Determination of Surface Dilational Viscosity Using the Oscillating Bubble Method

A new version of the oscillating bubble method enables us to determine the dilational modulus of fluid surfaces in the frequency range 1 Hz相似文献   

A study of kinetic molecular exchange processes in the medium frequency range by surface SHG on an oscillating bubble

The dilatational properties of fluid surfaces and interfaces have been comprehensively investigated in recent years. For example, an improved oscillating bubble device provided experimental results that allow for critical testing of established surface models, such as the Lucassen/van den Tempel (LvdT) model. The comparison of the LvdT model with the oscillating bubble experiments demonstrates a mismatch between the model parameters. For example, near the CMC or the limit of solubility the calculated parameters of surfactant solutions become unrealistically large. The deviation can be explained by the introduction of more detailed surface models, in particular by the modification of the effective thickness of the surface layer, its internal structure and the molecular exchange processes between these structures. For the verification of such processes an experimental setup was realized which allows for an independent determination of the instantaneous adsorption state at the surface of an oscillating bubble inside a surfactant solution. The setup utilizes the Second Harmonic Generation (SHG)--effect at the air-solution interface generated by the light of a pulsed LASER. The set-up is described in detail, and the results of a first experimental series are presented and discussed in this paper. As system, aqueous solutions of the fluortenside F381 were used.     

Transient aggregation during dry-down of solvent-borne dispersions

The present work investigates the role of changing solvency conditions on the stability of sterically stabilized colloids in evaporating solvent-borne dispersions and the recovery time for the redispersion of aggregates if destabilization occurs. Process conditions during the conversion of the coated fluid dispersion to a solid film must be carefully controlled to ensure that aggregation, leading to uneven pigment distribution, does not occur. Although a polymeric binder serves as a retention aid in a solvent-borne coating, it also act as a steric stabilizer during dry-down or curing of the coating. Because the dispersion medium in the coatings often is a mixture of solvents and nonsolvents, the binder's interactions with the dispersion medium govern whether aggregation is likely to occur. Tracking such an occurrence may be difficult due to rapidly changing solvent compositions. Light scattering is utilized herein to indicate if the conditions of a high speed coating and drying process (in this case, the production of magnetic data storage media) may lead to aggregation and, if aggregation is detected, the ability of aggregates to fully redisperse. Conformational changes of free polymeric binder chains in solution are followed to gauge the polymer-solvent interactions and, therefore, the binder's ability to prevent aggregation. A "danger zone" (conditions where aggregation is probable) is constructed from these measurements, and further dynamic light scattering measurements on binder-grafted-oxide particles detect aggregation in this zone. The aggregates are redispersed in a series of "good" solvents, and redispersion is found not to be instantaneous, suggesting that high-speed drying processes may not allow the flocculated colloids enough recovery time.     

Description of the kinetics of a model tribopolymerization process

A three‐dimensional system of ordinary differential equations of first order has been proposed to describe the kinetics of a model tribopolymerization process for which a linear coupling of the material surface dislocation density and polyreaction fields is assumed. Another important physical motivation on which the model is based stresses the predominant role of a highly rough surface under friction as a catalyst in the polyreaction process. In consequence, the fractal‐like chemical reaction concept has been applied. It is demonstrated that the system proposed is suitable for describing a variety of tribopolymerization processes and possesses non‐trivial time‐dependent behaviour even in some limiting analytical cases studied. As a specific result, a nonlinear equation generalizing the Vogel–Fulcher temperature‐dependence of the maximum time scale for the polyreaction caused by friction, characteristic of the selective transfer during friction process, has been recovered. Some comparison with an approach made earlier (G.P. Shpenkov, Friction Surface Phenomena (Elsevier, Amsterdam, 1995) pp. 165–172 and Ref. [239] therein) has been presented as well. Certain basic trends of modelling and experiment are in good agreement.     

