Dynamic behavior of polymer surface and the time dependence of contact angle

The increased attention has been focused on the re-searches of soft materials proposed by Pierre-Gilles de Gennes, a Nobel Prize Laureate in Physics. A special issue of “Science” on soft surfaces was published in 2002 to review specific surface properti…     

Advancing and receding wetting behavior of a droplet on a narrow rectangular plane

The contact angle (CA) measurements are generally performed on a large planar surface of a specific substrate with the width larger than the droplet size. In this study, the contact angle hysteresis on a narrow rectangular plane with a width smaller than the droplet size is experimentally studied through the inflation–deflation process by the needle–syringe method. The inflation process by stepwise addition of the liquid to the droplet leads to the contact line advancing outwardly along the major axis with advancing angle (θ_a). Although the droplet width is constrained by the edge of the plane, the CA along the minor axis (θ_w) increases and its value is greater than θ_a (θ_w > θ_a). Deflation process by stepwise withdrawal of liquid from the droplet results in the contact line retracting inwardly along the major axis as the CA reduces to receding angle (θ_r). In the meantime, the CA along the minor axis decreases as well. Both advancing and receding angles acquired from the narrow rectangular plane are confirmed with those obtained form the typical large surface of acrylic glass. On the basis of free energy minimization and liquid-induced defects model, Surface Evolver simulations are performed to reproduce the behavior of droplet on the narrow rectangular plane during the inflation–deflation process. The results of experiment and simulation agree with each other very well.     

Creation of low hysteresis superhydrophobic paper by deposition of hydrophilic diamond-like carbon films

The creation of low hysteresis superhydrophobic paper is reported using a combination of oxygen plasma etching and plasma deposition of an 80 nm non-fluorinated, hydrophilic diamond-like carbon (DLC) coating. The DLC has an equilibrium (flat surface) contact angle (θ _e ) of 68.2° ± 1.5°, which is well below the 90° contact angle that is typically believed to be a prerequisite for superhydrophobicity. Coating of paper substrates with the DLC film yields an advancing contact angle of 124.3° ± 4.1°, but the surface remains highly adhesive, with a receding contact angle <10°. After 60 min of plasma etching and DLC coating, a low hysteresis, superhydrophobic surface is formed with an advancing contact angle of 162.0° ± 6.3° and hysteresis of 8.7° ± 1.9°. To understand the increase in contact angle and decrease in hysteresis, atomic force microscopy and optical profilometry studies were performed. The data demonstrates that while little additional nanoscale roughness is imparted beyond the first 5 min of etching, the roughness at the microscale continually increases. The hierarchical structure provides the appropriate roughness to create low hysteresis superhydrophobic paper from a hydrophilic coating.     

Contact angle hysteresis

In thermodynamic equilibrium, the contact angle is related by Young's equation to the interfacial energies. Unfortunately, it is practically impossible to measure the equilibrium contact angle. When for example placing a drop on a surface its contact angle can assume any value between the advancing Θ_a and receding Θ_r contact angles, depending on how the drop is placed. Θ_a − Θ_r is called contact angle hysteresis. Contact angle hysteresis is essential for our daily life because it provides friction to drops. Many applications, such as coating, painting, flotation, would not be possible without contact angle hysteresis. Contact angle hysteresis is caused by the nanoscopic structure of the surfaces. Here, we review our current understanding of contact angle hysteresis with a focus on water as the liquid. We describe appropriate methods to measure it, discuss the causes of contact angle hysteresis, and describe the preparation of surfaces with low contact angle hysteresis.     

