Attractive Van Der Waals Forces Research Articles

Dynamic Charge Regulation Model for the Electrostatic Forces between Ionizable Materials

The electrostatic repulsion between similar surfaces with ionizable surface groups interacting across aqueous solutions is calculated for plane parallel and curved surfaces for the case in which the solution phase is at thermodynamic equilibrium, but the charged surface is not in equilibrium with the solution. Simulations are made for approaching and retracting oxidic surfaces for which the (equilibrium) surface chemistry is described by a standard 2-pK model. Population balances describe the concentration of each surface species as a function of the rates of the different reactions with the ions in solution. Approaching surfaces experience a higher repulsion than predicted by thermodynamic equilibrium, with the upper limit being the repulsion at a constant surface charge (at very high approach velocities). For retracting surfaces the electrostatic repulsion is lower than that for equilibrium and may decrease to zero when the surfaces are sufficiently discharged at contact and pulled apart fast enough. In that case, and with the attractive van der Waals force still operating, a net attractive force is predicted, which might explain the pull-off force (adhesion force) often measured in the atomic force microscope and the surface force apparatus.

Ultrathin Cross-Linked Networks at the Interface between Oil and Water: Structure, Properties, and Preparative Perspectives

The self-association process of surface-active monomers can be used to synthesize ultrathin cross-linked networks at fluid interfaces. We have systematically studied transient networks, which were stabilized and cross-linked by hydrogen bonds, Coulomb interactions, or van der Waals attraction forces. At the planar surface, we measured the rheological properties of these films. In addition to these experiments, we have also investigated typical structures of these networks by means of Brewster angle microscopy. The results of these investigations are that the surfactant molecules tend to build up two-dimensional aggregates, which are interconnected to form elongated network strands. Due to their simple synthesis, two-dimensional networks can serve as excellent model systems for advanced investigations of the stability and deformation properties of living cells. In addition to this, we analyzed the film-forming behavior of molecular tweezers, which are able to bind selectively electron deficient aromatic and aliphatic substrates.

The Electrical Double Layer on Gold Probed by Electrokinetic and Surface Force Measurements

Gold surfaces, obtained by vacuum deposition of 15-nm gold films on glass and silica wafers, were studied in aqueous solutions by streaming potential measurements and colloidal-probe AFM force measurements. In the force measurements both a bare and a gold-coated silica particle (6 μm in diameter) have been used as colloidal probes. From the streaming potential measurements we determined the zeta-potential of the gold surface, while from the force measurements the diffuse double-layer potential ψd was obtained by fitting the data to the DLVO theory or to the nonlinear Poisson–Boltzmann equation. Measured interactions were found to be entirely due to overlap of electric double layers with no indication of attractive Van der Waals forces. Results of both types of measurements are in good agreement. The double layer potential strongly depends on the pH, probably as a result of the presence of oxide species on the gold surface. Insight in the double layer potential of polarizable interfaces such as the gold/electrolyte solution interface is the first step for understanding the effect of externally applied potentials on the adsorption behavior of charged species.

Collision of drops with inertia effects in strongly sheared linear flow fields

The relative motion of drops in shear flows is responsible for collisions leading to the creation of larger drops. The collision of liquid drops in a gas is considered here. The drops are small enough for the Reynolds number to be low (negligible fluid motion inertia), yet large enough for the Stokes number to be possibly of order unity (non-negligible inertia in the motion of drops). Possible concurrent effects of Van der Waals attractive forces and drop inertia are taken into account.General expressions are first presented for the drag forces on two interacting drops of different sizes embedded in a general linear flow field. These expressions are obtained by superposition of solutions for the translation of drops and for steady drops in elementary linear flow fields (simple shear flows, pure straining motions). Earlier solutions adapted to the case of inertialess drops (by Zinchenko, Davis and coworkers) are completed here by the solution for a simple shear flow along the line of centres of the drops. A solution of this problem in bipolar coordinates is provided; it is consistent with another solution obtained as a superposition of other elementary flow fields.The collision efficiency of drops is calculated neglecting gravity effects, that is for strongly sheared linear flow fields. Results are presented for the cases of a simple linear shear flow and an axisymmetric pure straining motion. As expected, the collision efficiency increases with the Stokes numbers, that is with drop inertia. On the other hand, the collision efficiency in a simple shear flow becomes negligible below some value of the ratio of radii, regardless of drop inertia. The value of this threshold increases with decreasing Van der Waals forces. The concurrence between drop inertia and attractive van der Waals forces results in various anisotropic shapes of the collision cross-section. By comparison, results for the collision efficiency in an axisymmetric pure straining motion are more regular. This flow field induces axisymmetric sections of collision and strong inertial effects resulting in collision efficiencies larger than unity. Effects of van der Waals forces only appear when one of the drops has a very low Stokes number.