Oil coating of hydrophobic surfaces from aqueous media: formation and kinetic study

We perform oil coating of hydrophobic solid surfaces via aqueous media, from emulsions, and under the presence of a shear flow. The principle of such coating is based on the use of a system at the limit of aggregation to give rise to adhesion, with asymmetrical interfaces (oil droplet/water and solid surface/water) in order to favor the oil/surface adhesion in comparison to the oil/oil adhesion. This way, droplets stick to the solid substrate, whereas they are stable and homogeneously dispersed in the bulk. We have realized coatings from two systems of emulsions made of a mixture of hydroxy-terminated silicone oil and classical silicone oil and a mixture of sunflower oil and mineral oil. The kinetics of the coating is described by a Langmuir model where the adhesion between the oil particle and the surface is modeled as a first-order reaction. The resulting coatings are formed of oil droplets uniformly covering the solid surface. The coating density can vary with the nature of the experimental systems.     

Estimation of the dynamic size of fractal aggregates

A model is presented for an aggregation act occurring between two aggregates of any mass and fractal dimension. The kinetics of aggregation is also analyzed, as well as some previous works concerning the structure of fractal aggregates. As a result, a generalized curve is derived describing the normalized dynamic radius of clusters of spherical character as dependent on both the aggregate fractal dimension and the space dimension. It is shown how the curve may be utilized to determine the dynamic size of anisotropic aggregates. The obtained dependence can be used to estimate the dynamic size of fractal aggregates, to evaluate the prefactor in mass–radius relation and to model the aggregation kinetics.     

A generalized model for the stability of polymer colloids

A generalized model has been proposed to describe the stability of polymer colloids stabilized with ionic surfactants by accounting simultaneously for the interactions among three important physicochemical processes: colloidal interactions, surfactant adsorption equilibrium, and association equilibria of surface charge groups with counterions at the particle-liquid interface. A few Fuchs stability ratio values, determined experimentally for various salt types and concentrations through measurements of the doublet formation kinetics, are used to estimate the model parameters, such as the surfactant adsorption and counterion association parameters. With the estimated model parameters, the generalized model allows one to monitor the dynamics of surfactant partitioning between the particle surface and the disperse medium, to analyze the variation of surface charge density and potential as a function of the electrolyte type and concentration, and to predict the critical coagulant concentration for fast coagulation. Three fluorinated polymer colloids, stabilized by perfluoropolyether-based carboxylate surfactant, have been used to demonstrate the feasibility of the proposed colloidal stability model.     

An empirically validated analytical model of droplet dynamics in electrowetting on dielectric devices

Explicit analytical models that describe the capillary force on confined droplets actuated in electrowetting on dielectric devices and the reduction in that force by contact angle hysteresis as a function of the three-dimensional shape of the droplet interface are presented. These models are used to develop an analytical model for the transient position and velocity of the droplet. An order of magnitude analysis showed that droplet motion could be modeled using the driving capillary force opposed by contact angle hysteresis, wall shear, and contact line friction. Droplet dynamics were found to be a function of gap height, droplet radius, surface tension, fluid density, the initial and deformed contact angles, contact angle hysteresis, and friction coefficients pertaining to viscous wall friction and contact line friction. The first four parameters describe the device geometry and fluid properties; the remaining parameters were determined experimentally. Images of the droplet during motion were used to determine the evolution of the shape, position, and velocity of the droplet with time. Comparisons between the measured and predicted results show that the proposed model provides good accuracy over a range of practical voltages and droplet aspect ratios.     

Polymeric coatings on silver nanoparticles hinder autoaggregation but enhance attachment to uncoated surfaces

The propensity of silver nanoparticles (AgNPs) having two different polymer coatings (poly(vinylpyrrolidone), PVP, or gum arabic, GA) to aggregate, or to deposit to a reference surface (silica), was explored as a basis for differentiating the effect of surface coating on the stability of nanoparticles in aggregation and in deposition. Surface polymeric coatings stabilize nanoparticles against aggregation as shown by either an increased critical coagulation concentration as for PVP-coated AgNPs (AgPVP) or the absence of observable aggregation even at a high ionic strength as for GA-coated AgNPs (AgGA). In experiments of AgNPs deposition in a silica porous medium, dissimilar surfaces favored deposition, such as the case where polymer coatings were present on the AgNPs but were absent on the porous medium. The increased affinity of the AgNPs for the porous medium in this case may be explained by a shifted contact frontier where electrical double layer interaction is weaker. When coating polymers were introduced to the porous medium and allowed to preadsorb to the silica surfaces, the attachment efficiencies for both the AgPVP and AgGA were reduced due to steric and electrosteric stabilization, respectively. The results suggest that polymeric coatings that are usually deemed as stabilizers (as they indeed are in the case of autoaggregation) might not necessarily stabilize nanoparticles against deposition unless the collector surfaces are also coated with polymer.     

