Attractive Van Der Waals Forces Research Articles

Van der Waals Emulsions: Emulsions Stabilized by Surface‐Inactive, Hydrophilic Particles via van der Waals Attraction

AbstractSurface‐inactive, highly hydrophilic particles are utilized to effectively and reversibly stabilize oil‐in‐water emulsions. This is a result of attractive van der Waals forces between particles and oil droplets in water, which are sufficient to trap the particles in close proximity to oil–water interfaces when repulsive forces between particles and oil droplets are suppressed. The emulsifying efficiency of the highly hydrophilic particles is determined by van der Waals attraction between particle monolayer shells and oil droplets enclosed therein and is inversely proportional to the particle size, while their stabilizing efficiency is determined by van der Waals attraction between single particles and oil droplets, which is proportional to the particle size. This differentiation in mechanism between emulsification and stabilization will significantly advance our knowledge of emulsions, thus enabling better control and design of emulsion‐based technologies in practice.

Effects of DLVO, hydration and osmotic forces among soil particles on water infiltration

SummarySoil water infiltration is closely associated with the transport of nutrients and contaminants in soil. The physical mechanism by which electrolyte concentration influences water infiltration is not clear, however. Recent studies have shown that interaction forces between soil particles play a crucial role in the stability of soil aggregates and pores, which further affects soil water infiltration. In this research, column experiments were used to elucidate the effects of soil particle interaction forces on water infiltration in a permanently charged and a variably charged soil using different concentrations of KNO3 solutions (0.0001, 0.001, 0.01 and 0.1 mol l−1), and some interesting results on soil water infiltration were discovered. They indicated that: (i) four forces in soil (long‐range van der Waals attractive, electrostatic repulsive, hydration and osmotic repulsive forces) co‐determined the rate of soil water infiltration for both soil types tested, (ii) at smaller soil water content (large electrolyte concentration) the osmotic repulsive force became more important in soil water infiltration, and yet with a larger water content (small electrolyte concentration) the electrostatic repulsive force became more important, and (iii) the osmotic force affected water infiltration through its effect on particle interaction forces rather than its influence on the osmotic potential of water. This paper presents a quantitative description of how particle interaction forces affect water infiltration into soil.HighlightsElucidation of physical mechanisms of effects of clay charge and electrolyte concentration on soil water infiltration. Quantified effects of interaction forces of soil particles on soil water infiltration. Electrostatic repulsive and osmotic forces play a critical role in water infiltration. DLVO, hydration and osmotic forces among soil particles co‐determined soil water infiltration rate.

Morphology Studies on Mechanically Milled Aluminium Reinforced with B4C and CNTs

Carbon nanotubes (CNT) with exceptional mechanical and physical properties are the prime candidate materials for metal matrix composites. But the dispersion of CNTs in the metal matrix is affected by attractive van der Waals forces. In this study, we have adopted ultrasonication and mechanical alloying to achieve homogenous dispersion of multiple reinforcements namely B4C and CNTs within the commercial purity aluminium powder. The ultrsonication of all the starting materials was followed mechanical alloying process which was carried out in a planetary ball mill up to 480 min. The milling process was systematically studied by taking a small amount of milled powder at different milling time of 60, 120, 240 and 480 min respectively. The effect of ultrasonication and mechanical alloying on both morphology of the powders and dispersion of B4C and CNTs was studied using scanning electron microscopy. The results showed uniform dispersion of both B4C and CNTs within the pure aluminium powders without any agglomeration. Both the pure aluminium and hybrid nanocomposite powder showed decrease in particle size from 20.89 µm to 5.67 µm and 2.72 µm respectively after 480 minutes of ball milling.

