Surfactant Mass Transfer Research Articles

The overall‐process dynamic mass transfer research of surfactant at the three‐phase emulsions interface

AbstractSurfactant‐stabilized multiphase emulsions are widely used. However, current studies on the dynamic adsorption of emulsions during flow are mainly focused on the droplet formation stage rather than the flow stage. In this study, the mass transfer of surfactants in microchannels during the droplet formation and flow phases (the overall process) was investigated and a mass transfer diffusion model and an adsorption kinetic model for emulsions were developed, which explained the mass transfer process in microchannels for low and high surfactant concentrations, respectively. Subsequently, dynamic interfacial tension values between the two phases were calculated using two surfactants (SDS and DTAB) to validate these models and the results were in good agreement with the experimental values. This study contributes to a deeper understanding of how surfactants diffuse throughout three‐phase emulsions, thus shedding light on the mass transfer mechanisms inherent in these complex systems.

Effect of surfactant on dynamics and gas-liquid mass transfer for single carbon dioxide bubbles

In carbonation reaction, the impact of surfactants on the movement and mass transfer behavior of carbon dioxide bubbles were crucial for the production of high-quality silicon dioxide products. The study employed the Matlab image processing system to investigate the shapes, motion velocities, and rise path of single carbon dioxide bubbles within varying concentrations of cetyltrimethyl ammonium bromide (CTAB) solutions. This exploration aimed to elucidate the effect of CTAB on the movement and mass transfer coefficient (MTRC) of single carbon dioxide bubbles. Alterations in the CTAB solution concentration and bubble sizes significantly influenced the mass transfer and movement characteristics of single carbon dioxide bubbles. The conclusion was drawn through the computation of the MTRC of single carbon dioxide bubbles in deionized water and CTAB solutions: the addition of CTAB significantly reduced the mass transfer efficiency of carbon dioxide in deionized water, with the MTRC of CTAB solutions decreasing as the solution concentration increased. Utilizing the stagnant cap model, it can be concluded that CTAB's reduction of the MTRC of carbon dioxide was attributed to the high viscosity of the medium, low bubble velocity, stable flow conditions, and intensified adsorption degree of CTAB molecules at the gas-liquid interface. According to the significant periodic trajectories of single carbon dioxide bubbles in the stable range, the correlation between the trajectories of compressible bubbles in surfactant solution and mass transfer in liquid phase was established, and the previous method based on rigid sphere model was modified. The paper proposed the mechanism of CTAB's influence on single carbon dioxide bubbles, offering crucial theoretical insights into the industrial application of surfactants in the carbonation process for producing silicon dioxide.

Enhanced oil recovery potential analysis through simulation of a bio-based surfactant using CFD

Bio-based surfactant flooding is encouraged as an EORmethod, as it is obtained from natural sources and is economically attractive. In the present study, the EOR efficiency of a natural surfactant synthesized from rapseed oil has been simulated using ANSYS FLUENT. The process of oil recovery from a core by injection of surfactant has been simulated by incorporatingcore, brine, surfactant, and crude oil properties into the numerical model. Mass transfer of surfactant between the aqueous phase and crude oil has been modeled using species transport without a chemical reaction model. Species transport model has been coupled with the multiphase VOF model, where the flow of each phase is determined as per their respective viscous resistance. Experimental data of inletpressureand oil production has been used for history matchingand later used to visualize the oil flow through the porous media, concludingthat simulation of surfactant flooding can be achieved by species transport model.

Comparison of surfactant mass transfer with drop formation times from dynamic interfacial tension measurements in microchannels

Dynamic interfacial tension was studied experimentally during drop formation in a flow-focusing microchannel. A low viscosity silicone oil (4.6 mPa s) was the continuous phase and a mixture of 48% w/w water and 52% w/w glycerol was the dispersed phase. An anionic (sodium dodecylsulfate, SDS), a cationic (dodecyltrimethylammonium bromide, DTAB) and a non-ionic (Triton™ X-100, TX100) surfactant were added in the dispersed phase, at concentrations below and above the critical micelle concentration (CMC). For SDS and DTAB the drop size against continuous phase flowrate curves initially decreased with surfactant concentration and then collapsed to a single curve at concentrations above CMC. For TX100 the curves only collapsed at surfactant concentrations 8.6 times the CMC. From the collapsed curves a correlation of drop size with capillary number was derived, which was used to calculate the dynamic interfacial tension at times as low as 3 ms. The comparison of the surfactant mass transport and adsorption times to the interface against the drop formation times indicated that surfactant adsorption also contributes to the time required to reach equilibrium interfacial tension. Criteria were proposed for drop formation times to ensure that equilibrium interfacial tension has been reached and does not affect the drop formation.