Physical Principles of the Photostimulated Aggregation of Metal Sols

Physical mechanisms of the reasons for dramatic (up to 10⁸ times) acceleration of the aggregation of sol metals under the action of light are proposed for the first time. These mechanisms were classified with allowance for the procedure of sol stabilization and type of dispersion medium; their comparative analysis was performed using the main types of silver sols as the most convenient model medium. Mechanisms proposed are based on the current notions of the theory of coagulation kinetics and stability of sols with account of interfacial processes, as well as of the theory of photoeffect corrected for the influence of dispersion medium and the amplification of local electromagnetic fields inherent in colloidal structures with fractal geometry.     

The kinetics of overcoming the “entropy barrier”

The escape of a point particle from a cavity of an arbitrary configuration through a small orifice in a cavity wall is considered as a model of entropy barrier overcoming. The dynamics of the particle is determined by collisions with the wall and the dissipative action of the medium. As in reactions with overcoming energy barriers (the Kramers theory), three characteristic regimes can be identified depending on the intensity of friction: diffusion and intermediate regimes and the region of weak friction. For all of them, analytic dependences of rate constants on problem parameters were obtained. The procedure for sewing solutions together, similar to that employed in the Kramers theory, gives a unified equation for the transmission factor over the whole range of friction values. In the weak friction mode, overcoming the entropy barrier generally leads to a nonexponential kinetics.     

Porous media: A model system for the physics of disorder

We discuss several examples of applications of soft matter and statistical physics to various problems related to porous and fractured media. The structure of porous media often displays multiple scale features which can be analysed by neutron diffraction or electron microscopy and can be approached by fractal models. Such characteristics are also observed on the surface of natural fractures.The relation between various transport parameters such as permeability or conductivity introduce characteristic microscopic length scales which can sometimes be independently determined. In some cases, if the porous medium get clogged or if the number of flow channels or fractures is low enough so that threshold effects appear which can be analysed in terms of percolation models. Disorder physics approaches are particularly useful to analyse non miscible diphasic flows in some cases in which multiscale heterogeneities of the fluid mixture composition appear. This is for instance the case for very slow non wetting invasions and of the fast injection of a low viscosity fluid : these processes can be described respectively by the “invasion percolation” and the “diffusion limited aggregation” models. Finally tracer dispersion provides an application of random walk models to disordered systems : examples of the response of this measurement in partly saturated and double porosity media are presented.     

The kinetics of growth of semiconductor nanocrystals in a hot amphiphile matrix

The first comprehensive study on the kinetics of nanocrystal growth in a hot amphiphile medium is presented. An example is given with CdSe semiconductor nanocrystals grown after the injection of precursor (a mixture of Cd- and Se-reagents) in concentrated tri-octylphosphine oxide matrix (heated to more than 300 degrees C). The particle size distribution is reconstructed as a function of time from the absorption and photoluminescence spectra collected during the synthesis process. For this purpose a new expression is used relating the exciton energy due to quantum confinement with the nanocrystal radius. The growth kinetics is considered as a two-stage process in order to describe the time variation of nanoparticle size. During the first stage, called reaction-limited growth, the size of initial nucleus rapidly increases due to a sort of surface reaction exhausting the precursor in the nanoparticle vicinity. The growth in such conditions favors also a remarkable narrowing of the size distribution. The nanocrystal develops further on account of a slow precursor transfer from a distant space driven by the concentration gradient--classical diffusion-limited growth. The width of size distribution also increases proportional to the average particle size. Any growth will stop after the precursor concentration reaches a minimum value defining the limit for the final nanocrystal size in a batch. Solving the kinetic equations for the growth rate in each case of kinetics derives analytical expressions for the mean radius and variance of size distribution. Then the respective expressions are matched in a uniform solution valid during the entire synthesis. The theoretical model is in a good quantitative agreement with the experimental data for independent syntheses. Important characteristic scales of the processes (time-constant and length) and microscopic parameters of the reacting system (interfacial energy and reaction rate constant) are estimated from the data. It turns out that the fast reaction-limited growth is important to obtain well-defined nanocrystals of high optical quality by using less energy, time and consumable. However, to make them reproducibly uniform one should control also the ultra-fast nucleation process preceding the nanocrystal growth, which is still unknown. Nevertheless, our current findings allow the conceptual design of a new continuos-flow reactor for the manufacturing of a large amount of uniform nanocrystals.     