Surface characteristics of polysulfoalkyl methacrylates

Coated hydroxyethyl methacrylate-sodium sulfoalkyl methacrylate copolymer films were surface characterized. The contact angle hysteresis increases and the receding angle decreases with increasing alkyl side-chain length, while the advancing angle decreases with hydration time. It was found that the buoyancy slopes of the advancing (r_a) and receding (r_r) process determined by the Wilhelmy plate method were not parallel. The ratio of r_a to r_r was greater than 1, and increases with the alkyl side-chain length and the hydration time, contrary to that of polyhydroxyethyl methacrylate, where r_a/r_r was less than 1. The slope ratio would be suppressed in solution with added salt, revealing that the reorientation and expansion of the polymer chain in water is being suppressed. X-ray photoelectron spectroscopy (XPS) analysis of the surface of these copolymers showed a striking enrichment of the sulfonate groups in the surface. The zeta potential was between −40 and −50 mV as measured by the streaming potential method. During dehydration, along with a decrease in sulfur and sodium concentration in the surface, the carbon 1s peak at the high binding energy decreased and the alkyl carbon main peak increased. The surface tension of aqueous solutions of sulfoalkyl methacrylate monomers and homopolymers decreases with increasing alkyl side-chain length, which may contribute to the decrease in water-polymer film interfacial tension and thus the increase in the slope ratio.     

Wetting properties of aqueous solutions of hydrophobically modified inulin polymeric surfactant

The contact angles of aqueous solutions of a polymeric surfactant namely hydrophobically modified inulin (INUTEC®SP1) were measured on hydrophilic and hydrophobised quartz glass surfaces using the sessile drop technique. These measurements showed a large difference (>10°) between the advancing contact angle θ ₁ (that is measured immediately after placing the drop on the surface) and the constant contact angle θ ₂ (that is measured 30 minutes after placing the drop). In all the results only the contact angle θ ₂ was subsequently measured. θ versus INUTEC®SP1 concentration C _s curves were obtained at various NaCl concentrations both on hydrophilic and hydrophobic glass surfaces. On hydrophilic glass surface the θ versus C _s curves showed a maximum at a concentration range of 10^–6 to 2?×?10^–5 mol dm^-3 INUTEC®SP1. These curves were shifted to lower values as the NaCl concentration was increased. On such hydrophilic surface the INUTEC®SP1 molecule adsorbs with the polyfructose loops and tails oriented towards the surface leaving the alkyl chains in solution. Saturation adsorption with this orientation occurs at 2?×?10^–5 mol dm^-3 INUTEC®SP1. However, the contact angles remain quite small (<18°) indicating the presence of several hydrophilic glass patches uncovered by surfactant molecules. At C _s?>?2?×?10^–5 mol dm^-3 θ decreases with further increase of the INUTEC®SP1 concentration reaching 5° at the Critical Association Concentration (CAC) of the polymer. This indicates the formation of a bilayer of INUTEC®SP1 molecules with the alkyl chains hydrophobically attached to those of the first layer. On a hydrophobic glass surface, adsorption of INUTEC®SP1 occurs by multi-point attachment with the alkyl chains on the surface leaving the hydrophilic polyfructose loops and tails dangling in solution. This results in a gradual decrease of the contact angle with increase in INUTEC®SP1 concentration, reaching a plateau value (>85°) between 2?×?10^–5 and 2?×?10^–4 mol dm^-3. The large contact angles obtained on adsorption of the polymeric surfactant on a hydrophobic surface indicate the presence of several uncovered hydrophobic patches. These results give a reasonable picture of the adsorption and orientation of the INUTEC®SP1 molecules on both hydrophilic and hydrophobic solid surfaces.     

Experimental and theoretical studies of the topological and energy parameters in the crystal of 4,7-di-tert-butyl-2-phenyl-1,3,2-benzodioxaborole

The electron density distribution in the crystal of 4,7-di-tert-butyl-2-phenyl-1,3,2-benzodioxaborole (1) was examined both experimentally and theoretically. According to the theory “Atoms in Molecules”, the B-O bonds are “intermediate” interactions (?²γ(r) > 0, h _e(r) < 0), while the B-C(Ph), O-C, and C-C bonds are “shared” (?²γ(r) < 0, h _e(r) < 0). The energies of intra- and intermolecular interactions in the crystals were estimated. Compound 1 is inert to oxygen, which agrees with the low HOMO energy (?6.26 eV).     

