Surfactant-laden Interface Research Articles

Interfacial flow of a surfactant-laden interface under asymmetric shear flow

HypothesisThe shear stress of the axisymmetric flow field triggers a nonuniform distribution of the surfactants at the surface of a rising bubble, known as stagnant cap. The formation of the stagnant cap gives rise to Marangoni stresses that reduce the mobility of the interface, which in return reduces the rising velocity. However, the conditions in technological processes usually deviate from the linear rise of a single bubble in a quiescent unbounded liquid. Asymmetric shear can act on the bubble surface e.g. due to the vorticity in the surrounding flow, bubble–bubble interactions, or influence of the reactor wall. A different surfactant distribution at the interface is expected under asymmetric shear, which can change the hydrodynamic behavior of the interface drastically. ExperimentsHere we conduct model experiments with a bubble or a drop at the tip of a capillary placed in a defined flow field. Thereby we investigate the influence of asymmetric shear forces on the interface in the presence of surfactants. Microscopic particle tracking velocimetry is employed to measure the velocity of the surfactant-laden interface for different degrees of asymmetry in the surrounding liquid flow. FindingsWe show a direct experimental observation of the circulating flow at the interface under asymmetric shear, which prevents the formation of the typical stagnant cap. Additionally, we reveal that the interface remains mobile regardless of the surfactant concentration. Our results confirm that increasing the degree of asymmetry increases the shear forces and thus the interfacial velocity.

Hydrodynamic response of a surfactant-laden interface to a radial flow

We study the features of a radial Stokes flow due to a submerged jet directed toward a liquid-air interface. The presence of surface-active impurities confers to the interface an in-plane elasticity that resists the incident flow. Both analytical and numerical calculations show that a minute amount of surfactants is enough to profoundly alter the morphology of the flow. The hydrodynamic response of the interface is affected as well, shifting from slip to no-slip boundary condition as the surface compressibility decreases. We argue that the competition between the divergent outward flow and the elastic response of the interface may actually be used as a practical way to detect and quantify a small amount of impurities.

Reversible Adsorption of Nanoparticles at Surfactant-Laden Liquid-Liquid Interfaces.

In this study, we show that hydrophilic nanoparticles can readily desorb from liquid-liquid interfaces in the presence of surfactants that do not change the wettability of the particles. Our observations are based on a simple theoretical approach to assess the number of adsorbed particles at the surfactant-laden liquid-liquid interface. We test this approach by studying the interfacial self-assembly of equally charged particles and lipids dissolved in separate immiscible phases. Hence, we investigate the interfacial adsorption of aminated silica particles (80 nm) and octadecylamine to the decane/water interface by interfacial tension measurements, which are supplemented by interfacial rheology of the adsorbed interfacial films, scanning electron microscopy images of Langmuir-Blodgett films, and measurements of the three-phase contact angle of the particle surface in the presence of surfactants. The measurements show that particles adsorb at the surfactant-laden interface at all investigated surfactant concentrations and compete with the surfactants for interfacial coverage. Additionally, the wettability of the hydrophilic particles does not change in the presence of the lipids, except for the highest investigated lipid concentration. Comparing the adsorption energies of one particle and of the lipids as a function of the particle contact angle provides an estimate of the tendency for interfacial adsorption of particles from which the particle coverage can be assessed. Based on these findings, equally charged particles and lipids show a competitive behavior at the interface determined by the bulk surfactant concentration and the attachment energies of the particles at the interface. This leads to a simple mechanistic model demonstrating that particles can readily desorb from the interface due to direct displacement by surfactants, which are loosely adsorbed at the oil-facing particle side. This mechanism critically lowers the otherwise high interfacial energy barrier against particle desorption, which otherwise would lead to virtually irreversible particle attachment at the interface.

Swimming sheet near a plane surfactant-laden interface.

