Attractive Van Der Waals Forces Research Articles

PH-Induced Simultaneous Synthesis and Self-Assembly of 3D Layered β-FeOOH Nanorods

Higher-ordered architectures self-assembly of nanomaterials have recently attracted increasing attention. In this work, we report a spontaneous and efficient route to simultaneous synthesis and self-assembly of 3D layered beta-FeOOH nanorods depending on a pH-induced strategy, in which the continuous change of pH is achieved by hydrolysis of FeCl(3).6H(2)O in the presence of urea under hydrothermal conditions. The electron microscopy observations reveal that the square-prismic beta-FeOOH nanorods are self-assembled in a side-by-side fashion to form highly oriented 2D nanorod arrays, and the 2D nanorod arrays are further stacked in a face-to-face fashion to form the final 3D layered architectures. On the basis of time-dependent experiments, a multistage reaction mechanism for the formation of the 3D layered beta-FeOOH nanorods architecture is presented, involving the fast growth and synchronous self-assembly of the nanorods toward 1D, 2D, and 3D spontaneously. The experimental evidence further demonstrates that the urea-decomposition-dependent pH continuously changing in the solution, spontaneously altering the driving force competition between the electrostatic repulsive force and the attractive van der Waals force among the nanorods building blocks, is the essential factor to influence the self-assembly of the beta-FeOOH nanorods from 1D to 3D.

Simulation of dispersion and collection process of agglomerated particles in collision with fibers using discrete element method

The behavior of agglomerates in collision with fibers was simulated using the three-dimensional modified discrete element method and the influences of several factors on the fraction of collected particles were examined. Furthermore the single fiber collection efficiency for agglomerated particles was also investigated. In the case where gas velocity is quite low, agglomerates are only deformed but barely dispersed and thus collected as a single deformable particle. By contrast above some critical gas velocity, constituent particles are dispersed and at the same time partly collected on fibers. The fraction of collected particles first increases then decreases as the van der Waals attractive force between particle and fiber increases. The reason for the decrease in fraction of collected particles in strong adhesion region is that the smooth deformation of agglomerates along the fiber surface is inhibited by too strong adhesion. It was also suggested that there exists an optimum size ratio between the agglomerate size and fiber radius for the collection fraction. This is also closely related to the deformation of agglomerate along the fiber surface. In case of non-agglomerated particle collision, all the particles entering within the collision region are collected by fiber. By contrast in case of agglomerate collision, the dispersion of agglomerates as well as collection occurs in the same process and all the particles colliding with the fiber are not necessarily collected. Consequently the single fiber collection efficiency considerably decreases comparing to that for non-agglomerated particle collision.

Sodium citrate as an effective dispersant for the synthesis of inorganic–organic composites with a nanodispersed mineral phase

Although extensive efforts have been devoted to the development of polymer–ceramic composites for bone repair, those developed thus far were not able to mimic the nanostructure of bone, partly because of the aggregated, microscale organization of the mineral component. As a consequence, homogenization and intermixing of organic and inorganic components remain a major engineering challenge for the development of functional, biomimetic bone-substituting composites. In the current study, various dispersants were evaluated for their potential to be used as biocompatible dispersants in the synthesis of biomimetic composites with a nanodispersed mineral phase. Based on sedimentation experiments, tribasic sodium citrate was selected as the most effective dispersant for the stabilization of calcium phosphate (CaP) suspensions. Specific adsorption of citrate anions onto CaP nanocrystals was shown to result in a strong increase in the negative surface charge of the CaP particles and consequently increased repulsive interparticle forces that were able to overcome attractive van der Waals forces. Using sodium citrate as dispersant at a CaP/citrate ratio of 4.0, CaP–gelatin nanocomposites were fabricated which displayed a nanostructured mineral phase without occurrence of microscale CaP particles. Consequently, aggregation and sedimentation of CaP mineral phase was reduced considerably.

