Ionic liquid nanotribology: stiction suppression and surface induced shear thinning

The friction and adhesion between pairs of materials (silica, alumina, and polytetrafluoroethylene) have been studied and interpreted in terms of the long-ranged interactions present. In ambient laboratory air, the interactions are dominated by van der Waals attraction and strong adhesion leading to significant frictional forces. In the presence of the ionic liquid (IL) ethylammonium nitrate (EAN) the van der Waals interaction is suppressed and the attractive/adhesive interactions which lead to "stiction" are removed, resulting in an at least a 10-fold reduction in the friction force at large applied loads. The friction coefficient for each system was determined; coefficients obtained in air were significantly larger than those obtained in the presence of EAN (which ranged between 0.1 and 0.25), and variation in the friction coefficients between systems was correlated with changes in surface roughness. As the viscosity of ILs can be relatively high, which has implications for the lubricating properties, the hydrodynamic forces between the surfaces have therefore also been studied. The linear increase in repulsive force with speed, expected from hydrodynamic interactions, is clearly observed, and these forces further inhibit the potential for stiction. Remarkably, the viscosity extracted from the data is dramatically reduced compared to the bulk value, indicative of a surface ordering effect which significantly reduces viscous losses.     

Nonlinear evolution of thin liquid films dewetting near soft elastomeric layers

The nonlinear evolution of thin liquid films dewetting near soft elastomeric layers is examined in this work. Evolution equations are derived by applying the lubrication approximation and assuming that van der Waals forces in the liquid cause the dewetting and that the solid can be described as a linear viscoelastic material. Two cases are examined: (i) a liquid layer resting on an elastomer bounded from below by a rigid substrate, and (ii) an elastomer overlying a thin liquid film bounded from below by a rigid substrate. Linear stability analysis is carried out to obtain asymptotic relations which are then compared against solutions of the full characteristic equations. In the liquid-on-solid case, numerical solutions of the evolution equations show that van der Waals forces cause thinning of the liquid film and thickening of the elastomeric solid beneath film depressions. Inclusion of a short-range repulsive force suggests that regular patterns may form in which ridges of fluid rest on depressions in the solid. In the solid-on-liquid case, the van der Waals forces cause the solid layer to break up before the liquid film can dewet. The results presented here support the idea that the dewetting of thin liquid films might be exploited to create topographically patterned surfaces on soft polymeric solids.     

The bridging force between two plates by polyelectrolyte chains

The interaction between particles in a colloidal system can be significantly affected by their bridging by polyelectrolyte chains. In this paper, the bridging is investigated by using a self-consistent field approach which takes into account the van der Waals interactions between the segments of the polyelectrolyte molecules and the plates, as well as the electrostatic and volume exclusion interactions. A positive contribution to the force between two plates is generated by the van der Waals interactions between the segments and the plates. This positive (repulsive) contribution plays an important role in the force when the distances between the plates are small. With increasing van der Waals interaction strength between segments and plates, the force between the plates becomes more repulsive at small distances and more attractive at large distances. When the surfaces of the plates have a constant surface electrical potential and a charge sign opposite to that of the polyelectrolyte chains, the force between the two plates becomes less attractive as the bulk polyelectrolyte concentration increases. This behavior is due to a higher bulk counterion concentration dissociated from the polyelectrolyte molecules. At short distances, the force between plates is more repulsive for stiffer chains. A comparison between theoretical and experimental results regarding the contraction of the interlayer separation between the platelets of vermiculite clays against the concentration of poly(vinyl methyl ether) was made.     

