Surfactant Front Research Articles

Marangoni spreading time evolution and synergism in binary surfactant mixtures

HypothesisMarangoni spreading driven by localized surfactant solution deposition previously has been studied only for single surfactant systems. For binary surfactant mixtures, interactions that generate surface tension synergism, a thermodynamic effect, may also synergistically enhance Marangoni spreading dynamics, introducing the concept of Marangoni synergism. Spreading dynamics and possible Marangoni synergism should depend not only on thermodynamic properties but also kinetic properties of the binary system. Experiments and modelingTracer experiments that capture post-deposition surfactant front motion were performed in parallel with computational modeling, using binary surfactant pairs with varying interaction strengths. The model coupled the Navier-Stokes and advective diffusion equations with a Frumkin-type binary adsorption model. FindingsWe confirm the existence of Marangoni synergism. Stronger binary surfactant attraction favors synergism in both surface tension reduction and Marangoni spreading. Binary composition ranges over which surface tension synergism occurs differ from those for Marangoni synergism, indicating that the origins of the two synergistic effects are not identical. Analysis of model spreading velocities show that the thermodynamic spreading parameter is the controlling factor at early times for both single and binary surfactant systems, while the intrinsic adsorption and desorption kinetics influence spreading velocities and thus the occurrence of Marangoni synergism at later times.

Surfactant spreading on a deep subphase: Coupling of Marangoni flow and capillary waves

HypothesisSurfactant-driven Marangoni spreading generates a fluid flow characterized by an outwardly moving “Marangoni ridge”. Spreading on thin and/or high viscosity subphases, as most of the prior literature emphasizes, does not allow the formation of capillary waves. On deep, low viscosity subphases, Marangoni stresses may launch capillary waves coupled with the Marangoni ridge, and new dependencies emerge for key spreading characteristics on surfactant thermodynamic and kinetic properties. Experiments and modelingComputational and physical experiments were performed using a broad range of surfactants to report the post-deposition motion of the surfactant front and the deformation of the subphase surface. Modeling coupled the Navier-Stokes and advective diffusion equations with an adsorption model. Separate experiments employed tracer particles or an optical density method to track surfactant front motion or surface deformation, respectively. FindingsMarangoni stresses on thick subphases induce capillary waves, the slowest of which is co-mingled with the Marangoni ridge. Changing Marangoni stresses by varying the surfactant system alters the surfactant front velocity and the amplitude – but not the velocity – of the slowest capillary wave. As spreading progresses, the surfactant front and its associated surface deformation separate from the slowest moving capillary wave.

Transport of a partially wetted particle at the liquid/vapor interface under the influence of an externally imposed surfactant generated Marangoni stress

Marangoni flows offer an interesting and useful means to transport particles at fluid interfaces with potential applications such as dry powder pulmonary drug delivery. In this article, we investigate the transport of partially wetted particles at a liquid/vapor interface under the influence of Marangoni flows driven by gradients in the surface excess concentration of surfactants. We deposit a microliter drop of soluble (sodium dodecyl sulfate) aqueous surfactant solution or pure insoluble liquid (oleic acid) surfactant on a water subphase and observe the transport of a pre-deposited particle. Following the previous observation by Wang et al. [1] that a surfactant front rapidly advances ahead of the deposited drop contact line and initiates particle motion but then moves beyond the particle, we now characterize the two dominant, time- and position-dependent forces acting on the moving particle: (1) a surface tension force acting on the three-phase contact line around the particle periphery due to the surface tension gradient at the liquid/vapor interface which always accelerates the particle and (2) a viscous force acting on the immersed surface area of the particle which accelerates or decelerates the particle depending on the difference in the velocities of the liquid and particle. We find that the particle velocity evolves over time in two regimes. In the acceleration regime, the net force on the particle acts in the direction of particle motion, and the particle quickly accelerates and reaches a maximum velocity. In the deceleration regime, the net force on the particle reverses and the particle decelerates gradually and stops. We identify the parameters that affect the two forces acting on the particle, including the initial particle position relative to the surfactant drop, particle diameter, particle wettability, subphase thickness, and surfactant solubility. We systematically vary these parameters and probe the spatial and temporal evolution of the two forces acting on the particle as it moves along its trajectory in both regimes. We find that a larger particle always lags behind the smaller particle when placed at an equal initial distance from the drop. Similarly, particles more deeply engulfed in the subphase lag behind those less deeply engulfed. Further, the extent of particle transport is reduced as the subphase thickness decreases due to the larger velocity gradients in the subphase recirculation flows.

