Unusual wetting dynamics of aqueous surfactant solutions on polymer surfaces

Static and dynamic contact angles of aqueous solutions of three surfactants--anionic sodium dodecyl sulfate (SDS), cationic dodecyltrimethylammonium bromide (DTAB), and nonionic pentaethylene glycol monododecyl ether (C(12)E(5))--were measured in the pre- and micellar concentration ranges on polymer surfaces of different surface free energy. The influence of the degree of substrate hydrophobicity, concentration of the solution, and ionic/nonionic character of surfactant on the drop spreading was investigated. Evaporation losses due to relatively low humidity during measurements were taken into account as well. It was shown that, in contrast to the highly hydrophobic surfaces, contact angles for ionic surfactant solutions on the moderately hydrophobic surfaces strongly depend on time. As far as the nonionic surfactant is considered, it spreads well over all the hydrophobic polymer surfaces used. Moreover, the results obtained indicate that spreading (if it occurs) in the long-time regime is controlled not only by the diffusive transport of surfactant to the expanding liquid-vapor interface. Obviously, another process involving adsorption at the expanding solid-liquid interface (near the three-phase contact line), which goes more slowly than diffusion, has to be active.     

Kinetics of wetting and spreading by aqueous surfactant solutions

Interest in wetting dynamics processes has immensely increased during the past 10-15 years. In many industrial and medical applications, some strategies to control drop spreading on solid surfaces are being developed. One possibility is that a surfactant, a surface-active polymer, a polyelectrolyte or their mixture are added to a liquid (usually water). The main idea of the paper is to give an overview on some dynamic wetting and spreading phenomena in the presence of surfactants in the case of smooth or porous substrates, which can be either moderately or highly hydrophobic surfaces based on the literature data and the authors own investigations. Instability problems associated with spreading over dry or pre-wetted hydrophilic surfaces as well as over thin aqueous layers are briefly discussed. Toward a better understanding of the superspreading phenomenon, unusual wetting properties of trisiloxanes on hydrophobic surfaces are also discussed.     

Model and experimental studies for contact angles of surfactant solutions on rough and smooth hydrophobic surfaces

Despite the practical need, no models exist to predict contact angles or wetting mode of surfactant solutions on rough hydrophobic or superhydrophobic surfaces. Using Gibbs' adsorption equation and a literature isotherm, a new model is constructed based on the Wenzel and Cassie equations. Experimental data for aqueous solutions of sodium dodecyl sulfate (SDS) contact angles on smooth Teflon surfaces are fit to estimate values for the adsorption coefficients in the model. Using these coefficients, model predictions for contact angles as a function of topological f (Cassie) and r (Wenzel) factors and SDS concentration are made for different intrinsic contact angles. The model is also used to design/tune surface responses. It is found that: (1) predictions compare favorably to data for SDS solutions on five superhydrophobic surfaces. Further, the model predictions can determine which wetting mode (Wenzel or Cassie) occurred in each experiment. The unpenetrated or partially penetrated Cassie mode was the most common, suggesting that surfactants inhibit the penetration of liquids into rough hydrophobic surfaces. (2) The Wenzel roughness factor, r, amplifies the effect of surfactant adsorption, leading to larger changes in contact angles and promoting total wetting. (3) The Cassie solid area fraction, f, attenuates the lowering of contact angles on rough surfaces. (4) The amplification/attenuation is understood to be due to increased/decreased solid-liquid contact-area.     

Lattice model for the wetting transition of alkanes on aqueous surfactant solutions

Droplets of alkanes on aqueous solutions of the cationic surfactants C(n)TAB (CH3(CH2)(n-1)N+ (CH3)3Br-) exhibit a first-order wetting transition as the concentration of the surfactant is increased. A theoretical model is presented in which the surface free energy is broken down into a long-range dispersion interaction and a short-range interaction described by a 2D lattice gas, taking into account the interaction between oil and surfactant molecules. The model provides quantitative agreement with the observed wetting transitions and the variation in composition of the wetting film with bulk surfactant concentration. The behavior of oil drops on large reservoirs of dilute surfactant is discussed.     