Stick slip contact mechanics between dissimilar materials: effect of charging and large friction

Measurements of the contact radius as a function of applied force between a mica surface and a silica surface (mica/silica) in air are reported. The load/unload results show that the contact radius generally increases with applied force. Because of the presence of charging due to contact electrification, both a short-range van der Waals adhesion force and longer-range electrostatic adhesive interaction contribute to the measured force. The results indicate that approximately 20% of the pull-off force is due to van der Waals forces. The contact radius versus applied force results can be fit to Johnson-Kendall-Roberts (JKR) theory by considering that only the short-range van der Waals forces contribute to the work of adhesion and subtracting a constant longer-range electrostatic force. Also, an additional and unexpected step function is superimposed on the contact radius versus applied force curve. Thus, the contact diameter increases in a stepped dependence with increasing force. The stepped contact behavior is seen only for increasing force and is not observed when symmetric mica/mica or silica/silica contacts are measured. In humid conditions, the contact diameter of the mica/silica contact increases monotonically with applied force. Friction forces between the surfaces are also measured and the shear stress of a mica/silica interface is 100 times greater than the shear stress of a mica/mica interface. This large shear stress retards the increase in contact area as the force is increased and leads to the observed stepped contact mechanics behavior.     

Specific and nonspecific interaction forces between Escherichia coli and silicon nitride, determined by poisson statistical analysis

The nature of the physical interactions between Escherichia coli JM109 and a model surface (silicon nitride) was investigated in water via atomic force microscopy (AFM). AFM force measurements on bacteria can represent the combined effects of van der Waals and electrostatic forces, hydrogen bonding, steric interactions, and perhaps ligand-receptor type bonds. It can be difficult to decouple these forces into their individual components since both specific (chemical or short-range forces such as hydrogen bonding) and nonspecific (long-range colloidal) forces may be present in the overall profiles. An analysis is presented based on the application of Poisson statistics to AFM adhesion data, to decouple the specific and nonspecific interactions. Comparisons with classical DLVO theory and a modified form of a van der Waals expression for rough surfaces were made in order to help explain the nature of the interactions. The only specific forces in the system were due to hydrogen bonding, which from the Poisson analysis were found to be -0.125 nN. The nonspecific forces of 0.155 nN represent an overall repulsive interaction. These nonspecific forces are comparable to the forces calculated from DLVO theory, in which electrostatic-double layer interactions are added to van der Waals attractions calculated at the distance of closest approach, as long as the van der Waals model for "rough" spherical surfaces is used. Calculated electrostatic-double layer and van der Waals interactions summed to 0.116 nN. In contrast, if the classic (i.e., smooth) sphere-sphere model was used to predict the van der Waals forces, the sum of electrostatic and van der Waals forces was -7.11 nN, which appears to be a large overprediction. The Poisson statistical analysis of adhesion forces may be very useful in applications of bacterial adhesion, because it represents an easy way to determine the magnitude of hydrogen bonding in a given system and it allows the fundamental forces to be easily broken into their components.     

van der Waals interaction between internal aqueous droplets and the external aqueous phase in double emulsions

A mathematical model for analyzing the van der Waals interaction between the internal aqueous droplets (W(1)) and the external aqueous phase (W(2)) of double emulsions has been established. The effects of Hamaker constants of the materials forming the system, especially those of the two different adsorbed surfactant layers with uniform density (A(1) and A(2)), on the van der Waals interaction were investigated. The overall van der Waals interaction across the oil film is a combined result of four individual parts, that is, W(1)-W(2), A(1)-A(2), W(1)-A(1), and A(2)-W(2) van der Waals interaction, and it may be either attractive or repulsive depending on many factors. It was found that the overall van der Waals interaction is dominated by the W(1)-W(2) interaction at large separation distances between the W(1)/O and O/W(2) interfaces, while it is mostly determined by the A(1)-A(2) interaction when the two interfaces are extremely close. Specifically, in the cases when the value of the Hamaker constant of the oil phase is intermediate between those of W(1) and W(2) and there is a thick oil film separating the two interfaces, a weak repulsive overall van der Waals interaction will prevail. If the Hamaker constant of the oil phase is intermediate between those of A(1) and A(2) and the two interfaces are very close, the overall van der Waals interaction will be dominated by the strong repulsive A(1)-A(2) interaction. The repulsive van der Waals interaction at such cases helps stabilize the double emulsions.     