Measurement of Forces across Room Temperature Ionic Liquids between Mica Surfaces

Ionic liquids (ILs) are liquids that consist only of cations and anions. In spite of a plethora of emerging processes involving ionic liquids, there is no experimental measurement of the structure and dynamics of ILs at the solid−liquid interface comparable to the double layers of electrolyte solutions. Here, we report the direct measurements of the interactions (force−distance functions) between two atomically smooth mica surfaces across three different types of room temperature ILs. The forces are repulsive at all measured distances in all three cases, decaying exponentially with distance. Effective Debye screening lengths are found to be around 1−4 nm, much longer than expected from traditional theories, and strongly dependent on the size and molecular structure of the anion/cation. At small separations, in addition to the exponential repulsions, oscillatory forces are observed, while any monotonically attractive van der Waals forces, if present, are overwhelmed by the repulsive electrostatic and oscil...

Controlled free-form fabrication of nanowires by dielectrophoretic dispension of colloids

The vertical growth of conductive gold nanowires in ambient atmosphere through on-demand dielectrophoretically guided deposition of nanoparticle-laden colloids is demonstrated. Accelerated by dielectrophoresis, rapidly evaporating colloidal droplets leave a capillary nozzle (diameter of 500 nm) and are guided through an inhomogeneous electrostatic field. The nanoparticles in the colloidal droplets combine under the action of van der Waals attraction forces on the substrate to form highly flexible vertical nanowires with easily controllable lengths and diameters, here in the range of 150–800 nm. After bending to bridge an electrode pair and after annealing at 300 °C, the sample gold nanowires demonstrate excellent electrical conductivity considering their size.

Modeling the Adsorption and Coagulation of Fulvic Acids on Colloids by Brownian Dynamics Simulations

Humic substances (HS) play an important role in the reactivity and transport of colloids in natural environments. In particular, the presence of fulvic acids (FA) in natural waters modifies the interactions between inorganic particles and biopolymers and makes difficult to predict their stability with regard to aggregation processes. In this study, Brownian dynamics (BD) modeling is applied to quantify the interactions between negatively charged FA and (i) a positively charged inorganic particle and (ii) a rigid neutral polysaccharide in aqueous solutions. Hematite and schizophyllan are respectively used as model colloids. Modeling the adsorption of FA at the hematite particle surface and on the polysaccharide is based on van der Waals attractive forces and electrostatic interactions. Possible applications of the model, however, are not restricted to this system and any interaction potential or colloidal particle can be considered. The competition between FA adsorption and FA homocoagulation in solution is studied as function of the solution ionic strength. Results show that, under the conditions used, the amount of adsorbed FA is largely controlled by the solution ionic strength. At low ionic strength the amount of adsorbed FA is limited by the electrostatic repulsion between FA at the colloid surfaces and FA monolayers are formed. By increasing the ionic strength the number of adsorbed FA is found to increase. At a sufficiently large ionic strength, however, FA coagulation in solution may strongly compete with FA adsorption at the hematite and polysaccharide surfaces. FA aggregates then adsorb at the colloid surfaces to form extended and porous structures. Results also suggest that FA adsorption and structure of the adsorbed layers are mainly driven by the complex interplay between electrostatic attractive and repulsive interactions.

Towards surface quantum optics with Bose–Einstein condensates in evanescent waves

We present a surface trap which allows for studying the coherent interaction of ultracold atoms with evanescent waves. The trap combines a magnetic Joffe trap with a repulsive evanescent dipole potential. The position of the magnetic trap can be controlled with high precision which makes it possible to move ultracold atoms to the surface of a glass prism in a controlled way. The optical potential of the evanescent wave compensates for the strong attractive van der Waals forces and generates a potential barrier at only a few hundred nanometers from the surface. The trap is tested with Rb Bose-Einstein condensates (BEC), which are stably positioned at distances from the surfaces below one micrometer.

Ultraviolet Irradiation Suppresses Adhesion on TiO2

Environmental concerns have recently spurred a quest for materials that stay clean, such as TiO2, when subjected to the combined action of sunlight irradiation and exposure to rain. However, the fundamental mechanism that governs the self-cleaning properties of TiO2 still needs to be elucidated. TiO2 is known to be photocatalytic as well as to decompose organic adsorbents, but these properties do not explain its capacity to eliminate mineral contaminants. In the present paper, we report that hydrophilic UV-irradiated TiO2 layers are nonadhesive in the presence of water, thus preventing adhesion of mineral particles. Surface force measurements done using atomic force microscopy reveal the presence of an additional short-range repulsive force, which screens the van der Waals attractive forces, while long-range interactions are preserved. This additional short-range force does not originate from UV-induced trapping of surface charges or OH group creation, as we demonstrate by second harmonic generation and ambient pressure X-ray photoelectron spectroscopy investigations. This short-range repulsive force, which appears to be intrinsic to the TiO2 surface, is certainly a key phenomenon for a strong self-cleaning capacity.

