Attractive Van Der Waals Forces Research Articles

Interparticle Forces of a Native and Encapsulated Metal-Organic Framework and Their Effects on Colloidal Dispersion.

The colloidal properties of suspended metal-organic frameworks (MOFs) are critical for device fabrication and application. Herein, van der Waals attractive, electric double layer repulsive, and steric repulsive forces of a native and encapsulated MOF are quantified for the first time. The van der Waals attractive forces were investigated by conducting environmental ellipsometric porosimetry (EEP) and spectroscopic ellipsometry (SE) on submicron, optical-quality nanoparticle films. The repulsive forces were determined from colloid and material characterization measurements. These data were used to predict suspension properties via extended Derjaguin, Landau, Verwey, and Overbeek theory. The state of dispersion was quantified for comparison with theoretical predictions for nine solvents. The MOF encapsulated with a surface-selective modification showed superior suspension in hydrophobic solvents. These findings should expedite the formulation of MOF colloidal suspensions for future works.

Quantitative measurement of interaction strength between kaolinite and different oil fractions via atomic force microscopy: Implications for clay-controlled oil mobility

Clay–oil interaction in unconventional shale reservoir plays a critical role in oil migration, which further governs the exploration of petroleum resource. In this study, interaction strength between kaolinite and different oil fractions (saturates, aromatics, resins and asphaltenes) were quantitatively investigated by atomic force microscopy (AFM). The AFM tips with various oil fraction coatings were monitored to approach and retract from kaolinite surface, from which the force curves with adhesion information were derived. As a result, kaolinite-oil interaction curves are characterized with large hysteresis, which is due to the adhesion behavior between two phases caused by a meniscus formed at contact area. Adhesive strength, including force and work, increases from nonpolar to polar organics in order of saturates, aromatics, resins and asphaltenes. The clay-oil interaction could be well depicted by using a combined force model composed of attractive van der Waals force, capillary force and intermolecular repulsive force. With the application of infrared spectroscopy in oil fractions, the difference of adhesion among various oil fractions was analyzed from a viewpoint of binding types between functional groups in oil and kaolinite. Compared with nonpolar saturates, it is the aromatic CC group in aromatics, and the carboxy (-COOH) functional group in polar resins and asphaltenes, that contributes to their strong adhesion with clay surface, respectively. Quantitatively, adhesion force and work caused by aromatic CC and –COOH are an order of magnitude higher than aliphatic CH2+CH3. These observations on interaction strength between kaolinite and different oil fractions, especially from a view of chemistry, yield much information on the mechanism of clay-controlled oil mobility in shale from a new perspective.

Reconfigurable Chirality of DNA-Bridged Nanorod Dimers.

Gold nanorods assembled in a side-by-side chiral configuration have potential applications in sensing due to their strong chiroptical surface plasmon resonances. Recent experiments have shown that dimers of gold nanorods bridged by double-stranded DNA exhibit variable chiral configurations depending on the chemical and ionic properties of the solvent medium. Here, we uncover the underlying physics governing this intriguing chiral behavior of such DNA-bridged nanorods by theoretically evaluating their configurational free energy landscape. Our results reveal how chiral configurations emerge from an interplay between the twist-stretch coupling of the intervening DNA and the intermolecular interactions between the nanorods, with dimers exhibiting left-handed chirality when the interparticle interactions are dominated by attractive depletion or van der Waals forces and right-handed chirality when dominated by repulsive electrostatic or steric forces. We demonstrate how changes in the depletant or ion concentration of the solvent medium lead to different classes of configurational responses by the dimers, including chirality-switching behavior, in good agreement with experimental observations. Based on extensive analyses of how material properties like nanorod aspect ratio, DNA length, and graft height modulate the free energy landscape, we propose strategies for tuning the environmentally responsive reconfigurability of the nanorod dimers. Overall, this work should help control the chirality and related optical activity of nanoparticle dimers and higher-order assemblies for various applications.

