Der Waals Attraction Research Articles

The effectiveness of the coagulation sink of 3–10 nm atmospheric particles

Abstract. As a major source of ultrafine particles, new particle formation (NPF) occurs frequently in various environments. However, the survival of new particles and the frequent occurrence of NPF events in polluted environments have long been perplexing, since new particles are expected to be scavenged by high coagulation sinks. Towards solving these problems, we establish an experimental method and directly measure the effectiveness of the size-dependent coagulation sink of monodisperse 3–10 nm particles in well-controlled chamber experiments. Based on the chamber experiments and long-term atmospheric measurements from Beijing, we then discuss the survival of new particles in polluted environments. In the chamber experiments, the measured coagulation sink of 3–10 nm particles increases significantly with a decreasing particle size, whereas it is not sensitive to the compositions of test particles. Comparison between the measured coagulation coefficient with theoretical predictions shows that almost every coagulation leads to the scavenging of one particle, and the coagulation sink exceeds the hard-sphere kinetic limit due to van der Waals attractive force. For urban Beijing, the effectiveness of the coagulation sink and a moderate or high (e.g., > 3 nm h−1) growth rate of new particles can explain the occurrence of measured NPF events; the moderate growth rate further implies that, in addition to gaseous sulfuric acid, other gaseous precursors also contribute to the growth of new particles.

Comparison of different methods for assessing effects of soil interparticle forces on aggregate stability

Soil interparticle forces, involving in van der Waalsattractive force, and surface hydration and electrostatic repulsive forces, greatly influence the soil aggregate stability. However, current studies on methods for evaluating the impact of soil interparticle forces in aggregate stability are scarce. This research was aimed to examine the impact of soil interparticle forces on aggregate stability using diverse methods for different soil types. Soil aggregate stability was tested through the pipette method, wet sieving, and rainfall simulation, respectively characterized by aggregate stability index (ASI), mean weight diameter (MWD), and splash erosion mass (SE). Soil interparticle forces were adjusted by the changing concentrations of NaCl solution. The results showed that all three approaches can be applied to study the impact of soil interparticle forces on aggregate stability. The ASI, MWD, and SE showed little change below 10−2 mol L−1 NaCl concentration and then ASI and MWD increased at a high rate above 10−2 mol L−1 concentration of NaCl, while the SE showed the opposite trend. These results were as expected for soil interparticle forces. Moreover, for a single soil, a substantial correlation existed between the aggregate stability indicators obtained from three methods. However, the order of soil aggregate stability, measured by three methods were varied among soil types. Our results suggest that a single method cannot be applied to determine the aggregate stability of all soil types even if the breakdown mechanism was identical, because organic matter content and particle size distribution of soil are important factors influencing aggregate stability. Hence, in order to compare the difference in aggregate stability between various soils, multiple methods should be considered to investigate the impact of interparticle forces of soil on its aggregate stability. If a single method was to be chosen, wet sieving may be a good choice as it was not only relatively simple and time-saving but also reflected more comprehensive information about sizes and amount of fragments released from soil aggregates.

Effect of wrinkles on electrochemical performance of multiwalled carbon nanotubes as anode material for Li ion battery

A 1-D monohybrid of multiwalled carbon nanotubes and graphene sheets, graphene wrapped multiwalled carbon nanotubes (gC) structure, synthesized in a template-free simple chemical vapor deposition technique without any chemical functionalization, was employed as efficient anode material for Li ion battery. Graphene nanosheets affixed to the multiwalled carbon nanotubes (MWNTs) surface by van der Waal's attraction gives a wrinkled surface to the final 1-D gC configuration. The protrusions on the surface of the tube enhances the porosity of the system and also acts as defects, enhancing lithium adsorption sites while the inner MWNT core gives high electrical conductivity, resulting enhanced electrochemical performance of 373mAhg−1 at 100mAg−1 current density after 150 cycles.