Surfactants and wetting at superhydrophobic surfaces: water solutions and non aqueous liquids

Wetting studies regarding amphiphilic molecules and adsorption properties on highly water repellent solid surfaces play key roles in research and technology, with increasing interest both in fundamental and application fields. Nevertheless the wetting properties of aqueous surfactant solutions, non aqueous liquids or immiscible phases on superhydrophobic (SH) solid surfaces have been so far rarely investigated. In this work the authors give an overview on this topic reviewing the literature available together with preliminary results concerning the influence of the distribution properties of surfactants between two immiscible phases. Transition between wetting states can be also considered a possible development of these studies based on switching mechanisms.     

Physical properties of nanobubbles on hydrophobic surfaces in water and aqueous solutions

In recent years there has been an accumulation of evidence for the existence of nanobubbles on hydrophobic surfaces in water, despite predictions that such small bubbles should rapidly dissolve because of the high internal pressure associated with the interfacial curvature and the resulting increase in gas solubility. Nanobubbles are of interest among surface scientists because of their potential importance in the long-range hydrophobic attraction, microfluidics, and adsorption at hydrophobic surfaces. Here we employ recently developed techniques designed to induce nanobubbles, coupled with high-resolution tapping-mode atomic force microscopy (TM-AFM) to measure some of the physical properties of nanobubbles in a reliable and repeatable manner. We have reproduced the earlier findings reported by Hu and co-workers. We have also studied the effect of a wide range of solutes on the stability and morphology of these deliberately formed nanobubbles, including monovalent and multivalent salts, cationic, anionic, and nonionic surfactants, as well as solution pH. The measured physical properties of these nanobubbles are in broad agreement with those of macroscopic bubbles, with one notable exception: the contact angle. The nanobubble contact angle (measured through the denser aqueous phase) was found to be much larger than the macroscopic contact angle on the same substrate. The larger contact angle results in a larger radius of curvature and a commensurate decrease in the Laplace pressure. These findings provide further evidence that nanobubbles can be formed in water under some conditions. Once formed, these nanobubbles remain on hydrophobic surfaces for hours, and this apparent stability still remains a well-recognized mystery. The implications for sample preparation in surface science and in surface chemistry are discussed.     

Influence of aqueous electrolytes on the wetting behavior of hydrophobic solid polymers-low-rate dynamic liquid/fluid contact angle measurements using axisymmetric drop shape analysis

The interaction of inorganic ions with low-energy hydrophobic surfaces was examined using model systems of solid polymers without ionizable functional surface groups in aqueous electrolyte solutions. Low-rate dynamic contact angle measurements with captive bubbles in conjunction with axisymmetric drop shape analysis (ADSA) were performed to study the influence of electrolyte ions (in the aqueous test solutions) on the wettability of the polymers. When various types of ions were used, no significant change in advancing and receding contact angles was observed. The contact angle hysteresis was small. The zeta potential of the model polymers in aqueous electrolyte solutions was determined from streaming potential measurements. The variation of the zeta potential at different pH levels indicates preferential adsorption of hydroxyl ions at this interface. However, the presence of electrolytes at the interface between water and the different model polymers did not influence the macroscopic contact angle. The results may suggest the absence of any specific interaction between the ions and the solid polymer, as this should result in changes of hydrophobicity. Similar to the air/water interface, the composition and the potential of the polymer/water interface are obviously determined predominantly by the aqueous phase with only slight influence from the solid phase.     

Partial wetting of chemically patterned surfaces: the effect of drop size

Partial wetting of chemically heterogeneous substrates is simulated. Three-dimensional sessile drops in equilibrium with smooth surfaces supporting ordered chemical patterns are considered. Significant features are observed as a result of changing the drop volume. The number of equilibrated drops is found either to remain constant or to increase with growing drop volume. The shape of larger drops appears to approach that of a spherical cap and their three-phase contact line seems, on a larger scale, more circular in shape than that of smaller drops. In addition, as the volume is increased, the average contact angle of drops whose free energy is lowest among all equilibrium-shaped drops of the same volume appears to approach the angle predicted by Cassie. Finally, contrary to results obtained with two-dimensional drops, contact angle hysteresis observed in this system is shown to exhibit a degree of volume dependence in the advancing and receding angles. Qualitative differences in the wetting behavior associated with the two different chemical patterns considered here, as well as differences between results obtained with two-dimensional and three-dimensional drops, can possibly be attributed to variations in the level of constraint imposed on the drop by the different patterns and by the dimensionality of the system.     