Drying of a Colloidal Suspension Deposited on a Substrate: Experimental and Numerical Studies

We studied a colloidal suspension of polystyrene beads deposited on a glass substrate. The glass substrate contained either straight rough areas on the borders of an open channel or only straight rough areas. The drying of the suspension was observed with an optical microscope, the light bulb of which acted as an energy source to evaporate the suspension. Moreover, the light bulb of the microscope provided optical pressure due to light. We observed that the colloidal particles were trapped on the rough areas of the substrate and not in the open channel at the end of the drying process. In order to understand the experimental results, we modeled numerically the drying of the suspension using a Molecular Dynamics program. The forces imposed on the substrate by the particles are their weight, the optical pressure due to the light bulb of the optical microscope, the attractive Van der Waals force and the repulsive diffuse layer force. The forces acting between two particles are the attractive Van der Waals forces, the repulsive diffuse layer force and the capillary force. The Gaussian random force (linked to Brownian motion) and the particle liquid viscous drag force (also linked to Brownian motion) are horizontal and applied on one particle. The relation between the normal forces N (forces acting by the particles on the substrate) and the horizontal forces F is Amontons’ third law of friction F≤μkN; in rough areas of the substrate, μk is larger than in smooth areas. This explains that particles are trapped in the areas with high roughness.

Linear stability of a thin fluid film interacting with its surrounding bulk fluid

Thin films can become unstable when attractive van der Waals forces overcome the stabilizing effects of surface tension and viscous forces. In many applications, the effect of the surrounding bulk fluid cannot be neglected when considering a thin film subject to perturbations. In this work, we examine the two limits of potential flow and Stokes flow in the surrounding bulks to derive dispersion relations in each limit. We show that the effect of the surrounding bulks cannot be ignored for many film–bulk fluid pairings and film thicknesses and present conditions for the validity of each regime. In particular, the potential-flow regime exists when van der Waals forces are sufficiently strong, while the Stokes-flow regime exists when the bulk dynamic viscosity is sufficiently large. Due to the nature of the dispersion relation in the Stokes-flow limit, several distinct scenarios are identified in the corresponding stability diagram, each involving the interplay of different forces. For example, a novel instability regime involving capillary–viscous interactions is identified for large film thicknesses. Finally, by enlisting multiple realistic fluid pairings and film thicknesses wherein such instabilities can occur, we demonstrate the practical relevance of our theoretical findings. This work enables the extension of thin film stability theory to the analysis of antibubbles, splashing molten metals and ionic liquids, Mesler entrainment of microbubbles in breaking waves, and emulsion stability.

Molecular Dynamics Study on Graphene-Nanoflake Sensor Sandwiched Between Crossed Graphene-Nanoribbon Junctions.

We present a design of a nanoscale inertial measurement unit or a data archive using a graphene-nanoflake (GNF) sandwiched between crossed graphene-nanoribbon (GNR) junctions. When an external force applied is below the retracting force, the inertial force exerted on the movable GNF can telescope it. Then, the self-restoring force increases as the attractive van der Waals force between the GNF and the GNRs, which enables the GNF to automatically and fully retract back into the sandwich position immediately after the externally applied force is released. When the external force exceeds the retracting force, the GNF escapes from the crossed GNR junctions, which enables the device to be used as non-volatile memory. The heterostructure of GNR/h-BN/GNR can be considered as an advanced structure in the proposed scheme.

Multi-Walled Carbon Nanotubes Decorated with Guanidinylated Dendritic Molecular Transporters: An Efficient Platform for the Selective Anticancer Activity of Doxorubicin.

