Interparticle Repulsion Research Articles

Phase behavior under a noncentrosymmetric interaction: shifted-charge colloids investigated by Monte Carlo simulation.

Using Monte Carlo simulations, we investigate the structural characteristics of an interacting hard-sphere system with shifted charge to elucidate the effect of the noncentrosymmetric interaction on its phase behavior. Two different phase transitions are identified for this model system. With increasing volume fraction, an abrupt liquid-to-crystal transition first occurs at a significantly lower volume fraction than in centrosymmetrically charged systems. This is due to the stronger effective interparticle repulsion caused by the additional charge anisotropy. Moreover, within the crystal state at higher volume fraction, the system further undergoes a continuous disorder-to-order transition with respect to charge orientation. Detailed analyses in this work disclose the nature of these transitions, and orientation fluctuation can cause noncentrosymmetric unit cells. The dependence of crystal formation and orientational ordering on temperature was also examined. These findings indicate that the noncentrosymmetric interaction in this work results in additional freedoms to fine-tune the phase diagram and increase the functionalities of materials. Moreover, these model studies are essential to advance the future understanding regarding the fundamental physiochemical properties of novel Janus colloidal particles and protein crystallization conditions.

Explicit energy density functional for the Crandall two-electron model atom with harmonic confinement and inverse square law inter-particle repulsion

Abstract Though density functional theory is already developed in useful practical numerical forms, no explicit simple ground-state energy density functional exists. Here, towards establishing such a theory, we present the ground-state energy of the Crandall et al.'s two-electron spin-compensated model atom in terms of ∇ 2 ρ ( r ) / ρ ( r ) evaluated at r = 0 , where ρ ( r ) is the electron density.

Exchange bias of [formula omitted] and [formula omitted] core/shell nanoparticles

We compare here exchange bias (EB) properties of chemically synthesized core–shell nanoparticles (NPs), based either on a core of soft ferrite (MnFe2O4) or hard ferrite (CoFe2O4) protected by a maghemite shell (γ-Fe2O3). These NPs dispersed in acidic solutions are electrostatically stabilized, yielding to stable colloidal dispersions with a strong interparticle repulsion and negligible dipolar interactions in the probed range of temperatures. Field cooled (FC) magnetic hysteresis loops of non-textured frozen dispersions (with magnetic anisotropy axis of NPs distributed at random) and those of a powder based on the same NPs present a shift along the H-axis, expressing the coupling between the spin-ordered cores and the disordered surface layer of the NPs. The bias field is found to present a maximum, larger for NPs based on harder ferrite core. It is obtained for a cooling field of the order of one half of the anisotropy field, which is much larger for the CoFe2O4 cores than for MnFe2O4 ones. In powders, particles are in contact leading to an interparticle exchange which is not present in the dilute solutions where exchange bias properties are only due to an intraparticle exchange between core and surface. The thermal dependence of the bias field is well described by a reduced exponential behavior with a characteristic freezing temperature of about 8K.

Surface Pressure Isotherm for a Monolayer of Charged Colloidal Particles at a Water/Nonpolar-Fluid Interface: Experiment and Theoretical Model

Monolayers from electrically charged micrometer-sized silica particles, spread on the air/water interface, are investigated. Because of the electrostatic repulsion, the distances between the particles are considerably greater than their diameters, i.e., we are dealing with nondensely packed interfacial layers. The electrostatic repulsion between the particles occurs through the air phase. Surface pressure vs area isotherms were measured by Langmuir trough, and the monolayers' morphology was monitored by microscope. The mean area per particle is determined by Delaunay triangulation and Voronoi diagrams. In terms of mean area, the surface pressure for monolayers from polydisperse and monodisperse particles obeys the same law. The experiments show that Π ∝ L(-3) at large L, where Π is the surface pressure and L is the mean interparticle distance. A theoretical cell model is developed, which predicts not only the aforementioned asymptotic law but also the whole Π(L) dependence. The model presumes a periodic distribution of the surface charge density, which induces a corresponding electric field in the air phase. Then, the Maxwell pressure tensor of the electric field in the air phase is calculated and integrated according to the Bakker's formula to determine the surface pressure. Thus, all collective effects from the electrostatic interparticle interactions are taken into account as well as the effects from the particle finite size. By evaporation of water, the particle monolayers are deposited on a solid substrate placed on the bottom of the trough. The electrostatic interparticle repulsion is strong enough to withstand the attractive lateral capillary immersion forces that are operative during the drying of the monolayer on the substrate. The obtained experimental results and the developed theoretical model can be useful for prediction and control of the properties of nondensely packed interfacial monolayers from charged particles that find applications for producing micropatterned surfaces.

