Surfactant Concentration Gradient Research Articles

Experimental evaluation of Marangoni stress and surfactant concentration at interface of contaminated single spherical drop using spatiotemporal filter velocimetry

Spatiotemporal filter velocimetry (SFV) was extended to Lagrangian measurements with boundary-fitted measurement areas, and was applied to flows about single spherical drops of glycerol-water solution falling in stagnant silicon oil under clean and contaminated conditions to examine its applicability to the estimation of the Marangoni stress and surfactant concentration at a moving interface. Effects of bulk concentration of surfactant on the velocity field, the Marangoni stress and the surface concentration of surfactant were discussed from the measured data. As a result, we confirmed that accurate velocity distribution in the vicinity of the interface measured by SFV enables us to evaluate interfacial velocity and interfacial shear stresses and to estimate the Marangoni stress, interfacial tension and surfactant concentration at the interface with the assumption of negligible surface viscosity. The flow inside the drop and the interfacial velocity become weak due to the Marangoni stress caused by the gradient of surfactant concentration at the interface as the bulk concentration of surfactant increases. These results demonstrate that SFV is of great use in experimental analysis of adsorption and desorption kinetics at a moving interface.

Two-velocity flow containing surfactant

A hydrodynamic theory of two-velocity fluid with surfactant is constructed in the present paper. The model takes into account both surface effects and impact of the concentration gradient on relative velocity of the components. The dependence of the nonstationary two-phase flow regimes on the surface tension gradient and the dependence of the relative drop velocity on the gradient of the surfactant concentration are investigated numerically on the base of volume control method.

Tracing the Surfactant-Mediated Nucleation, Growth, and Superpacking of Gold Supercrystals Using Time and Spatially Resolved X-ray Scattering.

The nucleation and growth process of gold supercrystals in a surfactant diffusion approach is followed by simultaneous small- and wide-angle X-ray scattering (SAXS/WAXS), supplemented with scanning electron microscopy. The results indicate that supercrystal nucleation can be activated efficiently upon placing a concentrated surfactant solution of a nematic phase on top of a gold nanocrystal solution droplet trapped in the middle of a vertically oriented capillary tube. Supercrystal nuclei comprised of tens of gold nanocubes are observed nearly instantaneously in the broadened liquid-liquid interface zone of a steep gradient of surfactant concentration, revealing a diffusion-kinetics-controlled nucleation process. Once formed, the nuclei can sediment into the naoncrystal zone below, and grow efficiently into cubic or tetragonal supercrystals of ∼1 μm size within ∼100 min. Supercrystals matured during sedimentation in the capillary can accumulate and face-to-face align at the bottom liquid-air interface of the nanocrystal droplet. This is followed by superpacking of the supercrystals into highly oriented hierarchical sheets, with a huge number of gold nanocubes aligned for largely coherent crystallographic orientations.

Dynamics and Stability of Surface Waves with Surfactants

In this paper we consider a layer of incompressible viscous fluid lying above a flat periodic surface in a uniform gravitational field. The upper boundary of the fluid is free and evolves in time. We assume that a mass of surfactants resides on the free surface and evolves in time with the fluid. The surfactants dynamics couple to the fluid dynamics by adjusting the surface tension coefficient on the interface and also through tangential Marangoni stresses caused by gradients in surfactant concentration. We prove that small perturbations of equilibria give rise to global-in-time solutions in an appropriate functional space, and we prove that the solutions return to equilibrium exponentially fast. In particular, this proves the asymptotic stability of equilibria.

Structure of the fluid interface in an external magnetic field in the presence of a magnetizable surfactant

The structure of the flat interface between two conventional fluids in an external magnetic field in the presence of a magnetizable surfactant is investigated with account for the dependence of the free energy of the system on the surfactant concentration gradients and the bearing phase density. The dependence of the surface tension tensor components on the magnetic field strength is determined.

Comparison of Surfactant Distributions in Pressure-Sensitive Adhesive Films Dried from Dispersion under Lab-Scale and Industrial Drying Conditions.