Non-adsorbing polymers and yield stress of cement paste: Effect of depletion forces

In this work, we study the effect of non-adsorbing polymers with molar masses varying over several decades in both dilute and semi-dilute regimes on the yield stress of cement and calcite pastes. In both suspensions, we measure an increase of yield stress, the magnitude of which mainly depends on the polymer concentration regimes. From interparticle force calculations, we show that non-adsorbing polymers are at the origin of interparticle attractive depletion forces. In the absence of superplasticizers, Van der Waals attractive forces are the dominant flocculating mechanism in the system and depletion forces resulting from non-adsorbing polymers are negligible. The repulsive forces that result from superplasticizers' steric contribution increase the average interparticle distance and therefore decrease the magnitude of the attractive van der Waals forces. In this latter system, attractive depletion forces induced by non-adsorbing polymers become important and greatly contribute to the flocculation mechanism.

Role of Solute Attractive Forces in the Atomic-Scale Theory of Hydrophobic Effects.

The role that van der Waals (vdW) attractive forces play in the hydration and association of atomic hydrophobic solutes such as argon (Ar) in water is reanalyzed using the local molecular field (LMF) theory of those interactions. In this problem, solute vdW attractive forces can reduce or mask hydrophobic interactions as measured by contact peak heights of the ArAr correlation function compared to reference results for purely repulsive core solutes. Nevertheless, both systems exhibit a characteristic hydrophobic inverse temperature behavior in which hydrophobic association becomes stronger with increasing temperature through a moderate temperature range. The new theoretical approximation obtained here is remarkably simple and faithful to the statistical mechanical LMF assessment of the necessary force balance. Our results extend and significantly revise approximations made in a recent application of the LMF approach to this problem and, unexpectedly, support a theory of nearly 40 years ago.

Dynamic surface electronic reconstruction as symmetry-protected topological orders in topological insulator Bi2Se3

The layered narrow-band-gap semiconductor ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ is composed of heavy elements with strong spin-orbital coupling, which has been identified both as a good candidate for a thermoelectric material with high thermoelectric figure of merit $(ZT)$ and as a topological insulator of the ${Z}_{2}$ type with a gapless surface band in a Dirac-cone shape. The existence of a conjugated $\ensuremath{\pi}$-bond system on the surface of each ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ quintuple layer is proposed based on an extended valence bond model with valence electrons distributed in the hybridized orbitals. Supporting experimental evidence of a two-dimensional (2D) conjugated $\ensuremath{\pi}$-bond system on each quintuple layer of ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ is provided using electron energy-loss spectroscopy and electron density mapping through inverse Fourier transform of x-ray diffraction data. Quantum chemistry calculations support the $\ensuremath{\pi}$-bond existence between partially filled $4{p}_{z}$ orbitals of Se via side-to-side orbital overlap positively. The conjugated $\ensuremath{\pi}$-bond system on the surface of each quintuple ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ layer is proposed to be similar to that found in graphite (graphene) and responsible for the unique 2D conduction mechanism. The van der Waals (vdW) attractive force between quintuple layers is interpreted to be coming from the antiferroelectrically ordered effective electric dipoles, which are constructed with $\ensuremath{\pi}$-bond trimer pairs on Se layers across the vdW gap of minimized Coulomb repulsion.

Multiple Structural Transitions in Langmuir Monolayers of Charged Soft-Shell Nanoparticles.

We have investigated the morphologies of Langmuir layers of charged, polymeric hard-core/interlayer/soft-shell nanoparticles spread at the air-water interface. Depending on various mutual interactions, which are correlated to the areal densities of the deposited nanoparticles, we observed ordered patterns of nondense and closed-packed arrangements of core/interlayer/shell (CIS) nanoparticle ordering. At low areal densities, we found an almost regular distribution of the charged CIS nanoparticles on the water surface, which resulted from long-range repulsive electrostatic interactions between them. At higher areal densities, domains of more closely packed and ordered nanoparticles were formed, coexisting with regions of randomly and sparsely distributed nanoparticles. We relate these domains to the interplay of electrostatic repulsion and capillary attraction caused by the dipolar character of like-charged particles at the interface, allowing for a characteristic separation distance between nanoparticles of about 3-4 times the nanoparticle diameter. At relatively high areal densities, attractive van der Waals forces were finally capable of making nanoparticles to come in contact with each other, leading to densely packed patches of hexagonally ordered nanoparticles coexisting with regions of rather well-ordered nanoparticles separated by about 1 μm and regions of randomly and sparsely distributed nanoparticles. Intriguingly, upon re-expansion of the area available per nanoparticle, these densely packed patches disappeared, indicating that steric repulsion due to the presence of soft shells as well as long-range electrostatic repulsive forces were strong enough to assure reversibility of the morphological behavior.

