Fast dynamic wetting of polymer surfaces by miscible and immiscible liquids

The initial stages of spontaneous spreading of a solvent drop (toluene) on the surface of a soluble polymer (polystyrene) have been studied with a high-speed camera. For drops of 1–4 μL volume, the increase in contact radius r can be described by a power law r μ t^a r propto {t^{alpha }} , with the spreading exponent α = 0.50 and for the first ≈8 ms. Thereafter, the three-phase contact line was pinned leading to a macroscopic static contact angle of Θ₀ = 12–15°. The insoluble liquids ethanol (α = 0.47, Θ₀ = 0) and water (α = 0.35, Θ₀ = 90°) showed a slower spreading. We attribute the fast spreading of toluene to the strong interaction with the polymer, like in reactive wetting. The finite macroscopic contact angle indicates the formation of a ridge by softening of polystyrene due to permeated toluene and the subsequent plastic deformation by the surface tension of the liquid. This interpretation is supported by experiments on polymers grafted from a silicon wafer. Toluene completely wets polymer brush surfaces. Transport of toluene through the vapor phase plays a significant role.     

Dynamics and rheology of immiscible polymer-liquid-crystal systems

The morphology evolution in immiscible polymer-liquid-crystal systems is quite different from flexible polymer-polymer mixtures due to the anisotropic properties of liquid crystals. The dynamics and rheology of such system are discussed. A theoretical model is proposed to describe the dynamics of liquid-crystal droplets in a flow field and the rheological properties of immiscible liquid-crystal/polymer mixtures. The deformation of liquid-crystal droplet is found to be greatly dependent on the interfacial properties of polymer-liquid crystal. The scaling relationships of interfacial contribution to the stresses are found to be quite different from the isotropic mixtures with droplet morphology.     

Domain growth and wetting in phase-separating fluid mixtures

We present light scattering results from two different fluid mixtures undergoing phase separation next to a glass interface. We observe a characteristic length scale L, which coarsens as a function of time L ∼ t^3/2. This growth is faster than any observed previously in bulk samples. The physical process responsible for it is the formation of a wetting layer of one of the two coexisting phases next to the interface. We discuss recent theoretical attempts to explain the fast kinetic mode.     

Recent advances in studies of bubble-solid interactions and wetting film stability

Research in the area of bubble-solid interactions is reviewed and highlighted, with a focus on studies of wetting film drainage using theoretical approaches and experimental (interferometric) approaches, and also studies probing the stability of wetting films, where the stability has been affected by physical and chemical modification/factors. Significant advances have been made in recent years in the area of interferometry and force measurement of bubble-surface encounters, with multiple light wavelengths used to improve accuracy and certainty with regard to thickness of wetting films, as well as high speed interferometry. These advances have been accompanied with improvements to models to describe nonequilibrium aspects of opposing interfaces. Experimental studies of the influence of air bubbles and surface roughness have highlighted the importance of dissolved gas and surface condition in determining whether wetting films are stable. Finally, many new studies on the influence of polymer layers on wetting film stability and rupture have been published, and these are described in relation to the increase in our understanding of the role of adsorbed polymers in altering surface chemistry and physics of the underlying substrate.     

Features of wetting metal substrates in medium of neutral hydrocarbon

In order to eliminate the influence of atmospheric adsorption, the surface free energy of some metal substrates is determined under conditions of selective wetting. The dependence of obtained values on surface tension of used neutral hydrocarbon is found. A well-defined hydrophobicity of these surfaces before and after thermal oxidation is established.     

Transient responses of a wetting film to mechanical and electrical perturbations

This article reports real-time observations and detailed modeling of the transient response of thin aqueous films bounded by a deformable surface to external mechanical and electrical perturbations. Such films, tens to hundreds of nanometers thick, are confined between a molecularly smooth mica plate and a deformable mercury/electrolyte interface on a protuberant drop at a sealed capillary tube. When the mercury is negatively charged, the water forms a wetting film on mica, stabilized by electrical double layer forces. Mechanical perturbations are produced by driving the mica plate toward or by retracting the mica plate from the mercury surface. Electrical perturbations are applied to change the electrical double layer interaction between the mica and the mercury by imposing a step change of the bias voltage between the mercury and the bulk electrolyte. A theoretical model has been developed that can account for these observations quantitatively. Comparison between experiments and theory indicates that a no-slip hydrodynamic boundary condition holds at the molecularly smooth mica/electrolyte surface and at the deformable mercury/electrolyte interface. An analysis of the transient response based on the model elucidates the complex interplay between disjoining pressure, hydrodynamic forces, and surface deformations. This study also provides insight into the mechanism and process of droplet coalescence and reveals a novel, counterintuitive mechanism that can lead to film instability and collapse when an attempt is made to thicken the film by pulling the bounding mercury and mica phases apart.     

