Modeling receding contact lines on superhydrophobic surfaces

We use mesoscale simulations to study the depinning of a receding contact line on a superhydrophobic surface patterned by a regular array of posts. For the simulations to be feasible, we introduce a novel geometry where a column of liquid dewets a capillary bounded by a superhydrophobic plane that faces a smooth hydrophilic wall of variable contact angle. We present results for the dependence of the depinning angle on the shape and spacing of the posts and discuss the form of the meniscus at depinning. We find, in agreement with ref 17 , that the local post concentration is a primary factor in controlling the depinning angle and show that the numerical results agree well with recent experiments. We also present two examples of metastable pinned configurations where the posts are partially wet.     

Influence of fluid flow on the deposition of soluble surfactants through receding contact lines of volatile solvents

Soluble surfactants are often deposited from volatile solvents through moving contact lines. In this study, we demonstrate that altering the flow field near such a contact line fundamentally changes the deposited surfactant structure. At slow contact line speeds, the substrate emerges dry. A densely packed, tilted monolayer of surfactant is deposited along the solid-vapor interface from the rolling fluid motion at the contact line. At faster speeds, the substrate emerges with an evaporating thin film entrained on its surface. Surfactant is confined in the film in a constantly increasing concentration environment. Monodisperse crystalline islands nucleate and grow on the surface with sizes and shapes controlled by varying the deposition conditions. These results contrast with disordered deposits that result from evaporation at a pinned contact line. Our results suggest that dip-coating with control of dipping speed and evaporation rate may provide better control of deposition through contact lines of evaporating solvents.     

Comments on "Bubble motion in aqueous surfactant solutions"

The numerical simulations of bubble motion in dilute surfactant solutions reported previously by two of the authors contained a serious numerical inaccuracy. In agreement with experiments, single bubbles released from rest were predicted to reach a maximum speed before slowing to a terminal speed. However, subsequent experiments demonstrated that, in the simulations, the bubbles reached their terminal speed too quickly. The source of the discrepancy is an inaccuracy associated with the numerical algorithm used to solve the surfactant transport equation on the bubble surface. After correcting the problem, the simulations agree much better with the experiments.     

Study of the advancing and receding contact angles: liquid sorption as a cause of contact angle hysteresis

Two types of experiments were used to study the behavior of both advancing and receding contact angles, namely the dynamic one-cycle contact angle (DOCA) and the dynamic cycling contact angle (DCCA) experiments. For the preliminary study, DOCA measurements of different liquids on different solids were performed using an automated axisymmetric drop shape analysis-profile (ADSA-P). From these experimental results, four patterns of receding contact angle were observed: (1) time-dependent receding contact angle; (2) constant receding contact angle; (3) 'stick/slip'; (4) no receding contact angle. For the purpose of illustration, results from four different solid surfaces are shown. These solids are: FC-732-coated surface; poly(methyl methacrylate/n-butyl methacrylate) [P(MMA/nBMA)]; poly(lactic acid) (DL-PLA); and poly(lactic/glycolic acid) 50/50 (DL-PLGA 50/50). Since most of the surfaces in our studies exhibit time dependence in the receding contact angle, a more extended study was conducted using only FC-732-coated surfaces to better understand the possible causes of decreasing receding contact angle and contact angle hysteresis. Contact angle measurements of 21 liquids from two homologous series (i.e. n-alkanes and 1-alcohols) and octamethylcyclotetrasiloxane (OCMTS) on FC-732-coated surfaces were performed. It is apparent that the contact angle hysteresis decreases with the chain length of the liquid. It was found that the receding contact angle equals the advancing angle when the alkane molecules are infinitely large. These results strongly suggest that the chain length and size of the liquid molecule could contribute to contact angle hysteresis phenomena. Furthermore, DCCA measurements of six liquids from the two homologous series on FC-732-coated surfaces were performed. With these experimental results, one can construe that the time dependence of contact angle hysteresis on relatively smooth and homogeneous surfaces is mainly caused by liquid retention/sorption. The results also suggested that the contact angle hysteresis will eventually approach a steady state, where the rate of liquid retention-evaporation or sorption process would balance out each other. If the existence of contact angle hysteresis can be attributed to liquid sorption/retention, one should only use the advancing contact angles (measured on a dry surface) in conjunction with Young's equation for surface energetic calculations.     