Film-forming latex dispersions are an important class of material systems for a variety of applications, for example, pressure-sensitive adhesives, which are used for the manufacturing of adhesive tapes and labels. The mechanisms occurring during drying have been under intense investigations in a number of literature works. Of special interest is the distribution of surfactants during the film formation. However, most of the studies are performed at experimental conditions very different from those usually encountered in industrial processes. This leaves the impact of the drying conditions and the resulting influence on the film properties unclear. In this work, two different 2-ethylhexyl-acrylate (EHA)-based adhesives with varying characteristics regarding glass transition temperature, surfactants, and particle size distribution were investigated on two different substrates. The drying conditions, defined by film temperature and mass transfer in the gas phase, were varied to emulate typical conditions encountered in the laboratory and industrial processes. Extreme conditions equivalent to air temperatures up to 250 °C in a belt dryer and drying rates of 12 g/(m(2)·s) were realized. The surfactant distributions were measured by means of 3D confocal Raman spectroscopy in the dry film. The surfactant distributions were found to differ significantly with drying conditions at moderate film temperatures. At elevated film temperatures the surfactant distributions are independent of the investigated gas side transport coefficients: the heat and mass transfer coefficient. Coating on substrates with significantly different surface energies has a large impact on surfactant concentration gradients, as the equilibrium between surface and bulk concentration changes. Dispersions with higher colloidal stability showed more homogeneous lateral surfactant distributions. These results indicate that the choice of the drying conditions, colloidal stability, and substrates is crucial to control the surfactant distribution. Results obtained under lab-scale drying conditions cannot be transferred directly to the industrial application. The results were similar for both tested adhesive material systems, despite their different properties. This indicates that other properties, such as the particle size distribution and glass transition temperature, have surprisingly little effect on the development of the surfactant distribution.

Falling film on a flexible wall in the presence of insoluble surfactant

This work investigates the effect of an insoluble surfactant on the gravity-driven flow of a liquid film down a vertical flexible wall. The paper builds upon previous work [Matar et al., Phys Rev E 76(5):056301, 2007; Sisoev et al., Chem Eng Sci 65(2):950–961, 2010] to include the Marangoni effect attributable to the gradient of surfactant concentration on a free surface. Here we employ an integral method to derive a set of asymptotic evolution equations valid for a moderate flow rate, based on a long-wave approximation. A normal-mode approach is used to examine the linear stability of the system. Similar to the work presented by Matar et al., the results show that a flexible wall with weak damping acts to stabilize flow, while wall tension plays an unstable role. The insoluble surfactant, which acts to stabilize film flow, can reduce the effects of wall flexibility (wall damping and tension) on flow linear stability. The nonlinear evolution equations for the system are solved numerically for both a given initial perturbation wave packet and a periodic perturbation at the inlet boundary. The equations are mainly concerned with the evolution of the flow stability and wave interaction processes, during which solitary-like waveforms are observed. When wall damping is weak, it tends to deplete the ripples preceding the solitary-like humps. However, as wall damping increases in strength, the ripples intensify; a similar phenomenon is observed with an increase in wall tension. The surfactant, which reduces the amplitude and traveling speed of the solitary-like waveforms, acts to distinctly weaken the dispersion of the interfacial wave.

Stability Modeling of Water‐Based Surfactant Covered Micro‐bubble Fluids

AbstractCurrently, due to the decrease in easy access to crude oil reservoirs, oil and gas industries have focused on production from heavy oil and depleted reservoirs. In recent years, micro‐bubble fluids with surfactant and polymer layers around the bubbles are investigated as a part of drilling fluids and their positive effect on the formation damage is proven. Stability of the bubbles in the fluid is very important, and the optimum surfactant and polymer types should be chosen at optimum concentrations. In this work, an attempt is made to analyze the stability of bubbles of the drilling fluid, based on the diffusive mass transfer concept. Mass transfer and interfacial mass transfer coefficients become more important when surfactant concentration gradient exists in micro‐bubble layers. Interfacial mass transfer coefficients have an important effect on mass transfer phenomena in Aphron fluids system; so, the precise selection of these coefficients results in the conformity of modeling and experiments. We can say that reducing the mass transfer rate from bubble layers will result in stable bubbles in the fluid and, thus, the efficiency of the fluid during drilling will not decrease. It is shown that the interfacial mass transfer coefficient decreases with an increase in surfactant concentration.