Determination of Dynamic Interfacial Tension during the Generation of Tiny Droplets in the Liquid-Liquid Jetting Flow Regime.

Liquid-liquid dispersion coupled with droplet formation and mass transfer of surfactants is one of the most typical phenomena in many chemical processes. As in every aspect of this process, the interfacial tension variation caused by the unsaturated adsorption of surfactants on the droplet surface plays an important role. This article focuses on microdroplet formation and the dynamic interfacial behavior of surfactants in the jetting regime. In a capillary embedded step T-junction device, controllable preparation of monodisperse droplets is achieved, and a correlation for predicting droplet sizes is established. A method for measuring the dynamic interfacial tension is provided. Mass transfer coefficients are then calculated for Tween 20 during the droplet formation process by a semiempirical correlation. The results indicate that dynamic interfacial tensions are lower than those obtained when the surfactant is adsorbed to equilibrium. Based on the tip-streaming phenomenon, mass transfer coefficients for Tween 20 can reach up to ∼10-3 m/s, higher than those obtained in coaxial microfluidic devices. All the preliminary results shed light on the nature of droplet formation and will be of significance for application in industrial apparatuses.

Search for the Source of an Apparent Interfacial Resistance To Mass Transfer of CnEm Surfactants To the Water/Oil Interface.

Experiments have shown that relaxation of oil/water interfacial tension by adsorption of alkyl ethoxylate surfactants from water onto an oil droplet is delayed relative to diffusion-controlled adsorption. We examine possible causes of this delay, and we show that several are implausible. We find that redissolution of the surfactant in the oil droplet cannot explain the apparent interfacial resistance at short times because the interface will preferentially fill before any such redissolution occurs. We also perform umbrella sampling with molecular dynamics simulation and do not find any evidence of a free-energy barrier or low-diffusivity zone near the interface. Nor do we find evidence from the simulation that premicellar aggregation slows diffusion enough to cause the observed resistance to interfacial adsorption. We are therefore unable to pinpoint the cause of the resistance, but we suggest that "dead time" associated with the experimental method could be responsible-specifically a local depletion of surfactant by the ejected droplet when creating the fresh interface between the oil and water.

Effect of Marangoni Convection on Surfactant Transfer Between the Drop Connected to the Reservoir and Surrounding Liquid

The paper presents the results of comparative investigation of the interaction between the capillary and buoyant mechanisms of motion in a problem of surfactant mass transfer between an insoluble drop and surrounding fluid under different gravity conditions. The research was performed for the drop that is coupled with the reservoir filled with a source mixture through a long thin tube (needle). Visualization of the flow patterns and concentration fields has shown that surfactant diffusion from the needle at normal gravity leads to the onset of the oscillatory mode of the capillary convection in the drop. It has been found that the frequency of the Marangoni convection outbursts, the lifetime of the oscillatory flow modes and the amount of the source mixture involved in the process of mass transfer depend on the drop size and initial concentration of the surfactant. The obtained results are compared with the cases of surfactant diffusion from the isolated drop under terrestrial conditions and from the drop coupled with reservoir in microgravity. Additionally, a series of experiments were performed to investigate diffusion of a surfactant from the surrounding solution into a drop.

Microdroplet Generation with Dilute Surfactant Concentration in a Modified T-Junction Device

Emulsions with very low surfactant concentration or without surfactant are highly required in many fields. Microfluidics have proven to be a promising emulsion preparation method. However, for the fresh interface formation during microfluidic droplet generation, excessive amounts of surfactant are also demanded to speed up surfactant transfer and drive low interfacial tensions. Here we present a newly designed capillary embedded T-junction microdevice to intensify surfactant mass transfer for microdroplet generation. Within the device, surfactant molecules are concentrated onto the tip of the droplet breakup very fast by hydrodynamic forces, leading to extremely low dynamic IFTs temporarily. Microdroplets as small as 15 μm are generated with surfactant concentrations significantly lower than their CMC values, when the concentrations of SDS, Tween 20, and F68 are only 0.05 wt % (1/6 CMC), 0.20 wt % (4/7 CMC), and 0.20 wt % (2/3 CMC) individually. The novel method would be promising in fields of biocompatib...