Polyoxyalkylated diethylenetriamine polymeric surfactants: molecular structure effect on thin wetting films

Thin wetting films from aqueous solution of four polyoxyalkylated diethylenetriamine (DETA) polymeric surfactants (named A, B, C, and D) are studied. Surfactants A, B, and C have a star-like structure differing only by the number of polymeric branches: four, six, and nine in the mentioned order while the forth one, D, is of a dendritic type with four to six primary and two to three secondary branches. The receding contact angles θ _r of the solution on hydrophilic SiO₂ glass surface and the contact angle θ _aq of a drop of doubly distilled water on SiO₂ glass surface pretreated with DETA polymeric surfactant solution are measured. The θ _r values on hydrophobicity of SiO₂ glass surface, respectively, increase in the following order: surfactant A, surfactant C, surfactant B, and surfactant D. The equilibrium thickness h _eq of wetting films from DETA aqueous solution on hydrophilic SiO₂ glass surface is measured using the micro-interferometric method. Results show an unexpected course of the h _eq vs. C _s curves with a maximum. Results from the studies indicate that differences in polymeric surfactant molecular structure affect the properties exhibited at air/liquid and solid/liquid interfaces, such as the value of surface tension, degree of hydrophobicity of solid surface, equilibrium film thickness, etc.     

eaq− hydration energy and photoemission threshold potentials for mercury in contact with aqueous electrolytes

The meaning of the “red limit” potential in photoemission experiments is discussed. For mercury in contact with aqueous electrolytes, the energy of a photoemitted electron at the “red limit” is 0.6 eV higher than the solvation energy of e_aq^?. This difference is attributed to the solvent reorganization energy contribution to the hydration energy of e_aq^?.     

Anomalous contact angle hysteresis of a captive bubble: advancing contact line pinning

Contact angle hysteresis of a sessile drop on a substrate consists of continuous invasion of liquid phase with the advancing angle (θ(a)) and contact line pinning of liquid phase retreat until the receding angle (θ(r)) is reached. Receding pinning is generally attributed to localized defects that are more wettable than the rest of the surface. However, the defect model cannot explain advancing pinning of liquid phase invasion driven by a deflating bubble and continuous retreat of liquid phase driven by the inflating bubble. A simple thermodynamic model based on adhesion hysteresis is proposed to explain anomalous contact angle hysteresis of a captive bubble quantitatively. The adhesion model involves two solid–liquid interfacial tensions (γ(sl) > γ(sl)′). Young’s equation with γ(sl) gives the advancing angle θ(a) while that with γ(sl)′ due to surface rearrangement yields the receding angle θ(r). Our analytical analysis indicates that contact line pinning represents frustration in surface free energy, and the equilibrium shape corresponds to a nondifferential minimum instead of a local minimum. On the basis of our thermodynamic model, Surface Evolver simulations are performed to reproduce both advancing and receding behavior associated with a captive bubble on the acrylic glass.     

The absorption spectrum of carbon monoxide in the CO(3Π r,a) state between 215 nm and 285 nm

Bye-beam excitation of a He/CO mixture the CO(³Π _r ,a) state was sufficiently populated to allow the measurement of the absorption spectrum. The (0, 0), (1, 1), (2, 2) and (0, 1) bands of thec ³Π←a ³Π system of CO have been observed and the molecular constantsT _e =92036.0 cm^?1 (for the band head), ω _e =2249.5 cm^?1, ω _e x _e =29.5 cm^?1 have been derived for CO(c). A new electronic state withT _e =91854.3 cm^?1, ω _e =848.4 cm^?1, ω _e x _e =9.8 cm^?1,B _e =1.351 cm^?1, and α _e =0.021 cm^?1 was identified to be a³Σ state. It seems to be very likely that this state is the CO (3pσ,³Σ,j) state discussed in the literature. The results indicate a perturbation of the υ=1 levels of the new state by the CO (c,υ=0) levels. Another strong perturbation is found in the υ=4 levels. The three CO(³Σ,b,υ′=0,1,2)←CO (a,υ″=0) bands were also investigated yielding for CO(b):T _e =83778 cm^?1, ω _e =2335 cm^?1, ω _e x _e =59 cm^?1 andB _e =1.86 cm^?1.     