Nonmonotoic fluctuation-induced interactions between dielectric slabs carrying charge disorder

We investigate the effect of monopolar charge disorder on the classical fluctuation-induced interactions between randomly charged net-neutral dielectric slabs and discuss various generalizations of recent results [A. Naji et al., Phys. Rev. Lett. 104, 060601 (2010)] to highly inhomogeneous dielectric systems with and without statistical disorder correlations. We shall focus on the specific case of two generally dissimilar plane-parallel slabs, which interact across vacuum or an arbitrary intervening dielectric medium. Monopolar charge disorder is considered to be present on the bounding surfaces and/or in the bulk of the slabs, may be in general quenched or annealed and may possess a finite lateral correlation length reflecting possible "patchiness" of the random charge distribution. In the case of quenched disorder, the bulk disorder is shown to give rise to an additive long-range contribution to the total force, which decays as the inverse distance between the slabs and may be attractive or repulsive depending on the dielectric constants of the slabs. By contrast, the force induced by annealed disorder in general combines with the underlying van der Waals forces in a nonadditive fashion, and the net force decays as an inverse cube law at large separations. We show, however, that in the case of two dissimilar slabs, the net effect due to the interplay between the disorder-induced and the pure van der Waals interactions can lead to a variety of unusual nonmonotonic interaction profiles between the dielectric slabs. In particular, when the intervening medium has a larger dielectric constant than the two slabs, we find that the net interaction can become repulsive and exhibit a potential barrier, while the underlying van der Waals force is attractive. On the contrary, when the intervening medium has a dielectric constant between that of the two slabs, the net interaction can become attractive and exhibit a free energy minimum, while the pure van der Waals force is repulsive. Therefore, the charge disorder, if present, can drastically alter the effective interaction between net-neutral objects.     

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.     

A systematic approach to calibrate a transferable polarizable force field parameter set for primary alcohols

In this work, parameters are optimized for a charge‐on‐spring based polarizable force field for linear alcohols. We show that parameter transferability can be obtained using a systematic approach in which the effects of parameter changes on physico‐chemical properties calculated from simulation are predicted. Our previously described QM/MM calculations are used to attribute condensed‐phase polarizabilities, and starting from the non‐polarizable GROMOS 53A5/53A6 parameter set, van der Waals and Coulomb interaction parameters are optimized to reproduce pure‐liquid (thermodynamic, dielectric, and transport) properties, as well as hydration free energies. For a large set of models, which were obtained by combining small perturbations of 10 distinct parameters, values for pure‐liquid properties of the series methanol to butanol were close to experiment. From this large set of models, we selected 34 models without special repulsive van der Waals parameters to distinguish between hydrogen‐bonding and non‐hydrogen‐bonding atom pairs, to make the force field simple and transparent. © 2017 Wiley Periodicals, Inc.     

On the decisive role of finite chain extensibility and global interactions in networks

The stress-strain dependence of dry networks at unidirectional extension and compression is studied. The phenomenological van der Waals equation of state is compared with different molecular models in order to provide an interpretation of the van der Waals corrections. It is shown that the stress-strain behavior predicted by the phantom, Langevin, and constrained junction fluctuation models are altogether covered by the van der Waals approach. The relationship between the suppression of junction fluctuation parameter introduced by Dossin and Graessley and the van der Waals corrections has been worked out. The effect of junction functionality on the small strain modulus as well as on the second Mooney-Rivlin coefficient is also presented.     

A modeling approach to describe the adhesion of rough, asymmetric particles to surfaces

A combined theoretical and experimental study of the adhesion of alumina particles and polystyrene latex spheres to silicon dioxide surfaces was performed. A boundary element technique was used to model electrostatic interactions between micron-scale particles and planar surfaces when the particles and surfaces were in contact. This method allows quantitative evaluation of the effects of particle geometry and surface roughness on the electrostatic interaction. The electrostatic interactions are combined with a previously developed model for van der Waals forces in particle adhesion. The combined model accounts for the effects of particle and substrate geometry, surface roughness and asperity deformation on the adhesion force. Predictions from the combined model are compared with experimental measurements made with an atomic force microscope. Measurements are made in aqueous solutions of varying ionic strength and solution pH. While van der Waals forces are generally dominant when particles are in contact with surfaces, results obtained here indicate that electrostatic interactions contribute to the overall adhesion force in certain cases. Specifically, alumina particles with complex geometries were found to adhere to surfaces due to both electrostatic and van der Waals interactions, while polystyrene latex spheres were not affected by electrostatic forces when in contact with various surfaces.     