Influence of surfactants on unsaturated water flow and solute transport

AbstractSurfactants can reduce soil water retention by changing the surface tension of water and the contact angle between the liquid and solid phases. As a result, water flow and solute transport in unsaturated soil may be altered in the presence of surfactants. In this study, the effects of a representative nonionic surfactant, Triton X‐100, on coupled water flow and nonreactive solute transport during unsaturated flow conditions were evaluated. Batch reactor experiments were conducted to measure the surfactant sorption characteristics, while unsaturated transport experiments were performed in columns packed with 40–270 mesh Ottawa sand at five initial water contents. Following the introduction of surfactant solution, the rate of water percolation through the sand increased; however, this period of rapid water drainage was followed by decreased water percolation due to the reduction in soil water content and the corresponding decrease in unsaturated hydraulic conductivity behind the surfactant front. The observed changes in water percolation occurred sequentially, and resulted in faster nonreactive solute transport than was observed in the absence of surfactant. A one‐dimensional mathematical model accurately described coupled water flow, surfactant, and solute transport under most experimental conditions. Differences between model predictions and experimental data were observed in the column study performed at the lowest water content (0.115 cm3/cm3), which was attributed to surfactant adsorption at the air‐water interface. These findings demonstrate the potential influence of surfactants additives on unsaturated water flow and solute transport in soils, and demonstrate a methodology to couple these processes in a predictive modeling tool.

Effects of added surfactant on swelling and molecular transport in drug-loaded tablets based on hydrophobically modified poly(acrylic acid).

A combination of NMR chemical shift imaging and self-diffusion experiments is shown to give a detailed molecular picture of the events that occur when tablets of hydrophobically modified poly(acrylic acid) loaded with a drug (griseofulvin) swell in water in the presence or absence of surfactant (sodium octylbenzenesulfonate). The hydrophobic substituents on the polymer bind and trap the surfactant molecules in mixed micelles, leading to a slow effective surfactant transport that occurs via a small fraction of individually dissolved surfactant molecules in the water domain. Because of the efficient binding of surfactant, the penetrating water is found to diffuse past the penetrating surfactant into the polymer matrix, pushing the surfactant front outward as the matrix swells. The added surfactant has little effect on the transport of drug because both undissolved solid drug and surfactant-solubilized drug function as reservoirs that essentially follow the polymer as it swells. However, the added surfactant nevertheless has a strong indirect effect on the release of griseofulvin, through the effect of the surfactant on the solubility and erosion of the polymer matrix. The surfactant effectively solubilizes the hydrophobically modified polymer, making it fully miscible with water, leading to a more pronounced swelling and a slower erosion of the polymer matrix.

Soluble surfactant spreading on spatially confined thin liquid films

We studied the spreading of soluble surfactants on spatially confined thin liquid films by means of comprehensive experiments and numerical simulations. We determined the time evolution of the liquid film thickness both from interference microscopy measurements and finite element calculations. A characteristic rim develops ahead of the spreading surfactant front. Within certain time intervals, the rim position can be well represented by a power-law relation xrim ∼ tα. The corresponding spreading exponent α depends on the method of surfactant deposition and the numerical values deduced from experiments and simulations quantitatively agree. Depth-resolved simulations that account for domain deformability using the Arbitrary Lagrangian–Eulerian method show that shear-induced concentration non-uniformities across the rim film thickness tend to reduce the rim height. Fingering instabilities that are frequently observed in experiments were qualitatively reproduced in the simulations.

Influence of the Type of Surfactant on the Mobility of Flushing Fluids for Electro-Remediation Processes

The mobility of four flushing fluids in an electro-remediation process has been studied. A mock-up plant with two rows of three electrodes positioned in semipermeable electrolyte wells was used in this study. Two different cases were considered: first an initial state when the soil was wet with water and the addition of surfactant starts, and second a second state when the surfactant front had passed through the soil. The results show that the electro-osmotic flux strongly depends on the nature of the flushing fluid and on the voltage gradient applied between electrode rows. The presence of ionic surfactants (both cationic and anionic) led to lower electro-osmotic flow-rates. The concentration of surfactants in the drain tanks increased in the case of the flushing fluid with the non-ionic surfactant because the surfactant transport was more efficient. Conversely, the anionic surfactant accumulated more in the injection well. This finding is related to the effect of migration, which in the reception wells was opposite to the electro-osmotic flux.