In this work we analyze the velocity of a swimming sheet near a plane surfactant-laden interface by assuming the Reynolds number and the sheet's deformation to be small. We observe a nonmonotonic dependence of the sheet's velocity on the Marangoni number (Ma) and the surface Péclet number (Pe_{s}). For a sheet passing only transverse waves, the swimming velocity increases with an increase in Ma for any fixed Pe_{s}. When Pe_{s} is increasing, on the other hand, the swimming velocity of the same sheet either increases (at large Ma) or it initially increases and then decreases (at small Ma). This dependence of the swimming velocity on Ma and Pe_{s} is altered if the sheet is passing longitudinal waves in addition to the transverse waves along its surface.

On the interfacial roughness scale in turbulent stratified two-phase flow: 3D lattice Boltzmann numerical simulations with forced turbulence and surfactant

Numerical 3D simulations of turbulent, stratified two-phase shear flow with a surfactant laden interface were used to test and develop a phenomenological interfacial roughness scale model where the energy required to deform the interface (buoyancy, interfacial tension, and viscous work) is proportional to the turbulent kinetic energy adjacent to the interface.The turbulence was forced in the upper and lower liquids in the simulations, to emulate the interfacial dynamics without requiring (prohibitively) large simulation domains and Reynolds numbers. The addition of surfactant lead to an increased roughness scale (for the same turbulent kinetic energy) due to the introduction of interfacial dilatational elasticity that suppressed horizontal motion parallel to the interface, and enhanced the vertical motion.The phenomenological roughness scale model was not fully developed for dilatational elasticity in this work, but we proposed a source term that represents surfactant induced pressure fluctuations near the interface. This source term should be developed further to account for the relation between surfactant density fluctuations and turbulence adjacent to the interface. We foresee that the roughness scale model can be used as a basis for more general interfacial closure relations in Reynolds averaged turbulence models, where also mobile surfactant is accounted for.

Emulsion-Assisted Nanoparticle Precipitation: Time Scale Analysis and Dynamic Simulation

This contribution provides a model-based approach of a nonspecific emulsion-assisted precipitation process in order to analyze the influence of process parameters on the particle size and size distribution of primary nanoparticles. The here presented process route comprises mass transfer of reactant molecules from the continuous phase of the emulsion into the dispersed droplets in a stirred tank reactor. In a time scale analysis, the penetration of the reactant through the surfactant-laden interface can be identified as the controlling process mechanism. With regard to the rate-controlling process step, a mathematical model based on the population balance approach is derived. A high resolution discretization scheme extended by a flux limiter function is applied to solve the population balance equations. The dynamics of the particle formation process and the final particle size distribution are investigated in numerical simulations by variations of the droplet size, the initial concentration level of the d...

SCATTERING OF LIGHT ON SURFACTANT-LADEN LIQUID-LIQUID CRYSTAL INTERFACE

The spectrum of surface quasi-elastic light scattering (SQELS) on a surfactant-laden interface between nematic liquid crystal and isotropic fluid is calculated for the case when the surfactant film induces homeotropic anchoring of the nematic director. The result can be used for determination of the surface tension, compressional modulus, and a combination of the surface viscosity coefficients from the experimental SQELS data.

DNA Hybridization-Induced Reorientation of Liquid Crystal Anchoring at the Nematic Liquid Crystal/Aqueous Interface

Interactions between DNA and an adsorbed cationic surfactant at the nematic liquid crystal (LC)/aqueous interface were investigated using polarized and fluorescence microscopy. The adsorption of octadecyltrimethylammonium bromide (OTAB) surfactant to the LC/aqueous interface resulted in homeotropic (untilted) LC alignment. Subsequent adsorption of single-stranded DNA (ssDNA) to the surfactant-laden interface modified the interfacial structure, resulting in a reorientation of the LC from homeotropic alignment to an intermediate tilt angle. Exposure of the ssDNA/OTAB interfacial complex to its ssDNA complement induced a second change in the interfacial structure characterized by the nucleation, growth, and coalescence of lateral regions that induced homeotropic LC alignment. Fluorescence microscopy showed explicitly that the complement was colocalized in the same regions as the homeotropic domains. Exposure to noncomplementary ssDNA caused no such response, suggesting that the homeotropic regions were due to DNA hybridization. This hybridization occurred in the vicinity of the interface despite the fact that the conditions in bulk solution were such that hybridization did not occur (high stringency), suggesting that the presence of the cationic surfactant neutralized electrostatic repulsion and allowed for hydrogen bonding between DNA complements. This system has potential for label-less and portable DNA detection. Indeed, LC response to ssDNA target was detected with a lower limit of approximately 50 fmol of complement and was sufficiently selective to differentiate a one-base-pair mismatch in a 16-mer target.