VOF simulations of the contact angle dynamics during the drop spreading: Standard models and a new wetting force model

Introduction

In this study,a novel numerical implementation for the adhesion of liquid droplets impacting normally on solid dry surfaces is presented. The advantage of this new approach, compared to the majority of existing models, is that the dynamic contact angle forming during the surface wetting process is not inserted as a boundary condition, but is derived implicitly by the induced fluid flow characteristics (interface shape) and the adhesion physics of the gas–liquid-surface interface (triple line), starting only from the advancing and receding equilibrium contact angles. These angles are required in order to define the wetting properties of liquid phases when interacting with a solid surface.

Methodology

The physical model is implemented as a source term in the momentum equation of a Navier-Stokes CFD flow solver as an “adhesion-like” force which acts at the triple-phase contact line as a result of capillary interactions between the liquid drop and the solid substrate. The numerical simulations capture the liquid–air interface movement by considering the volume of fluid (VOF) method and utilizing an automatic local grid refinement technique in order to increase the accuracy of the predictions at the area of interest, and simultaneously minimize numerical diffusion of the interface.

Results

The proposed model is validated against previously reported experimental data of normal impingement of water droplets on dry surfaces at room temperature. A wide range of impact velocities, i.e. Weber numbers from as low as 0.2 up to 117, both for hydrophilic (θ_adv = 10° – 70°) and hydrophobic (θ_adv = 105° – 120°) surfaces, has been examined. Predictions include in addition to droplet spreading dynamics, the estimation of the dynamic contact angle; the latter is found in reasonable agreement against available experimental measurements.

Conclusion

It is thus concluded that theimplementation of this model is an effective approach for overcoming the need of a pre-defined dynamic contact angle law, frequently adopted as an approximate boundary condition for such simulations. Clearly, this model is mostly influential during the spreading phase for the cases of low We number impacts (We < ˜80) since for high impact velocities, inertia dominates significantly over capillary forces in the initial phase of spreading.     

Equilibrium and kinetics of wetting hydrophobic microporous membrane in sodium dodecyl benzene sulphonate and diethanolamine aqueous solutions

Liquid penetrations into hydrophobic capillaries are controlled by the adsorption and diffusion of the solute in the vicinity of the moving meniscus. The wetting process of microporous hydrophobic polyvinylidenefluoride (PVDF) and polytetrafluoroethylene (PTFE) membrane was investigated in both sodium dodecyl benzene sulphonate (SDBS) and diethanolamine (DEA) aqueous solutions. The experimental results revealed that wetting both the PVDF and PTFE membranes in SDBS solutions at high concentrations proceeded in two stages: rapid wetting and slow wetting, but this transition in the wetting rate was not observed during the membrane wetting at low SDBS concentration and in DEA solutions. The membrane wetting process was accelerated by increasing the solution temperature.     

Molecular dynamics simulation of aqueous NaF and NaI solutions near a hydrophobic surface

We present results from molecular dynamics simulation of aqueous solutions of alkali halide salts (NaI and NaF) at the interface with hydrophobic objects. The primary objective of this study is to investigate the structural properties of the salt solutions at the hydrophobic surface. An alkane crystal has been taken as the parent model for a hydrophobic surface. A hexagonal hole was created on it, which was half a nm deep and 2.5 nm wide. The density distributions of different species (water, anions, and cations) are studied as a function of distance from the surface. While iodide prefers the interface, the fluoride ions stay inside the bulk water region. The higher concentration of iodide ions at the interface drags sodium counterions to the interface. It also decreases the water density at the interface because of steric effects of the iodide ions. The number of contacts between the surface carbons and water decreases in the case of NaI solutions but is unchanged for NaF solutions. The orientation of the water-ion and the water-water hydrogen bond vector orientations near the interface is discussed in detail.     