An efficient doxorubicin (DOX) drug delivery system with specificity against tumor cells was developed, based on multi-walled carbon nanotubes (MWCNTs) functionalized with guanidinylated dendritic molecular transporters. Acid-treated MWCNTs (oxCNTs) interacted both electrostatically and through hydrogen bonding and van der Waals attraction forces with guanidinylated derivatives of 5000 and 25,000 Da molecular weight hyperbranched polyethyleneimine (GPEI5K and GPEI25K). Chemical characterization of these GPEI-functionalized oxCNTs revealed successful decoration with GPEIs all over the oxCNTs sidewalls, which, due to the presence of guanidinium groups, gave them aqueous compatibility and, thus, exceptional colloidal stability. These GPEI-functionalized CNTs were subsequently loaded with DOX for selective anticancer activity, yielding systems of high DOX loading, up to 99.5% encapsulation efficiency, while the DOX-loaded systems exhibited pH-triggered release and higher therapeutic efficacy compared to that of free DOX. Most importantly, the oxCNTs@GPEI5K-DOX system caused high and selective toxicity against cancer cells in a non-apoptotic, fast and catastrophic manner that cancer cells cannot recover from. Therefore, the oxCNTs@GPEI5K nanocarrier was found to be a potent and efficient nanoscale DOX delivery system, exhibiting high selectivity against cancerous cells, thus constituting a promising candidate for cancer therapy.

Improved CO2 adsorption capacity and fluidization behavior of silica-coated amine-functionalized multi-walled carbon nanotubes

The adsorption capacity and fluidization behavior of silica-coated amine-functionalized multi-walled carbon nanotubes (MWCNTs) were studied in this work. The coupling of both studies is fundamental for the practical application of CO2 sorbents in fluidized beds. Sorbents were prepared by hydrolysis of tetraethylorthosilicate (TEOS) onto acid-treated multi-walled carbon nanotubes followed by impregnation with monoethanolamine (MEA). The effects of different loading amounts of MEA and TEOS on CO2 uptake were investigated using thermogravimetric analysis under a simulated flue gas composed of 15%CO2/85% N2. At the optimal loadings of TEOS (30 ml) and MEA (30 wt%) on MWCNTs, 2.67 wt% of CO2 was captured at 25 °C, while the adsorption capacity of the pristine MWCNTs before any modification was negligible. Adsorption/desorption tests repeated 25 times also showed that the prepared sorbent has excellent stability during the cyclic operation; 9.42% reduction of the uptake was reported. Fluidizability of the most promising sorbent was also assessed when adding hydrophobic silica nanoparticles (NPs). Maximum bed expansion of 3.9 was achieved in the presence of 7.5 wt% silica NPs which was comparable by the amount of 2.24 belonged to raw MWCNTs. Enhancement of fluidity may be attributed to the decreased van der Waals attraction forces between the particles of sorbent. Besides, the CO2 adsorption capacity of the sorbent mixed with 5 wt% silica NPs was surprisingly increased by approximately 8%, reaching ~3 wt% of CO2.

Dispersion stability of zinc oxide nanoparticles in an electroless bath with various surfactants

Stable dispersion of nanoparticles in electrolytes is essential for preparing a wide range of advanced electroless nanocomposites. However, nanoparticles agglomerate if the van der Waals attractive forces prevail over the electrostatic repulsive forces. As a result, various surfactants (anionic, cationic, and non-ionic) were used to prevent nanoparticle agglomeration in the electroless plating bath. This study specifically examines the dispersion stability of zinc oxide (ZnO) nanoparticles in an electroless bath with different surfactants, including anionic sodium dodecyl sulfate (SDS), cationic cetyltrimethylammonium bromide (CTAB), and non-ionic polyvinylpyrrolidone (PVP). The zeta potential and hydrodynamic diameter measurements were utilized as an indicator of nanodispersion stability. The results showed a better-dispersed state of ZnO nanoparticles with SDS surfactant with a small average size of the agglomerate. Surfactants SDS and CTAB improved stability by electrostatic repulsion between ZnO nanoparticles whereas PVP by steric inhibition towards other particles. In addition, ultraviolet–visible absorbance spectroscopy (UV–Vis) was used to examine the dispersion stability of ZnO nanoparticles with different surfactants. The nano-ZnO distributed with SDS displayed a maximum percentage of absorbance, suggesting the homogeneous dispersion of ZnO nanoparticles.