Experimental evaluation and numerical modeling of adhesion phenomena in polysilicon MEMS

Adhesion phenomena in poly-silicon Micro-Electro-Mechanical Systems (MEMS) are here studied by means of experimental characterization and numerical simulation. In the experimental part, the adhesion energy is measured on-chip by means of on-purpose designed and fabricated devices; moreover, the surface roughness is characterized through the Atomic Force Microscope. For the numerical part, a specific method is developed to simulate the process of adhesion based on a numerical description of surface roughness, on simplified modeling of adhesion forces due to van der Waals and capillary attraction and on non-linear constitutive modeling for polysilicon surfaces.

Reducing Fines Migration by Use of Nanofluids Injection - An Experimental Study

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 144196, ’Reduction of Fines Migration by Nanofluids Injection - An Experimental Study,’ by A. Habibi, SPE, M. Ahmadi, and P. Pourafshary, SPE, University of Tehran, and Sh. Ayatollahi, SPE, Shiraz University, prepared for the 2011 SPE European Formation Damage Conference, Noordwijk, The Netherlands, 7-10 June. The paper has not been peer reviewed. Formation damage because of fines migration is a major reason for productivity decline. Many studies have characterized fines and their migration effect on permeability reduction. Nanofluids that contain nanoparticles (NPs) show specific properties including a high tendency for adsorption and being a good candidate for injection into the near-wellbore region because of the very small NP sizes. The study indicates that fines could adhere to the matrix grains, hindering their migration, when the porous materials are soaked with nanofluids. Introduction Fines are loose unconsolidated particles (smaller than 37 µm) that move with fluid flow and cause formation damage because of the filtering action of the porous media. The biggest drawbacks of this process are pore plugging and productivity-index reduction. Various surface forces have been found to be responsible for fines detachment and release from the pore surfaces. London/van der Waals attraction, double-layer and Born repulsion, and hydrodynamic forces are the dominant forces in the detachment of fines from porous media. When the total interaction energy between fines and pore surface becomes positive, the repulsive forces are bigger than attractive forces and fines detachment occurs. NP size ranges from 1 to 100 nm, and NPs have high specific surface area and unique properties, such as very high adsorption potential and heat conductivity. NPs have been used for formation-damage control, enhancing oil recovery, and wettability alteration. In the proppant packs, NPs strengthened the attractive forces and fixed the suspended fines in the porous media. In this experimental study, porous media were soaked with nanofluid for 24 hours and then the suspended fines were passed through porous media to determine the most efficient component. In the next step, a glass-bead-packed column containing uniformly distributed fines in the bed was flooded with distilled water. To investigate the main parameters in this process, the NP concentration and fluid-injection rate were investigated. The zeta potential of the treated models was measured, and the total interaction energy was calculated to verify the results. Finally, scanning-electron-microscope (SEM) images of the surface were obtained for qualitative observation of fines attachment to the pore surfaces.

ENERGY STORAGE, RELEASE, AND DISSIPATION IN THE GECKO ADHESION SYSTEM

ABSTRACTDifferent types of biological adhesion can be categorized according to the length scales, structures, and materials involved. The setal adhesion system of the gekkonid lizards occupies a hierarchy of scales from the toes (~ 1 cm) to the terminal spatular pads on the setal branches (~ 100 nm). This unique combination of scale and foot-hair morphology allow the animal robust, controllable, and near-universal adhesion via van der Waals attraction, but it is also apparent that the mechanical behavior of the β-keratin plays an important role in an animal’s climbing ability. Experimental results show a four-fold increase in the viscoelastic loss tangent of β-keratin, alongside a substantial increase in adhesion of setal arrays, over a range of relative humidity from 10 to 80%. A model of single-spatular deformation predicts that the elastic energy stored in the setal branches, energy which is not completely recovered on detachment, is strongly influenced by these properties changes. The enhanced dissipation characteristics of the system explain the effects of environmental humidity on the clinging ability of geckos.

