Interfacial Bending Research Articles

Effect of membrane bending stiffness on the axisymmetric deformation of capsules in uniaxial extensional flow

The effect of interface bending stiffness on the axisymmetric deformation of liquid capsules enclosed by elastic membranes subject to uniaxial extensional flow is considered. Flow-induced deformation causes the development of membrane in-plane elastic tensions and bending moments due to the noninfinitesimal thickness of the membrane or to a preferred equilibrium configuration of an interfacial molecular network, accompanied by transverse shear tensions. The elastic tensions are related to the surface deformation by means of Mooney’s linear constitutive law for thin elastic sheets, and the bending moments are related to the membrane resting shape and to the instantaneous principal curvatures by means of constitutive equations. Interfacial force and torque balances are used to relate the jump in hydrodynamic traction across the interface to the elastic tensions and bending moments. A numerical procedure is implemented for simulating the capsule deformation in uniaxial extensional Stokes flow based on a boundary-integral method. Results on the transient and asymptotic deformation for capsules with spherical unstressed shapes illustrate the effect of bending stiffness expressed by an interface modulus of bending for a broad range of elasticity capillary numbers. It is shown that bending stiffness, however large, is not able to restrain continued deformation beyond a critical capillary number, and its main effect is to cause highly deformed capsules with pointed shapes to develop instead nearly cylindrical shapes with rounded caps.

Ionic Strength Effects of Electrolytes on Solubilized States of Water in AOT Reversed Micelles

The limiting amounts of solubilized aqueous NaCl, NaNO3, MgCl2, and AlCl3 in sodium 1,2-bis-(2-ethylhexyloxycarbonyl)-1-ethanesulfonate (aerosol OT)/isooctane solutions have been measured as a function of the ionic strength of the electrolytes. In any system, the limiting amounts increased up to the optimal ionic strength (Ioopt), and afterward, as ionic strength increased, they decreased and were followed by a constant. These increased and decreased curves of water solubilization below and above Ioopt, called, respectively, salting-in and salting-out curves, could be interpreted, respectively, from the counteracting effects of attractive intermicellar interaction and interfacial bending stress. The effects of electrolytes on the solubilized states of water by micelles were examined using NMR and near-infrared spectroscopic techniques. Consequently, two types of waters, i.e., water bound directly to the ionic head groups of the surfactant in reversed micelles and water interacting with the hydrated head groups in swollen micelles through hydrogen bonds, were almost unaffected by any electrolytes, whereas much greater effects were observed for bulklike water in W/O microemulsions. The order of the degree of lowering of salting-out curves in W/O microemulsion regions in each system was found to agree with the Hofmeister series.

Enhanced supercurrent density in polycrystalline YBa2Cu3O(7-delta) at 77 K from calcium doping of grain boundaries

With the discovery of high-temperature superconductivity, it seemed that the vision of superconducting power cables operating at the boiling point of liquid nitrogen (77 K) was close to realization. But it was soon found that the critical current density Jc of the supercurrents that can pass through these polycrystalline materials without destroying superconductivity is remarkably small. In many materials, Jc is suppressed at grain boundaries, by phenomena such as interface charging and bending of the electronic band structure. Partial replacement ('doping') of the yttrium in YBa2Cu3O(7-delta) with calcium has been used to increase grain-boundary Jc values substantially, but only at temperatures much lower than 77 K (ref. 9). Here we show that preferentially overdoping the grain boundaries, relative to the grains themselves, yields values of Jc at 77 K that far exceed previously published values. Our results indicate that grain-boundary doping is a viable approach for producing a practical, cost-effective superconducting power cable operating at liquid-nitrogen temperatures.

Syndiotactic polystyrene/high-density polyethylene blends compatibilized with SEBS copolymer: thermal, morphological, tensile, dynamic-mechanical, and ultrasonic characterization

A series of syndiotactic polystyrene (sPS)/ high-density polyethylene (HDPE) binary blends were prepared in the melt and then compression molded. Afterwards the blends were characterized by means of DSC, SEM and TEM microscopy, tensile tests, DMTA and acoustic measurements. The morphological, thermal and dynamic-mechanical analyses carried out on the binary blends evidence a clear phase separation between the components at all compositions and a lack of adhesion at the interface, Coherently, tensile texts show no improvement in the toughness of sPS when blended with HDPE. Upon addition of polystyrene-block-poly(ethylene-ranbutylene)-bloch-polystyrene (SEBS) copolymer to the binary blends as compatibilizer, a fine dispersion of HDPE within the sPS matrix and a good adhesion at the imerface is found. Although the good adhesion is confirmed by DMTA, tensile tests reveal no improvement of the mechanical properties of these ternary blends. This result is explained by assuming that the crystaltization of sPS at the interface favors the disentanglement, of poly-styrene end-groups of SEBS out of the sPS domain and therefore a weakening of the interfacial bending strength.