A study on the mechanism of calcium ion in promoting the sedimentation of illite particles

Clay minerals are the main reason why coal slime water is difficult to settle. For the exploration of the influence of Ca2+ on the sedimentation effect of illite, the adsorption characteristics of illite were investigated under different Ca2+ concentrations. After adsorption equilibrium, the sedimentation test was carried out, and the changes in the surface properties of illite particles were analyzed by measuring Zeta potential and contact angle. The Theory of EDLVO was adopted to calculate the interaction energy between illite particles. The results showed that with the increase of Ca2+ concentration, the equilibrium adsorption capacity of illite increased, and the adsorption equilibrium was basically achieved at 60 min. The adsorption process was in accordance with the quasi-second-order kinetic model and Langmuir isothermal adsorption model. The sedimentation effect of illite was obviously improved after adsorption of Ca2+. When the concentration of Ca2+ was 10 mmol/L and pH = 8, the sedimentation rate was 92.5%. With the increase of Ca2+ concentration, the absolute value of the Zeta potential of illite after adsorption equilibrium decreased continuously, and the contact angle increased continuously. According to the Theory of EDLVO, the polar force is the dominant force affecting the interaction between particles. As the concentration of Ca2+ increases, the van der Waals attraction force between illite particles increases, the electrostatic and polar repulsive forces weaken, and the total interaction energy gradually decreases, indicating that Ca2+ can promote the agglomeration of illite particles, which is of benefit to the clarification of coal slime water.

Motion in the Restricted Three-Body Problem at the Nanoscale

This paper studies the classical restricted three-body problem of a carbon atom in the vicinity of two carbon 60 fullerenes ( fullerenes) at the nanoscale. The total molecular energy between the two fullerenes is determined analytically by approximating the pairwise potential energies between the carbon atoms on the fullerenes by a continuous approach. Using software MATHEMATICA, we compute the positions of the stationary points and their stability for a carbon atom at the nanosacle and it is observed that for each set of values, there exists at least one complex root with the positive real part and hence in the Lyapunov sense, the stationary points are unstable. Since only attractive Van der Waals forces contribute to the orbiting behavior, no orbiting phenomenon can be observed for , where the Van der Waals forces becomes repulsive. Although the orbital is speculative in nature and also presents exciting possibilities, there are still many practical challenges that would need to be overcome before the orbital might be realized. However, the present theoretical study is a necessary precursor to any of such developments.

Motion in the Restricted Three-Body Problem at the Nanoscale

This paper studies the classical restricted three-body problem of a carbon atom in the vicinity of two carbon 60 fullerenes ( fullerenes) at the nanoscale. The total molecular energy between the two fullerenes is determined analytically by approximating the pairwise potential energies between the carbon atoms on the fullerenes by a continuous approach. Using software MATHEMATICA, we compute the positions of the stationary points and their stability for a carbon atom at the nanosacle and it is observed that for each set of values, there exists at least one complex root with the positive real part and hence in the Lyapunov sense, the stationary points are unstable. Since only attractive Van der Waals forces contribute to the orbiting behavior, no orbiting phenomenon can be observed for , where the Van der Waals forces becomes repulsive. Although the orbital is speculative in nature and also presents exciting possibilities, there are still many practical challenges that would need to be overcome before the orbital might be realized. However, the present theoretical study is a necessary precursor to any of such developments.

Tuning wettability and surface order of MWCNTs by functionalization for water desalination

While multiwall carbon nanotubes (MWCNTs) attract a lot of attention for their remarkable physical and mechanical properties, it is necessary to modify their surface chemistry and wettability with functionalization for successful application of these materials. Vertical and spaghetti MWCNTs were produced and functionalized with H2 and SF6 plasma treatments. Morphological and chemical characterizations were conducted. Molecular dynamics (MD) simulations demonstrated apparent unbundling of the functionalized vertical MWCNTs with the repulsive electrostatic forces between functional agents dominating the attractive van der Waals forces between the CNTs. MD simulations of hydrogenated spaghetti showed an increase in the structural order. Raman scattering provided evidence of the role played by the functional groups in increasing the graphitic character of MWCNTs. Contact angle measurements revealed the wettability tuning of the hydrogenated MWCNTs with a pronounced correlation with surface order. Hydrophobic-hydrophilic transitions of fluorinated MWCNTs with fluorination time also revealed the exchange of nonpolar and polar bonds on the side walls of the MWCNTs. Electrical conductivity measurements reflected the potential of functionalized MWCNT surfaces in holding back the salt, allowing fast transport of water, cancelling the conventional trade-off between selectivity and permeability, and presenting these materials as candidates for new generation of nanomaterials in water desalination.