Calculation model of uniform media filtration capacity

Filtration is an efficient method for turbidity removal and widely used for water and gas purification. The process of filtration is complicated, and although the basic principles concerning filtration were brought forward decades ago, there is a gap between the theories and their application. Filtration is still an active field for theoretical and experimental research, and there is plenty of room for improvement in this area. In this paper, through the analysis of the uniform media filtration, the capillary model is used to determine the flow pattern in the packed bed, and then the fluid shear force distribution in the capillary is formulated. The physical-chemical forces including van der Waals attraction force, electric double-layer force, Born repulsion force and structural force between the particle and the capillary wall are calculated by analytical equations. By equilibrium analysis of the forces acting on a particle adhered on the capillary wall and appropriate simplification, the analytical equation of the porosity in the saturated filter bed is obtained, from which the maximum entrapment capacity during the filtration process can be achieved. Moreover, the filtration capacity under certain operations are also presented and discussed.

DLVO Interactions of Tungsten Oxide and Cobalt Oxide Surfaces Measured with the Colloidal Probe Technique

We have investigated the DLVO surface forces of oxidized tungsten and cobalt surfaces using the atomic force microscope (AFM) colloidal probe technique. It was shown by X-ray photoelectron spectroscopy (XPS) and electrokinetic measurements that this model system is representative of industrial tungsten carbide (WC) and cobalt powders used in the production of hard metals. We found that the attractive van der Waals forces are well described by Hamaker constants, calculated from optical data for WO3 and CoOOH. The repulsive electrostatic double layer forces between WO3 surfaces increase with increasing pH due to an increasingly negative surface potential. This surface potential decreases with increasing ionic strength at pH 7.5. The electrostatic interaction between WO3 and CoOOH is attractive at pH 10, suggesting a positively charged CoOOH surface.

Numerical simulation of the microstructure and compression behavior of Eckernförde Bay sediments

Transmission electron microscope (TEM) studies conducted on sediments recovered using diver-collected cores and gravity cores from Eckernförde Bay in the Baltic Sea, Germany, reveal that the sediments consist of silt- to clay-sized sediment, both free and bound into fecal pellets, organic matter, biogenic materials, methane gas and microorganisms. The microstructure of the upper 2 m of the sediment is complex, with the clay particles arranged in a very open structure with variable inter- and intra-domain pore space. In an effort to understand the relationship between microscopic variables (e.g., microfabric) and macroscopic geotechnical properties (e.g., compressibility), a numerical study was performed using the discrete element method. The objective of this study was to develop an understanding of the micro- to macro-linkage of mechanical properties. Specifically, one-dimensional compression behavior of the sediment was numerically simulated and compared with experimental data. Compositional parameters of the sediment were obtained from specific tests conducted at the Naval Research Laboratory and from tests conducted by other researchers participating in the project. Image analysis was used to quantify the microfabric parameters from TEM images of the sediment. The effect of organic matter was simulated by introducing inter-particle contact cementation. Results indicate that inter-particle bonding due to inter-particle cementation and/or the van der Waals attractive force (1) contributes to the apparent overconsolidation observed in the laboratory and (2) holds the highly porous, unstable microstructure together until a threshold stress level is reached. Change in void ratio during compression is due to inter-, rather than intra-domain movement.

Mechanism of hydrodynamic separation of biological objects in microchannel devices.