Wetting behavior of pulmonary surfactant aqueous solutions

The wetting properties of pulmonary surfactant aqueous solutions with respect to solid surfaces with different degree of hydrophobicity have been studied. The contact angles θ of drops from a pulmonary surfactant solution onto SiO₂-glass surfaces have been measured as a function of their degree of hydrophobicity θ _w. The completely hydrophilic SiO₂-glass surface is essentially hydrophobized by the animal-derived pulmonary surfactant Curosurf. The hydrophobization depends on the surfactant concentration—the contact angles increase with increasing the Curosurf concentration C _s in the low concentration range, but they remain almost constant in a wide range of C _s >90 μg/ml. Additions like NaCl and bovine serum albumin influence the θ-values. The contact angles θ naturally increase with increasing θ _w but this dependence is not linear—the curve steepens at larger θ _w values. The thickness h of the wetting thin liquid films from Curosurf aqueous solutions depends on the hydrophobicity θ _w of the solid surface and the h(θ _w) curves always pass a minimum. The h-values, as well as the h(θ _w) curve, are mainly determined by the steric and hydrophobic disjoining pressures, which depend on the orientations and conformations of the molecules adsorbed on the solid surface from the very complicated multi-component aqueous solutions.     

Dynamic Contact Angles of Water in Ultrathin Capillaries

Velocities of motion V of advancing meniscus of water in quartz capillaries with radii from 45 to 270 nm was directly measured using an optical microscope. In the case, when the meniscus advanced over the wetting film that is remained after the previous meniscus receding, hysteresis was not observed, and the wetting was complete. When the meniscus advanced over the yet unwetted surface, the dynamic contact angle _d greatly depended on V, this dependence was the more pronounced, the smaller the r value. As the velocity V increases to 10^–3 cm s^–1, the value of _d rises to 60°–70° reaching the plateau. Preliminary adsorption of water vapors on the capillary surface markedly decreases the values of _d. The results obtained cannot be explained in terms of hydrodynamic and barrier theories of the contact angle. It was assumed that the controlling factor is the kinetics of vapor adsorption on the capillary surface in front of advancing meniscus.     

Adsorption of oil into surfactant monolayers and structure of mixed surfactant+oil films

We describe a tensiometric method for determining the adsorption isotherm of an oil on a surfactant monolayer adsorbed at the air–water surface. The method involves measuring the surface pressure of oil, π_oil, as a function of its activity, a_oil, varied by changing the relative vapour pressure. We compare the isotherm of dodecane adsorption onto a C₁₂E₅ monolayer determined in this way with that measured directly using neutron reflectivity. The agreement between the two allows us to conclude that, at least for this system, addition of oil does not result in a change in the chemical potential of the surfactant. Structural analysis of the dodecane+C₁₂E₅ mixed film has been performed with neutron reflectivity using two contrasts. In one, only the surfactant chain region is highlighted, whilst in the other only the oil film is visible. We document, for the first time, changes in thickness and packing density of both the oil and surfactant chains in the mixed surfactant+oil layer upon increasing oil content. For a diverse range of other oil+surfactant systems, we have determined the initial (S_i) and equilibrium (S_e) spreading coefficients of oil by measuring π_oil following addition of liquid oil (at unit activity) to surfactant solution surfaces. Those systems in which S_e is close to zero display a repulsive van der Waals component of the disjoining pressure-oil film thickness isotherm, whilst with toluene as oil the calculated isotherm is attractive, consistent with non-spreading observed for this oil.     

Nonbonded interactions. 1. Anisotropic hydrogen-hydrogen interactions

The hydrogen-hydrogen nonbonded potential which may be derived from the calculated interactions between hydrogen molecules has been obtained. The best three-parameter Buckingham function gave an RMS error of 0.18 kcal/mol in fitting Price and Stone's ab initio data for 130 pairs of hydrogen molecules, which may be compared with an RMS error of 0.74 kcal/mol using the parameters in the MM2 force field. Burton's basis set is also considered. A better fit to these data requires that the angular relationship between the bonds be included. The data for hydrogen as well as experimental data for chlorine show that these atoms appear “larger” normal to the bond axis than along the axis, and this is probably also the case for other atoms. When simple angular terms are added it is possible to fit the Price and Stone data set with an RMS error of less than 0.06 kcal/mol. The preferred function was: V = [a₀ + a₁(sin θ₁ + sin θ₂)⁴ + a₂r]e^?3r ? [c₀ + c₁(1 + sinθ₁sinθ₂)]/r⁵. Deficiencies in the current ab initio data are discussed.     

