Interaction forces between condensed bodies in contact

Interaction forces at very small distances have mainly been investigated by extrapolation from larger distances, and by wetting and contact angle studies. The main result is a satisfactory understanding of the contribution of Van der Waals forces. Other types of forces, in particular those that may arise at distances of atomic dimensions, are not understood. The paper attempts to investigate the “other” forces by combining evidence from contact angle studies with results of direct measurements of the force of adhesion between a small, spherical particle and a flat substrate.     

Control of adhesion and surface forces via potential-dependent adsorption of pyridine

The orientation and extent of adsorption of pyridine on a gold electrode is known to depend on applied potential and is well characterized. By use of the electrochemical surface forces apparatus, we measured the potential dependence of the double-layer interactions and adhesive forces between a gold electrode and a mica surface for different pyridine concentrations. We observed that, unlike mica-mica interactions, the gold-mica interactions were strongly affected by the presence of small concentrations of pyridine. We are able to reach high negative surface potentials (as determined by applying Derjaguin-Landau-Verway-Overbeek theory to our force measurements), which is similar to what is observed in the absence of pyridine. This demonstrates the electronic nature of the forces measured and shows that pyridine does not displace potential-determining ions on the surface. At positive potentials, where the interaction between gold and mica is attractive, pull-off measurements are a strong function of applied potential. The major effect of the presence of pyridine is on the observed shift in the potential of zero force (PZF), moving it to more negative potentials. This effect is caused by the strong dipole of the pyridine molecule. When the applied potential is cast as a deviation from the PZF, the effect of pyridine is to reduce adhesion between gold and mica. We modeled the potential-dependent adhesion of this system using an electrocapillary framework developed previously, and in doing so, we establish the relationship between the gold-liquid and gold-mica surface energies. In addition, we show that pyridine adsorption affects the capacitance of the gold-mica interface.     

Lateral capillary forces between solid bodies on liquid surface: a lattice Boltzmann study

When two solid bodies are placed on the surface of a dense liquid under gravitation, they deform the liquid surface to experience a lateral capillary force between themselves that can be attractive and repulsive, depending on the wettabilities and weights of the bodies. In the present study, the lateral capillary force between two square bodies at a liquid-vapor interface has been examined using numerical simulations based on a two-dimensional two-phase lattice Boltzmann (LB) method. The particular situations were simulated, where every body was vertically constrained and had the fixed triple points at its upper or lower corners. Here, the triple point indicates the place at which vapor, liquid, and solid phases meet. The interaction force between these two bodies was calculated as a function of the separation distance, the interfacial tension, and the gravitational acceleration. The simulation results agree well with the analytical expression of the lateral capillary interaction, indicating that our LB method can reproduce the interaction force between two bodies of various wettabilities at a liquid-vapor interface in mechanical equilibrium.     

Dusty-gas model. Allowance for surface forces

The dusty-gas model has been extended to the case of nanoporous media, in which the action of surface forces must be taken into account. A basic set of transport equations underlying the model has been derived proceeding from a set of kinetic equations for an ordinary gas and dust particles. In the kinetic equations, the interaction between the gas and dust particles is represented as a sum of a long-range (analog of surface forces) and short-range components. The contribution of the long-range component has been taken into account in the self-consistent approximation, while the short-range component has been considered in the standard manner. Allowance for the surface forces has been shown to result in a substantial modification of the equation for gas transport through porous bodies, with this modification being most pronounced at nonuniform temperatures.     

Interaction forces measured between poly(N-isopropylacrylamide) grafted surface and hydrophobic particle

The interaction forces between poly(N-isopropylacrylamide) (PNIPAAm)-grafted surfaces and colloidal particles in an aqueous solution were investigated using an atomic force microscope. Measurements were conducted between smooth silicon wafers on which PNIPAAm was terminally grafted and silica particles hydrophobized with a silanating reagent in an aqueous electrolyte solution under controlled temperature. Below the lower critical solution temperature (LCST) of PNIPAAm, there were large repulsive forces between the surfaces, both on approach and separation of the surfaces. On the other hand, above LCST, attractive forces were observed both in approaching and in separating force curves. When surface hydrophobicity of the particles increased, the maximum attractive force tended to increase. The changes of hydration state of the grafted PNIPAAm chains depending on temperature are considered to greatly alter the interaction force properties. The role of the intermolecular interaction between the PNIPAAm chains and the hydrophobic particles in the interaction forces is discussed.     