In this study, the separation mechanism employed in hydrodynamic chromatography in microchannel devices is analyzed. The main purpose of this work is to provide a methodology to develop a predictive model for hydrodynamic chromatography for biological macromolecules in microchannels and to assess the importance of various phenomenological coefficients. A theoretical model for the hydrodynamic chromatography of particles in a microchannel is investigated herein. A fully developed concentration profile for non-reactive particles in a microchannel was obtained to elucidate the hydrodynamic chromatography of these particles. The external forces acting on the particles considered in this model include the van der Waals attractive force, double-layer force as well as the gravitational force. The surface forces, such as van der Waals attractive force as well as the double-layer repulsive force, can either enhance or hinder the average velocity of the macromolecular particles. The average velocity of the particles decreases with the molecular radius because the van der Waals attractive force increases the concentration of the particles near the channel surface, which is the low-velocity region. The transport velocity of the particles is dominated by the gravity and the higher density enlarges the effect caused by gravity.

Pressure-Induced Freezing of the Hydrophobic Core Leads to a L1 → H1 Phase Transition for C12E5 Micelles in D2O

We apply small-angle neutron scattering (SANS) to study the effect of pressure on micelle structure in a solution of 1% by weight pentaethylene glycol mono-n-dodecyl ether (C 1 2 E 5 ) in D 2 O at 20 °C and pressures up to ∼3000 bar. At ambient pressure, the structure is a network of branched, semiflexible, cylindrical micelles with the branch points comprised ofthree-armedjunctions. Our SANS results reveal that pressure induces a phase transition from this network of threadlike micelles to hexagonally ordered bundles of cylindrical micelles. Using geometric packing constraints for three-arm junctions and cylinders, we show that the formation of three-arm junctions becomes increasingly unfavorable with increasing pressure due to the compression of the micelle hydrophobic core, and as such, the network becomes unstable at pressures close to those observed in our SANS experiments. We also measured the temperature dependence of the transition pressure and find that it follows the pressure-temperature freezing curves for liquid n-alkanes of comparable hydrocarbon chain length. These observations lead us to propose that the phase transition is related to a loss of flexibility or conformational entropy of the C 1 2 E 5 micelles upon the pressure-induced freezing of the micelle hydrophobic core to form an amorphous solid. The formation of hexagonally ordered bundles of cylindrical micelles follows as attractive van der Waals forces between the micelles are not offset by the loss of repulsive undulation forces arising from the fluidity of the hydrophobic core.

Hydrodynamics of subphase entrainment during Langmuir–Blodgett film deposition

The phenomenon of subphase entrainment that accompanies Langmuir–Blodgett film deposition is examined in terms of the hydrodynamics of dip coating coupled with mass transport of the depositing monolayer. The governing equations for film thickness and for concentration of the monolayer material have been derived based on the lubrication approximation theory for steady-state conditions, subject to viscous, surface tension, gravity, intermolecular and Marangoni forces. The equations have been solved by numerical methods. In the absence of the monolayer, the model reduces to various forms of the case of substrate withdrawal from a pure liquid — the well known dip coating problem. Effect of several parameters related to material properties and operating conditions have been examined. We consider two mechanisms for subphase entrainment: the flow driven by the motion of the solid substrate, and Marangoni driven flow caused by surface tension gradient due to surface concentration gradient. Attractive van der Waals forces and Marangoni flow are responsible for thickening of the entrained subphase film. Gravity drainage and repulsive van der Waals forces thin the entrained subphase film. The entrained subphase film thickness measurement is done by using a dual channel spectrometer in conjunction with a software to obtain thickness from spectral information. Our numerical results are in good agreement with the extent of entrainment predicted by the model.

Brownian flocculation of polymer colloids in the presence of a secondary minimum.

Most analyses of Brownian flocculation apply to conditions where London-van der Waals attractive forces cause particles to be strongly bound in a deep interparticle potential well. In this paper, results are reported that show the interaction between primary- and secondary-minimum flocculation when the interparticle potential curve reflects both attractive and electrostatic repulsive forces. The process is highly time-dependent because of transfer of particles from secondary- to primary-minimum flocculation. Essential features of the analysis are corroborated by experiments with 0.80-microm polystyrene spheres suspended in aqueous solutions of NaCl over a range of ionic strengths. In all cases, experiments were restricted to the initial stage of coagulation, where singlets and doublets predominate.