Thiophene/Graphene Interface Peculiarities for Potential Organic Electronic Applications

Interfacial study between thiophene molecule and graphene surface is presented by employing density functional theory methods. To do so, interfacing separation distance is varied from 1.00A to 2.50A. Our reported HOMO-LUMO energy gap values, adsorption energy as well as binding energy show the existence of intermolecular forces accumulated from the attractive van der Waals and Pauli repulsion forces. It is noted subsequently that the growing intermolecular forces are very sensitive even to relatively a small change in the interfacing separation distance between the molecule and surface. In the electronic density of states, dense electrons population of the thiophene/graphene system is found with appearance of spinpolarization at energy Fermi level. Moreover, a slight magnetic behaviour on thiophene molecule, accompanied by a decrease in the magnetization of graphene surface, is observed in the presence of the molecule near to the surface.

Shear-induced aggregation or disaggregation in edible oils: Models, computer simulation, and USAXS measurements

The effects of shear upon the aggregation of solid objects formed from solid triacylglycerols (TAGs) immersed in liquid TAG oils were modeled using Dissipative Particle Dynamics (DPD) and the predictions compared to experimental data using Ultra-Small Angle X-ray Scattering (USAXS). The solid components were represented by spheres interacting via attractive van der Waals forces and short range repulsive forces. A velocity was applied to the liquid particles nearest to the boundary, and Lees-Edwards boundary conditions were used to transmit this motion to non-boundary layers via dissipative interactions. The shear was created through the dissipative forces acting between liquid particles. Translational diffusion was simulated, and the Stokes-Einstein equation was used to relate DPD length and time scales to SI units for comparison with USAXS results. The SI values depended on how large the spherical particles were (250 nm vs. 25 nm). Aggregation was studied by (a) computing the Structure Function and (b) quantifying the number of pairs of solid spheres formed. Solid aggregation was found to be enhanced by low shear rates. As the shear rate was increased, a transition shear region was manifested in which aggregation was inhibited and shear banding was observed. Aggregation was inhibited, and eventually eliminated, by further increases in the shear rate. The magnitude of the transition region shear, γ̇t, depended on the size of the solid particles, which was confirmed experimentally.

Effective particle size from molecular dynamics simulations in fluids

We report molecular dynamics simulations designed to investigate the effective size of colloidal particles suspended in a fluid in the vicinity of a rigid wall where all interactions are defined by smooth atomic potential functions. These simulations are used to assess how the behavior of this system at the atomistic length scale compares to continuum mechanics models. In order to determine the effective size of the particles, we calculate the solvent forces on spherical particles of different radii as a function of different positions near and overlapping with the atomistically defined wall and compare them to continuum models. This procedure also then determines the effective position of the wall. Our analysis is based solely on forces that the particles sense, ensuring self-consistency of the method. The simulations were carried out using both Weeks–Chandler–Andersen and modified Lennard-Jones (LJ) potentials to identify the different contributions of simple repulsion and van der Waals attractive forces. Upon correction for behavior arising the discreteness of the atomic system, the underlying continuum physics analysis appeared to be correct down to much less than the particle radius. For both particle types, the effective radius was found to be sim 0.75sigma , where sigma defines the length scale of the force interaction (the LJ diameter). The effective “hydrodynamic” radii determined by this means are distinct from commonly assumed values of 0.5sigma and 1.0sigma , but agree with a value developed from the atomistic analysis of the viscosity of such systems.