Dispersion of functionalized silver nanoparticles in polymer matrices: Stability, characterization, and physical properties

AbstractThree types of silver/polymer nanocomposite films were prepared: (1) silver/polymethyl methacrylate, (2) silver/polystyrene, and (3) silver/polyt‐butyl methacrylate by mixing colloidal solution of functionalized Ag nanoparticles with solution of pre‐made polymers. The nanoparticles are randomly dispersed into the matrices. The desired surface functionality of Ag nanoparticle was achieved by using specific amphiphilic block copolymer (specific hydrophobic block part) as stabilizer. The stability of nanoparticles in these host hydrophobic polymer matrices was determined after annealing the nanocomposite films at high temperature. The nanoparticles remained stable and no agglomeration occurred upon annealing up to 120°C. Up to 20 wt% of block copolymer stabilized Ag nanoparticles (11% Ag) was successfully loaded in the polymer matrix. Such a dispersion of Ag nanoparticles into a hydrophobic polymer matrix significantly enhances the mechanical properties of the nanocomposite films. The storage and loss modulus increase with increasing Ag nanoparticles content. Compared with a polymer matrix, the polymer/Ag nanocomposites film (containing 0.65% of Ag) prepared by this method displayed storage modulus that are up to 130% higher. This is attributed to the high surface area of the polymer coated Ag nanoparticles in the polymer matrix and van der Waal's attraction force between surface of Ag nanoparticles (polymer brush) and host polymer matrix. Comparison of thermal properties of polymer and polymer/Ag nanocomposite films showed that the thermal stability of nanocomposites increases by about 20°C. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers

1412 コロイドのオリフィス流れ安定化効果 : 流路寸法への依存性(S30-2 複雑流体の流動現象(2),S30 複雑流体の流動現象)

The flow fluctuation of an aqueous colloid through a small orifice was measured; the solution included fine particles of approximately 100nm in diameter. Colloids of 1% and 2% volume fraction were investigated using two types of flow channels. In these volume fractions, the surface potential of the particles were composed of electro-static repulsions and a van der Waal's attraction which affected flow property. In this experiment, the electro-static repulsion was controlled by the addition of salt. In the case of flow channel #1, an increase in salt concentration suppressed the flow instability. However in flow channel #2, salt concentration had no effect on the orifice flow. This experiment has shown that orifice flow stabilization effects depend on the channel dimensions.

Fluctuation-induced long-range interactions in polymer systems

We discover a new universal long-range interaction between solid objects in polymermedia. This polymer-induced interaction is directly opposite to the van der Waalsattraction. The predicted effect is deeply related to the classical Casimir interactions,providing a unique example of universal fluctuation-induced repulsion ratherthan normal attraction. This universal repulsion comes from the subtracted softfluctuation modes in the ideal counterpart of the real polymer system. The effect canalso be interpreted in terms of subtracted (ghost) large-scale polymer loops. Weestablish the general expressions for the energy of polymer-induced interactionsfor arbitrary solid particles in a concentrated polymer system. We find that thecorrelation function of the polymer density in a concentrated solution of very longchains follows a scaling law rather than an exponential decay at large distances.These novel universal long-range interactions can be of importance in variouspolymer systems. We discuss the ways to observe/simulate these fluctuation-inducedeffects.

Contactless electrostatic handling of microcomponents

Microdevices are every day becoming more sophisticated and flexible. Common manufacturing techniques based on semiconductor technology do not fully meet the requirements brought up by the development of new applications or the necessity of increased performances in actual systems. In order to obtain three-dimensional structures with high mechanical properties, new manufacturing approaches based on the exploitation of different materials are necessary and the assembly of these microparts becomes a key issue to solve. In the assembly process, because of the low mass of the microcomponents, adhesive forces (surface tension, van der Waals and electrostatic attraction) cause the component to stick, with the gripper making the releasing phase very complex. Therefore microassembly requires new handling techniques such as, for example, electrostatic contactless handling, which appears to be very promising. Through a comparative analysis between theoretical results and simulations, the electrostatic forces on which the alignment and movement of microparts is based have been estimated and a demonstrative device has been designed and tested. In this paper electrostatic contactless handling is discussed, the developed prototype is presented and the feasibility of the transportation and positioning of mini- and micro-objects (as predicted by theory) is demonstrated through a set of experiments.