Surfactant transport onto a foam lamella

A modelling study of surfactant evolution on a film surface coupled to lamellar film drainage has been carried out. This study simulates the surfactant transport onto a lamella in a foam fractionation column with reflux: such columns offer greater control over enrichment and recovery of the surface active material. Insoluble surfactant is assumed during the simulation as such surfactants potentially derive more benefit from a reflux system. There are two terms involved in the equation for surfactant flux which are the Marangoni flow – from the Plateau border to the centre of the lamella – and the film drainage – which is directed from the centre of the lamella to the Plateau border. The extent of film drainage is bounded using two extremal assumptions of mobile or rigid interfaces. On a mobile interface, the film drainage dominates the Marangoni effect, while on a rigid interface, the Marangoni effect is dominant. The numerical simulation was carried out using a material point method followed by a bookkeeping operation to regrid the film. Analytical solutions for the case of no drainage and for a (quasi) steady state in which Marangoni flows and drainage are balanced were used to verify the numerical simulation. From the simulations, it can be concluded that the film drainage obtained using surfactant with a mobile interface is much faster than that determined using surfactant with a rigid interface, meaning that surfactant tends to be washed out of the film in the mobile case. The desirable condition in a foam fractionation column is however where the Marangoni flow dominates the liquid drainage which can be achieved when using surfactant that gives a rigid interface. The (quasi) steady state solution verifies the simulation result at later time for the case of film with a rigid interface. An asymptotic solution in the case of film with a mobile interface gives a prediction of the surface concentration of surfactant at the centre of the film, but also predicts growing surfactant concentration gradients and growing Marangoni stresses near the film's edge, which may then invalidate model assumptions.

Dynamics of surfactant-enhanced oil mobilization and solubilization in porous media: Experiments and numerical modeling

A fast and accurate technique based on multipoint electrical measurements is used to monitor the hydrocarbon mobilization and solubilization in a porous medium during the rate-controlled injection of a surfactant solution. The residual n-decane (n-C10) remaining in a soil column after one cycle of n-decane/NaCl solution drainage-imbibition is flushed by NaCl and sodium dodecyl sulfate (SDS) solution. The transient response of the water saturation averaged over five successive segments of the soil column is determined by monitoring the electrical resistance between successive ring electrodes, whereas the dissolved n-C10 concentration in effluent is measured with solid phase micro-extraction (SPME) and Gas Chromatography–Flame Ionization Detector (GC–FID). The oil ganglia mobilization and solubilization processes are modelled by using 1-dimensional mass balances coupled with a 2-parameter dispersion-like equation describing the rate of oil ganglia mobilization driven by the surfactant concentration gradient. The inverse modeling of experimental datasets allows the estimation of the parameters quantifying the rates of oil mobilization and solubilization. Visualization experiments of solitary n-C10 ganglia solubilization in flowing SDS/NaCl aqueous solution are employed to estimate approximately the local (pore-scale) mass-transfer coefficient of solubilization and assess the corresponding overall coefficient estimated from soil column test.

Controlling thread formation during tipstreaming through an active feedback control loop

Microscale tipstreaming is a hydrodynamic phenomenon capable of producing submicron sized droplets within a microfluidic device. The tipstreaming process results in the drawing of a thin thread from a highly curved interface and occurs as a result of interfacial surfactant concentration gradients that develop due to elongational flows generated within flow focusing geometries. However, in conventional microfluidic devices, the thread formation is periodically interrupted by the formation of larger primary droplets. This study presents an active feedback control loop capable of eliminating the production of primary droplets and producing a continuous thread, and therefore a continuous droplet stream. A proportional controller is used to successfully control the position of the interface and generate a continuous thread. A derivative component is incorporated in an attempt to increase controller stability, but this component is found to be ineffective. Analysis of the tip position as a function of time is performed to determine the optimal proportional gain constant and set point value for the proportional controller that minimize fluctuations in the produced droplet sizes. The generation of a continuous thread facilitates the use of tipstreaming in several applications, including nanoparticle synthesis, chemical detection, and enzyme activity studies.

Shape oscillations of an oil drop rising in water: effect of surface contamination

AbstractInertial shape oscillations of heptane drops rising in water are investigated experimentally. Diameters from 0.59 to 3.52 mm are considered, corresponding to a regime where the rising motion should not affect shape oscillations for pure immiscible fluids. The interface, however, turns out to be contaminated. The drag coefficient is considerably increased compared to that of a clean drop due to the well-known Marangoni effect resulting from a gradient of surfactant concentration generated by the fluid motion along the interface. Thanks to the decomposition of the shape into spherical harmonics, the eigenfrequencies and the damping rates of oscillation modes$n= 2$, 3, 4 and 5 have been measured. Frequencies are not affected by contamination, while damping rates are increased by a considerable amount that depends neither on drop instantaneous velocity nor on diameter. This augmentation, however, depends on the mode number: it is maximum for mode two (multiplied by 2.4) and then relaxes towards the value of a clean drop as$n$increases. A previous similar investigation of a drop attached to a capillary has not revealed such an increase of the damping rates, indicating that the coupling between rising motion and surface contamination is responsible for this effect.