Instability of ultrathin viscoelastic freestanding films

The linear stability of freestanding thin films under the influence of attractive van der Waals forces is investigated for three rheological models, viz., Newtonian viscous films, viscoelastic solid films, and Jeffreys viscoelastic liquid films, with the aim of studying the role of rheology on the instability. Thin freestanding viscous films are unconditionally unstable, whereas the shear modulus in thin freestanding solid viscoelastic films governs the onset of instability. Interestingly, elasticity plays a dual role with regard to the stability of freestanding solid and liquid films: while it has a stabilizing influence on the former, it is destabilizing in the latter. Linear stability results of Jeffreys viscoelastic freestanding films are compared with those from supported films in the inertialess limit. The instability of Jeffreys viscoelastic freestanding film is unaffected by the relaxation time, but is enhanced with decrease in the viscosity ratio (μr, the ratio of solvent viscosity to total viscosity). The dominant length scale of instability in Jeffreys viscoelastic freestanding film shifts toward shorter wavelengths with decrease in μr. For μr→0, the maximum growth rate remains bounded in a freestanding viscoelastic film in the presence of inertia, but diverges in its absence, similar to supported viscoelastic films. In general, our results show that freestanding thin films exhibit faster dynamics than supported thin films. The mode of deformation of the freestanding film (viz., bending or squeezing) is not imposed a priori in our analysis and is found to be a squeezing (symmetric) mode with equal amplitudes at the interfaces.

Neural Network for Surface-Profile Estimation of Atomic Force Microscope

This paper describes a methodology for implementing an identification algorithm for an atomic force microscope to estimate a surface profile of a sample. A microbeam of the atomic force microscope is modeled as an Euler-Bernoulli beam. A tip-sample interaction force used here is a piecewise function described by the attractive van der Waals force and the repulsive Derjaguin-Muller-Toporov force which can represent the indentation made by a tip of the microbeam into the sample surface. The identification of the instantaneous gap between the tip of the undeformed microbeam and the sample surface is implemented on line based on a trained input-output recurrent network. With the measured cantilevered-beam-tip motion supplied to the recurrent network, the recurrent network is going to compute the estimated gap. The approach being used in this study is in contrast to the conventional approach in that it is based on the signal in the time domain instead of in the frequency domain; therefore, the number of steps involved is fewer. When comparing the proposed approach with the adaptive-tracking controllers based on adaptive control Lyapunov functions, the proposed approach outperforms the adaptive-tracking controllers significantly. Whereas the adaptive-tracking controllers require much more measured signals to supply to the controllers and the parameter estimator. Instead, the proposed approach has to store two units in the tapped-delay-line memory while the identification algorithm is running.

Quantitative Estimation of Biocapped Surface Chemistry Driven Interparticle Interactions and Growth Kinetics of Gold Nanoparticles