STRUCTURE AND DYNAMICS OF CARBON NANOTUBES IN CONTACT WITH GRAPHITE SURFACE AND OTHER CONCENTRIC NANOTUBES

Rigid carbon nanotubes in single and double walled formations, placed on a graphite surface, are bounced, rotated, slid and rolled. Various features associated with these motions are studied by assuming a 6-exp form of interaction (Van der Waal's attraction and Born–Mayer repulsion) among the C-atoms. Calculations reported here are for tubes of diameter around 14 Å, for which rigid tube approximation is known to work well. The oscillatory motion corresponding to rolling has the softest mode, whereas the one with highest frequency corresponds to bouncing. The energy barriers corresponding to these motions are also reported in this paper. The rotational and translational energy barriers for the movement of one nanotube with respect to the other one, in a double walled nanotube, have also been studied and it turns out that these tubes rotate and slide freely at room temperature. The translational energy barrier, in case of zigzag tubes, is interestingly, an order of magnitude higher than that of armchair tubes. In case of rotation, the case is reverse. Furthermore, it turns out that any drag of a concentric nanotube along the long axis direction is coupled with rotation, indicating easy screw motion instead of a simple drag. We also describe the dynamics of translational telescopic motion of a multiwalled nanotube assembly where a core oscillates within an open ended outer shell assembly.

4-(E)-2-[3-(3-[(E)-2-(4-cyanophenyl)-1-diazenyl]perhydrobenzo[d]imidazol-1-ylmethyl)perhydrobenzo[d]imidazol-1-yl]-1-diazenylbenzonitrile: X-ray crystal structure

4-(E)-2-[3-(3-[(E)-2-(4-Cyanophenyl)-1-diazenyl]perhydrobenzo[d]imidazol-1-ylmethyl) perhydrobenzo[d]imidazol-1-yl]-1-diazenylbenzonitrile (1) has been synthesized by reac- tion of p-cyanobenzene diazonium chloride with a mixture of formaldehyde and trans-1,2-cyclohexanediamine. The crystal structure has been determined by single crystal X-ray diffraction analysis. The bis-triazene (1) does not adopt a folded conformation, unlike previously studied ethylenediamine analogues, and there is no indication of π-stacking in the crystal packing. The dominant interaction between molecules is the van der Waal's attraction between cyclohexane rings. This result establishes the structure of the product of this diazonium coupling reaction as the 1-(1-imidazolidinylmethyl)imidazolidine derivative and not the alternate tetraazabicyclo[4.4.1]undecane derivative. Crystal data: 1 C29H34N10, orthorhombic, space group Pbca, a = 17.946(1), b = 13.106(1), c = 24.108(1) A, V = 5670.6(3) A3, for Z = 8.

Dispersion Stability of SiC, SiCN and Si<sub>3</sub>N<sub>4</sub> Ultrafine Powders in Aqueous and Nonaqueous Media

A successful colloidal processing of ultrafine powders to produce high quality ceramic materials gives demand for stable, high degree particle dispersions. The stability of ceramic suspensions on the basis of SiC, Si 3 N 4 and SiCN nanopowders in water and organic solvents (octane, benzene, toluene, benzyl alcohol and 1-octanol) was examined within 24 hours by means of screen sedimentation tests. Experimental results of the settling velocities were related to the calculated values of the total interaction potential energy between the particles interacting over the solvent molecules-the dispersion (van der Waals) attraction in case of organic liquids in the absence of dispersants, and a combination of dispersion attraction and electrostatic forces in case of water, and finally to the physical properties of the used solvents.

Grana formation: entropy-assisted local order in chloroplasts?

Grana formation is a prominent feature of the ultrastructure of chlorophyll b-containing chloroplasts, serving to fine-tune photosynthetic efficiency. This paper examines the physical forces that determine the state of minimum free energy of the chloroplast as manifested by grana formation. In particular, it considers the interplay of van der Waals attraction, electrostatic repulsion and short-range hydration repulsion between thylakoid membranes. In addition to these interactions as determinants of the free energy of the system, it is also proposed that ordering of thylakoid membranes and of intramembrane protein complexes is driven by an increase in the overall entropy of the system. This local order may partly come about by an increase in entropy associated with a greater free volume for diffusion of membrane and stromal components.