Mechanical theory of structural disjoining pressure in liquid crystal films

A mechanical theory of structural disjoining pressure in nematic liquid crystal films is developed based on Laplace's interfacial stress balance equation. Identification of the interfacial stresses in nematic liquid crystal interfaces leads to two contributions to the structural disjoining pressure. It is shown that tensor order parameter gradients across the film give rise to bulk Ericksen stresses, whose normal component results in a disjoining pressure that tends to stabilize the film. In addition, tangential gradients in the tensor order parameter give rise to gradients in interfacial bending stresses whose normal component results in a film pressure that may be disjoining or conjoining. Phenomenological expressions for the two structural disjoining pressures are obtained using the classical equations of liquid crystal bulk and surface elasticity.

Gaussian Curvature Modulus of an Amphiphilic Monolayer

In this paper the Helfrich ansatz for bending an amphiphilic monolayer is re-expressed in terms of the deviations in curvature away from a preferred value. Using this expression and a simple model of the response of amphiphiles to interfacial bending, it is demonstrated that there are strict limits to the value of the ratio of the Gaussian curvature modulus to the mean curvature modulus (κ G /κ). We have made an estimate of κ G /κ from measurements of the swelling behavior in water of inverse bicontinuous cubic mesophases in a system composed of 1-monoolein, dioleoylphosphatidylcholine, and dioleoylphosphatidylethanolamine. The estimate, -0.75 ± 0.08, is in agreement with the limits set by our model of -1 < κ G /κ < 0. This determination is the first to be made on an inverse bicontinuous cubic phase which is sufficiently swollen to be in a regime where first-order curvature elastic energetics should be sufficient to describe the state of the mesophase and hence provide a reliable estimate of κ G /κ.

Minimization of the Free Energy of Arbitrarily Curved Interfaces

The minimization of the free energy of a two-phase system with an interface of arbitrary curvature leads to an extremum (Laplace) condition containing the pressure difference, ΔP, between the two sides of the interface. The expression for ΔPis a function of the normal curvatures and of the resulting bending moments which are themselves functions of the normal curvatures, the mathematical form of which depends on the particular model for the interfacial bending energy that has been employed. On this basis, conclusions can be drawn about the equilibrium shape and curvatures of an interface, e.g., for bicontinuous microemulsions and vesicles. In addition, the pressure difference between the inside and the outside of surfactant-laden interfaces can be calculated. This pressure difference influences the work of formation of microemulsion droplets. A section devoted to the boundary conditions has also been included where in particular the case of a liquid meniscus attached to a cylindrically shaped solid surface is treated.

BASIC STUDY ON ULTRA-THIN WHITETOPPING

This paper covers the basic study on ultra-thin white topping that is also been developed in European countries and U. S. A. The aims are to find out its applicability through making mini model experiments, research on the interface shear stress under various temperature and traffic loads by FEM program. As the results, the characteristics of slab temperature and underlying existing asphalt pavement, the effects of the pavement temperature on the interface shear stress and bending stress, and fatigue damage are revealed.

Electric Component of the Interfacial Bending Moment and the Curvature Elastic Moduli

The interfacial bending moment and curvature elastic moduli are related theoretically to the surface potential, ΔV, which is liable to direct measurement. The dependence of the interfacial bending properties on the position of the Gibbs dividing surface is investigated. As an application, experimental data for the surface potential of micellar surfactant solutions containing Al3+ions are analyzed. The changes in the bending moment, as determined from ΔVpotential, correlate with the transition from spherical to cylindrical micelles induced by the Al3+ions. The results can be important for interpretation of data for formation of microemulsions, flocculation in emulsions, fluctuation capillary waves at interfaces and biomembranes, interactions between inclusions in lipid bilayers, etc.