Crossover in the wetting behavior at surfactant-laden liquid-crystal–water interfaces: Experiment and theory

The behavior of a nematic liquid crystal at a surfactant-laden interface to an aqueous phase is studied under the condition of homeotropic anchoring. It is shown that with decreasing surfactant concentration the system shifts from surface-enhanced to surface-decreased order, i.e., the behavior changes from complete nematic wetting when the nematic-isotropic phase transition is approached from above to a different wetting behavior below the transition, characterized by a considerably decreased Maier-Saupe order parameter at the interface. The experimental behavior is analyzed within the framework of the Landau-de Gennes theory supplemented by a surface free energy, in which the wetting behavior is controlled by the magnitude of the anchoring strength and the preferred surface order parameter in comparison to the bulk order parameter. The theoretical modeling is able to account for all experimental observations.

Interfacial Relaxation in Polymer Blends and Gibbs Elasticity

We have investigated the influence of different amounts of symmetric block copolymers such as poly(styrene-b-methyl methacrylate) on the form and interfacial relaxation times of a polymer blend containing mainly 7.5% polystyrene as a dispersed phase in a poly(methyl methacrylate) matrix. Special emphasis was put on the investigation of the effect of the compatibilizer molecular weight on these times. For this purpose, we combined predictions of the extended Palierne model with assumptions on the changes of interfacial tension with the amount of compatibilizer. It turned out that the identification of the interfacial modulus in Palierne's model with the Gibbs elasticity modulus as the intrinsic property of a surfactant-laden interface, responsible for the reconstitution of an equilibrium surfactant distribution, is the key point for a quantitative understanding of the ongoing processes and the interfacial relaxation time scaling. On the basis of the assumption that Gibbs elasticity is the important interfa...

On the anomalous behavior of surfactant contaminated drops

In a previous paper ( S. Padmanabhan and A. Bose, J. Colloid Interface Sci., 123, 494, 1988 ), experimental dimensionless sessile drop volumes for solutions of strong surfactant (large variation of surface tension with concentration) at the point of incipient motion on an inclined substrate were found to be several times the analytical prediction by Dussan V. ( J. Fluid Mech., 151, 1, 1985 ). These experiments were conducted by placing a drop on a horizontal substrate, then tilting further to the critical configuration. The equilibrium surface tension was used to evaluate the nondimensional drop volume at critically. In experiments reported here, drops were placed on substrates pretilted to different initial angles, with subsequent tilting to criticality. The key results are that as pretilt angle is increased, the experimental drop volumes converge to their corresponding analytical predictions, while the critical inclination angle decreases. Additionally, if the equilibrium surface tension is used for computing the capillary force, the capillary force equals the gravitational force only for the most pretilted substrate. At each pretilt angle, an average adjusted surface tension allows experimentally and analytically predicted drop volumes to match. If this surface tension is used to compute the capillary force, the capillary force equals the gravitational force at criticality for each angle of pretilt. These results suggest that morphological changes that occur when the drop goes from its initial to its final state cause a “nonequilibrium” surface tension to develop on the liquid—vapor interface. Adsorption—desorption barriers or diffusional resistances to transport of surfactants prevent equilibration within the time scale of the experiments. Furthermore, these morphological changes are opposed by the hydrodynamic rigidity of the surfactant-laden interface. The role of these barriers as well as that of surfactant elasticity is progressively reduced with increasing substrate pretilt. These experiments validate the predictive power of the analysis by Dussan V. even for contaminated liquids and show that a similar analysis could be used in other geometries as well. However, to avoid artifacts, the proper procedure for obtaining critical inclination angles is to increasingly pretilt the substrate until the drop slides spontaneously and pick the final configuration for subsequent analysis.