Inhibation of crystal growth during drying in gels derived from a cheap,mixed metal oxide precursor

The influence of addition of small amounts of either citric acid or lactic acid on the formation of crystalline matter in dried gels derived from a multi-component industrial sol–gel silica precursor has been studied. The sols were water-based and had formic acid as the main acid constituent. A pronounced decrease in the extent of crystallization was observed for both acids, with citric acid being more effective than lactic acid. The results are discussed based on the complexation behavior of the corresponding acids under the studied conditions, and the complexation behavior in solution can be directly linked to the extent of crystallization in the dried gels. However, the sol–gel kinetics followed that expected for a purely silica-based sol, which suggests that the kinetics is mainly controlled by the silica portion of the sol. The results are suggested to be of importance for the industrial use of these sols as binders, as pronounced crystallization in the gels upon drying may lead to mechanical stresses, and thus to a decreased binder performance.     

Dynamic mechanical properties of immiscible polymer systems with and without compatibilizer

Blends of polyamide12 (PA12) and isotactic polypropylene (PP) were prepared by melt mixing, in an internal mixer, in the presence and absence of compatibiliser. The compatibiliser used was maleic anhydride grafted PP (PP-g-MA). The dynamic mechanical properties of the blends with and without compatibiliser were studied. Although compatibilization shifted the glass transition temperatures (T_g's) of component polymers only marginally, it significantly enhanced the storage modulus of the blends. The storage moduli of the uncompatibilised blends were compared with those predicted by theoretical models. Correlation between the dynamic mechanical properties of both compatibilised and uncompatibilised blends and their phase morphology was made.     

Effect of geometry on steady wetting kinetics and critical velocity of film entrainment

The velocity dependence of receding dynamic contact, angles θ_r/ U for siliconized cylinders of different, radii withdrawn from a glycerol-water mixture [19] show an independence on geometry and substantial influence of the material properties of the solid surface. These data are compared with the results of Ngan and Dussan [16a,b] for advancing angles θ_α / U (silicon oil displacing air), which suggest a considerable effect of geometry. A similar asymmetry of the effects of geometry and material properties on the critical velocities of liquid and air film entrainment follows from the Juxtaposition of our previous results for u_cr^R[19,20] with literature data on U_cr^A onto different solid substrates.The experimental data are interpreted on the basis of the equations of Cox-Voinov [21,22], describing the data of Ngan and Dussan quantitatively. The data for the receding meniscus can be represented quantitatively only by a combined Blake-Haynes-Voinov equation taking into account the dissipation in the three-phase contact zone and in the bulk liquid.     

Continuous fabrication of biocatalyst immobilized microparticles using photopolymerization and immiscible liquids in microfluidic systems

We report a novel technique for manufacturing polymeric microparticles containing biocatalysts by the behavior of immiscible liquids in microfluidic systems and in situ photopolymerization. The approach utilizes a UV-polymerizable hydrogel/enzyme solution and an immiscible oil solution. The oil and hydrogel solutions form emulsions in pressure-driven flow in microchannels at high values of the dimensionless capillary number (Ca). The resultant hydrogel droplets are then polymerized in situ via exposure to 365 nm UV light. This technique allows for the generation of monodisperse particles whose size can be controlled by the regulation of flow rates. In addition, both manufacturing microparticles and immobilizing biocatalysts can be performed simultaneously and continuously.     

Transient nucleation and bubble growth in immiscible liquid composites induced by counterdiffusion of gases

Supersaturation, homogeneous nucleation, and subsequent bubble growth and motion in immiscible liquid layers induced by counterdiffusion of gases at different temperatures are studied analytically. The range of the critical embryo size to initiate transient nucleation is determined between spontaneous and infinitely-slow nucleation. In addition, the ranges of bubble departure size and terminal rising velocity are evaluated together with the degree of superheat required for heat transfer to be the controlling mechanism. Numerical results are obtained for two special cases: (i) a composite with negligible surface resistances to heat and mass transfer, and (ii) a composite with one side insulated. The mechanics of microexplosion of emulsions is explained.     

Application of thin metal film elements in bioanalysis

Advanced metal deposition and microfabrication techniques enable preparation of metal surfaces with high precision and excellent control over their size and shape with subnanometer resolution. Thin metal films of different types and functions can be found in many analytical instruments. Surfaces with high optical quality serve as mirrors, beam splitters, antireflective coatings etc. Smooth metal coating is crucial in electron microscopy. Unique properties of the thin metal films are widely used in optical systems, as tools for sample manipulation but also for chemical sensing and detection. While some of the applications are widespread and belong to the basic curriculum in analytical chemistry, the newer or less common uses of thin metal films are well known only to the experts in the field. The purpose of this critical review is to highlight the role of thin metal films in bioanalysis and summarize some of their main applications in current bioanalytical instrumentation.     