Film thickness formation in nanoscale due to effects of elastohydrodynamic, electrostatic and surface force of solvation and van der Waals

PurposeThe purpose of this is to study the mechanism of an oil film thickness formation in the nanoscale. A polar lubricant of propylene carbonate is used as the intervening liquid between contiguous bodies in concentrated contacts. A pressure caused by the hydrodynamic viscous action in addition to the double-layer electrostatic force, van der Waals inter-molecular forces and solvation pressure owing to inter-surface forces is considered when calculating the ultrathin lubricating films.Design/methodology/approachUsing the Newton–Raphson iteration technique applied for the convergence of the hydrodynamic pressure, a numerical solution has been ascertained.FindingsThe results show that, at separations beyond about five molecular diameters of the intervening liquid, the formation of a lubricant film thickness is governed by the combined effects of viscous action and surface force of an attractive van der Waals force and a repulsive double-layer force. At smaller separations below five molecular diameters of the intervening liquid, the effect of the solvation force is dominant in determining the oil film thickness.Originality/valueThis paper fulfils an identified need to study the behavior of polar lubricants in concentrated contacts in ultrathin conjunctions. The effect of the hydrodynamic action, electrostatic force and surface action of van der Waals and solvation forces is considered when calculating the lubricant oil film thickness.

Plasticisation and compatibilisation of poly(propylene) with poly(lauryl acrylate) surface modified MWCNTs

Poly(lauryl acrylate) P[LA] synthesised using Cu(0)-mediated polymerisation was used to non-covalently functionalise multi-walled carbon nanotubes (MWCNTs) achieved via combined CH-π interactions and physical wrapping. The inclusion of P[LA] competed with the attractive van der Waals forces between MWCNTs altering interfacial interactions between neighbouring CNTs and, assisted their dispersion and distribution in a poly(propylene) (PP) matrix, confirmed from SEM/HR-TEM imaging and oscillatory rheology measurements. A rheological percolation threshold measured for composites of unmodified MWCNTs and PP at ∼0.5 wt% was shifted to lower MWCNT concentrations (∼0.25 wt%) for P[LA] functionalised MWCNTs. Addition of P[LA] also reduced the nucleating effect of MWCNTs for PP as a coating of P[LA] on the nanotube surfaces hindered trans-crystallisation of the PP. Evidence from static and dynamic mechanical measurements confirmed the P[LA] plasticised PP as both an increase in ductility and a decrease in the glass transition temperature (Tg) of PP of up to 25 °C was obtained. Addition of unmodified MWCNTs to PP resulted in an increase in the DC electrical conductivity of PP of 15 orders of magnitude, from 10−14 S/m to 101 S/m for a MWCNT loading of 5 wt%. Non-covalent functionalisation of MWCNTs utilising P[LA] only degraded electrical conductivity by about 2 orders of magnitude across the composition range.

Understanding the exfoliation and dispersion of MoS2 nanosheets in pure water

Exfoliation and dispersion of two dimensional (2D) transition metal dichacogenides (TMDCs) such as MoS2 in water are highly demanded. In this paper, we exfoliate MoS2 nanosheets in pure water via ultrasonication and attempt to understand the exfoliation process and the dispersion behavior of hydrophobic MoS2. Cavitation-induced exfoliation leads to thinning and fragmentation of MoS2 flakes. Formation of mesoporous MoS2 sheets suggests that the exfoliation initiates from the basal planes. As for the dispersion of MoS2 nanosheets in water, we find that higher centrifugation rate and thus smaller lateral size, results in improved stability. This can be attributed to the enhanced edge effects. Fragmentation of MoS2 flakes generates many edges, to which hydrophilic and ionizable groups are attached. Herein, the edge effects include two aspects. On the one hand, edge-attached polar and hydrophilic groups promote interactions of MoS2 nanosheets with water molecules and result in better wettability, which can be explained by the hemi-wicking model. On the other hand, dissociation of these groups makes MoS2 sheets negatively charged. It is found that edge charge dominates. As the flake size reduces, attractive van der Waals force and hydrophobic force can be overcome by electrostatic repulsions, which prevent MoS2 nanosheets from aggregation. This study opens a new vista on exfoliating and dispersing hydrophobic 2D nanomaterials in water.