Interaction forces in thin liquid films stabilized by hydrophobically modified inulin polymeric surfactant. 2. Emulsion films

The interaction forces in emulsion films stabilized using hydrophobically modified inulin (INUTEC SP1) were investigated by measuring the disjoining pressure of a microscopic horizontal film between two macroscopic emulsion drops of isoparaffinic oil (Isopar M). A special measuring cell was used for this purpose whereby the disjoining pressure Pi was measured as a function of the equivalent film thickness hw. The latter was determined using an interferometric method. In this way Pi-hw curves were established at a constant INUTEC SP1 concentration of 2x10(-5) mol.dm-3 and at various NaCl concentrations. At a constant disjoining pressure of 36 Pa, a constant temperature of 22 degrees C, and a film radius of 100 microm, hw decreased with an increase in the NaCl concentration, Cel, and reached a constant value of 11 nm at Cel=5x10(-2) mol.dm-3. This reduction in film thickness is due to the compression of the electrical double layer, and at the above critical NaCl concentration any electrostatic repulsion is removed and only steric interaction remains. This critical electrolyte concentration represents the transition from electrostatic to steric interaction. At a NaCl concentration of 2x10(-4) mol.dm-3 the Pi-hw isotherms showed a gradual decrease in hw with an increase in capillary pressure, after which there was a jump in hw from approximately 30 to approximately 7.2 nm when Pi reached a high value of 2-5.5 kPa. This jump is due to the formation of a Newton black film (NBF), giving a layer thickness of the polyfructose loops of approximately 3.6 nm. The film thickness did not change further when the pressure reached 45 kPa, indicating the high stability of the film. Pi-hw isotherms were obtained at various NaCl concentrations, namely, 5x10(-2), 5x10(-1), 1.0, and 2.0 mol.dm-3. The initial thicknesses are within the range 9-11 nm, after which a transition zone starts, corresponding to a pressure of about 0.5 kPa. In this zone all films transform to an NBF with a jump, after which the thickness remains constant with a further increase in the disjoining pressure up to 45 kPa, with no film rupture. This indicates the very high stability of the NBF in the presence of high electrolyte concentrations. The high emulsion film stability (due to strong steric repulsions between the strongly hydrated loops of polyfructose) is correlated with the bulk emulsion stability.     

Interaction forces and lubrication of dendronized surfaces

Dendron brushes provide excellent opportunities to tune mechanical properties of interfacial layers. Compared to linear brushes the dendronic version is more compact and thus gives more steep repulsive barriers upon brush overlap. Provided that the range is sufficient to overcome the Van der Waals forces, the many free ends may be used to functionalize the colloidal particles for ‘smart’ delivery applications. Brushes show little interpenetration upon lateral compression and therefore brushed surfaces feature low friction coefficients. Dendron brushes have comparatively lower interpenetration lengths and hence are expected to protect surfaces even better against wear. We argue that one can still improve on this: when the free ends of star-like or dendronic molecules are (chemically) linked to each other to form a brush with a peripheral layer of macrocycles, the interpenetration length is even less. The latter is shown to be the case in recent computer simulations and is reproduced here using the numerical self-consistent field method. We conclude that the role of branching of molecules, possibly in combination with charges in the brush, should be further investigated to come to a deeper understanding of lubrication and wear protection in aqueous media. The recent progress that is reviewed below has a small bias to theoretical developments. It contains solid stepping stones for more systematic experimental verifications and detailed computer simulations.     

Chaotic dynamics and entropy elastic forces of chain molecules

The dynamic behaviour of a phantom molecular chain, consisting of mass points connected by mass-less rigid rods, is investigated in two dimensions. The end points of the chain are fixed in space. The dynamics of the molecule are determined by Newton's equations of motion. Their numerical solution is represented in form of Poincaré sections and shows deterministic chaos. It is shown that the forces of constraint, acting on the fixed end points, fluctuate in time. The time average of these forces is calculated as a function of the extension of the chain.     

通过静电相互作用制备CdTe/SiO2纳米复合物

以四乙氧基硅烷(TEOS)为原料,在乙醇相中制备了尺寸为80nmSiO₂纳米粒子,用3-氨基丙基三乙氧基硅烷对SiO₂纳米粒子进行了氨基化.以巯基乙酸为配体,在水相中合成了CdTe纳米晶.通过静电相互作用,CdTe纳米晶被吸附到氨基化的二氧化硅球的表面上.研究了两种粒子复合后引起的CdTe纳米晶发光光谱的变化.     

Simulations of surface forces in polyelectrolyte solutions

We have simulated interactions between charged surfaces in the presence of oppositely charged polyelectrolytes by coupling perturbations in the isotension ensemble to a free energy variance minimization scheme. For polymeric systems, this method completely outperforms configurationally biased versions of grand canonical simulations. Proper diffusive equilibrium between bulk and slit has been established for polyelectrolytes with up to 60 monomers per chain. A consequence of imposing diffusive equilibrium conditions, in contrast to previous more restricted models, is the possibility of surface charge inversion; ion-ion correlation and the cooperativity of monomer adsorption drive the formation of a polyion layer close to the surface, that overcompensates the nominal surface charge. This is observed even at modest surface charge densities, and leads to a build up of a long ranged electrostatic barrier. In addition, the onset of charge inversion requires very low bulk polymer densities. Due to screening effects, this leads to a higher and more long-ranged free energy barrier at low, compared to high, bulk densities. Oscillatory forces, reminiscent of those found in simple hard sphere systems, are resolved in the high concentration regime. As a consequence of a second surface charge inversion, the system "stratifies" to form a stable polyelectrolyte layer in the central part of the slit, stabilized by the adsorbed surface layers.     