Measurements of the stabilisation of liquid film flow by the soluble surfactant sodium dodecyl sulfate (SDS)

Gravity-driven film flow of aqueous solutions of SDS is studied experimentally and the evolution of small-amplitude, regular inlet disturbances is investigated. With the addition of SDS, strong attenuation of non-linear growth is observed, with traveling waves remaining relatively small in height and near-sinusoidal over an impressive parametric range. The critical Reynolds number of the primary instability rises by an order of magnitude. Maximum stabilization is observed at small surfactant loadings (characterized by surface tension 60–65mN/m) and the critical Reynolds number gradually decreases with further addition of surfactant. Observations are interpreted by the competing effects of surface elasticity -which increases with the adsorbed SDS and intensifies Marangoni stresses- and surfactant mass transfer between bulk and interface -which also increases with the amount of SDS and mitigates interfacial gradients and Marangoni stresses.

Mass-Transfer-Controlled Dynamic Interfacial Tension in Microfluidic Emulsification Processes.

Varied interfacial tension caused by the unsaturated adsorption of surfactants on dripping droplet surfaces is experimentally studied. The mass transfer and adsorption of surfactants, as well as the generation of fresh interfaces, are considered the main factors dominating the surfactant adsorption ratio on droplet surfaces. The diffusion and convective mass transfer of the surfactants are first distinguished by comparing the adsorption depth and the mass flux boundary layer thickness. A characterized mass transfer time is then calculated by introducing an effective diffusion coefficient. A time ratio is furthermore defined by dividing the droplet generation time by the characteristic mass transfer time, t/tm, in order to compare the rates of surfactant mass transfer and droplet generation. Different control mechanisms for different surfactants are analyzed based on the range of t/t(m), and a criterion time ratio using a simplified characteristic mass transfer time, t(m)*, is finally proposed for predicting the appearance of dynamic interfacial tension.

Numerical study of surfactant effects on the buoyancy-driven motion of a drop in a tube

We present results of our numerical study of the effect of surfactants on buoyancy-driven motion of drops in a tube at intermediate Reynolds numbers. The drop and bulk phases are treated as incompressible Newtonian fluids and simulated using a front-tracking method. The steady shapes and velocity–volume curves for drops ranging in drop size from 0.2 to 1.3 of tube radius are determined numerically. At small Bond numbers, the velocity–volume curve shows a maximum before the velocity plateaus for large drops. As the Bond number increases, the maximum in the velocity–volume curve disappears with elongated, more streamlined drop shapes consistent with previous experimental studies. For increasing Weber numbers, the rear of the drop shows a flattening followed by the development of a negative curvature. Surfactants are modeled using a Langmuir equation of state in the adsorption–desorption limit and the effect of surfactant mass transfer, fractional coverage of surfactants, and the interfacial Peclet number on the velocity–volume curve is determined. Marangoni stress generated due to the non-uniform distribution of surfactants at the interface reduces drop mobility. Reduced drop mobility is more prominent for drop sizes that are comparable to the tube diameter and is maximum when mass transfer to and from the interface is inhibited. As the fractional coverage of soluble surfactants, or the interfacial Peclet number increases, larger Marangoni stresses are generated along the interface that lead to greater retardation of the drop motion.

Surfactant flow between a Plateau border and a film during foam fractionation

A fluid mechanics problem relevant to foam fractionation processes is analysed. Specifically the fluid flow field transporting surfactant from foam Plateau borders (fed with surfactant-rich material) towards comparatively surfactant-lean foam films is considered. The extent to which this surfactant mass transfer is limited by surface viscous effects is studied. Previous work (Vitasari et al., 2016) made assumptions about the likely flow field along the Plateau border surface. These assumptions suggested that ‘high’ surface viscosity (measured by a suitable dimensionless parameter) led to strong suppression of the rate of surfactant mass transfer from Plateau border to film, whereas ‘low’ surface viscosity did not suppress this mass transfer rate in any significant way. More detailed fluid mechanical calculations which are carried out here corroborate the aforementioned assumptions in the ‘high’ surface viscosity regime. However the calculations suggest that in the ‘low’ surface viscosity regime, in contrast to the findings from the previous assumptions, moderate reductions in the rate of surfactant mass transfer are also possible. Counterintuitively these moderate reductions in mass transfer rate potentially have more negative impact on fractionation processes than the aforementioned strong suppression. This is because they tend to arise under conditions for which the efficiency of the fractionation system is particularly sensitive to any reduction whatsoever in the surfactant mass transfer rate.