Predicting gas diffusion regime within pores of different size, shape and composition

The ability to separate mixtures of molecules is a vital technology in a world that emits excess carbon dioxide into the atmosphere, needs purified water, desires artificial kidneys and requires hydrogen for sustainable energy alternatives. Membranes are composed of angstrom and nanometer-sized pores which may be designed to separate a gas, vapor or liquid mixture. In this paper we employ mathematical modeling, using the Lennard-Jones interactions between the gas molecule and the pore wall, to determine the gas diffusion regime occurring within pores of different size, shape and composition. This novel approach is used to predict the transport of light gases, namely, He, H 2, CO 2, O 2, N 2 and CH 4, through carbon tubes, carbon slits, silica tubes and silica slits. Minimum pore size for barrier-free transport ( d min) and the minimum pore size for Knudsen diffusion ( d K) are calculated for each gas and a mechanism for the intermediate region is suggested in which the attractive van der Waals forces cause an accelerated entrance velocity of the gas at the pore opening. Experimental results for gas transport in carbon nanotube, carbon molecular sieving and molecular sieving silica membranes are explained well by the model. The aim of this work is to provide the guidelines for tailoring porosity in membranes and adsorbents, such that desired separations are achieved.

Transition Force Measurement between Two Negligibly Charged Surfaces: A New Perspective on Nanoparticle Halos

Since 2001, silica microspheres have been reported to be stabilized by highly charged hydrous ZrO(2) nanoparticles which form halos around the microspheres at pH 1.5. However, the exact mechanisms behind this novel stabilization method in terms of the relevant interaction forces remain unclear. In order to gain a greater insight into this mechanism, the interaction between a silica flat and a silica sphere in different ZrO(2) nanoparticle suspensions was investigated by the colloid probe technique. The interaction force between a silica flat and a 600 nm silica sphere was first investigated in a ZrO(2) nanoparticle (D approximately 8 nm) suspension with volume fractions of 10(-3), 10(-4), 10(-5), and 10(-6). When the volume fraction of ZrO(2) is 10(-6), only a purely attractive van der Waals force was observed between the silica surfaces. With an increase in the ZrO(2) nanoparticle volume fraction, a peak was detected on the transition force curve at a ZrO(2) volume fraction of 10(-5) while a purely repulsion force was observed for ZrO(2) volume fractions of 10(-4) and 10(-3). The average distance difference between the peak and the zero distance point on the transition force curve which should define the distance between the halo on the microsphere is approximately 2.3 nm. Additionally, the repulsion increases with the effective zeta potential of the binary composite sphere (BCS, the entity of the silica sphere and the surrounding zirconia particles) on an increase of the nanoparticle volume fraction while the adhesion force decreases, which indicates a denser nanoparticle halo.

Highly loaded UV curable nanosilica dispersions for rapid prototyping applications

Silica microcomponents are attractive for applications requiring high temperature stability and chemical durability such as in microreactors. Rapid prototyping methods of forming ceramic dispersions hold great potential for manufacturing of microreactors. Stereolithography for example can benefit from highly solid loaded, low viscosity and high transparency dispersions. This article presents the development of >40 vol.%, UV curable transparent silica nanodispersions having viscosities suitable for stereolithography applications. The main parameters enabling these high loading dispersions are minimization of the Van der Waals attractive forces and affinity of monomer end group for the silica surface (e.g. hydrogen bonding). The minimization of the Van der Waals attraction is achieved by refractive index matching of the nanosilica and UV curable acrylate monomers. This results also in highly transparent dispersions having curing depths in the order of 10 mm in the UVA range. The transformation of selected dispersions to self-supporting silica glass sheets through UV curing, debinding and sintering was investigated by TGA/DTA, dilatometry and XRD.