Calculation of wetting angles in crude oil/water/quartz systems

A method for the determination of water-advancing wetting angles has been developed and tested. The method allows measurements in black oils, as opposed to traditional techniques which substitute transparent model oils prior to measurements. The method is based on the Laplace equation and axisymmetric drop shape analysis. The main source of error is the determination of the drop volume. Results in transparent systems are comparable to results using other techniques. Wetting angles are determined for water in two different crude oil systems, using quartz as the substrate. The quartz surfaces are water wet over large pH ranges, but it is possible to accurately identify pH intervals where the surfaces are intermediate or oil wet.     

Dewetting behavior of a block copolymer/homopolymer thin film on an immiscible homopolymer substrate

Numerous previous studies have established that the addition of a microphase-ordered AB diblock copolymer to a thin homopolymer A (hA) film can slow, if not altogether prevent, film rupture and subsequent film dewetting on a hard substrate such as silica. However, only a few reports have examined comparable phenomena when the hA/AB blend resides on a soft B-selective surface, such as homopolymer B (hB). In this work, the dewetting kinetics of thin films composed of polystyrene (PS) and a symmetric poly(styrene-b-methyl methacrylate) (SM) diblock copolymer on a poly(methyl methacrylate) substrate is investigated by hot-stage light microscopy. Without the SM copolymer, the dewetting rate of the PS layer is constant under isothermal conditions and exhibits Arrhenius behavior with an apparent activation energy of approximately 180 kJ/mol. Addition of the copolymer promotes a crossover from early- to late-stage dewetting kinetics, as evidenced by measurably different dewetting rates. Transmission electron microscopy reveals the morphological characteristics of dewetted PS/SM films as functions of film thickness and SM concentration.     

Thermal behavior and film formation from an organogermanium polymer/nanoparticle precursor

In situ high-resolution transmission electron microscopy (HRTEM) was used to investigate the effect of heating on an organo-Ge polymer/nanoparticle composite material containing 4-8 nm diameter alkyl-terminated Ge nanoparticles. The product was obtained from the reduction of GeCl4 with Na(naphthalide) with subsequent capping of the -Cl surface with n-butyl Grignard reagent. The in situ HRTEM micrographs show that the product undergoes significant changes upon heating from room temperature to 600 degrees C. Two pronounced effects were observed: (i) Ge nanoparticles coalesce and remain crystalline throughout the entire temperature range, and (ii) the organo-Ge polymer acts as a source for the in situ formation of additional Ge nanoparticles. The in situ-formed Ge nanoparticles are approximately 2-3 nm in diameter. These in situ-formed nanoparticles (2-3 nm) are so dense that, together with the original ones, they build up an almost continuous crystalline film in the temperatures between 300 and 500 degrees C. Above 480 degrees C, melting of the in situ formed Ge nanoparticles (2-3 nm) is observed, while nanoparticles greater than 5 nm remain crystalline. After cooling to room temperature, the 2-3 nm Ge nanoparticles recrystallized.     

Nanosize metal clusters in polymer systems

Three synthetic methods have been developed for embedding nanosize metal clusters into polymers: (i) in situ synthesis of silver nanoparticles in cross-linked poly-acrylamide gels, (ii) implantation of cryochemically produced silver nanoparticles into poly-acrylamide gels, and (iii) encapsulation of metal nanoparticles in poly-p-xylylene films. All methods allow one to produce 3–20 nm stable metal clusters embedded into bulk materials, thin films or fine particles dispersed in organic solvents or water. Results on some physical properties of the metal-polymer systems thus obtained are presented.     

Direct measurement of the viscous force between two spherical particles trapped in a thin wetting film

Here we present the first direct measurement of the viscous drag force between two spherical particles of millimeter size trapped in a thin wetting film. Each particle is constrained by the liquid/air interface and the solid substrate. The viscous force is counterbalanced by another known force, the attractive capillary immersion force between identical particles protruding from the film surface. The results of the measurements provide evidence for an increased hydrodynamic force due to a non-Stokesian resistance to the particle motion. Our findings can be applied to the self-assembly of colloidal particles in a two-dimensional array for coating and to the friction between small species and a solid. Received: 19 March 1999 Accepted in revised form: 11 May 1999     

Interfacial tension and wetting behavior of air/oil/ionic liquid systems

We measured the interfacial tension and the density of air/n-hexane, n-decane, 1-perfluorohexane/1-hexyl-3-methyl-imidazolium hexafluorophosphate systems as a function of temperature. From the air/ionic liquid surface tension values, it was suggested that Coulombic interaction between imidazolium cations and counter anions are not so much different between the surface and bulk. The density values indicated that the decrease of surface tension by saturating organics was closely correlated to the mutual solubility between ionic liquid and organics. Interfacial tension at the oil/ionic liquid interfaces suggested that ionic liquid molecules were more ordered at the oil/ionic liquid interfaces compared to the air/ionic liquid interfaces, but the decrease of the entropy due to the interfacial orientation of ionic liquid was compensated by the increase of the entropy due to the contact of different chemical species. The initial spreading coefficients and the Hamaker constants indicated that all the oil phases spread at the air/ionic liquid interfaces spontaneously, and form the complete wetting films.