Spontaneous Curvature-Induced Rayleigh-like Instability in Swollen Cylindrical Micelles

We study a Rayleigh-like instability of an oil in water cylindrical “droplet” in the presence of excess surfactant. The surfactant monolayer is modeled by an interfacial bending energy with a nonvanishing spontaneous curvature C0. We find that, even when surfactant reduces the interfacial tension to a vanishingly small value, the bending energy with spontaneous curvature can destabilize the cylinder if its radius R is greater than C0-1. The cylinder deforms into “beads” which can eventually break up into spherical droplets whose radius we predict. Unlike in the classical Rayleigh case, we find that the total interfacial area is usually increasing. Using our theory, we discuss the kinetics of the transition from a microemulsion phase of swollen cylindrical micelles to a droplet phase, following a sudden increase in the spontaneous curvature parameter.

Scaling properties of the capillary-wave model with interfacial bending rigidity

The scaling properties of interfacial correlations in the extended capillary-wave model (Romero-Rochin, V., Varea, C., and Robledo, A., 1992, Physica A, 184, 367) are analysed and the procedure developed previously by Weeks and collaborators for studying interfacial correlations is confirmed to apply to the extended case and to reproduce most of the earlier features. In addition to this it is found that: (i) the extended model is consistent with the expression for the bending rigidity κ given by the fourth moment of the direct correlation function; (ii) the scaling properties of the extended model differ from those of the ordinary model and we find that for d = 3 dimensions the correlations obey scaling on length scales less than the capillary length L c. The consequences for κ and the interfacial width W when scaling occurs at all length scales are examined, and it is suggested that their limiting values as L c → ∞ depend on the range of the molecular interactions. When these are sufficiently long ranged...

One-dimensional quantum electron states in n-AlxGa1−xAs/u-GaAs modulation-doped corrugated heterojunctions

New quasi-one-dimensional quantum electron states are proposed in modulation-doped n-AlxGa1−xAs/u-GaAs heterostructures with periodic interface bending. Unlike usual flat heterointerfaces, the investigated system is sawtooth corrugated by bendings with a period of about 850 Å and a bending angle 90°. Quantum-mechanical simulations yield energy spectrum with a single level below the Fermi energy. The probability distribution of electrons shows formation of a densely packed quasi-one-dimensional electron gas in the convex corner of the GaAs region. The ground level is as high as 100 meV above the deepest bottom of the conduction band potential and the excited state is separated by a significant energy gap of 16 meV.

Polydispersity of AOT droplets measured by time-resolved fluorescence quenching

The size and polydispersity of AOT water/oil microemulsions were measured by time-resolved fluorescence quenching. From the measured volume polydispersity the radius polydispersity was derived and calculated. An average polydispersity of 15.7% was obtained. No trends in polydispersity values at different droplet sizes or different solvent chain lengths were observed. The polydispersity values obtained are compared with values obtained by a new approach to theoretically intimate the polydispersity, and approximate values for the interfacial tension, γ∞ of the planar oil/water interface and the interfacial bending constant, K, were computed

Possible new structure for one-dimensional electron-gas systems by interface bending of n-AlGaAs/u-GaAs heterojunctions

A new concept in one-dimensional electron-gas (1DEG) systems is proposed by introducing periodic bending of the heterointerface of an n-AlGaAs/u-GaAs modulation-doped structure. The carrier densities and the electrostatic potential of this system are numerically analyzed based on the classical drift-diffusion model where the doping concentration ND of n-AlzG1−zAs is 1.0×1018 cm−3, the aluminum molar fraction z of AlzGa1−zAs is 0.3, the carrier density of p-type GaAs is 1.0×1014 cm−3, and the bending angle of the heterointerface is 90°. We found that electrons are more accumulated in convex regions of the u-GaAs layer than in concave ones, and the electron density in a convex region is about twice that in the conventional two-dimensional electron-gas structure. We can treat this high density of electrons as a 1DEG channel for field-electron transistors (FETs) when the period of the bending interface is about 850 Å.

