhancement in the rate of spreading in most cases. The spreading rate is found to be higher for bulkier surfactants with stronger repulsive interactions. With the exception of monolayers with strong cohesive interactions which tend to retard the spreading process, the overall effect of an insoluble monolayer is to increase the rate of drop spreading. Simulation results for small Bond numbers indicate the existence of a power-law region for the time-dependence of the basal radius of the drop, consistent with experimental measurements.     

Distributions of ionic concentrations and electric field around the three-phase contact at high rates of Langmuir-Blodgett deposition

A mathematical problem is formulated and numerically solved for addressing the electric field and ionic concentration distributions developing around the three-phase contact line during the Langmuir-Blodgett deposition of charged monolayers. Compared to a previous paper dealing with the same effect (J. Phys. Chem. B 2004, 108, 13449), the present analysis is not restricted to the case of low deposition rates and small concentration changes. The obtained results show that, for sufficiently high deposition rates, the subphase composition substantially changes in the immediate vicinity of the three-phase contact line. It is shown that the predicted changes in the subphase composition can drastically affect the adhesion work and the dynamic contact angle. On this basis, the influence of the concentration polarization effect on meniscus behavior is discussed.     

Role of the meniscus shape in large-area convective particle assembly

Dense and uniform particle films are deposited using a robust version of the convective particle assembly process. We analyze how the shape of the gas-liquid interface and the three-phase contact line govern the stability of convective deposition and, thus, the achievable quality of films. Interference microscopy indicates that a highly curved meniscus cannot compensate for the ubiquitous perturbation during deposition. A moderately curved meniscus provides flexibility to compensate and localize perturbation and enables reliable homogeneous deposition. We analyze which setup geometry and meniscus velocity yield appropriate meniscus shapes. The quality of the resulting films is analyzed and compared to the deposition conditions. Uniform films over areas beyond the centimeter range are accessible using the optimized process, which is suitable for functional particle coatings and templates for microstructured materials.     

Induced Collapse in the Formation of Alternating Langmuir–Blodgett Films

Two compounds whose molecules have large dipole moments, strereoregular perfluorinated alkylmethacrylate-co-methacrylic acid (A) and p-octadecylaminoazobenzene-p"-sulfamide (B), were used to assemble Langmuir–Blodgett polar films by the alternating method to achieve high polarization. According to estimates, the dipole moment of the dimeric unit of copolymer A is equal to about 4 D, the dipole moment of compound B, to 12 D. Unusually high (several units) transfer ratio was observed for the monolayer B when preparing the structures of (AB) _n type. The monolayers were stable during their formation process. Upon the pause of a substrate when passing through the monolayer B, the monolayer area remained strictly constant. According to the model proposed, the monolayer looses its stability and locally collapses in the meniscus zone at the contact line of the monolayer B and substrate due to an increase in the electrostatic repulsion between the dipoles of molecules B. The validity of this model is confirmed by the data of small-angle X-ray diffraction and atomic force microscopy for various alternating Langmuir–Blodgett films.     

Molecular relaxation dynamics of self-assembled monolayers

Dielectric relaxation spectroscopy is used to quantify molecular motion in alkylsilane SAMs coated on porous glass over a broad temperature range, -30 to -150 degrees C. Systematic measurements using SAMs with variable coating densities allow us to determine the effect of monolayer disorder on molecular mobility in thin molecular films. A relaxation process with an activation energy of approximately 25 kJ/mol is found to dominate dynamics of SAM-chain segments near the substrate. By introducing polar CN groups at the ends of the chain, we show that the relaxation process in the monolayer canopy can be isolated and studied. This approach can be generalized to other substituent polar groups to probe localized relaxation dynamics in surface-grafted monolayer films.     