The key role of electron–nuclear potential energy in determining the ground-state energy of inhomogeneous electron liquids in both real and model atoms

Recent density functional theory (DFT) work of Gál and March (GM) on the ground-state energy E of a two-electron model atom (like He but with inverse square law interparticle repulsion) related E to the electron–nuclear potential energy by . Also the model of GM satisfies , but now with harmonic confinement. While modern non-relativistic DFT requires numerical treatment of real atoms, in the exact limit of DFT at large Z, the Thomas–Fermi (TF) theory is regained, where much analytical work can be done. This yields, as , the non-relativistic energy of such neutral atoms as . The correlated electron density is finally considered briefly in the two models cited above.

Two-dimensional array of particles originating from dipole-dipole interaction as evidenced by potential curve measurements at vertical oil/water interfaces.

We propose a new method to evaluate the interaction potential energy between the particles adsorbed at an oil/water interface as a function of interparticle distance. The method is based on the measurement of the interparticle distance at a vertical oil/water interface, at which the gravitational force is naturally applied to compress the particle monolayer in the in-plane direction. We verified the method by examining whether we obtained the same potential curve upon varying the gravitational acceleration by tilting the interface. The present method is applicable in the force range from ∼0.1 to ∼100 pN, determined by the effective weight of the particles at the interface. The method gives a rather simple procedure to estimate a long range interaction among the particles adsorbed at oil/water interfaces. We applied this method to polystyrene particles at the decane/aqueous surfactant solution interface, and obtained the interparticle potential curves. All the potential curves obtained by the present method indicated that the interparticle repulsion is due to the electrical dipole-dipole interaction based on the negative charge of the particles. The mechanism of the dipole-dipole interaction is further discussed on the basis of the effects of surfactants.

Quantum Statistical Derivation of a Ginzburg-Landau Equation

The pairon field operator ψ(r,t) evolves, following Heisenberg’s equation of motion. If the Hamiltonian H contains a condensation energy α0(<0) and a repulsive point-like interparticle interaction , , the evolution equation for ψ is non-linear, from which we derive the Ginzburg-Landau (GL) equation: for the GL wave function where σdenotes the state of the condensed Cooper pairs (pairons), and n the pairon density operator (u and are kind of square root density operators). The GL equation with holds for all temperatures (T) below the critical temperature Tc, where eg(T) is the T-dependent pairon energy gap. Its solution yields the condensed pairon density . The T-dependence of the expansion parameters near Tc obtained by GL: constant is confirmed.

Lipid-based nanodiscs as models for studying mesoscale coalescence – a transport limited case

Lipid-based nanodiscs (bicelles) are able to form in mixtures of long- and short-chain lipids. Initially, they are of uniform size but grow upon dilution. Previously, nanodisc growth kinetics have been studied using time-resolved small angle neutron scattering (SANS), a technique which is not well suited for probing their change in size immediately after dilution. To address this, we have used dynamic light scattering (DLS), a technique which permits the collection of useful data in a short span of time after dilution of the system. The DLS data indicate that the negatively charged lipids in nanodiscs play a significant role in disc stability and growth. Specifically, the charged lipids are most likely drawn out from the nanodiscs into solution, thereby reducing interparticle repulsion and enabling the discs to grow. We describe a population balance model, which takes into account Coulombic interactions and adequately predicts the initial growth of nanodiscs with a single parameter - i.e., surface potential. The results presented here strongly support the notion that the disc coalescence rate strongly depends on nanoparticle charge density. The present system containing low-polydispersity lipid nanodiscs serves as a good model for understanding how charged discoidal micelles coalesce.