Interfacial Dynamics of Water-in-Oil Dropliets: a Temperature-Jump Investigation

AbstractThe dynamic response of water-in-oil AOT microemulsion droplets to perturbations caused by rapid energy transfer has been interpreted in terms of the surfactant interface bending rigidity. The Iodine Laser Temperature Jump technique was used to rapidly (1 μs) increase the microemulsion temperature and thus disturb the system equilibrium. The dynamic relaxation to the new equilibrium position was monitored by measuring the turbidity of the microemulsion over a I ms time interval immediately after the temperature jump. The rate and direction of the turbidity change depended on the initial temperature, size and salt content of the water droplets. We observed characteristic times of 2 to 10 μs and attributed them to the relaxation of the surfactant interface. A bending modulus of 0.4 kT was derived for the AOT interface, using a model for the shape perturbation of a spherical interface.

Microscopic expressions for interfacial bending constants and spontaneous curvature.

We derive the interfacial-curvature free energy for a simple fluid from density-functional theory, and find a form matching that for a two-dimensional shearless elastic media. The fourth moment of the direct correlation function and the density gradient determine the bending moduli \ensuremath{\kappa} and \ensuremath{\kappa}\ifmmode\bar\else\textasciimacron\fi{}, while the spontaneous curvature ${\mathit{c}}_{0}$ is given by asymmetry in the density. We obtain the critical indices of these quantities and corrections to the Laplace equation for curved interfaces.

The van der Waals component of interfacial bending moment 2. Model development and numerical results

A model for calculation of the pressure tensor components is developed on the basis of some general results in the thermodynamics and statistical mechanics of spherical interfaces. The van der Waals intermolecular forces are taken into consideration. The model contains three parameters which are to be found from experiments. These are the interfacial tension of a flat interface and the Hamaker constants of the two neighboring phases. Then the model allows calculation of the interfacial bending moment, of the Tolman length and of the curvature dependence of the interfacial tension. Numerical results are obtained for various liquid-gas and water-oil interfaces at different radii of curvature. The results agree well with statistical-mechanical calculations and computer simulations by other authors. The calculated mechanical and thermodynamical interfacial tensions turn out to differ appreciably for drop radii below 1000 Å owing to the bending moment effect. The results can be applied for calculation of the size distribution and other properties of different fluid dispersions, including microemulsions.

The van der Waals component of the interfacial bending moment 1. Contribution of the pressure tensor tails

The main purpose of this study is to find explicit expressions for the interfacial bending moment and for the Tolman length in terms of the interfacial tension at the flat interface and of the Hamaker constants of the two neighboring fluid phases. For that purpose a model expression for the anisotropy of the pressure tensor in the vicinity of a spherical interface is obtained by utilizing the Irving and Kirkwood formula in conjunction with some general thermodynamic restrictions. In particular, asymptotic expressions for the pressure tensor tails are derived. The results allow calculation of the curvature dependence of the interfacial tension due to the van der Waals interaction. They are applicable to microemulsions, critical emulsions and homogeneous nucleation.

The interfacial bending moment: Thermodynamics and contributions of the electrostatic interactions

The problem concerning the magnitude and the sign of the interfacial bending moment of the droplets in fluid disperse systems is considered on the basis of a thermodynamic approach. It is demonstrated that the total bending moment can be expressed as a superposition of contributions connected with the different components in the system. Expressions for the contributions of the electric double layer, of the interactions between adsorbed dipoles and of the electrolyte excess osmotic pressure to the value of the bending moment are derived. The effect of a possible incomplete dissociation of the adsorbed ionic surfactant monolayer is also taken into account. The results show that the van der Waals and the electric double layer interactions provide significant contributions to the bending moment, both of them of the order of 10 pN. Even at high electrolyte concentrations, the electrostatic bending moment can be important owing to the contribution of the Stern layer. The results can be applied to study the curvature dependence of the interfacial tension in microemulsions and in liquid-gas dispersions.

A soluble seven-vertex model for clusters with interfacial bending energy

A model describing an ensemble of disjoint clusters on a square lattice is introduced. In addition to the usual surface energy terms, in x and in y, the model includes a bending energy in c per corner in the interface. It is formulated as a seven-vertex model which is in general not solvable. For beta in c=ln(2)/2, however, this seven-vertex model becomes free fermionic one and an exact expression for its free energy is derived. In the extreme anisotropic ('Hamiltonian') limit the operator content coincides with that of the 1D Ising model in a transverse field. Three critical lines with Ising-like behaviour are found.