Fractional-Flow Theory of Foam Displacements With Oil

Summary Fractional-flow theory provides key insights into complex foam enhanced-oil-recovery (EOR) displacements and acts as a benchmark for foam simulators. In some cases with mobile oil present, the process can be represented as a two-phase displacement. We examine three such cases. A first-contact-miscible (FCM) gasflood with foam injection includes a chemical shock defining the surfactant front and a miscible shock defining the gas front. The optimal water fraction for the foam, that which gives the fastest oil recovery in 1D, maintains the gas front slightly ahead of the foam (surfactant) front. The success of a foam process with FCM CO2 and surfactant dissolved in the (supercritical) CO2 depends on the strength of foam at very low water fractional flow, such as for a surfactant- alternating-gas (SAG) process with surfactant dissolved in water. The speed of propagation of the foam front depends on surfactant adsorption on rock and on the partitioning of surfactant between water and CO2 but is always less than the velocity of the foam front in a SAG flood with surfactant ahead of the gas. A foam with surfactant that partitions preferentially into water rather than into CO2 would propagate slowly, regardless of the surfactant's absolute solubility or the level of adsorption on rock. An aqueous surfactant preflush can speed the advance of foam, however. An idealized model of a surfactant flood pushed by foam suggests that it is best to inject a relatively high water content into the foam to ensure that the gas front remains behind the surfactant front as the flood proceeds. Any gas that passes ahead of the surfactant front would finger through the oil and be wasted. We present simulations to verify the solutions obtained with fractional-flow methods and illustrate the challenges of accurate simulation of these processes.

On the Importance of Hysteresis in Numerical Modeling of Surfactant-Induced Unsaturated Flow

It is known that surfactants can induce flow in unsaturated porous media due to the dependence of capillary pressure on surface tension. A commonly observed feature in systems with surfactant-induced flow is a transient wetting/drying/wetting sequence associated with the propagation of a surfactant solute front under monotonic flow conditions. Previous efforts to model surfactant-induced flow in relatively complex (e.g., two-dimensional systems) have not successfully incorporated hysteretic moisture retention properties. In this research, hysteretic, two-dimensional simulations of surfactant-induced flow were performed to assess the potential importance of considering hysteresis in such simulations. Hysteretic simulation results were compared to experimental data and to non-hysteretic simulations. The results suggest that the inclusion of hysteresis in numerical simulations can improve the match between simulated and experimental results in systems with surfactant-induced unsaturated flow. Furthermore, the inclusion of hysteresis in numerical simulations played a significant role in predicting the distribution of the contaminant and correct pressure head/moisture condition at the end of the experiment.

Thermal structure of clean and contaminated free-surfaces subject to an impinging gas jet

The thermal structure of clean and contaminated free-surfaces subject to the transient flow of a gas jet were investigated experimentally. The interface and near-surface flow were examined using optical high-speed (HS) motion analysis, infrared (IR) imagery, and laser-induced fluorescence (LIF). IR imagery revealed an instability in the form of thermal scars on the expanding circular surfactant front. The nature of this instability was explored by performing experiments with both clean and contaminated surfaces. LIF visualization techniques were used to gain insight into the nature of the near-surface flow field. This revealed the presence of a vortex ring that underwent an instability in which ringlets surrounded the primary core. Using simultaneous IR/LIF imaging of a fixed spatial region, it is shown that the thermal scars are spatially and temporally correlated with the near-surface ringlet structures, suggesting that the scars are a surface manifestation of the near-surface structures.

