Dynamics of a climbing surfactant-laden film--I: base-state flow

The dynamics of a surfactant-laden film climbing up an inclined plane is investigated through a two-dimensional (2-D), nonlinear evolution equation for the interface coupled to convective-diffusion equations for the surfactant, derived using lubrication theory. One-dimensional (1-D) solutions, representing the base-state flow, are investigated for constant flux and constant volume configurations; these flows are parameterised by capillarity, gravity, convection-diffusion ratios (represented by Péclét numbers at the surface and bulk), a solubility parameter, sorption kinetics constants, the number of surfactant monomers in a micelle, and the nonlinearity of the surfactant equation of state. In both configurations studied, a front develops spreading up the substrate against the direction of gravity whereby the leading edge of the front follows a power-law as a function of time. The effect of system parameters on the base-state flow is explored through an extensive parametric study, while the stability of the above-mentioned system to spanwise perturbations is the focus of Part II.     

Linear stability of a surfactant-laden annular film in a time-periodic pressure-driven flow through a capillary

This paper analyzes the effect of surfactant on the linear stability of an annular film in a capillary undergoing a time-periodic pressure gradient force. The annular film is thin compared to the radius of the tube. An asymptotic analysis yields a coupled set of equations with time-periodic coefficients for the perturbed fluid-fluid interface and the interfacial surfactant concentration. Wei and Rumschitzki (submitted for publication) previously showed that the interaction between a surfactant and a steady base flow could induce a more severe instability than a stationary base state. The present work demonstrates that time-periodic base flows can modify the features of the steady-flow-based instability, depending on surface tension, surfactant activity, and oscillatory frequency. For an oscillatory base flow (with zero mean), the growth rate decreases monotonically as the frequency increases. In the low-frequency limit, the growth rate approaches a maximum corresponding to the growth rate of a steady base flow having the same amplitude. In the high-frequency limit, the growth rate reaches a minimum corresponding to the growth rate in the limit of a stationary base state. The underlying mechanisms are explained in detail, and extension to other time-periodic forms is further exploited.     

Pressure-driven motion of surfactant-laden drops through cylindrical capillaries: effect of surfactant solubility

The effect of bulk-soluble surfactants on the dynamics of a drop translating through a cylindrical tube under low-Reynolds-number conditions is investigated. Interfacial surfactant adsorption/desorption is modeled according to the Frumkin adsorption framework, and the bulk-insoluble surfactant limit is recovered as the rate of surfactant sorption becomes large compared to that of bulk diffusion. As the equilibrium surface coverage is increased, the mechanism by which drop mobility is reduced changes from uniform retardation at low surface coverage to the formation of a stagnant cap at high surface coverage. For large capillary numbers, the drop does not achieve a steady shape, and eventually it breaks up either through the formation of a penetrating viscous jet of suspending fluid, or by continuous elongation and pinch-off. Surfactants have a destabilizing effect on transient drop shapes by accelerating the formation and development of the penetrating viscous jet that leads to drop breakup. The critical conditions for drop breakup, as well as the mode of breakup, depend on the manner in which the strength of the flow (i.e., the capillary number) is increased.     

Dynamics of terrace formation in a nanostructured thin block copolymer film

We have used dynamic self-consistent field (DSCF) theory to investigate the structural evolution of an ABA block copolymer thin film placed between a solid substrate and a free surface. In line with the few existing theoretical studies for pure homopolymers and mixtures, the free interface is introduced by a void component. In our calculations, the free surface experiences surface roughening and eventually the formation of terraces, as in the experiments. The kinetic pathway of the microstructures was compared to findings of an existing detailed experimental study (Knoll, A.; Lyakhova, K. S.; Horvat, A.; Krausch, G.; Sevink, G. J. A.; Zvelindovsky, A. V.; Magerle, R. Nat. Mater. 2004, 3, 886) and was found to be equivalent in detail. This corroborates our assumption in this earlier work that the pathway due to changing film thickness is similar to a pathway due to changing surface energetics. Moreover, our calculations show for the first time that microstructural transitions are a driving force of polymer/air interface curving and the formation of terraces.     

Nonlinear stability analysis of a two-layer thin liquid film: dewetting and autophobic behavior

The nonlinear stability analysis of a liquid film composed of two superposed thin layers of immiscible liquids resting on a solid substrate is performed. It is shown that the coupling of van der Waals interactions in the two layers can lead to an autophobic behavior in the form of spinodal decomposition of two planar liquid layers into a system of localized drops divided by almost planar wetting layers. The results of the weakly nonlinear analysis near the instability threshold are confirmed by the numerical solution of a system of two strongly nonlinear evolution equations for the liquid-liquid and liquid-gas interfaces. The kinetics of the drop coarsening at late stages is studied and is found to be close to that reported for a one-layer film. It is also shown that gravity effects can become significant even for very thin two-layer films.     