The effect of adsorption modeling on the stability of surfactant-laden liquid film flow

The primary instability of gravity-driven, liquid film flow is examined when the liquid contains a soluble surfactant. It is shown that the model adopted for the description of adsorption equilibrium affects significantly the prediction of the critical Reynolds number, \(Re_\mathrm{c} \). Weak attractive/repulsive van der Waals forces between adsorbed molecules stabilize/destabilize the film, whereas strong, attractive van der Waals forces result in non-monotonic variation of \(Re_\mathrm{c} \) with surface coverage. Ionic surfactants form an electric double layer at the interface, whose repulsive potential destabilizes the film. The intensity of electrostatic effects varies inversely with the solution ionic strength, and two asymptotic limits—zero and high concentration of an indifferent salt—are examined. The competition between electrostatic and van der Waals forces results in a very rich behavior, with the critical Reynolds number increasing or decreasing with surface coverage. The parametric variation of \(Re_\mathrm{c} \) is physically understood in terms of the effectiveness of surfactant mass exchange (between the interface and the bulk) in short-circuiting the basic stabilization mechanism, i.e., Marangoni stresses.

Electrodeposition of bismuth telluride thin films containing silica nanoparticles for thermoelectric applications

The electrodeposition of bismuth telluride (Bi2Te3) with incorporated silica nanoparticles (diameter = 16 nm) was studied in ethylene glycol using a rotating disk electrode. "By matching the refractive index of the silica nanoparticles to the refractive index of the electrolyte i.e. "refractive index matching", the attractive van der Waals force between particles was minimized. Refractive index matching was used to avoid agglomeration of the silica particles in solution due to the attractive van der Waals forces and resulted in Bi2Te3 coatings that contain up to 8vol% of individual silica particles as verified by TEM. The concentration of the incorporated silica nanoparticles was found to be influenced by several parameters: the applied current density, rotation speeds and silica concentration in the electrolyte. A model was developed to predict the amount of incorporated particles. The effect of each parameter was predicted by this model and verified experimentally. The Seebeck coefficient and thermal diffusivity of bismuth telluride films with and without silica nanoparticles were measured.

Powder disperser for the continuous aerosolizing of dry powdered nanoparticles

A new method has been developed for the continuous aerosolization of dry powdered nanoparticles using a commercially available and cost-effective vacuum generator. A commercial doser was used to supply nanoparticles continuously and precisely. A novel device (called a pressure equilibrium unit) was designed to connect the doser and the vacuum generator to make sure both parts avoid suffering from pressure shock. This system is denoted asa continuous vacuum generator disperser (CVGD). The dosing rate of the CVGD for different particle materials tends to a constant value. All the aerosolized engineered nanoparticles (ENPs) of varying materials and primary diameter obtained through the CVGD distribute identically log-normally asa whole. It is more difficult to separate the nanoparticles of smaller primary diameter using the CVGD, due to the stronger van der Waals attractive force between them. The total particle concentration can be adjusted easily by changing the motor speed of the doser (Dr), which was directly proportional to the dosing rate. As Dr increased, the PSDs shifted to a larger diameter to some extent. For sufficient large dosing rates, the CVGD performance could be improved significantly by increasing the inlet pressure of the vacuum generator (Pj).

Surface instability of imperfectly bonded multi-layered curved films under van der Waals forces

Abstract Using a linear stability analysis and the transfer matrix method, we investigate the surface instability of an imperfectly bonded multi-layered curved film interacting with a curved rigid contactor, another imperfectly bonded multi-layered curved film or an imperfectly bonded multi-layered simply-supported cylindrical shell in each case through the action of attractive van der Waals forces. The imperfect interface is modelled as a linear spring layer with vanishing thickness characterized by normal and tangential imperfect interface parameters. Detailed numerical results are presented to demonstrate the resulting analytical solutions.