One-pass separation of single-wall carbon nanotubes by gel chromatography with a gradient of surfactant concentration

We have investigated the diameter-selective separation of carbon nanotubes by one-pass gel chromatography with a gradient of surfactant concentration. The formation of surfactant gradient in a column was successfully measured and is explained by a simple diffusion process even in the gel. We found that the diameter of eluted nanotubes is inversely proportional to the surfactant concentration of eluate. The detailed analysis of the movement of the nanotubes in the gel revealed that the separation mechanism was qualitatively explained by a model based on the trapping and de-trapping events of the nanotube–surfactant micelle on the gel surface, where the probability of the trapping and de-trapping events is proportional to the product of the diameter of the nanotubes and the surfactant concentration.

Simulation of spreading surfactant on a thin liquid film

The spreading of insoluble surfactant on a thin liquid film is modeled by a pair of nonlinear partial differential equations for the height of the free surface and the surfactant concentration. A numerical method is developed in which the leading edge of the surfactant is tracked. In the absence of higher order regularization the system becomes hyperbolic/degenerate-parabolic, introducing jumps in the height of the free surface and the surfactant concentration gradient. We compare numerical simulations to those of a hybrid Godunov method in which the height equation is treated as a scalar conservation law and a parabolic solver is used for the surfactant equation. We show how the tracking method applies to the full equations with realistic gravity and capillarity terms included, even though the disturbance in the height of the free surface extends beyond the support of the surfactant concentration.

Lateral Transport of Solutes in Microfluidic Channels Using Electrochemically Generated Gradients in Redox-Active Surfactants

We report principles for a continuous flow process that can separate solutes based on a driving force for selective transport that is generated by a lateral concentration gradient of a redox-active surfactant across a microfluidic channel. Microfluidic channels fabricated with gold electrodes lining each vertical wall were used to electrochemically generate concentration gradients of the redox-active surfactant 11-ferrocenylundecyl-trimethylammonium bromide (FTMA) in a direction perpendicular to the flow. The interactions of three solutes (a hydrophobic dye, 1-phenylazo-2-naphthylamine (yellow AB), an amphiphilic molecule, 2-(4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-s-indacene-3-pentanoyl)-1-hexadecanoyl-sn-glycero-3-phosphocholine (BODIPY C(5)-HPC), and an organic salt, 1-methylpyridinium-3-sulfonate (MPS)) with the lateral gradients in surfactant/micelle concentration were shown to drive the formation of solute-specific concentration gradients. Two distinct physical mechanisms were identified to lead to the solute concentration gradients: solubilization of solutes by micelles and differential adsorption of the solutes onto the walls of the microchannels in the presence of the surfactant concentration gradient. These two mechanisms were used to demonstrate delipidation of a mixture of BODIPY C(5)-HPC (lipid) and MPS and purification of BODIPY C(5)-HPC from a mixture of BODIPY C(5)-HPC and yellow AB. Overall, the results of this study demonstrate that lateral concentration gradients of redox-active surfactants formed within microfluidic channels can be used to transport solutes across the microfluidic channels in a solute-dependent manner. The approach employs electrical potentials (<1 V) that are sufficiently small to avoid electrolysis of water, can be performed in solutions having high ionic strength (>0.1M), and offers the basis of continuous processes for the purification or separation of solutes in microscale systems.

Convective and diffusive surfactant transfer in multiphase liquid systems

Laser interferometry was used to investigate diffusive and convective mass transfer in a multicomponent fluid mixture with a liquid–liquid or liquid–gas interface. For this purpose, an immobile gas bubble or insoluble fluid droplet, having the shape of a short cylinder with a free lateral surface, was inserted into a thin liquid layer. In the case of non-uniform distribution of the dissolved surfactant component, the Marangoni convection near the drop/bubble was initiated by the surface tension inhomogeneities, depending on the surfactant concentration. The applied experimental techniques allowed us to study the structure and evolution of the convective flows and concentration fields in a liquid layer, which due to its small thickness were nearly two-dimensional. Making use of both the vertical and horizontal orientation of the liquid layer, we investigated the mass transfer process at different levels of the interaction between gravity and capillary forces. During the experiments, we detected new solutocapillary phenomena, which were found to be caused by oscillatory regimes of solutal convection occurring around air bubbles and chlorobenzene drops in heterogeneous aqueous solutions of alcohol with a vertical surfactant concentration gradient. The role of the oscillatory instability in the processes of drop saturation by the surfactant from its water solution and an inverse process of surfactant extraction from the drop into the surrounding homogeneous fluid (water) was determined. A reasonable explanation for the driving mechanisms of the discovered effects has been proposed.