Quantum systems of ultracold bosons with customized interparticle interactions

Recent progress in cooling and trapping of polarized clouds of chromium ${}^{52}\text{Cr}$, dysprosium ${}^{164}\text{Dy}$, and erbium ${}^{168}\text{Er}$ opens a roadmap to quantum systems where the shapes of interparticle interactions can be customized. The main purpose of this work is to get deeper insight into the role the overall shape of the interparticle interaction plays in the context of trapped ultracold bosons. We show that strong interparticle repulsion inevitably leads to multihump fragmentation of the ground state. The fragmentation phenomenon is universal---it takes place in traps of different dimensionalities and topologies and for very broad classes of repulsive interparticle potentials. The physics behind this is identified and explained.

Ordering of Polystyrene Nanoparticles on Substrates Pre-Coated with Different Polyelectrolyte Architectures

Adjusting the inter-particle distances in ordered nanoparticle arrays can create new nano-devices and is of increasing importance to a number of applications such as nanoelectronics and optical devices. The assembly of negatively charged polystyrene (PS) nanoparticles (NPs) on Poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) brushes, quaternized PDMAEMA brushes and Si/PEI/(PSS/PAH)2, was studied using dip- and spin-coating techniques. By dip-coating, two dimensional (2-D), randomly distributed non-close packed particle arrays were assembled on Si/PEI/(PSS/PAH)2 and PDMAEMA brushes. The inter-particle repulsion leads to lateral mobility of the particles on these surfaces. The 200 nm diameter PS NPs tended to an inter-particle distance of 350 to 400 nm (center to center). On quaternized PDMAEMA brushes, the strong attractive interaction between the NPs and the brush dominated, leading to clustering of the particles on the brush surface. Particle deposition using spin-coating at low spin rates resulted in hexagonal close-packed multilayer structures on Si/PEI/(PSS/PAH)2. Close-packed assemblies with more pronounced defects are also observed on PDMAEMA brushes and QPDMAEMA brushes. In contrast, randomly distributed monolayer NP arrays were achieved at higher spin rates on all polyelectrolyte architectures. The area fraction of the particles decreased with increasing spin rate.

Forces acting on dielectric colloidal spheres at a water/nonpolar fluid interface in an external electric field. 2. Charged particles

Here, we calculate the electric forces acting on charged dielectric colloidal particles, which are attached to the interface between a nonpolar fluid (air and oil) and water in the presence of applied uniform external electric field, E0, directed normal to the interface. Electric charges are present on the particle–nonpolar fluid interface. The solution to the problem represents a superposition of the solutions of two simpler problems: (i) charged particle in the absence of external field and (ii) uncharged particle in the presence of external field. Because the external field can be directed upward or downward, it enhances or opposes the effect of the particle surface charges. As a result, the vertical (electrodipping) force vs. E0 may have a maximum or minimum and can be positive or negative depending on the particle contact angle and dielectric constant. In contrast, the lateral electric force between two identical charged floating particles is always positive (repulsive), but it can vary by many orders of magnitude with E0. This is because at a certain value of E0, the net dipolar moment of the particle becomes zero. Then, the interparticle force is governed by the octupolar moment, which leads to a much weaker and short-range repulsion. In the vicinity of this special value of E0, the interparticle repulsion is very sensitive to the variations in the external field. These effects can be used for a fine control of the lattice spacing in non-densely packed interfacial colloidal crystals of regular hexagonal packing for producing lithographic masks with various applications in nanotechnology.