Linear stability of a draining film squeezed between two approaching droplets

In the present paper we analyze the effect of infinitesimal non-axisymmetric perturbations in determining the critical gap thickness at which a draining, finite radius thin-film becomes unstable. The film is part of the suspending fluid trapped between two approaching deformable drops under the action of a flow field. We carry out a linear stability analysis in the context of a quasi-static approximation where the rate of growth of the disturbances is assumed to be much faster than the rate of film drainage. An analytical solution is derived for the model in the special case of a uniformly thick film, for two types of perturbation: fixed-end and free-end. It is shown, for this special case, when the hydrodynamic force pushing the drops together from the external flow is constant, that the four most unstable disturbances are of the free-end kind, associated with the lowest frequency modes of azimuthal variation in the film thickness. Higher modes are stabilized by surface tension. Our analysis also shows that adopting the unretarded form of the van der Waals disjoining pressure yields results similar to the analysis when electromagnetic retardation effects are included in the calculation. A second case is analyzed where the film is also of uniform thickness but its lateral extent and the gap thickness are both time-dependent. This case was included to extend the predictions to glancing drop-collisions where the external hydrodynamic force is time-dependent. We find that there is a maximum capillary number below which the film becomes unstable, and that there is range of angles in the trajectory where the film becomes unstable, but that outside this range the film is stable.     

Multiple emulsion stability: pressure balance and interfacial film strength

The objective of this study was to investigate the significance of inner and outer phase pressure, as well as interfacial film strength on W/O/W multiple emulsion stability using microscopy and long-term stability tests. It was observed that immediately upon applying a coverslip to samples the multiple droplets deformed and there was coalescence of the inner aqueous droplets. Under certain conditions (such as lipophilic surfactant concentration and internal phase osmotic pressure) the destabilized multiple emulsions formed unique metastable structures that had a "dimpled" appearance. The formation of these metastable structures correlated with the real-time instability of the W/O/W multiple emulsions investigated. Multiple emulsion stability also correlated with the interfacial film strength (measured by interfacial elasticity) of the hydrophobic surfactant at the mineral oil/external continuous aqueous phase interface. The formation of the metastable dimpled structures and the long-term stability of the multiple emulsions were dependent on the osmotic pressure of the inner droplets and the Laplace curvature pressure as described by the Walstra Equation (P. Walstra, "Encyclopedia of Emulsion Technology" (P. Becher, Ed.), Vol. 4. Dekker, New York, 1996). It appears that the effect of coverslip pressure on multiple emulsions may be useful as an accelerated stability testing method or for initial formulation screening.     

Wetting film stability and flotation kinetics

Single bubble experiments performed with different size fractions of quartz particles and different, but known, contact angles revealed two modes of flotation dynamics in superclean water. (1.) A monotonic increase of collection efficiency Ecoll with increasing particle size was observed at high particle hydrophobicity and, correspondingly, a low wetting film stability (WFS). (2.) At low particle hydrophobicity and, correspondingly, high WFS, an extreme dependence of Ecoll on particle size was observed. The use of superclean water in our experiments prevented the retardation of bubble surface movement caused by surfactants or other impurities that is usual for other investigations and where particle-bubble inertial hydrodynamic interactions are suppressed. In the present study the free movement of the bubble surface enhances particle-bubble inertial interaction, creating conditions for different flotation modes, dependent on WFS. At the instant of inertial impact, a particle deforms the bubble surface, which may cause its rebound. Where the stability of the thin water film, formed between opposing surfaces of a bubble and a particle, is low, its rupture is accompanied with three phase contact line extension and contact angle formation before rebound. This prevents rebound, i.e. the first collision is accompanied by attachment. A high WFS prevents rupture during an impact. As a result, a contact angle does not arise and rebound is not prevented. However, rebound is accompanied by a second collision, the kinetic energy of which is smaller and can cause attachment at repetitive collision. These qualitative considerations are confirmed by the model quantification and comparison with measured Ecoll. For the first time the Sutherland equation (SE) for Ecoll is confirmed by experiment for smaller particle sizes and, correspondingly, very small Stokes numbers. The larger the particle size, the larger is the measured deviation from the SE. The SE is generalized, accounting for the centrifugal force, pressing hydrodynamic force and drainage in the low WFS case and, correspondingly, attachment occurs at first collision or during sliding. The derived generalized Sutherland equation (GSE) describes experimental data at low WFS. However, its application without account for possible rebound does not explain the measured extreme dependence in the case of high WFS. The theory for drainage during particle impact and the beginning of rebound enables conditions for either attachment or rebound in terms of the normal component of the impact velocity and the critical film thickness to be derived. Combining this condition with the GSE allowed the equation for Ecoll to be derived, accounting for attachment area shrinkage and attachment during a repetitive collision. This equation predicts the extreme dependence. Thus the WFS determines the modes of flotation dynamics and, in turn, probably affects the mechanisms, which control the flotation domain. At low WFS its upper boundary is controlled by the stability of the particle-bubble aggregate. At high WFS the upper boundary can be controlled by rebound because the latter reduces the attachment efficiency by a factor of 30 or more even with repetitive collision.     