Van der waals interactions and decrease of the rotational barrier of methyl-sized rotators: a theoretical study

Rotational barriers of methyl-sized molecular rotators are investigated theoretically using ab initio and empirical force field calculations in molecular models simulating various environmental conditions experienced by the molecular rotors. Calculations on neopentane surrounded by methyl groups suggest that the neopentane's methyl rotational potential energy barrier can be reduced by up to an order of magnitude by locating satellite functional groups around the rotator at a geometry that destabilizes the staggered conformation of the rotator through van der Waals repulsive interactions and reduces the staggered/eclipsed relative energy difference. Molecular mechanics and molecular dynamics calculations indicate that this barrier-reducing geometry can also be found in molecular rotators surface mounted on graphite surfaces or carbon nanotube models. In these models, molecular dynamics simulations show that the rotation of methyl-sized functional groups can be catalyzed by van der Waals interactions, thus making very rigid rotators become thermally activated at room temperature. These results are discussed in the context of design of nanostructures and use of methyl groups as markers for microenvironmental conditions.     

An Extension of the Consistent Valence Force Field (CVFF) with the Aim to Simulate the Structures of Vanadium Phosphorus Oxides and the Adsorption of n‐Butane and of 1‐Butene on their Crystal Planes

A specific force field of Consistent Valence Force Field type was developed with the aim to simulate the structures of catalysts of vanadium phosphorus oxide type and the reversible adsorption of organic compounds on specific crystallographic planes of such catalysts by molecular modeling. The appropriate parameters were derived for the bonded (stretching, bending, and torsional deformations) and nonbonded (attractive and repulsive van der Waals and Coulomb forces) atomic interactions for V—O and P—O bonds in typical fragments of these catalysts with the vanadium atom in the oxidation state IV. The parameters for bonded interactions were computed from Hessian matrices, supplied by the program DMol for performing Density Functional Theory, by means of a program for non‐linear regression. The DMol program was applied to energy minimize structures of known vanadium phosphorus oxides, which were compared with X‐ray structures, and to obtain their Hessian matrices as a basis for the force constants needed. Some hypothetical structural models had to be added. The van der Waals parameters were estimated by means of correlations between van der Waals radii and the repulsive parameters and between polarizabilities and the dispersive parameters from the literature. The force field obtained was applied to simulate the crystal structure of vanadyl pyrophosphate and to compute the heat of adsorption of n‐butane and of 1‐butene on its (100) plane (computer codes of company Biosym/MSI/Accelrys). The experimental crystal structure and the adsorption energies were fairly well reproduced, except that the a lattice constant proves somewhat too large.     

Trajectory Analysis and Collision Efficiency during Microbubble Flotation

The hydrodynamic interaction between a rising bubble and a sedimenting particle during microbubble flotation is considered. The effects of attractive van der Waals forces and attractive or repulsive electrostatic forces are included. A mathematical model is presented which is used to perform a trajectory analysis and to calculate collision efficiencies between the bubble and particle. It is shown that collision efficiencies and the nature of the bubble-particle interactions are strongly dependent on the relative strengths of the van der Waals and electrostatic forces and on the lengthscales over which these forces act. It is demonstrated that optimal operating conditions can be suggested to achieve efficient microbubble flotation by correctly accounting for the interaction of van der Waals, electrostatic, and hydrodynamic forces. Copyright 1999 Academic Press.     