Simulation of self-assembly in an evaporating droplet of colloidal solution by dissipative particle dynamics

A method of Brownian dissipative dynamics of charged colloidal nanoparticles in microdroplet of solution deposited on plane substrate is proposed for investigation of self-assembly and self-ordering of colloids during solute evaporation. Method is based on the numerical solution of multi-scale Langevin equation for each particle, the hydrodynamic microflows approach, and droplet evaporation model. The method takes into account the DLVO-forces between the particles, their interaction with the substrate (adhesion, friction, roughness); Stokes’, Brownian, and capillary forces (wetting and depinning, outflow angles, surface tension). The self-assembled pattern morphology dependence on the model parameters is investigated. The nature of coffee ring effect was studied. It is shown that the hexagonal domain ordering of particles ensemble in pattern can be formed onto plane substrate as a result of interparticle repulsion and the capillary compression during evaporation of solvent. Numerical results are in good agreement with experiments on self-assembly in colloidal droplet deposited by inkjet technology.

Development of a slow polycolloid-releasing substrate (SPRS) biobarrier to remediate TCE-contaminated aquifers

In this study, an in situ slow polycolloid-releasing substrate (SPRS) biobarrier system was developed to continuously provide biodegradable substrates for the enhancement of trichloroethylene (TCE) reductive dechlorination. The produced SPRS contained vegetable oil (used as a slow-released substrate), cane molasses [used as an early-stage (fast-degradable) substrate], and surfactants [Simple Green™ (SG) and soya lecithin (SL)]. An emulsification study was performed to evaluate the globule droplet size and stability of SPRS. The distribution and migration of the SPRS were evaluated in a column experiment, and an anaerobic microcosm study was performed to assess the capability of SPRS to serve as a slow and long-term carbon-releasing substrate for TCE dechlorination. The results show that a stable oil-in-water (W/O, 50/50) emulsion (SPRS) with uniformly small droplets (D10, 0.93μm) has been produced, continuously supplying primary substrates. The emulsion containing the surfactant mixture (with 72mg/L SL and 71mg/L SG) had a small absolute value of the zeta potential, which reduced the inter-particle repulsion, leading the emulsion droplets to adhere to one another after collision. The addition of SPRS creates anaerobic conditions and leads to a more complete and thorough removal of TCE through biodegradation and sorption mechanisms.

The central role of the von Weizsäcker kinetic energy density in determining the energy functional of a model two-electron atom with harmonic confinement and inverse square interparticle repulsion

This study uses an analytically known expression for the relative motion wave function \(\Psi _{RM}(r)\) for the Crandall et al. two-electron model atom with harmonic confinement plus inverse square law interparticle repulsion. This wavefunction is then employed to calculate the kinetic energy density for chosen values of both the harmonic force constant and the strength of the interaction coupling. Further consideration is then given to the Gal–March energy density functional which is shown to depend on the von Weizsacker kinetic energy density in a fundamental manner.

Coulomb expansion: Analytical solutions

Exact and approximate analytical solutions are presented, describing expansion of a cloud of charged particles in one, two, and three dimensions (assuming the planar, axial, and spherical symmetries, respectively). The expansion occurs in a gas or dilute plasma, where the screening is unimportant, so that particles interact with each other via Coulomb repulsive forces. It is shown that, irrespective of dimensionality, the density distribution remains homogeneous across the cloud and the velocity increases linearly towards the cloud boundary. The density evolution obeys a universal dependence, asymptotically decreasing with time as t(-1). It is also shown that in the presence of an inhomogeneous external field the interparticle repulsion becomes negligible at an early stage of expansion and then the density decreases with time exponentially.