Spontaneous pattern formation by dip coating of colloidal suspensions on homogeneous surfaces

We study the slow withdrawal of a partially wet vertical plate at velocity U from a suspension of well-wet particles. Periodic horizontal striped assemblies form spontaneously at the three-phase contact line on energetically uniform surfaces. Stripe width and spacing depend on the withdrawal velocity U relative to a transition velocity Ut. Thick stripes separated by large spaces form for UUt, thin stripes separated by small spaces form. The stripe spacing is reduced by an order of magnitude and varies weakly with U until a maximum velocity is reached at which the stripes fail to form. A partially wet surface can entrain a meniscus. For UUt, we infer that a film of thickness h is entrained above the meniscus. When h is smaller than the particle diameter D, particles aggregate where the entrained film thickens to match up to the wetting meniscus. When an entrained particle becomes exposed to air by evaporation, it becomes the new pinning site from which the next film is entrained. The film thickness h increases with U; at some velocity, h becomes comparable to D. Particles flow into the film and deposit there in a disordered manner. A diagram summarizing particle deposition is developed as a function of D, U, and h.     

Transition from homogeneous Langmuir-Blodgett monolayers to striped bilayers driven by a wetting instability in octadecylsiloxane monolayers

We show that two dips of an oxidized silicon substrate through a prepolymerized n-octadecylsiloxane monolayer at an air-water interface in a rapid succession produces periodic, linear striped patterns in film morphology extending over macroscopic area of the substrate surface. Langmuir monolayers of n-octadecyltrimethoxysilane were prepared at the surface of an acidic subphase (pH 2) maintained at room temperature (22 +/- 2 degrees C) under relative humidities of 50-70%. The substrate was first withdrawn at a high dipping rate from the quiescent aqueous subphase (upstroke) maintained at several surface pressures corresponding to a condensed monolayer state and lowered soon after at the same rate into the monolayer covered subphase (downstroke). The film structure and morphology were characterized using a combination of optical microscopy, imaging ellipsometry, and Fourier transform infrared spectroscopy. An extended striped pattern, perpendicular to the pushing direction of the second stroke, resulted for all surface pressures when the dipping rate exceeded a threshold value of 40 mm min(-1). Below this threshold value, uniform deposition characterizing formation of a bimolecular film was obtained. Under conditions that favored striped deposition during the downstroke through the monolayer-covered interface, we observed a periodic auto-oscillatory behavior of the meniscus. The stripes appear to be formed by a highly correlated reorganization and/or exchange of the first monolayer, mediated by the Langmuir monolayer at the air-water interface. This mechanism appears distinctly different from nanometer scale stripes observed recently in single transfers of phospholipid monolayers maintained near a phase boundary. The stripes further exhibit wettability patterns useful for spatially selective functionalization, as demonstrated by directed adsorptions of an organic dye (fluorescein) and an oil (hexadecane).     

Convectively assembled asymmetric dimer-based colloidal crystals

Monolayer films from polystyrene asymmetric dimer colloidal particles were formed on a silicon substrate using a heat assisted vertical deposition technique. In dilute particle suspensions of systematically varied concentrations, the system maximizes the packing efficiency within a thin meniscus region. Structures with positional order and orientational order in and out of the substrate plane were observed in surface and cross-sectional scanning electron microscopy (SEM) images. The confining effect of the meniscus height drove the formation of the resulting oblique and hexagonal lattices with controlled orientation. The crystals exhibited features similar to the planes of the boron nitride and zinc sulfide atomic structures. The diffraction properties of both colloidal crystal structures were demonstrated via selected area diffraction for laser light in the visible region.     

Effect of chain length of self-assembled monolayers in dip-pen nanolithography using molecular dynamics simulations

The pattern transfer mechanism of an alkanethiol self-assembled monolayer (SAM) with different chain lengths during the dip-pen nanolithography (DPN) process and pattern characterizations are studied using molecular dynamics (MD) simulations. The mechanisms of molecular transference, alkanethiol meniscus characteristics, surface adsorbed energy, transfer number, and pattern formation are evaluated during the DPN process at room temperature. The simulation results clearly show that the molecular transfer ability in DPN is strongly dependent on the chain length. Shorter molecules have significantly better transport and diffusion abilities between the meniscus and substrate surface, and the transport period can be maintained longer. The magnitude of adsorbed energy increases with chain length, so many more molecules can be transferred to the surface when shorter molecules are used. After deposition, the magnitude of the adsorbed area and pattern height decrease with increasing chain length.     