Influence of surface roughness on dispersion forces

Surface roughness occurs in a wide variety of processes where it is both difficult to avoid and control. When two bodies are separated by a small distance the roughness starts to play an important role in the interaction between the bodies, their adhesion, and friction. Control of this short-distance interaction is crucial for micro and nanoelectromechanical devices, microfluidics, and for micro and nanotechnology. An important short-distance interaction is the dispersion forces, which are omnipresent due to their quantum origin. These forces between flat bodies can be described by the Lifshitz theory that takes into account the actual optical properties of interacting materials. However, this theory cannot describe rough bodies. The problem is complicated by the nonadditivity of the dispersion forces. Evaluation of the roughness effect becomes extremely difficult when roughness is comparable with the distance between bodies. In this paper we review the current state of the problem. Introduction for non-experts to physical origin of the dispersion forces is given in the paper. Critical experiments demonstrating the nonadditivity of the forces and strong influence of roughness on the interaction between bodies are reviewed. We also describe existing theoretical approaches to the problem. Recent advances in understanding the role of high asperities on the forces at distances close to contact are emphasized. Finally, some opinions about currently unsolved problems are also presented.     

Interaction forces between a deformable air bubble and a spherical particle of tuneable hydrophobicity and surface charge in aqueous solutions

Unsupported nanosized MoS(2) and CoMo-sulfide catalysts were synthesized, and their catalytic performances for the deep hydrodesulfurization (HDS) of treated gas oil were investigated as compared with that of a CoMo/Al(2)O(3) catalyst. The HDS reactions were carried out in a batch autoclave reactor at 340 °C and 3 MPa H(2). The CoMo-sulfide catalyst shows the highest activity and can reduce the sulfur content to less than 10 ppm. The decrease in total sulfur content as a function of reaction time was found to follow pseudo-second order kinetics (empirical form). The change in the concentration of some individual representative sulfur-containing species in gas oil as a function of time was found to follow pseudo-first-order kinetics. However, the change in combined concentration of these species in the gas oil during HDS with the reaction time was found to corroborate pseudo-second-order kinetics. A kinetic model approach was proposed from which an estimation of the intrinsic kinetic data can be achieved. The model fitted the obtained data reasonably well, suggesting its potential for better assessment of the catalytic activity in the HDS of real feedstock. The study reveals that ranking of catalyst activities using model refractory sulfur-containing compounds does not necessarily imply a typical rank in case of investigating the real feedstocks.     

Control of surface forces through hydrated boundary layers

Hydration layers surrounding charges or zwitterionic moieties have long been known to play important roles in areas including antifouling and colloidal stability, and particularly over the past 15 years or so, their role in boundary lubrication has been widely investigated. Hydration repulsion because of hydrated ions or polar groups present on surfaces may dominate their interactions at high electrolyte concentrations, so that Derjaguin–Landau–Verwey–Overbeek theory does not apply. Hydration shells, strongly held by the charges they surround, can sustain large pressures without being squeezed out, while by rapidly relaxing, and they behave like a fluid during shear; this may lead to their acting as lubrication vectors with outstanding friction–reduction properties. This review considers hydration layers around trapped ions, polymer brushes, and amphiphiles (surfactants and phosphatidylcholines), focusing on their lubrication properties. Finally, we suggest some prospects for further development of current hydrated vectors and designing new hydrated vectors for modifying surface interactions.     