In phytosynthesis of gold nanoparticles (AuNPs), biomolecules play a vital role in biocapping the surface of particles and generating the electrostatic repulsive forces to inhibit their growth kinetics. However, estimation of bioactive compounds influencing their surface characteristics through formation of electric repulsive forces ( $$V_{elec}$$ ), Van der Waals attraction forces ( $$V_{vdw}$$ ) and ultimately hindering their growth is still in the phase of obscurity. Current study, based on surface chemistry approach has been performed for identification of bioactive compounds in Elaeis guineensis leaves (EGL/OPL), acting as biocapping agents and directing the growth of AuNPs over a period of time. The quantitative estimation of interparticle interactions and modification in Ostwald ripening (MOR) model were also done to correlate the growth kinetic of AuNPs. The X-ray photoelectron spectroscopy (XPS) showed the major contribution of oxygen, carbon and nitrogen elements, corresponding to polyphenolic, carboxylic and amides, in biocapping the surface of AuNPs and directing their interparticle interactions associated with growth kinetics. The $$V_{elec}$$ forces were reduced with an enhancement in the $$V_{vdw}$$ forces, depicting their major role in impeding growth of AuNPs. The MOR model exhibited an excellent agreement of predicted growth with experimental size enlargements of AuNPs, having 4.8% average absolute relative percentage error.

Coalescence of Droplets in a Microwell Driven by Surface Acoustic Waves.

Microwell arrays are amongst the most commonly used platforms for biochemical assays. However, the coalescence of droplets that constitute the dispersed phase of suspensions housed within microwells has not received much attention to date. Herein, we study the coalescence of droplets in a two-phase system in a microwell driven by surface acoustic waves (SAWs). The microwell structure, together with symmetric exposure to SAW irradiation, coupled from beneath the microwell via a piezoelectric substrate, gives rise to the formation of a pair of counter-rotating vortices that enable droplet transport, trapping, and coalescence. We elucidate the physics of the coalescence phenomenon using a scaling analysis of the relevant forces, namely, the acoustic streaming-induced drag force, the capillary and viscous forces associated with the drainage of the thin continuous phase film between the droplets and the van der Waals attraction force. We confirm that droplet-droplet interface contact is established through the formation of a liquid bridge, whose neck radius grows linearly in time in the preceding viscous regime and proportionally with the square root of time in the subsequent inertial regime. Further, we investigate the influence of the input SAW power and droplet size on the film drainage time and demarcate the coalescence and non-coalescence regimes to derive a criterion for the onset of coalescence. The distinct deformation patterns observed for a pair of contacting droplets in both the regimes are elucidated and the possibility for driving concurrent coalescence of multiple droplets is demonstrated. We expect the study will find relevance in the demulsification of immiscible phases and the mixing of samples/reagents within microwells for a variety of biochemical applications.

Influence of initial film radius and film thickness on the rupture of foam films

The thinning dynamics and film lifetime between two bubbles depend on the extent to which the interfaces are flattened upon close contact, characterized by the film initial radius and thickness. Approaches that account for both dimple formation and attractive van der Waals forces have not been used to reveal film lifetime dependence on these initial features. Here we address this gap with numerical simulations of the classical thin film equation, formulated so that the film initial radius and thickness are direct input parameters. We find different thinning behavior for large and small radii films and characterize it using known analytical solutions from the literature.

Effect of mechanical and electro-chemical contacts on the particle orientation of clay minerals during swelling and sedimentation: A DEM simulation

The importance and quality of particle orientation on the performance of clay has been well-documented experimentally. This paper aims to quantitatively evaluate clayey mineral reorientation during swelling/sedimentation according to the balance of mechanical contacts and electrochemical forces. A DEM code has been developed and the behavior of montmorillonite, kaolinite and illite, each having different pore fluid characteristics, has been simulated as for platy-shaped particles. Nonlinear contact law was considered for the induced net force as it relates to the mechanical contacts, Van der Waals attractive force, and diffuse double-layer (DDL) repulsive force. Good compatibility was observed between the DEM simulation and DDL theory. The results showed that the particle weight, magnitude of the repulsive force, and number of DDL contacts were effective parameters controlling particle reorientation. At a low swelling pressure, the particles rotated easily such that an edge-to-face fabric was created and the potential for overlap between particles as well as the mechanical contacts increased. The number of the mechanical contacts versus the time of sedimentation was not linear, but bell-shaped. Kaolinite ranked higher on the DDL repulsive contact list; however, it had a higher deposition rate due to its higher particle weight and lower repulsion force.

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.