Probing of polyelectrolyte monolayers by zeta potential and wettability measurements

Detection of the very first step of polyelectrolyte adsorption onto a solid support is of great importance for understanding mechanisms of solid surface modification. It was shown that streaming potential and contact angle measurements can be successfully used for polyelectrolyte (PE) adsorption characterization in a broad range of surface coverage. Cationic polyallylamine hydrochloride (PAH) was used for the formation of the layer. The electrokinetic characteristics of the substrate covered by the PAH layer were compared with contact angles measured under wet (captive air bubble/substrate in water) and dry (sessile water droplet/dried substrate) conditions. It has been demonstrated that contact angle values determined under both conditions are in good agreement. The observed rapid increase in the contact angle from zero for the bare mica surface to the value close to one characteristic of the PAH monolayer appears in the same PAH coverage range as zeta potential value changes due to adsorption. These results show that wettability can be as sensitive to the presence of small amounts of adsorbed species as electrokinetic measurements.     

Spreading, evaporation, and contact line dynamics of surfactant-laden microdrops

An optical technique based on the reflectivity measurements of a thin film was used to experimentally study the spreading, evaporation, contact line motion, and thin film characteristics of drops consisting of a water-surfactant (polyalkyleneoxide-modified heptamethyltrisiloxane, called superspreader) solution on a fused silica surface. On the basis of the experimental observations, we concluded that the surfactant adsorbs primarily at the solid-liquid and liquid-vapor interfaces near the contact line region. At equilibrium, the completely wetting corner meniscus was associated with a flat adsorbed film having a thickness of approximately 31 nm. The calculated Hamaker constant, A = -4.47 x 10(-)(20) J, shows that this thin film was stable under equilibrium conditions. During a subsequent evaporation/condensation phase-change process, the thin film of the surfactant solution was unstable, and it broke into microdrops having a finite contact angle. The thickness of the adsorbed film associated with the drops was lower than that of the equilibrium meniscus. The drop profiles were experimentally measured and analyzed during the phase-change process as the contact line advanced and receded. The apparent contact angle, the maximum concave curvature near the contact line region, and the convex curvature of the drop increased as the drop grew during condensation, whereas these quantities decreased during evaporation. The position of the maximum concave curvature of the drop moved toward the center of the drop during condensation, whereas it moved away from the center during evaporation. The contact line velocity was correlated to the observed experimental results and was compared with the results of the drops of a pure alcohol. The experimentally obtained thickness profiles, contact angle profiles, and curvature profiles of the drops explain how the surfactant adsorption affects the contact line motion. We found that there was an abrupt change in the velocity of the contact line when the adsorbed film of the surfactant solution was just hydrated or desiccated during the phase-change processes. This result shows the effect of vesicles and aggregates of the surfactant on the shape evolution of the drops. For these surfactant-laden water drops, we found that the apparent contact angle increased during condensation and decreased during evaporation. However, for the drop of a pure liquid (n-butanol and 2-propanol) the apparent contact angle remained constant at a constant velocity during condensation and evaporation. The contact line was pinned during the evaporation and spreading of the surfactant-laden water drops, but it was not pinned for a drop of a pure alcohol (self-similar shape evolution).     

Substrate surface energy dependent morphology and dewetting in an ABC triblock copolymer film

A gradient combinatorial approach was used to examine the effect of substrate surface energy on the morphology and stability of films of a poly(isoprene-b-styrene-b-ethylene oxide) triblock copolymer that exhibits an alternating gyroid morphology in the bulk. Atomic force microscopy data across our surface energy (water contact angle) library suggest a transformation to predominantly surface parallel lamellae with an antisymmetric ordering. For substrate water contact angles below 70 degrees the film exhibited autophobic dewetting from an adsorbed half-period triblock copolymer monolayer at longer annealing times. X-ray photoelectron spectroscopy and near edge X-ray absorption fine structure analysis along gradient specimens indicated that the substrate surface energy governed the composition profile of the monolayer, and this variation in chemical expression was key to whether the film was stable or autophobically dewet. These observations demonstrate that enthalpic interactions, in addition to entropic considerations, can play a major role in autophobic dewetting of block copolymer films.     