How Post‐Translational Modifications Influence Amyloid Formation: A Systematic Study of Phosphorylation and Glycosylation in Model Peptides

A reciprocal relationship between phosphorylation and O‐glycosylation has been reported for many cellular processes and human diseases. The accumulated evidence points to the significant role these post‐translational modifications play in aggregation and fibril formation. Simplified peptide model systems provide a means for investigating the molecular changes associated with protein aggregation. In this study, by using an amyloid‐forming model peptide, we show that phosphorylation and glycosylation can affect folding and aggregation kinetics differently. Incorporation of phosphoserines, regardless of their quantity and position, turned out to be most efficient in preventing amyloid formation, whereas O‐glycosylation has a more subtle effect. The introduction of a single β‐galactose does not change the folding behavior of the model peptide, but does alter the aggregation kinetics in a site‐specific manner. The presence of multiple galactose residues has an effect similar to that of phosphorylation.     

Peptide Self-Assembly into Amyloid Fibrils at Hard and Soft Interfaces—From Corona Formation to Membrane Activity

Peptides and proteins are exposed to a variety of interfaces in a physiological environment, such as cell membranes, protein nanoparticles (NPs), or viruses. These interfaces have a significant impact on the interaction, self-assembly, and aggregation mechanisms of biomolecular systems. Peptide self-assembly, particularly amyloid fibril formation, is associated with a wide range of functions; however, there is a link with neurodegenerative diseases, such as Alzheimer's disease. This review highlights how interfaces affect peptide structure and the kinetics of aggregation leading to fibril formation. In nature, many surfaces are nanostructures, such as liposomes, viruses, or synthetic NPs. Once exposed to a biological medium, nanostructures are coated with a corona, which then determines their activity. Both accelerating and inhibiting effects on peptide self-assembly have been observed. When amyloid peptides adsorb to a surface, they typically concentrate locally, which promotes aggregation into insoluble fibrils. Starting from a combined experimental and theoretical approach, models that allow for a better understanding of peptide self-assembly near hard and soft matter interfaces are introduced and reviewed. Research results from recent years are presented and relationships between biological interfaces, such as membranes and viruses, and amyloid fibril formation are proposed.     

Surface diffusion in reversed-phase liquid chromatography

More than 40 years ago, Giddings pointed out in “Dynamics of Chromatography” that surface diffusion should become an important research topic in the kinetics of chromatographic phenomena. However, few studies on surface diffusion in adsorbents used in chromatography were published since then. Most scientists use ordinary rate equations to study mass transfer kinetics in chromatography. They take no account of surface diffusion and overlook the significant contributions of this mass transfer process to chromatographic behavior and to column efficiency at high mobile phase flow rate. Only recently did the significance of surface diffusion in separation processes begin to be recognized in connection with the development of new techniques of fast flow, high efficiency chromatography. In this review, we revisit the reports on experimental data on surface diffusion and introduce a surface-restricted molecular diffusion model, derived as a first approximation for the mechanism of surface diffusion, on the basis of the absolute rate theory. We also explain how this model accounts for many intrinsic characteristics of surface diffusion that cannot properly be explained by the conventional models of surface diffusion.     

基于含氟表面活性剂修饰的金纳米粒子测定卡托普利

在较高离子强度和一定温度下,卡托普利能引起非离子表面活性剂FSN-100修饰的14nm金纳米粒子胶体溶液快速聚集,引起金纳米粒子在519nm处的吸光度降低,而在640nm处的相对吸光度成线性增加.据此,建立了一种光度测定卡托普利的新方法.卡托普利药片制剂中常见赋形剂,如淀粉、糊精、葡萄糖、果糖、麦芽糖、蔗糖、明胶、山梨醇、乳糖等对本实验没有干扰.本方法具有快速、简便、选择性高等优点,线性范围为2.0～12.5μg/mL,卡托普利检出限(S/N=3)为1.25μg/mL.该方法成功用于药片制剂中卡托普利的测定,回收率在99%到103%之间.