Conformational Mechanics of Polymer Adsorption Transitions at Attractive Substrates

Conformational phases of a semiflexible off-lattice homopolymer model near an attractive substrate are investigated by means of multicanonical computer simulations. In our polymer-substrate model, nonbonded pairs of monomers as well as monomers and the substrate interact via attractive van der Waals forces. To characterize conformational phases of this hybrid system, we analyze thermal fluctuations of energetic and structural quantities, as well as adequate docking parameters. Introducing a solvent parameter related to the strength of the surface attraction, we construct and discuss the complete solubility-temperature phase diagram. Apart from the main phases of adsorbed and desorbed conformations, we identify several other phase transitions such as the freezing transition between energy-dominated crystalline low-temperature structures and globular entropy-dominated conformations.

A low temperature scanning probe microscope using a quartz tuning fork

A frequency modulation-atomic force microscope (FM-AFM) designed to measure atomic scale topography at low temperatures is developed. Piezoelectric quartz tuning forks are employed as a force sensor for low temperatures use. In order to perform high-resolution measurements, detection of attractive forces between a tip and a sample is important. Measurements of attractive van der Waals forces on a SrTiO3 substrate is successfully performed down to 4.2 K by minimizing an amplitude of the tuning fork. Topographic imaging of atomic steps of the SrTiO3 substrate is also achieved at room temperatures in vacuum (10-3 Pa).

Measurements of Electrostatic Interactions between Charged Membranes

Neighboring lipid membranes experience attractive and repulsive forces. The attractive (van der Waals) force is due to induced change fluctuations and it acts even between neutral membranes. Repulsive forces arise from the tendency of lipids to stay hydrated and can include electrostatic interactions due to accumulation of membrane surface charge. When the attractive vdW force is balanced by repulsive forces, an equilibrium spacing between lamellae is established, as measured by X-ray scattering. This is often the case for neutral membranes. By contrast, lipid bilayers made entirely of charged lipid species such as the negatively charge phosphatidylserine (PS) swell indefinitely (“unbind”) in the absence of salt indicating the dominance of electrostatic repulsion. To measure the strength of electrostatic interactions in solution, we manipulate membrane charge in two simple ways: by adsorbing “sticky” ions such as bromide onto neutral phosphatidylcholine lipids, and by mixing-in charged lipid species. We will present results and discuss in what conditions membrane electrostatics can lead to unbinding.

Theoretical study of 1-(4-hexylcyclohexyl)-4-isothiocyanatobenzene: molecular properties and spectral characteristics

The mesogenic species 4-(4-hexylcyclohexyl) isothiocyanatobenzene (6CHBT) was studied with density functional theory and molecular mechanics in order to investigate the molecular properties, interactions between dimers and to interpret the IR spectrum. Two types of calculations were performed for model systems containing single and double molecules of 6CHBT. Calculations (involving conformation analysis) for isolated species indicated that the trans isomer, in the equatorial-equatorial conformation, is the most energetically stable. The 6CHBT molecule is polar, with a rather high (4.43 D) dipole moment with negatively charged isothiocyanato (NCS) ligand. The dimer-dimer interaction energies show that the head-to-head configuration (where van der Waals attraction forces play the major role) is the most energetically stable. Vibrational analysis provided detailed assignment of the experimental infra-red (IR) spectrum.

Improving Aerosol Dispersion through Processing and Dissemination Techniques

Abstract The state of dispersion is a vital aspect of powder technology impacting both traditional and emerging technologies in many diverse areas of interest including health care, industry, the environment, and the military. In such applications, the properties and quality of the resultant product will be directly and noticeably affected by the degree to which the particles are dispersed. In any operation where satisfactory particle dispersion is imperative, a reduction in dispersion, or increased agglomeration, leads to poor efficiency and ultimately results in lower yields and increased cost. As powder technology moves into the nano-age, the challenges of maintaining and improving particle dispersion become increasingly important. Decreasing particle size into the submicron range can significantly increases the effect of surface forces leading to an increase in the cohesive forces of the powder, thus, making an understanding of dispersion even more essential. In order to develop the knowledge base to understand and control aerosol dispersion, a fundamental study into significant powder characteristics and environmental conditions that influence dispersion has been conducted. This paper discusses the results of testing various techniques employed to improve the dispersion of sub-micron aluminum flakes such as using a fumed silica spacer to reduce interparticle contact area and, in turn, attractive van der Waals forces, as well as examining the effect of dissemination pressure on the state of dispersion of the particles. This research was conducted using statistical experimental design in order to efficiently study significant factors.