The dynamic adsorption of different surfactants on droplet formation in coaxial microfluidic devices

In this work, the dynamic adsorption of different surfactants is studied during droplet formation in microfluidic devices. For a macromolecule surfactant (Tween 20 and TX 100) and micromolecule surfactant (SDS and CTAB), the tendency of dynamic interfacial tension at the rupturing moment to decrease with increasing concentrations is contradicted. However, turning points are noted for the different surfactant systems. Using a particular curve to fit the experimental data, turning concentrations are determined to approximate CMC values. The results presented in this work provide a more in-depth understanding of the dynamic mass transfer of different surfactants outside the droplet during the formation of a droplet in microfluidic devices.

Visualization of Multiscale Processes - Bubble Dynamics in Surface Active Colloids

The growth dynamics of isolated gas bubbles from a submerged capillary-tube orifice in a pool of aqueous solution of Cetyl Trimethyl Ammonium Bromide (CTAB) was studied by multi-scale modeling. The macro-scale bubble ebullience is controlled by the molecular scale surfactant adsorption/desorption on the liquid-gas interface. Molecular dynamics simulations were carried out to predict the interfacial adsorption/desorption kinetics. The results of the molecular dynamics simulations were input to the volume-of-fluid based macro-scale computations. The size and shape of bubbles from incipience to departure were measured using high speed videography for model validation. Predictions of the multi-scale model agree with the experimental measurements of bubble size evolution and bubble diameter at departure. The surfactant mass transfer and adsorption on the liquid gas interface gives rise to dynamic surface tension. As a result of the surfactant presence, the bubble departure diameters were smaller in CTAB solution compared to pure water. Furthermore, dynamic surface tension behavior of CTAB makes the bubble departure diameter a function of bubble Reynolds number (Re based on the orifice diameter and air flow rate). At low flow rates or low Re, the bubble departure diameters are smaller than those in water. As the air flow rate increases, the bubble departure diameters tend towards those in pure water. The authors gratefully acknowledge funding from AFOSR Thermal Science Program and AFRL DoD Supercomputing Resource Center for computing time and resources.

The dynamic mass transfer of surfactants upon droplet formation in coaxial microfluidic devices

In this work, the dynamic adsorption of surfactants is used to evaluate the mass transfer coefficient during droplet formation. As surfactants exist in only one phase and at the interface, the resistance of mass transfer inside and outside the droplet can be decoupled. By exchanging the continuous phase and dispersed phase (N-hexane/water-Tween 80), the mass transfer coefficients inside and outside the droplet were evaluated. A semi-empirical equation with modified parameters was developed to incorporate the effects of convection and the experimental conditions on the mass transfer coefficient during droplet formation in micro-channels. The results presented in this work provide a more in-depth understanding of the dynamic mass transfer of surfactants inside and outside the droplet during the formation of a droplet in microfluidic devices.

Experimental and theoretical analysis of emulsification characteristics using a high porosity microscreen under oscillatory shear conditions

Application of oscillations for production of oil-in-water emulsion using high porosity woven metal micro-screen (WMMS) has been investigated. An analytical model has been developed for predicting the average droplet size that took into consideration the dynamic interaction between the dispersed phase flow and oscillation parameters. The effect of deviation from simple Stokes oscillatory flow analysis on the hydrodynamic and diffusion layer has been considered. It was found that increasing oscillation intensities decreases the average droplet size due to both the increase in the detachment forces as well as the increase in surfactant mass transfer rate, which decreases the interfacial tension holding force. On the other hand, increasing the dispersed phase flow rate had dual opposite effects. First, it increases the rate of surfactant depletion, which increases the interfacial tension holding force leading to larger droplet size. Second, it increases the active pore fraction and the probability of steric hindrance between droplets forming at adjacent holes which creates an additional detachment “push-off” force that leads to smaller droplet size. The model predictions agreed satisfactory with the experimental data of this investigation as well as selected data from previous investigations by different authors with R2=0.96.