In-situ fluorescence imaging of sol-gel thin film deposition

Pyranine was used as a fluorescence probe to monitor the chemical evolution in-situ during thin film deposition by the dip coating process. The sensitivity of the pyranine luminescence to protonation/deprotonation effects was used to quantify changes in the water/alcohol ratio in real time within the depositing film as the substrate was withdrawn from the coating reservoir. The spatially resolved spectral results clearly showed that preferential evaporation of alcohol occurred with increasing distance from the reservoir and that the maximum water content reached rather high values near the drying line. Correlation of the luminescence results with the interference pattern of the drawn films allows the solvent composition in the film to be mapped as a function of film thickness. These experiments demonstrate for the first time that luminescent organic molecules may be applied to the processing science of sol-gel thin film deposition.     

On the shape of a hydrostatic meniscus attached to a corrugated plate or wavy cylinder

The shape of a hydrostatic meniscus attached at a fixed contact angle to a vertical plate or circular cylinder with periodic corrugations is studied by analytical and numerical methods, and the effect of wall irregularities on the shape of the contact line and vertical component of the capillary force is discussed. An asymptotic analysis for a plate with small-amplitude sinusoidal corrugations is carried out to first order with respect to the corrugation amplitude, and a boundary-value problem is formulated and solved by a shooting method to determine the meniscus shape and elevation of the contact line. The meniscus attached to a corrugated plate with rounded corners produced by a Schwarz-Christoffel mapping function for a triangular wave is considered by numerical methods. The Laplace-Young equation determining the meniscus shape is solved in orthogonal curvilinear coordinates generated by conformal mapping using a finite-difference method. The numerical results are successfully compared with the predictions of the perturbation expansion for small amplitudes and discussed with reference to the rise of a meniscus inside a dihedral angle for large amplitudes. A companion asymptotic analysis is presented for a meniscus outside a vertical circular cylinder with small-amplitude sinusoidal corrugations. The analytical predictions are successfully compared with numerical solutions of the Laplace-Young equation for a meniscus outside an elliptical cylinder with aspect ratio near unity, regarded as a deformed circle.     

Fluorescence microscopy studies of layer/substrate interaction during the Langmuir-Blodgett transfer: Fractional condensation and local layer modification in lipid monolayers at the three-phase line

Transfer fluorescence microscopy reveals the substrate-mediated fractional condensation and phase-selective deposition of dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylethanolamine (DMPE) monolayers during the LB-transfer. Preferentially the higher ordered liquid-condensed (LC) state is transferred onto the substrate during the transfer of a monolayer in the LC/LE (liquid/expanded) coexistence state on the water subphase. This is manifested in the directly observable attraction of LC-domains towards the three-phase line and observation of a domain-free gap as consequence of the segregation of the fluorescent probe into the floating monolayer adjacent to the three-phase line. Fingering domain growth nucleating at the three-phase line and the substrate-mediated pressure deposition of probe-free material corroborate the preference of the solid substrate for the higher condensed phase. These observations are caused by changes in the free energy of the monolayer due to the replacement of the aqueous interface by the solid substrate surface.     

Influence of ion transfer kinetics on the composition of langmuir-blodgett films

The effect of ion transfer kinetics on the ionic composition of Langmuir-Blodgett (LB) films formed by charged monolayers is analyzed. The dynamic regimes of the LB deposition are considered by taking into account the competitive adsorption of several counterions having different diffusivities, valences, binding constants, and bulk concentrations. It is shown that the composition of deposited films should change with the deposition rate. At lower deposition rates, the ion with higher binding constant is more represented within the deposited monolayer in comparison to the higher deposition rates. At low deposition rates, the ratio of counterion amounts within the LB films is the same as that within the floating monolayer excluding the ions within the diffuse layer. At high deposition rates, the ratio of the counterion amounts is the same as that within the floating monolayer when the potential-determining counterions within the diffuse layer are taken into account.