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).     

Oxygen reduction on polycobaltprotoporphyrin film: I. Catalytic activity and stability of polycobaltprotoporphyrin film toward oxygen reduction

The catalytic activity of polycobaltprotoporphyrin (PCoPP) was compared with adsorbed cobaltprotoporphyrin monolayer. The results have shown that PCoPP film shows higher catalytic activity and stability than monolayer on glass carbon electrode in both alkaline and acid solution. Catalytic activity of PCoPP goes through a maximum with increase of film thickness. A model was proposed to explain such dependence. The effect of film thickness and solution pH on the stability of PCoPP film was studied.     

Fundamentals of Trapped Ion Mobility Spectrometry Part II: Fluid Dynamics

Trapped ion mobility spectrometry (TIMS) is a new high resolution (R up to ~300) separation technique that utilizes an electric field to hold ions stationary against a moving gas. Recently, an analytical model for TIMS was derived and, in part, experimentally verified. A central, but not yet fully explored, component of the model involves the fluid dynamics at work. The present study characterizes the fluid dynamics in TIMS using simulations and ion mobility experiments. Results indicate that subsonic laminar flow develops in the analyzer, with pressure-dependent gas velocities between ~120 and 170 m/s measured at the position of ion elution. One of the key philosophical questions addressed is: how can mobility be measured in a dynamic system wherein the gas is expanding and its velocity is changing? We noted previously that the analytically useful work is primarily done on ions as they traverse the electric field gradient plateau in the analyzer. In the present work, we show that the position-dependent change in gas velocity on the plateau is balanced by a change in pressure and temperature, ultimately resulting in near position-independent drag force. That the drag force, and related variables, are nearly constant allows for the use of relatively simple equations to describe TIMS behavior. Nonetheless, we derive a more comprehensive model, which accounts for the spatial dependence of the flow variables. Experimental resolving power trends were found to be in close agreement with the theoretical dependence of the drag force, thus validating another principal component of TIMS theory.

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Dynamics and stability of dispersions of polyelectrolyte-filled multilayer microcapsules

The authors report dynamic and coagulation properties of a dispersion of polyelectrolyte multilayer microcapsules filled with solutions of a strong polyelectrolyte. Microcapsules are shown to take a charge of the sign of encapsulated polyions and are characterized by a nonuniform distribution of inner polyions, which indicates a semipermeability of the shell and a leakage of counterions. The capsule self-diffusion coefficient in the vicinity of the similarly charged wall is measured using a particle tracking procedure from confocal images of the dispersion. The diffusion of capsules in the force field suggests that the effective interaction potential contains an electrostatic barrier, so that we deal with the same types of interaction forces as for solid particles. The theoretical estimates of the authors show that when microcapsules are in close proximity, their interaction should even be quantitatively the same as that of colloids with the same surface potential. However, due to the mobility of inner polyions they might repel stronger at large distances. The authors thus conclude that the encapsulation of charged polymers is an important factor in determining the adhesion and interaction properties of multilayer microcapsules.     