Ultrasonic nebulization in aqueous solutions and the role of interfacial adsorption dynamics in surfactant enrichment

High-density micron-sized aerosols from liquid surfaces were generated using an ultrasonic (frequency = 1056 kHz) nebulization technique in the absence and presence of a number of surfactants. The surfactants included cationic surfactants, cetylpyridinium chloride and dodecylpyridinium chloride, and anionic surfactants, sodium dodecylbenzenesulfonate and sodium benzenesulfonate. The nebulization process generated aerosols of a narrow size distribution with a number mean diameter of about 3.4 mum, which is close to the theoretical value suggested by the Lang Equation. The aerosol droplets are enriched in surfactant as a consequence of the large interfacial area. The enrichment factor varied for different surfactants, depending on their surface activity. The extent of enrichment can be related to the rate of mass transfer of surfactant to the liquid surface. Surface concentrations of between 15 and 30% of the equilibrium value are observed, indicating turbulent mass transfer is the rate limiting step.     

一种新型有机硅表面活性剂提高药液润湿性能的研究

研究了w(阿维菌素)=3%微乳剂,w(仲丁威)=15%微乳剂稀释液在一种含葡萄糖酰胺基的有机硅表面活性剂存在时的表面张力和接触角。对于阿维菌素微乳剂稀释液,当有机硅表面活性剂质量分数由0增加到0.0014%时,表面张力由32.4mN/m迅速降低到20.4mN/m。对于仲丁威微乳剂稀释液,当有机硅表面活性剂质量分数由0增加到0.014%时,表面张力由30.4 mN/m迅速降低到22.1mN/m,之后继续增加有机硅表面活性剂质量分数,表面张力基本不变。阿维菌素微乳剂稀释液接触小麦叶面瞬间的接触角(95°),比在梨树叶面的接触角(64°)高。稀释液在植物叶面的接触角随时间的推移和有机硅表面活性剂质量分数的增加而减小,并且稀释液在梨树叶面比小麦叶面润湿铺展快得多。     

Dynamic wetting and spreading characteristics of a liquid droplet impinging on hydrophobic textured surfaces

We report on the wetting dynamics of a 4.3 μL deionized (DI) water droplet impinging on microtextured aluminum (Al 6061) surfaces, including microhole arrays (hole diameter 125 μm and hole depth 125 μm) fabricated using a conventional microcomputer numerically controlled (μ-CNC) milling machine. This study examines the influence of the texture area fraction ?(s) and drop impact velocity on the spreading characteristics from the measurement of the apparent equilibrium contact angle, dynamic contact angle, and maximum spreading diameter. We found that for textured surfaces the measured apparent contact angle (CA) takes on values of up to 125.83°, compared to a CA of approximately 80.59° for a nontextured bare surface, and that the spreading factor decreases with the increased texture area fraction because of increased hydrophobicity, partial penetration of the liquid, and viscous dissipation. In particular, on the basis of the model of Ukiwe and Kwok (Ukiwe, C.; Kwok, D. Y. Langmuir 2005, 21, 666), we suggest a modified equation for predicting the maximum spreading factor by considering various texturing effects and wetting states. Compared with predictions by using earlier published models, the present model shows better agreement with experimental measurements of the maximum spreading factor.     

Unsteady motion of receding contact lines of surfactant solutions: the role of surfactant re-self-assembly

Re-self-assembly of surfactant molecules must occur at moving contact lines of soluble surfactant solutions. Molecules are transported into and out of the contact line region from four sources: the three interfaces meeting at the contact line and the fluid confined between the solid-liquid and liquid-vapor interfaces. As molecules move among these sources at the contact line, they must rearrange. The dynamics of this re-self-assembly has been shown to have a dominating effect on the structure of advancing contact lines, causing unsteady motion and complex structure of the contact line. It might be assumed that the re-self-assembly for receding contact lines leads to more steady contact line movement. However, in this article we show that for a wide variety of systems this is not true. Quasi-static distortions of the contact line occur as it retreats because of the inability of the surfactant to completely re-self-assemble at localized positions along the contact line.     