Unnatural proteins for the control of surface forces

We introduce a new method for the stabilization of colloidal particles via the synthesis and adsorption of unnatural proteins. Biosynthesis of protein-based polymers offers the advantages of preparation of complex sequences through control of the primary sequence, monodisperse polymers, ease of combinatorial search for anchor blocks, environmentally friendly synthesis, use of water as the solvent, and incorporation of a palette of known natural proteins. We have synthesized an unnatural protein with the sequence thioredoxin-Pro(39)Glu(10) for modification of the forces between alumina particles. The polyglutamate sequence, Glu(10), is anionic (pH > 3) and is designed to anchor the protein to positively charged solids, e.g. alumina in water (pH < 9). The polyproline sequence, Pro(39), is neutral. The thioredoxin is a recombinant form of the natural globular protein with a histidine patch (His-patch-thioredoxin) and is zwitterionic. The combined thioredoxin-Pro(39) sequence is hydrophilic with pI approximately 6.3. This block is designed to remain in solution, thereby providing a steric barrier to the approach of two particles in a range of salt and pH conditions. Ellipsometry experiments show that thioredoxin-Pro(39)Glu(10) does adsorb to alumina. Force measurements with the atomic force microscopy (AFM) colloid probe technique show that adsorption of thioredoxin-Pro(39)Glu(10) leads to repulsive forces that decay exponentially with the separation between the surfaces and are independent of salt concentration in the range 0.001-0.1 M KNO(3). This demonstrates that the repulsive forces are not electrostatic. We hypothesize that the repulsion is due to confinement and loss of solvent for the adsorbed polymer; the forces are similar to those expected for a polymer brush. Force measurements between thioredoxin-coated alumina surfaces also show a repulsive force, but the force has a decay length that is consistent with electrostatic double-layer forces: the thioredoxin has not neutralized the surface charge of the underlying alumina. Our results point to interesting future experiments where recombinant DNA technology could be used to synthesize fusion proteins containing useful natural proteins and an anchor. This may allow preparation, via single-step aqueous self-assembly, of anchored proteins that maintain their natural structure. Our technique is not limited to homopolymer blocks; more complex primary sequences can be used.     

2D Plasmonic Molecules via Hydrogen Bond Interaction between Polymer-Grafted Nanoparticles

The use of macromolecular design features to regulate non-covalent bonding on the nanoscale is a young and emerging fabrication strategy for advanced nanostructures. For the first time, we describe a self-assembly method to create a series of 2D plasmonic molecules (PMs) using hydrogen-bond interaction between a pair of polymer-capped gold nanoparticles (hydrogen-bond donor and acceptor). Due to the nature of hydrogen-bond interaction, we found that polymer interaction and solvation compete with each other during the self-assembly process, which turns out to be the most important condition for controlling the coordination number of PMs. We have conducted an extensive study on the solvent effect, which has helped us to design and fabricate a series of precise PMs with high symmetry.     

The role of surface forces in mineral flotation

Flotation is an interfacial separation technique, which plays a major role in mineral processing industry. It separates particles according to their wetting properties. In flotation pulp, particles and bubbles are highly dispersed in aqueous medium and in the presence of various flotation reagents. Almost all interfacial interactions including inter-particle, inter-bubble, and bubble-particle interactions in the complex pulp medium are driven by surface forces. Therefore, a fundamental understanding of the role of surface forces in flotation is a prerequisite to enhance practical flotation performance and adapt it for treatment of complex and refractory ores. In this paper, recent advances in the field of surface forces encountered in mineral flotation are reviewed. In particular, we highlight the latest progress in the attachment mechanism between bubble and particle with the aid of atomic force microscope and interference microscope. The current knowledge gap and future directions are also discussed.     

Interaction of NO2 with TiO2 surface under UV irradiation: products study

The interaction of NO(2) with TiO(2) solid films was studied under UV irradiation using a low pressure flow reactor (1-10 Torr) combined with a modulated molecular beam mass spectrometer for monitoring of the gaseous species involved. HONO, NO, and N(2)O were observed as the products of the reactive uptake of NO(2) to the illuminated TiO(2) surface with the sum of their yields corresponding nearly to 100% of the nitrogen mass balance. The yield of the products was measured as a function of different parameters such as irradiance intensity, relative humidity (RH), temperature, and concentrations of NO(2) and O(2). The yield of N(2)O was found to be 0.15 ± 0.05 independent of the experimental conditions. The distribution of the products between NO and HONO was found to be independent of temperature in the range T = 280-320 K and was governed by relative humidity: increase in RH led to lower NO and higher HONO yield, with a maximum of nearly 65% reached at ~5% RH. Presence of molecular oxygen was shown to shift the HONO/NO distribution to HONO at low RH (<5%) with no effect at higher RH where the HONO yield is maximum. The following values for the yield of the products of NO(2) interaction with pure TiO(2) under real atmospheric conditions can be recommended from this work: 0.65 ± 0.10, 0.05 ± 0.05, and 0.15 ± 0.05 for HONO, NO, and N(2)O, respectively. The mechanism of the photoinitiated heterogeneous reaction and possible atmospheric implications of the obtained results are discussed.     

Modern state of the investigation of long-range surface forces

Development of the concept of surface long-range forces and, in particular, the equilibrium disjoining pressure of liquid and gaseous interlayers has been set forth. Considered are the molecular, adsorption, electrical, structural, and electronic components of disjoining pressure. The contribution of the disjoining pressure to the hydrodynamics of thin layers is considered. The first theory of the frost heaving of soils has been formulated. Stated are the investigations of surface forces, in particular, in the processes of the formation of new interfaces and arising phenomena of the emission of electrons, ions, photons, and neutrons.