Transport of exogenous surfactants on a thin viscous film within an axisymmetric airway

The transport of exogenous surfactant in an axisymmetric pulmonary airway lining is analyzed using the model based on lubrication theory to understand the parameters involved in surfactant replacement therapy for neonatal respiratory distress syndrome, including the ratio of film thickness-to-airway radius ε, and the initial exogenous and endogenous surfactant distributions. Numerical simulation of the model indicates that the film disturbance induced by Marangoni flow in low generation airways (small ε, such as trachea) would be more significant than that in high generation airways (large ε, such as bronchioles) for the same amount of instilled surfactant. The non-dimensional transport properties of the exogenous surfactant, however, are virtually identical in various airway generations considered. Increasing the initial local exogenous concentration enhances the surfactant transport and also intensifies the film disturbance. The presence of endogenous surfactant tends to moderate the shock-like film disturbance and also prolongs the extent of surfactant monolayer front. This, however, does not mean that the spread of exogenous surfactant is enhanced with the presence of endogenous surfactant. The extended monolayer is attributed to the compression of endogenous surfactant concentration in regions ahead of the advancing exogenous surfactant front.

The spreading of surfactant solutions on thin liquid films

The spreading of a surfactant solution on a water film at first glance seems a trivial problem. However, in the last 30 years or so this has been shown to be anything like the case. There have been numerous studies which show that Marongoni driven fingering of the spreading surfactant front exists. In this paper this work has been reviewed and an attempt has been made to rationalise the results. The paper concludes with some recent observations of ours concerning the spreading of sodium dodecyl sulfate over relatively thick water films, 200 μm or less.

Effects of ethanol addition on micellar solubilization and plume migration during surfactant enhanced recovery of tetrachloroethene

Alcohol addition has been suggested for use in combination with surfactant flushing to enhance solubilization kinetics and permit density control of dense non-aqueous phase liquid (DNAPL)-laden surfactant plumes. This study examined the effects of adding ethanol (EtOH) to a 4% Tween 80 (polyoxyethylene (20) sorbitan monooleate) solution used to flush tetrachloroethene (PCE)-contaminated porous media. The influence of EtOH concentration, subsurface layering and scale on flushing solution delivery and PCE recovery was investigated through a combination of experimental and mathematical modeling studies. Results of batch experiments demonstrated that the addition of 2.5%, 5% and 10% (wt.) EtOH incrementally increased the PCE solubilization capacity and viscosity of the surfactant solution, while reducing solution density from 1.002 to 0.986 g/cm 3. Effluent concentration data obtained from one-dimensional (1-D) column experiments were used to characterize rate-limited micellar solubilization of residual PCE, which was strongly dependent upon flow velocity and weakly dependent upon EtOH concentration. Two-dimensional (2-D) box studies illustrated that minor differences (0.008 g/cm 3) between flushing and resident solution density can strongly influence surfactant front propagation. A two-dimensional multiphase simulator, MISER, was used to model the influence of EtOH composition on the aqueous flow field and PCE mass recovery. The ability of the numerical simulator to predict effluent concentrations and front propagation was demonstrated for both 1-D columns and 2-D boxes flushed with EtOH-amended Tween 80 solutions. Results of this study quantify the potential influence of alcohol addition on surfactant solution properties and solubilization capacity, and demonstrate the importance of considering small density variations in remedial design.

Surfactant-Influenced Gas–Liquid Interfaces: Nonlinear Equation of State and Finite Surface Viscosities

A canonical flow geometry was utilized for a fundamental study of the coupling between bulk flow and a Newtonian gas–liquid interface in the presence of an insoluble surfactant. We develop a Navier–Stokes numerical model of the flow in the deep-channel surface viscometer geometry, which consists of stationary inner and outer cylinders, a floor rotating at a constant angular velocity, and an interface covered initially by a uniformly distributed surfactant. Here, the floor of the annular channel is rotated fast enough so the flow is nonlinear and drives the film toward the inner cylinder. The boundary conditions at the interface are functions of the surface tension, surface shear viscosity, and surface dilatational viscosity, as described by the Boussinesq–Scriven surface model. A physical surfactant system, namely hemicyanine, an insoluble monolayer on an air–water interface, with measured values of surface tension and surface shear viscosity versus concentration, was used in this study. We find that a surfactant front can form, depending on the Reynolds number and the initial surfactant concentration. The stress balance in the radial direction was found to be dominated by the Marangoni stress, but the azimuthal stress was only due to the surface shear viscosity. Numerical studies are presented comparing results of surfactant-influenced interface cases implementing the derived viscoelastic interfacial stress balance with those using a number of idealized stress balances, as well as a rigid no-slip surface, providing added insight into the altered dynamics that result from the presence of a surfactant monolayer.