Observations of coagulation in isotropic turbulence

Turbulent-shear-induced coagulation of monodisperse particles was examined experimentally in the nearly isotropic, spatially decaying turbulence generated by an oscillating grid. The 3.9 μm polystyrene microspheres used in the experiments were made neutrally buoyant and unstable by suspending them in a density-matched saline solution. In this way, particle settling, double-layer repulsion and particle inertia were negligible and the effect of turbulent shear was isolated. The coagulation rate was measured by monitoring the loss of singlet particles as a function of time and reactor turbulence intensity. By restricting consideration to experimental conditions where the singlet concentration was in excess, the effect of higher-order aggregate (i.e. triplet) formation was negligible and nonlinear regression using an integral rate expression that included terms for doublet formation and breakup was used to obtain the turbulent coagulation rate constant. The strength of the van der Waals attractions was characterized with the Hamaker constant obtained from Brownian coagulation experiments. Since particle bulk mixing was fast compared to the coagulation rate, the observed coagulation rate constants were averages over the local coagulation rates within the grid-stirred reactor. Knowledge of the spatial variation of turbulence within the reactor was necessary for quantitative prediction of the experiments because model predictions for the coagulation rate are nonlinear functions of shear rate. The investigation was conducted with particles smaller than the length scales of turbulence and since the smallest turbulent length scales, the Kolmogorov scales, have the highest shear rate they controlled the rate of particle aggregation. The distribution of the Kolmogorov shear rate at various grid oscillation frequencies was obtained by measuring the turbulent kinetic energy (E) using acoustic Doppler velocimetry and relating E to the Kolmogorov shear rate using scaling arguments. The experimentally measured turbulent coagulation rate constants were significantly lower than theoretical predictions that neglect interparticle interactions; however, simulations that included particle interactions showed excellent agreement with the experimental results. The favourable comparison provides evidence that the computer simulations capture the important physics of turbulent coagulation. That is, particle transport on length scales comparable to the particle radius controls the rate of turbulent shear coagulation and particle interactions are significant.

Turbulent coagulation of colloidal particles

Theoretical predictions for the coagulation rate induced by turbulent shear have often been based on the hypothesis that the turbulent velocity gradient is persistent (Saffman & Turner 1956) and that hydrodynamic and interparticle interactions (van der Waals attraction and electrostatic double-layer repulsion) between colloidal particles can be neglected. In the present work we consider the effects of interparticle forces on the turbulent coagulation rate, and we explore the response of the coagulation rate to changes in the Lagrangian velocity gradient correlation time (i.e. the characteristic evolution time for the velocity gradient in a reference frame following the fluid motion). Stokes equations of motion apply to the relative motion of the particles whose radii are much smaller than the lengthscales of turbulence (i.e. small particle Reynolds numbers). We express the fluid motion in the vicinity of a pair of particles as a locally linear flow with a temporally varying velocity gradient. The fluctuating velocity gradient is assumed to be isotropic and Gaussian with statistics taken from published direct numerical simulations of turbulence (DNS). Numerical calculations of particle trajectories are used to determine the rate of turbulent coagulation in the presence and absence of particle interactions. Results from the numerical simulations correctly reproduce calculated coagulation rates for the asymptotic limits of small and large total strain where total strain is a term used to describe the product of the characteristic strain rate and its correlation time. Recent DNS indicate that the correlation times for the fluctuating strain and rotation rate are of the same order as the Kolmogorov time (Pope 1990), suggesting theories that assume either small or large total strain may poorly approximate the turbulent coagulation rate. Indeed, simulations for isotropic random flows with intermediate total strain indicate that the coagulation rate in turbulence is significantly different from the analytical limits for large and small total strain. The turbulent coagulation rate constant for non-interacting monodisperse particles scaled with the Kolmogorov time and the particle radius is 8.62±0.02, whereas the commonly used model of Saffman & Turner (1956) predicts a value of 10.35 for non-rotational flows in the limit of persistent turbulent velocity gradients. Additional simulations incorporating hydrodynamic interactions and van der Waals attraction were used to estimate the actual rate of particle coagulation. For typical values of these parameters, particle interactions reduced the coagulation rate constant by at least 50%. In general, the collision efficiency (the ratio of coagulation with particle interactions to that without) decreased with increasing particle size and Kolmogorov shear rate.