Vesicle protrusions shaped by actin–membrane interactions

When the surface to volume ratio of an actin filled vesicle is increased, for instance by temperature elevation, small protrusions are found to emerge from the initially spherical vesicle. We suggest here that these structures result from a balance between membrane–actin filaments interactions and the membrane curvature elasticity, their diameter λc minimizing the sum of these two energies. When the actin–membrane interaction is the superposition of the repulsive undulation force of the membrane, the attractive Van der Waals force between membrane and filaments, and the bending energy of filaments, the protrusion diameter is found to be between 0.1 μm and 1 μm, depending on parameters such as the vesicle radius, the monomeric actin concentration, the membrane tension and the thickness of the actin sheet.

Attractive Forces between Surfaces: What Can and Cannot Be Learned from a Jump-In Study with the Surface Forces Apparatus?

We study theoretically the dynamics of film thinning under the action of an attractive surface force near the point of a jump instability. Our approach is illustrated by modeling van der Waals and hydrophobic attractive forces. The main result is that with the hydrophobic force law reported previously it is often impossible to establish the jump separation with any certainty. The surfaces instead approach slowly from a distance which is much larger than the point where an actual jump is expected. We conclude that an attractive force measured by the static jump technique is overestimated, and we formulate principles of a new dynamic jump method. The use of this new technique would permit direct measurements of attractive forces at separations below the static jump distance down to contact of the surfaces.

Modeling of Hydrodynamic Chromatography for Colloid Migration in Fractured Rock

The role of colloids in the migration of radionuclides in the geosphere has been emphasized in the performance assessment of high-level radioactive waste disposal. The literature indicates that the colloid velocity may not be equal to the velocity of groundwater owing to hydrodynamic chromatography. A theoretical model for hydrodynamic chromatography of colloid migration in the fracture is proposed in the present work. In this model, the colloids are treated as nonreactive and the external forces acting on colloidal particles are considered including the inertial force, the van der Waals attractive force, and the electrical double-layer repulsive force, as well as the gravitational force. A fully developed concentration profile for colloids is obtained to elucidate migration behavior for colloids in the fracture. The effects of parameters governing these forces and the aperture of the fracture are determined using a theoretical model.

Liquid-crystal phases of capped carbon nanotubes

sociated with the electronic properties of carbon nanotubes. Depending on their helicity, single-wall nanotubes are either metallic or semiconducting, and therefore have the potential of forming the basis of a future all-carbon, nanotube-based microelectronics. 2 It is also clear that finite-sized carbon nanotubes are variable-length rods, which by virtue of their shape and size can act as liquid crystals. 3 To examine the possibility of nanotubes forming tailored liquid crystals for application purposes, we investigated the phase diagrams of capped nanotubes as a function of nanotube length and diameter in two limits: in the presence of the attractive van der Waals forces, and in the hard-rod limit characteristic of nanotubes in which such interactions are completely screened. Our results show that when attractive interactions are present, the columnar phase preempts all others at room temperatures.

Effects of hydrodynamic chromatography on colloid-facilitated migration of radionuclides in the fractured rock

The performance assessment of high level radioactive waste disposal has emphasized the role of colloids in the migration of radionuclides in the geosphere. Previous literature [Nagasaki S, Tanaka S, Suzuki A. Fast transport of colloidal particles through quartz-packed columns. J. Nucl. Sci. Technol. 1975;30(11):1136] indicates that owing to hydrodynamic chromatography the colloid velocity may not be equal to that of groundwater. Using hydrodynamic chromatography, this work investigates the effects of the size of colloidal particles on the radionuclide migration facilitated by colloids in a single fractured porous rock. Also, a methodology is proposed to develop a predictive model to assess transport within the fracture rock as well as various other phenomenological coefficients, particularly the size of colloidal particles. In addition, a fully developed concentration profile for non-reactive colloids in the fracture is developed to elucidate hydrodynamic chromatography of colloids in geological media. The external forces acting on colloidal particles hypothesized in the model proposed herein include inertial force, van der Waals attractive force, double layer force as well as gravitational force. The dispersion coefficient of colloids and the distribution coefficient for radionuclides with colloids are also considered as they pertain to the size of the colloid. In addition, the size distributions of colloids are utilized to investigate the effects of polydispersed colloids.