Spreading of liquid drops over dry surfaces

Spreading of thin, axisymmetric, non-volatile, Newtonian liquid drops over a dry, smooth, flat solid surface is considered both theoretically and experimentally in the case of complete wetting. The drop profile is solved analytically by matching the “outer” solution for large film thicknesses, where only the capillary effects are important, with the “inner” solution for small film thicknesses, where the viscous and disjoining pressure effects are comparable to capillary effects. It is shown that the apparent radius of the wetted spot, the apex height of the drop, and the apparent advancing dynamic contact angle follow different power laws in time and the advancing dynamic contact angle follows a power law in capillary number. Both the prefactor and the exponent of each power law are derived theoretically. Good agreement between the theory predictions and experimental measurements is shown for both the prefactor and exponent of each power law. It is necessary to emphasize that the theory suggested does not include any fitting parameters.     

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.     

Effect of hydrophobicity on the stability of the wetting films of water formed on gold surfaces

We have developed a methodology that can be used to determine disjoining pressures (Π) in both stable and unstable wetting films from the spatial and temporal profiles of dynamic wetting films. The results show that wetting films drain initially by the capillary pressure created by the changes in curvature at the air/water interface and subsequently by the disjoining pressure created by surface forces. The drainage rate of the film formed on a gold surface with a receding contact angle (θ(r)) of 17° decreases with film thickness due to a corresponding increase in positive Π, resulting in the formation of a stable film. The wetting film formed on a hydrophobic gold with θ(r)=81° drains much faster due to the presence of negative Π in the film, resulting in film rupture. Analysis of the experimental data using the Frumkin-Derjaguin isotherm suggests that short-range hydrophobic forces are responsible for film rupture and long-range hydrophobic forces accelerate film thinning.     

Carbon-13 NMR spectra of some chlorins and other chlorophyll degradation products

The ¹³C NMR spectra of the methyl esters of phaeophorbides-a and -b, mesophaeophorbides-a and -b, pyrophaeophorbide-a, mesopyrophaeophorbide-a, chlorin-e₆, mesochlorin-e₆, chlorin-p₆, rhodin-g₇, mesorhodin-g₇, phaeoporphyrin-a₅, 2-vinylphaeoporphyrin-a₅, rhodoporphyrin-XV, and 2-vinylrhodoporphyrin-XV, and of trans-octaethylchlorin, in deuteriochloroform and/or trifluoroacetic acid solution are reported. On the basis of comparisons within this comprehensive series and proton off-resonance decoupled spectra, assignments of most resonances are made; complete assignment of the quaternary “pyrrole” ring carbons was difficult to accomplish. A downfield shift of the α- and β-meso-carbons of chlorins in trifluoroacetic acid relative to deuteriochloroform is used to confirm that the Chlorobium chlorophylls (660) from Chloropseudomonas ethylicum are meso-methylated at the δ-position.     

Properties of PAN-TBP extraction chromatographic material

Three production routes of the preparation of a solid extractant based on tributylphosphate (TBP) embedded in the polyacrylonitrile matrix (PAN) have been studied. The method of direct PAN coagulation with TBP was found to be not viable due to the significant TBP solubility in the coagulation bath. The most suitable PAN-TBP solid extractant was prepared by the well-known impregnation method of ready-made neat PAN beads. The kinetics of uranium extraction from 3 mol L^?1 HNO₃, the effect of nitrate and nitric acids concentrations on the value of weight distribution coefficients D _g as well as the uranium “extraction isotherm” were determined for this material. Uranium extraction was rather fast, approximately 1 h was sufficient for the equilibrium achievement. Capacity for the uranium uptake, measured in batch experiments on PAN-TBP for 0.048 mol L^?1 of uranium in 3 mol L^?1 nitric acid, was found to be q = 0.363 mmol g^?1 (58 % of the theoretical capacity). It was concluded that PAN-TBP material behaves like TBP in liquid–liquid extraction. Extraction capacity determined in column experiments was lower (by about 23 %) than expected from the “extraction isotherm” due to the TBP leaching out of the column. The thus prepared material is therefore not very suitable for multicycle extraction and stripping and can be used once, particularly for the analytical purposes.