Advancing and receding motion of droplets on ultrahydrophobic post surfaces

We have fabricated a range of silicon post surfaces where post width and spacing have been systematically varied. As one subset, we have generated surfaces where the post spacings in x and y assume different values. On these surfaces, the dynamic contact angles become anisotropic. A fluoropolymer monolayer is photochemically attached to the microstructured silicon, leading to the appearance of ultrahydrophobic properties. On one side, the advancing contact angles on these surfaces are not affected by variations in the geometric parameters. This furthers the conclusion that, during the advancing motion, a true contact angle of 180 degrees is reached. On the other side, the receding angles are strongly influenced by the post size and spacing. We quantitatively analyze this dependence and relate variations in the receding angle to the shape and movement of the three-phase contact line. It is suggested that during the receding motion the meniscus successively dewets from one post at a time, with a step function running along the contact line until it has receded from a row of posts over its entire length.     

Ultrafiltration of surfactant solutions

The ultrafiltration of colloid solutions containing hexadecyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and alkylpolyglucoside (APG) through hydrophilic membranes with a 10,000 mol wt cut-off from regenerated cellulose was studied. The effects of experimental conditions on the permeate flux and secondary resistance were determined. It was found that both CTAB and APG were convenient surfactants for ultrafiltration, as high permeability of their solutions was observed. The secondary resistance was always significantly lower than the resistance of the membrane. Additionally, electrolytes had a relatively weak negative effect upon ultrafiltration fluxes. SDS was the least convenient surfactant due to formation of a gel layer, susceptibility of its colloid solutions to electrolyte content, and a high secondary resistance. The concentration of the surfactant in the permeate could increase above critical micelle concentration, especially under conditions inducing high polarization. Migration of CTAB on the surface of pores seemed responsible for that transfer.     

The dynamic contact angle I. Dependence of the receding contact angle on velocity in the surfactant-containing three-phase system

Spontaneous three-phase contact (tpc) motion is investigated in order to determine the dependence of the static contact angle on tpc velocity in surfactant-containing systems after recession. To interpret the experimental results, the molecular-kinetic sitechanging theory and the hydrodynamic theory were considered. It is shown that, especially at very high tpc velocities, the experimental results are not thoroughly described by these theories. The deviations are explained as a surfactant transfer from the liquid/gas to the solid/gas interface which, under insufficient afterdiffusion, leads to an increase in surface tension and to a changed surface rheology. This mechanism could be governed by a model.     

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.     

On the properties of surfactant aqueous solutions

Summary From studies of aqueous solutions of dodecylammoniumnitrate an association mechanism has been proposed involving multiequilibrium. In the concentration range considered we can differentiate between two aspects: ranges of marked qualitative and quantitative changes and formation of differently structured surfactant species.

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Zusammenfassung Aus der Untersuchung wäßriger Lösungen von Dodecylammoniumnitrat würde ein Assoziationsmechanismus abgeleitet, welcher ein Multiequilibrium beinhaltet. Im untersuchten Konzentrationsbereich können zwei Bereiche unterschieden werden: Bereiche mit ausgeprägten qualitativen und quantitativen Änderungen und Bereiche mit der Bildung von verschieden strukturierten oberflächenaktiven Spezies.

     

Computer simulation of surfactant solutions

Major advances have been made at several levels of computer simulation of surfactant solutions. Atomistic level studies of preassembled surfactant structures have become fairly routine. The development of structure in surfactant solutions has now been studied using atomistic, coarse grain and mesoscopic models. Coarse grain and mesoscopic simulations have been used to determine phase diagrams. The challenges involved in treating complex surfactant solutions will continue to drive this field forward.     

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.     