Surfactant enhanced removal of PCE in a nominally two-dimensional, saturated, stratified porous medium

Although surfactant enhanced remediation of nonaqueous phase liquids (NAPLs) by pump-and-treat technology has been studied extensively in the laboratory with one-dimensional columns, very few multi-dimensional investigations have been reported. In this study we focus on the removal of perchloroethylene (PCE) from a two-dimensional, saturated porous medium containing a low permeability sand layer situated in an otherwise high permeability sand. A PCE spill was applied at the surface of the porous medium and allowed to redistribute until static equilibrium was achieved. The porous medium was then flushed with various surfactant and co-solvent formulations injected at the PCE source location and extracted at the bottom of the porous medium using a configuration similar to that of Abdul and Ang [Abdul, S.A., Ang, C.C., 1994. In situ surfactant washing of polychlorinated biphenyls and oils from a contaminated field site: Phase II. Pilot study. Ground Water 32, 727–734]. Effluent samples were analyzed for dissolved PCE concentrations. Volumetric water and PCE content values were determined at a number of locations by means of dual-energy gamma radiation measurements. Once surfactant flushing had started, PCE moved as a distinct separate phase ahead of the surfactant front. Most of this downward moving PCE accumulated on top of the low permeability sand layer. Some PCE, however, passed quickly through this layer and subsequently through the high permeability sand below it. Movement of some of the PCE into and through the low permeability sand layer was attributed to local heterogeneities combined with reduced interfacial tensions associated with the surfactant formulation. Clean-up of PCE in most of the high permeability sand was considered to be effective. PCE accumulated on top of the fine layer, however, posed a significant challenge to remediation and required several pumping configurations and surfactant/co-solvent formulations before most of it was removed.

Interaction of exogenous and endogenous surfactant: spreading-rate effects

The spreading rate of an exogenous surfactant monolayer due to surface tension gradients is examined by using our previously reported theoretical analysis, with particular attention given to the effects of endogenous surfactant. It is found that the presence of an endogenous surfactant reduces the spreading rate of exogenous surfactant and that, in certain circumstances, the spreading may be halted. A recently published paper (F. F. Espinosa, A. H. Shapiro, J. J. Fredberg, and R. D. Kamm. J. Appl. Physiol. 75: 2028-2039, 1993) reaches the opposite conclusion about the effect of endogenous surfactant, i.e., that the presence of an endogenous surfactant increases the spreading rate of the exogenous surfactant. This communication discusses the relevant issues associated with these different results and what the implications may be for surfactant replacement therapy. It is found that the endogenous surfactant, which is ahead of the advancing exogenous surfactant front, undergoes a concentration increase due to surface area compression of the air-liquid interface. Hence the spreading exogenous surfactant can raise surfactant concentrations in regions distal to its own location, and this is a previously unrecognized potential therapeutic mechanism of instilled surfactants. After initial instillations of intratracheal boluses of exogenous surfactant, additional surfactant may better reach the desired target site if delivered by aerosol. Predictions of surfactant and piggy-backed drug-delivery times through the lung are also discussed.

Marangoni-type surface flow on an undeformable free drop

The Marangoni flow caused by interfacial tension gradients is experimentally and theoretically investigated in the case of an undeformable free drop suspended in an immiscible liquid. The spreading of the surfactant on the interface—as a result of the Marangoni effect—is approximated to a succession of quasisteady states; integration of the hydrodynamic equations proposed in the authors' theoretical model yields the velocities in the outer liquid, in the inner one, and on the surface of the drop. Experimentally, as much undeformable free drops are obtained if, made of paraffin oil and immersed in an ethanol-water or methanol-water equidense mixture, they also have large viscosities (36–80 cP) as compared with that of the continuous medium (1–3 cP). The interfacial tension differences of only 2.3–6.7 dyn cm −1 have been established by injecting with a surfactant (propanol-water or isopropanol-water mixture dyed with methylene blue). The advance velocities of the surfactant front have been determined by filming (with 500 images/sec) and they satisfactorily agree with the theoretical values, particularly for high grades of covering of the drop surface.