Detergency in a tube

We study the motion of a drop of oil inside a capillary tube induced by a gradient of surfactant concentration. We first show that the wetting condition selects whether the drop moves towards or away from the surfactant. We then focus on the non-wetting case, for which the drop eventually escapes from the tube, and explore the different dynamics of propulsion. On a long time scale, we observe two regimes: either the drop reaches a constant velocity, or it moves intermittently, by successive jumps and stops. As a paradoxical consequence, viscous drops can be as mobile or even faster than inviscid ones.

Linear instability of two-fluid Taylor–Couette flow in the presence of surfactant

The effect of an insoluble surfactant on the centrifugal and shear instability of a pair of radially stratified immiscible liquids in the annular gap between concentric two-fluid Taylor–Couette flow is investigated by a normal-mode linear analysis and complementary energy analysis. The interface is assumed to be concentric with the cylinders. The gravitational effects are ignored. Influences of density and viscosity stratification, surface tension, surfactant concentration distribution and Taylor–Couette shearing are considered comprehensively. The instability characteristics due to competition and interaction between various physical instability mechanisms are of principal concern. Neutral curves with upper and lower branches in the Reynolds number (Re1)/axial wavenumber (k) plane are obtained. A window of parameters is identified in which the flow is linearly stable. The Marangoni traction force caused by the gradient of surfactant concentration stabilizes the axisymmetric perturbations but initiates an instability corresponding to non-axisymmetric modes in the presence of basic Couette shearing flow. Co-rotation of the outer cylinder has a stabilizing effect in expanding the stable region, which dwindles in the counter-rotation situation.

On a simple model for surfactant-dominated contact-line movement

Abstract This paper presents a simple approximate continuum model for contact-line advance from Newtonian aqueous surfactant solution into oil. Geometrical complexity of the underlying solid surface and the liquid/liquid interface is included in the general formulation of model equations. A much simpler two-dimensional model for slow flow where interfaces are plane, the oil is inviscid and the contact angle is small, is then developed and analysed. In both models, the mechanics of the flow is dominated by surfactant concentration gradients in the three interfaces. It is concluded that quantitative predictions at the several length scales characterising the problem will require a hierarchy of models, and that a role will be played by molecular dynamic, Brownian dynamic, Lattice–Boltzmann and fully continuum-level FD/FE/FV simulations.

Electrochemical Generation of Gradients in Surfactant Concentration Across Microfluidic Channels

We report the generation and manipulation of spatial gradients in surfactant and micelle concentration across microfluidic channels by combining use of a redox-active surfactant with electrochemical methods. The approach is founded on the observation that 11-ferrocenylundecyltrimethylammonium bromide (FTMA) behaves as a surfactant in aqueous solution (e.g., self-assembles to form micelles at a critical concentration of 0.1 mM in aqueous 0.1 M Li(2)SO(4)) whereas oxidized FTMA remains dispersed in a monomeric state up to concentrations of at least 30 mM. By flowing aqueous FTMA solutions through microfluidic channels (width of 80 microm, depth of 72 microm, and length of 42 mm) and by applying potentials of 0 V (vs Ag|AgCl; cathode) and +0.3 V (vs Ag|AgCl; anode) to gold electrodes lining both side-walls of the microfluidic channels, we measured lateral gradients in concentration of oxidized FTMA and reduced FTMA to be generated across the microfluidic channels by splitting the exiting stream into four channels. These measurements revealed the lateral concentration profile of FTMA to be consistent with the presence of slowly diffusing micelles of FTMA in a spatially localized region near the cathode and monomeric FTMA only near the anode. The lateral concentration profiles of reduced and oxidized FTMA, and thus the patterning of micelles within the microfluidic channels, were manipulated via changes in the inlet FTMA concentration, potentials applied to the electrodes, and flow rate. These experimental measurements were compared to a simple model, which assumed fast electrode kinetics, lateral transport of FTMA by diffusion only (no migration), and local micelle-monomer equilibrium within the bulk solution. This comparison revealed qualitative but not quantitative agreement between model and experiment. Calculations of ionic conductivity and associated experimental measurements support the proposition that Ohmic resistance to the passage of current along the channel (between the working and the counter electrodes) contribute, in part, to the lack of quantitative agreement between the model and the measurements. The capability to generate and manipulate lateral concentration profiles of surfactants and micelles across microfluidic channels, as demonstrated by the results presented in this paper, offers the basis of new principles for continuous separation processes and microanalytical systems, and more broadly, new methods to generate gradients in concentration of analytes that interact with surfactants.