溶致液晶中金属纳米粒子的掺杂及其作用机制研究

在用琥珀酸二异辛酯磺酸钠(AOT)构建的具有长程有序结构的层状溶致液晶内, 用不同方式导入预制的亲油或亲水贵金属纳米粒子, 可得到纳米粒子分布在不同介观空间内的无机/有机杂合体. 依据小角X射线散射和偏光显微镜结果, 通过分析掺杂纳米粒子与液晶模板的相互作用, 对掺杂前后体系结构的变化及制得杂合体的稳定性进行了表征. 结果表明, 除考虑掺杂粒子与层状模板空间的匹配外, 体系中静电斥力、范德华引力和Helfrich涨落力之间的平衡是维持液晶结构稳定的基本条件.     

Forces between a rigid probe particle and a liquid interface. II. The general case

The semianalytic theory developed previously (Chan, D. Y. C., Dagastine, R. R., and White, L. R., J. Colloid Interface Sci. 236, 141 (2001)) to predict the force curve of an AFM measurement at a liquid interface using a colloidal probe has been expanded to incorporate a general force law with both attractive and repulsive forces. Expressions for the gradient of the force curve are developed to calculate the point at which the probe particle on the cantilever will spontaneously jump in toward the liquid interface. The calculation of the jump instability is reduced to a straightforward embroidery of the simple algorithms presented in Chan et al. In a variety of sample calculations using force laws including van der Waals, electrostatic, and hydrophobic forces for both oil/water and bubble/water interfaces, we have duplicated the general behaviors observed in several AFM investigations at liquid interfaces. The behavior of the drop as a Hookean spring and the numerical difficulties of a full numerical calculation of F(deltaX) are also discussed.     

Nano‐adhesion and friction of multi‐asperity contact: a molecular dynamics simulation study

In our study, the contact and sliding processes between a flat plate and a substrate with multiple asperities are studied by molecular dynamics (MD) simulations, and how the number of asperities and asperity height influence the adhesion force and friction force are investigated thoroughly. The normal force versus the separation distance curve during contact processes is analyzed completely and from which the van der Waals (vdW) force (F_vdW) and the adhesion force (F_adh) are obtained and compared with the Katainen model. The adhesion force and the friction force increase linearly as the increase of the number of asperities (i.e. real contact area) with same asperity height. With the identical number of asperities, the adhesion force and the friction force decrease with the increase of the asperity height at first. However the reductions of the adhesion force and the friction force become less obvious, when the asperity height is larger than a critical value (20 Å for our simulation parameters). Copyright © 2015 John Wiley & Sons, Ltd.     

Exact Non-Linear Relationship Between Exponential-6 and Lennard-Jones (12-6) Potential Functions

The relationship between two of the most frequently adopted van der Waals potential functions – Exponential-6 and Lennard-Jones (12-6) – is shown to be faulty upon comparison of the repulsive terms. By using the Maclaurin's series expansions, an exact relationship between both the potential functions is obtained by means of a non-linear correction factor. A purely Exponential-6 form is recovered when the correction factor is taken at zeroth order, while a purely Lennard-Jones (12-6) form is attained as the order of the correction factor tends to infinity. For real van der Waals systems that are bounded by the Exponential-6 and Lennard-Jones (12-6) potential functions, a positive integer order of the correction factor may possibly provide good curve-fitting to experimental data.     

Unstable Supramolecular Structure of [Bmim][BF4] in Aqueous Solution

To unravel the exact composition and structure of aggregates in an aqueous solution of 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF₄]), we performed static and dynamic light-scattering measurements, as well as transmission electron microscopy (TEM). Results from this work show that the aggregates are vesicles and unstable; herein, we discuss the driving force behind the self-assembly. Apart from the van der Waals forces and repulsive electrostatic interactions between adjacent cation clusters, the hydrogen-bonding forces as well as counterion effects might also contribute to this driving force. The information obtained here is useful for a better understanding of the vital role that aggregation behavior plays in the field of ionic liquid recovery, and its potential use in controlled release, drug delivery, and petroleum recovery.