Closed‐Loops of liquid‐liquid immiscibility in binary mixtures of equal sized molecules predicted with a simple theoretical equation of state

AbstractSeveral types of closed‐loop liquid‐liquid immiscibility behavior in binary mixtures have been found with a simple isotropic model fluid. The calculations are based on an equation of state composed by the Carnahan‐Starling hard sphere repulsion and the van der Waals‐attraction. The extension to binary mixtures is accomplished with the usual quadratic one‐fluid mixing rules. The results are presented in a global phase diagram as introduced by van Konynenburg and Scott. Several closed‐loop phase diagrams similar to those discovered by Boshkov ten years ago with a Lennard‐Jones equation of state have been found. The results are discussed in the context of the topological and physical origin of closed‐loop behavior in systems of isotropic mixtures.

Colloidal stability of protein-polymer systems: A possible explanation by hydration forces

In this paper the stability domains of immunoglobulin ~IgG fragment! F~ab8) 2-polymer systems have been examined using a low-angle scattering technique. The rates of aggregate formation are expressed in terms of a stability ratio as a function of electrolyte concentration. After the usual rapid aggregation achieved at a certain ionic strength ~critical coagulation concentration!, an abnormal stabilization is observed with increasing ionic strength. This exceptional stability at high electrolyte concentration cannot be explained by the Derjaguin, Landau, Verwey, and Overbeek @B. V. Derjaguin and L. Landau, Acta Physicochim. USSR 14, 633 ~1941! ;E . J. W. Verwey and J. Th. G. Overbeek, Theory of the Stability of Lyophobic Colloids~Elsevier, Amsterdam, 1952!, Vols. 1 and 2# theory, which attributes the colloidal stability to the London‐van der Waals attraction and the electrostatic repulsion. Effects of electrolyte concentration, counterion valence, pH, protein coverage, and time on the experimental stability are investigated. A possible explanation based on the so-called ‘‘hydration forces’’ is proposed. @S1063-651X~97!05804-2# PACS number~s!: 82.70.Dd, 47.20.Ma, 82.65.Dp

Quantitative analysis of spatially resolved valence electron energy-loss spectra for electronic structure studies of grain boundaries and thin intergranular films

The spatial variation of the electronic structure at interfaces is critical to both interatomic bonding at atomically abrupt interfaces such as grain boundaries and also to the development of van der Waals (vdW) attraction forces at partially wetted interfaces. This interfacial electronic structure, as represented by the interband transition strength , can be determined by Kramers Kronig (KK) analysis of either vacuum ultraviolet (VUV) optical reflectance spectra or spatially resolved valence electron energy loss (SR-VEEL) spectra. Quantitative analysis of SR-VEELS requires accurate spectral line shapes coupled with single scattering deconvolution, convergence correction, and KK analysis. Both the energy loss functions (Fig. 1) and the interband transitions (Fig. 2) determined for α-Al2O3 using SR-VEELS compare well with the VUV results. In addition the use of the spectral line scan method, whereby typically 200 SR-VEEL spectra are acquired along a scan line of 20 nm, helps overcome many uncertainties in the data acquisition and analysis.

Nonlinear theory of film rupture with viscosity variation

A nonlinear kinematic equation for thin liquid film thickness which takes into account the effects of viscosity variation and London/van der Waals attractions is derived to study the mechanism of film rupture. After appropriate rescaling of time variable by a suitable factor, we show that this equation could be converted into a constant-viscosity equation. Then without further analysis we can deduce directly, by examination of the rescaling factor, that a cooling (heating) from the wall will increase (reduce) the rupture time.