Modelling relaxation following T1 transformations of foams incorporating surfactant mass transfer by the Marangoni effect

The dynamics following T1 transformation of foams, during which certain foam films shrink and others grow, was modelled. Variation in surface tension due both to change in surfactant concentration and viscous drag was taken into account. Surfactant transfer was also taken into account, which was assumed to take place between connected films and to be caused by the Marangoni effect. Two models were considered in the first instance, one treating the two sides of shrinking films to be identical and another treating them to be different. Numerical results show plausible behaviours in certain parameter regimes, however with several potential issues. In particular, in one model, singularity was observed above a certain value of the surfactant mass transport coefficient, while such an event was avoided in the other model, as large tension differences between adjacent films are then suppressed. Analysing the results and comparing the models, with a view to applying them to a hexagonal honeycomb foam (rather than just to an isolated set of films), led to a possibility of further improvement; namely incorporating an inter-system surfactant transfer and variation in both surface tension and surfactant concentration along individual films, on top of the mechanisms already considered. Inter-system surfactant transfer was readily incorporated into a third model, which again displayed plausible behaviour.

On the dissolution of microbubble contrast agents in an ultrasound field.

Accurate understanding of the behavior of ultrasound contrast particles driven by an external sound source is crucial to enabling the development of better therapeutic and imaging techniques. Unfortunately current models are unable to fully explain, amongst other factors, changes in bubble characteristics following pulsed excitation. One aspect that has hitherto received little attention is the time dependent nature of the microbubble coating, specifically its gas permeability and variable surface tension. We show initially through nonlinear numerical simulations that acoustically driven microbubbles can dissolve more rapidly than stationary bubbles. However, the increased rate observed seems insufficient to account for the significant changes in bubble response over a few pulse cycles observed experimentally. Then, in an attempt to explain this continued discrepancy between models and experiments, we consider the impact of changes in surfactant concentration on the bubble behavior and gas diffusion. In particular, a model of surfactant mass transfer, including shedding, between the surface of the bubble and the bulk liquid is developed and subsequently coupled to a nonlinear bubble equation. Results from this model are presented that can replicate and potentially explain bubble response changes which may occur, leading to improvements in propagation modeling and quantitative imaging software.

Viscous froth simulations with surfactant mass transfer and Marangoni effects: Deviations from Plateau's rules

The viscous froth model is a rheological model for dry, “two-dimensional” foams, such as a monolayer of bubbles confined between two glass plates. The model is typically out of mechanical equilibrium due to viscous dissipation by drag along the confining plates. By introducing variable local surfactant coverages and variable local surface tensions, we modify the model such that, in addition to being out of mechanical equilibrium, foam structures can also be out of physicochemical equilibrium. We include effects accounting for spatially and temporally varying Marangoni forces and surfactant transport, and we investigate the effects on a simple, periodic honeycomb lattice under shear. It is found that surfactant coverage can vary substantially between and within films, with surfactant becoming highly depleted on film edges that are subjected to rapid direct shear. Moreover substantial deviations from Plateau's laws occur at flowing three-fold vertices an effect previously noted in experiments but without any definitive explanation in theory or models. For large enough values of the governing dimensionless groups (capillary, Deborah, and Marangoni numbers), angles may locally vary from 120° by 10° or more. Correspondingly, the change in surface tensions at a three-fold vertex leads to a change in the time required to induce a topological rearrangement (T1) in a sheared foam sample. In this sense, variable surfactant coverage effects help to preserve the structure of a flowing foam, but also lead to an increase in energy storage within the foam.

Interfacial Rheology Through Microfluidics

The bulk properties and structural characteristics of emulsions arise substantially from their interfacial rheology, which depends strongly on surfactant mass transfer and its coupling to flow. Typical methods used to measure such properties often employ simpler flows and larger drops than those encountered in typical processing applications. Mass transfer mechanisms are governed by droplet size; therefore experimentation at length scales typical of those encountered in applications is desired. Utilizing a microfluidic approach allows high-throughput experimentation at relevant length scales and with adjustable flow dynamics. Using a microfluidic device that facilitates the measurement of interfacial tension in two-phase droplet flows, particle tracers are also used to determine the droplet internal circulation velocity as a measure of interfacial mobility. Combining these measurements in a single device, the coupling between interfacial tension, interfacial retardation, and surfactant mass transfer is explored and mass transfer coefficients and interfacial mobility are measured for a two-phase system containing a diffusing surfactant. Such a device is also used to probe the deformability of elastic capsules and viscoelastic biological cells.