The role of the surfactant head group in the emulsification process: Single surfactant systems

To clarify the effect of the surfactant head group on the emulsification process, dilute dodecane in water emulsions were prepared in a small flow-through cell with three surfactants which had the same hydrocarbon tail length but different head groups. The different surfactants types were (a) a nonionic, hexa(ethyleneglycol) mono n-dodecyl ether (C12E6), (b) an anionic, sodium dodecyl sulfate (SDS), and (c) a cationic, n-dodecyl pyridinium chloride (DPC), and the emulsions were prepared under the same conditions. From dynamic light scattering measurements, it was shown that the mean steady state droplet size of the emulsions (obtained after 20 min dispersion) could be related to the interfacial tension at concentrations in the region of the cmc. This result was in agreement with laminar and turbulent viscous flow theory. However, the particle size versus surface tension data for the different surfactant systems did not fall on a single line. This behavior suggested that the surfactant played a secondary role in defining the droplet size (in addition to reducing the interfacial tension) possibly through diffusion and relaxation, during deformation of the interface. In addition, it was found that the values of the equilibrium "surfactant packing densities" of the different surfactants at the oil/water interface were almost equal near the cmc, but the mean droplet size and the interfacial tension at the cmc decreased following the order DPC>SDS>C12E6 .     

Line energy and the relation between advancing, receding, and young contact angles

The line energy associated with the triple phase contact line is a function of local surface defects (chemical and topographical); however, it can still be calculated from the advancing and receding contact angles to which those defects give rise. In this study an expression for the line energy associated with the triple phase contact line is developed. The expression relates the line energy to the drop volume, the interfacial energies, and the actual contact angle (be it advancing, receding, or in between). From the expression we can back calculate the equilibrium Young contact angle, theta0, as a function of the maximal advancing, thetaA, and minimal receding, thetaR, contact angles. To keep a certain maximal hysteresis between advancing and receding angles, different line energies are required depending on the three interfacial energies and the drop's volume V. We learn from the obtained expressions that the hysteresis is determined by some dimensionless parameter, K, which is some normalized line energy. The value of K required to keep a constant hysteresis (thetaA-thetaR) rises to infinity as we get closer to theta0 = 90 degrees.     

Thermodynamics of aqueous carbohydrate surfactant solutions

The paper deals with the application of the micelle formation theory, developed by Nagarajan and Ruckenstein [R. Nagarajan, E. Ruckenstein, Langmuir 7 (1991) 2934–2969] and Nagarajan [R. Nagarajan, in: K. Esumi (Ed.), Structure–Performance Relationships in Surfactants, Dekker, New York, 1997, pp. 1–81; R. Nagarajan, Adv. Colloid Interface Sci. 26 (1986) 205-264] to various n-alkyl-β-d-glucopyranoside surfactants, differing in the surfactant tail length (n-octyl-β-d-glucopyranoside C₈G₁, n-decyl-β-d-glucopyranoside C₁₀G₁ and dodecyl-β-d-glucopyranoside C₁₂G₁). The model predicts that the carbohydrate surfactant molecules assemble for energetic reasons in spherical bilayer vesicles. The critical micellar concentration as function of the temperature shows a minimum value. The formed micellar aggregates exhibit a broad distribution of sizes. It is demonstrated in this study that the thermodynamic theory in combination with phase separation thermodynamics can be used successfully to described the phase separation, which occurs for the system C₁₀G₁+water and C₁₂G₁+water at low surfactant concentrations.     

To improve alignment of isoindigo-based conjugated polymer film by controlling contact line receding velocity

The macroscopic alignment of conjugated polymers with low grain boundary is essential to carrier transport. During film forming process, the match between contact line receding velocity and the critical alignment velocity is essential to get the alignment polymer film. In this paper, the contact line receding velocity of a D-A conjugated polymer film, isoindigo and bithiophene (ⅡDDT-C3), was adjusted by solvent vapor content and film-formation temperature. Only when solvent vapor content was 0.3 mL and the film-formation temperature was 90℃, the contact line receding velocity was in accordance with the critical alignment velocity, and the highest degree of alignment was attained in the ⅡDDT-C3 film, with the dichroic ratio up to 4.08. Fibers were aligned parallel with the direction of the contact line receding and the molecules of ⅡDDT-C3 adopted an edge-on orientation with the backbone parallel with the direction of fiber long axis. The π-π stacking distance between adjacent molecules was 3.63 Å.     

The contact angles of surfactant solution on the quartz surface

Summary The contact angles formed on the quartz surface by 0,1 N NaCl aqueous solution at different pH were measured. The effect of surfactants such as anion-active sodium dodecyl sulphate (NaDS) and cation-active cetyltrimethylammonium bromide (CTAB) was investigated. The results are interpreted in terms of the Frumkin-Derjaguin theory of wetting.With 4 figures and 1 table