S1603-1-4 凹凸を有する走行面上の3自由度浮上浮動ヘッドの分子気体潤滑解析(情報機器コンピュータメカニクス)

Recently, the space between the flying head slider and the disk has been reduced to I Onm or less. Consequently, recording media with grooves such as discrete track media (DTM) and bit-patterned media (BPM), are considered to be some of the most promising media for achieving ultrahigh track densities. Thus, it is becoming increasingly important to analyze the static and dynamic characteristics of flying head sliders over DTM/BPM media using the molecular gas-film lubrication (MGL) equation. In this paper, we first use the perturbation method in the frequency domain to obtain not only the stiffness and damping produced by the air film but also the negative stiffness caused by the attractive van der Waals (vdW) force. The perturbation method was applied to a slider with 3 degrees-of-freedom (3-DOF) over an asymmetric groove-slider configuration, which caused a rolling motion in addition to translational and pitching motions. The dependence of the dynamic characteristics such as spacing fluctuations on the groove wave number (frequency) were numerically obtamed.

P-HDI-05 MOLECULAR GAS-FILM LUBRICATION ANALYSES OF A SLIDER OVER A DISK WITH GROOVES : STATIC AND DYNAMIC FLYING CHARACTERISTICS OF A 3-DOF SLIDER(Head/Disk Interface and Tribology,Technical Program of Poster Session)

Recently, the space between the flying head slider and the disk has been reduced to 10nm or less. Consequently, recording media with grooves such as discrete track media (DTM) and bit-patterned media (BPM), are considered to be some of the most promising media for achieving ultrahigh track densities. Thus, it is becoming increasingly important to analyze the static and dynamic characteristics of flying head sliders over DTM/BPM media using the molecular gas-film lubrication (MGL) equation. In this paper, we first use the perturbation method in the frequency domain to obtain not only the stiffness and damping produced by the air film but also the negative stiffness caused by the attractive van der Waals (vdW) force. The perturbation method was applied to a slider with 3 degrees-of-freedom (3-DOF) over an asymmetric groove-slider configuration, which caused a rolling motion in addition to translational and pitching motions. The dependence of the dynamic characteristics such as spacing fluctuations on the groove depth and the frequency were numerically obtained.

Direct evidence on reduced adhesion of Salbutamol sulphate particles due to L-leucine coating

Attractive van der Waals forces are responsible for unwanted agglomeration of fine dry powder particles which can cause problems e.g. with inhalable drugs in the treatment of asthma. In this article we report on experimental evidence of reduced adhesion of salbutamol sulphate particles when they are coated with L-leucine. Particle production and coating was performed using an aerosol flow reactor. The L-leucine-containing particles were fully covered with L-leucine as evidenced by X-Ray Photoelectron Spectroscopy and Scanning Electron Microscopy findings. With a suitable concentration of precursor solution the particles become rough and adhesion diminished to one fifth of the adhesion of uncoated particles. The main reason for this reduced adhesion was an increase in surface roughness, which reduced the contact area and the number of contact points thus increasing the effective separation between particles.

The Role of Attractive van der Waals Forces in the Catalysis of Michael Addition by a Phenyl Decorated Uranyl-Salophen Complex

The Et(3)N-assisted addition of beta-ketoester 3 to MVK in chloroform is catalyzed with high turnover efficiency by the phenyl-substituted uranyl-salophen compound 2b but not by the parent compound 1b. A plausible mechanism is suggested, involving concomitant nucleophilic attack at the beta-carbon and hydrogen bonding between the Et(3)NH(+) countercation and the carbonyl oxygen of the s-cis conformation of the enone reactant. The role of the van der Waals interactions with the aromatic side arm of 2b as a crucial driving force for catalysis is discussed.