Dispersing carbon nanotubes in aqueous solutions by a silicon surfactant: Experimental and molecular dynamics simulation study

The dispersion effect of carbon nanotubes (CNTs) in aqueous solutions by a silicon surfactant (ethoxy modified trisiloxane, named Ag-64) was investigated in detail using experimental method and molecular dynamics simulation. The Si–O–Si chain of silicon surfactant was flexible due to long Si–C bond and it could easily wrap onto the surface of CNTs through hydrophobic and other intermolecular interactions. The hydrophilic part of PEO provided the CNTs dispersed in the aqueous solution and prevented CNTs from aggregating in water through steric stabilization. It was found that Ag-64 could disperse CNTs with different diameters and it was an effective dispersing agent. The results of molecular dynamics simulation indicated that Ag-64 molecules could wrap onto the surface of CNTs leading to steric stabilization so that it could well disperse CNTs, and Van der Waals attraction was the dominating force of Ag-64 adsorbing onto CNTs. Our study may provide experimental and theoretical basis for using silicon surfactants to disperse CNTs, which can open the avenue of new applications for silicon surfactants.     

Impact of surface forces on wetting of hierarchical surfaces and contact angle hysteresis

The influence of the long-range surface forces on the wetting of multi-scale partially wetted surfaces is discussed. The possibility of partial wetting is stipulated by a specific form of the Derjaguin isotherm. Equilibrium of a liquid meniscus inside a cylindrical capillary is used as a model. The interplay of capillary and disjoining pressures governs the equilibrium of the liquid in the nano- and micrometrically scaled pores constituting the relief of the surface. It is shown that capillaries with a radius smaller than a critical one will be completely filled by water, whereas the larger capillaries will be filled only partially. Thus, small capillaries will show the Wenzel type of wetting behavior, while the same liquid inside the large capillaries will promote the Cassie-Baxter type of wetting. Consideration of disjoining/conjoining pressure allows explaining of the “rose petal effect”, when a high apparent contact angle is accompanied with a high contact angle hysteresis.     

The hydrophobic effect: molecular dynamics simulations of water confined between extended hydrophobic and hydrophilic surfaces

Structural and dynamic properties of water confined between two parallel, extended, either hydrophobic or hydrophilic crystalline surfaces of n-alkane C(36)H(74) or n-alcohol C(35)H(71)OH, are studied by molecular dynamics simulations. Electron density profiles, directly compared with corresponding experimental data from x-ray reflectivity measurements, reveal a uniform weak de-wetting characteristic for the extended hydrophobic surface, while the hydrophilic surface is weakly wetted. These microscopic data are consistent with macroscopic contact angle measurements. Specific water orientation is present at both surfaces. The ordering is characteristically different between the surfaces and of longer range at the hydrophilic surface. Furthermore, the dynamic properties of water are different at the two surfaces and different from the bulk behavior. In particular, at the hydrophobic surface, time-correlation functions reveal that water molecules have characteristic diffusive behavior and orientational ordering due to the lack of hydrogen bonding interactions with the surface. These observations suggest that the altered dynamical properties of water in contact with extended hydrophobic surfaces together with a partial drying of the surfaces are more indicative of the hydrophobic effect than structural ordering, which we suggest to be independent of surface topology.     

Oil coating of hydrophobic surfaces from aqueous media: formation and kinetic study

We perform oil coating of hydrophobic solid surfaces via aqueous media, from emulsions, and under the presence of a shear flow. The principle of such coating is based on the use of a system at the limit of aggregation to give rise to adhesion, with asymmetrical interfaces (oil droplet/water and solid surface/water) in order to favor the oil/surface adhesion in comparison to the oil/oil adhesion. This way, droplets stick to the solid substrate, whereas they are stable and homogeneously dispersed in the bulk. We have realized coatings from two systems of emulsions made of a mixture of hydroxy-terminated silicone oil and classical silicone oil and a mixture of sunflower oil and mineral oil. The kinetics of the coating is described by a Langmuir model where the adhesion between the oil particle and the surface is modeled as a first-order reaction. The resulting coatings are formed of oil droplets uniformly covering the solid surface. The coating density can vary with the nature of the experimental systems.