吸湿シクロヘキサン中におけるシリカ表面間相互作用力

Interaction forces between silica surfaces in wet and dry cyclohexane are measured using atomic force microscopy. A strange (strong?) attractive force acts between the surfaces with increasing water concentration, which is much further and stronger than the van der Waals attractive force. The attractive force decreases monotonically as the approaching velocity between two surfaces increases. The following results are obtained concerning the adhesion force: i) the probability for strong adhesion force increases with water concentration, ii) a finite contact time is required for the appearance of strong adhesion force, iii) hydrophobilization of one surface brings a sharp decrease in adhesion force. These results suggest that the strong adhesion force is caused by the formation of water bridge.

Effect of Additive NaCl on Rheological Characteristics of Synthetic Hectorite Aqueous Suspensions.

The rheological characteristics of synthetic hectorite/water suspensions have been measured by a cone-plate rheometer as a function of additive NaCl concentration (CNaCl). The hectorite particles, dialyzed preliminarily to remove electrolytic impurities involved, were utilized for preparing a 2.0 wt% aqueous suspension. The apparent viscosities of the suspensions decreased monotonically with increasing CNaCl; these changes in the viscosities came from the primary electroviscous effect. In dynamic viscoelastic measurements of the suspensions, we found that the critical strain also decreased with increasing CNaCl. By the use of a Cryo-SEM, we observed that different types of the particle-coagulated structures in the various suspensions with respective NaCl concentrations. These findings can be explained by the different interaction modes originated from the electrostatic charge near the surface of hectorite particles, as in the following situations : (1) in the low CNaCl region, the electrostatic repulsion among the particles leads effectively to form an expanded network structure in the system; (2) in the high CNaCl region, the van der Waals attraction force among the particles governs the interaction mode in the system, thus resulting to form a dense-packed coagulation structures.

Membrane properties of sodium 2- and 6-(poly)prenyl-substituted polyprenyl phosphates

The amphiphilic 2- and 6-(poly)prenyl-substituted polyprenyl phosphates and their isomers, spontaneously form vesicles. Vesicle formation depends on the pH of the buffer and the structural features of the hydrophobic chains such as their lengths and the double bond positions. The compactness of the vesicles was estimated by measuring their water permeability, evaluating the intramolecular attractive van der Waals forces between the highly branched chains and measuring the spin–lattice relaxation times in 13C NMR spectra.

The role of fuzzy interfaces in the nucleation, growth and agglomeration of aluminum hydroxide in concentrated caustic solutions

Fundamental crystal growth theory relies on classical concepts of monomeric addition at step sites on crystal surfaces. The nucleation and growth of crystalline aluminium hydroxide from concentrated caustic solutions does not follow classical crystal growth mechanistic pathways. Numerous techniques including static and dynamic light scattering, small angle X-ray and neutron scattering, cryovitirification transmission electron microscopy, rheology and atomic force microscopy have been employed in the study of aluminium hydroxide crystallisation from concentrated caustic solutions. The observations from these techniques have been interpreted on the basis polymer crystal growth theory, thermodynamic phase inversions analysis and entropically driven insolubility. The experimental observations can be interpreted on the basis that aluminium hydroxide nuclei and crystals are surrounded by a diffuse interface which grades in density from the crystalline aluminium hydroxide particle core to the surrounding solution. A mechanism for the nucleation and growth mechanisms of aluminium hydroxide has been proposed: initial solution formation of a loose polymeric network; clustering of this network followed by gradual densification to form amorphous nuclei; further densification of the core of the nuclei to form crystallites and gradual densification but not crystallisation of the still amorphous diffuse interface. The presence of this diffuse interface enables the slow agglomeration behaviour of aluminium hydroxides particles in concentrated caustic liquors to be explained. In liquors of very high ionic strength (in this case up to 6 M NaOH) particulate agglomeration would be expected to be rapid due to the small double layer thickness as predicted by DLVO theory. During rapid growth the diffuse interface inhibits the sufficiently close approach of the dense part of the particles to the point where attractive inter-particulate van der Waals forces would dominate and agglomeration would take place. As supersaturation is depleted and the growth rate of the diffuse interface decreases but densification is still occurring the particles can approach more closely and agglomeration will occur. Thus it is probable that the observed agglomeration behaviour is supersaturation and growth rate related.