Effect of surfactants on the stability of thin liquid film flow on a rotating disk

The effect of surfactants on surface instabilities of thin liquid film flow on a rotating disk was studied at different flow rates Q (0.5相似文献   

Spacer layer screening effect: a novel fluorescent film sensor for organic copper(II) salts

A fluorescent film sensor was prepared by chemical assembly of pyrene on a glass plate surface via a long flexible spacer. It was found that the film is highly selective for some organic Cu2+ salts, such as copper acetate and copper propionate. The presence of inorganic Cu2+ salts and other metal(II) acetates, including Ni2+, Co2+, Pb2+, Cd2+, Zn2+, etc., had little effect upon the sensing behavior of the film for copper acetate or copper propionate. The observation was explained by employing a proposed "two-dimensional solution" model. The quenching by copper acetate of the emission of the film is static in nature due to complexation of the spacers to the metal ions. Furthermore, the response of the film sensor to copper acetate is fully reversible. To the best of our knowledge, this film sensor may be the first one that can differentiate greasy copper salts from inorganic copper salts.     

Electropoly merization: direct film formation on metal substrates II locus of polymerization

Films of polyacrylonitrile and polymethacrylonitrile can be produced in situ on metal electrodes by the electrolysis of a conducting solution of the monomer. Electroanalytical techniques have been used to investigate the locus of the polymerization and also to attempt to determine the rate constants for propagation and termination. Differential capacitance measurements indicate that there may be some adsorption of these monomers at potentials where they are electroactive when NaN03 is used as electrolyte but no adsorption of the polymer produced was detected. When the electrolyte contains quaternary alkyl ammonium ions, they are preferentially adsorbed. Sweep voltammetry indicates that direct reduction of both monomers occurs by a two electron transfer step which precedes a fast follow-up reaction which may be either addition of a second monomer molecule or protonation. Because of practical difficulties imposed by the system, the use of rotating disc electrode was limited but, when used in conjunction with a digital simulation technique, it allowed calculation of kp and k, for the anionic polymerization based on the electrolysis of acrylonitrile containing tetraethylammonium p-toluene sulphonate.     

Column Dynamics of Ternary Ion Exchange Part II: Solution Mass Transfer Controlling

The nonequilibrium theory for column dynamics of multicomponent ion exchange has been evaluated using the ternary system Ag-Na-H at a total solution concentration of 0.05 N (0.05 M) so that solution mass transfer controls the exchange.Ternary mass transfer coefficients have been related to binary values by both simple (constant) and improved (concentration-dependent) Fick's law models, as well as by simple and improved Nernst-Planck models. Binary coefficients of all four types were obtained directly from constant-pattern binary breakthroughs and the individual-component Nernst-Planck coefficients were derived from the binary coefficients.Using each of the four models, ternary effluent concentration histories (ECH) were predicted using the method of characteristics. All the models predicted effluent concentration histories that matched closely the experimental ones, indicating that either model may be used satisfactorily for prediction; the simple models are preferred to the improved ones since they contain fewer parameters.     

CFD evaluation of drop retraction methods for the measurement of interfacial tension of surfactant-laden drops

Drop retraction methods are popular means of measuring the interfacial tension between immiscible polymers. Experiments show that two different drop retraction methods, imbedded fiber retraction (IFR) and deformed drop retraction (DDR), give inconsistent results when a surfactant is present on the surface of the drop. These inconsistencies are deemed to be due to dilution of the surfactant and due to gradients in interfacial concentration of surfactant along the drop surface. This physical picture is quantified for the simple case of a Newtonian drop in a Newtonian matrix, with an insoluble, nondiffusive surfactant at the interface. The drop is deformed in computational fluid dynamics simulations by shearing the matrix, and then allowed to retract. Dilution and interfacial tension gradients effects are found to be especially large at the early stages of retraction, making IFR unsuitable for measuring the interfacial tension of surfactant-laden interfaces. The effects of surfactant dilution and gradients are found to persist even at late stages of retraction, causing the DDR method to underestimate the equilibrium interfacial tension significantly. The largest underestimates occur when the drop viscosity is lower than the matrix viscosity.     

Electrodeposition of silver nanoparticles on a zinc oxide film: improvement of amperometric sensing sensitivity and stability for hydrogen peroxide determination

A strategy to fabricate a hydrogen peroxide (HP) sensor is developed by electrodepositing silver nanoparticles (Ag NPs) on a modified glassy carbon electrode (GCE) with a zinc oxide (ZnO) film. The Ag NPs/ZnO/GCE has been characterized by scanning electron microscopy, cyclic voltammetry, and chronoamperometry. It has been found that the Ag NPs synthesized in the presence of ZnO film provide an electrode with enhanced sensitivity and excellent stability. The sensitivity to HP is enhanced 3-fold by using Ag NPs/ZnO/GCE compared to Ag NPs/GCE. The HP sensor exhibits good linear behavior in the concentration range 2 µM to 5.5 mM for the quantitative analysis of HP with a detection limit of 0.42 µM (S/N?=?3).