The coupling effects of soil organic matter and particle interaction forces on soil aggregate stability

Soil organic matter (SOM) and particle interaction forces (electrostatic repulsive force (ERF), van der Waals (vdW) attractive force and surface hydration repulsive force) profoundly influence the stability of soil aggregates. However, few studies have investigated the coupling effects of SOM and particle interaction forces on soil aggregate stability. Therefore, the objectives of this study were to: (1) quantitatively determine the particle surface properties before (control) and after SOM removal and after straw incubation for 240days, and then calculate the particle interaction forces based on the measured data; and (2) quantitatively evaluate the coupling effects of SOM and particle interaction forces on the stability of soil aggregates by measuring the quantities of released small particles after aggregate breakdown. Compared to the control treatment, SOM removal decreased the surface charge number, specific surface area and the effective Hamaker constant and increased the surface charge density, which, in turn, decreased the vdW attractive force and increased the ERF, DLVO repulsive force (ERF+vdW attractive force) and the net repulsive force between two adjacent soil particles, while straw incubation had opposite effects. Compared to the control treatment, SOM removal increased the mass percentage of released small particles (<10, <5, and <2μm) w (<d)%, while straw incubation decreased the w (<d)%. Hence, we concluded that aggregate stability is influenced by particle interaction forces, while organo-mineral interactions can increase the vdW attractive force between two adjacent particles within soil aggregates and decrease the ERF, DLVO repulsive force, and net repulsive force, resulting in increased stability of soil aggregates.

Thiocarbohydrates on Gold Nanoparticles: Strong Influence of Stereocenters on Binding Affinity and Interparticle Forces.

Carbohydrates carrying thiol groups at the C-2 position have been attached to gold nanoparticles (AuNPs) with stereocenters in close proximity to the surface for the first time. Their configurations can be clearly distinguished by the tendency of particle aggregation. AuNP surface plasmon resonance (SPR), X-ray photoelectron spectroscopy (XPS), and IR spectroscopy indicate that the thiocarbohydrates replace citrate molecules at different rates, causing aggregation and eventually precipitation. A quantitative formulation of this aggregation process shows that reactivities can vary by several magnitudes. Adsorption isotherms and kinetics also demonstrate that the number of thiocarbohydrates varies by a factor of two. Molecular mechanics force field (MMFF) calculations reveal their relative orientations. Based on these models, the different binding behavior can be ascribed to attractive van der Waals forces and hydrogen bonds. Such interactions occur either between the carbohydrate and AuNPs, by lateral intermolecular forces at the surface, or by interparticle attraction, in analogy to cell-surface carbohydrates of biological recognition systems. Aggregation of NPs therefore act as an indicator to differentiate between various carbohydrates with defined configurations.

Molecular Considerations for Mesophase Interaction and Alignment of Lyotropic Liquid Crystalline Semiconducting Polymers

Intermolecular interactions in conjugated polymers influence crystallinity, self-assembly, and packing motif, factors which in turn crucially impact charge transport properties such as carrier mobility in organic electronic devices. Correlated alignment of molecular and crystalline morphologies provides direct pathways for charge carriers to follow; however, the role of intermolecular interactions in achieving this is unexplored. Herein, we synthesize a series of lyotropic liquid crystalline conjugated polymers with variable side-chain structure to lend distinct steric repulsion and van der Waals attractive forces to each mesophase. We use this to investigate the role of intermolecular interactions on mesophase alignment. The strength of intermolecular interaction for each mesophase is compared by measuring melting temperature, π-stacking distance, and the Maier-Saupe interaction parameter. In general we find that side-chain structure can impact interaction strength by varying steric repulsion and backbone attractions and that the Maier-Saupe interaction parameters correlate with higher degrees of alignment after shearing, achieving a dichroic absorbance ratio of up to 2. This observation is used to develop equilibrium processing methods for fabricating macroscopically aligned polymer substrates used in transistors, improving mobility by a factor of 3 compared to spin-coated devices.