Controlled Fabrication of Hexagonally Close-Packed Langmuir–Blodgett Silica Particulate Monolayers from Binary Surfactant and Solvent Systems

We describe a controllable method to fabricate hexagonally close-packed Langmuir-Blodgett (LB) monolayers with stearic acid (SA) as co-surfactant and methanol as co-solvent. The optimal SA concentrations and volume ratios of chloroform to methanol are 0.8 mg/mL and 3:1 for particles of 140 nm, 0.50 mg/mL and 4:1 for particles of 300 nm, and 0.05 mg/mL and 5:1 for particles of 550 nm, respectively. Additionally, SEM detections of the monolayers transferred at different surface pressures indicate that the monolayers deposited from the binary systems are more compressible. The experimental results indicate that the interparticle repulsions and particle-water interactions can be enhanced without decreasing the particle hydrophobicity by adding SA and methanol; thus, particulate monolayers with large hexagonally close-packed domains composed of small silica particles can be successfully fabricated using LB technique. We propose that the enhanced interparticle repulsion is attributed to the Columbic repulsion resulting from the attachment of SA molecules to the CTAB modified particles around the three phase contact line.

Size‐Dependent Electrostatic Chain Growth of pH‐Sensitive Hairy Nanoparticles

Big and small, short and tall: Nanoparticles (NPs) coated with pH-sensitive polymers underwent reversible chain growth as a result of varying electrostatic interparticle repulsion as determined by the pH value. Tuning of the pH value also enabled the coupling of differently sized NPs to form linear composite chains reminiscent of block copolymers (see picture).

Role of interparticle attraction in the yielding response of microgel suspensions

We use rheological measurements to examine the yielding behavior of a microgel system spanning the range from soft jammed glassy suspensions dominated by inter-particle repulsion to colloidal gels produced by attractive interactions. Under repulsive conditions, the suspensions display a prototypical soft glassy yielding response in which the shear loss modulus exhibits a single peak on increasing strain during the crossover from elastic to viscous behavior. By contrast, under fully attractive conditions the colloidal gel displays a more complex yielding, with two distinct peaks in the loss modulus in the vicinity of the yield strain. It is apparent that the gels yield initially by network rupture, followed by shear induced densification which leads to the formation of compact clusters. We show that the second peak in the loss modulus is consistent with the subsequent breakup of these dense clusters. We quantitatively map the steady progression from simple glassy yielding to the more complex gel response on increasing attraction strength by the evolution of peak locations, magnitudes and frequency dependencies. Notably, the peak locations diverge as the network becomes more fragile and spatially heterogeneous with increasing attraction strength. There is little frequency dependence in the peak positions, but the amplitude of the second yielding peak shows a non-monotonic dependence with a maximum near 5 rad s−1. Time-resolved measurements and decreasing strain sweeps highlight pronounced differences in the reversibility of the network rupture and cluster breakup processes. Correspondingly, the linear viscoelastic properties of the gel are strongly dependent on mechanical history whereas the glass exhibits no such dependence.

The oxidative toxicity of Ag and ZnO nanoparticles towards the aquatic plant Spirodela punctuta and the role of testing media parameters

The toxicity effects of silver (nAg) and zinc oxide (nZnO) engineered nanoparticles (ENPs) on the duckweed Spirodela punctuta were studied to investigate the potential risks posed by these ENPs towards higher aquatic plants. The influence of media abiotic factors on the stability of the ENPs was also evaluated. Marked agglomeration of ENPs was observed after introduction into testing media whereby large particles settled out of suspension and accumulated at the bottom of testing vessels. The high ionic strength (IS) promoted agglomeration of ENPs because it reduced the inter-particle repulsion caused by a reduction in their surface charge. Low dissolution was observed for nAg, reaching only 0.015% at 1000 mg L(-1), whilst improved dissolution was observed for nZnO, only falling below analytical quantification at 0.1 mg L(-1) and lower. The quantification of free radicals namely, reactive oxygen and nitrogen species (ROS/RNS) and hydrogen peroxide (H2O2), indicated the induction of oxidative stress in plants exposed to the ENPs. A definite dose influence was observed for ROS/RNS volumes in plants exposed to nZnO for 14 days, a response not always observed. The total antioxidant capacity (TAC) and superoxide dismutase (SOD) activity in plants indicated varying degrees of oxidative toxicity caused by exposure to ENPs. This toxicity was driven mainly by particulates in plants exposed to nAg, whilst dissolved Zn(2+) was the main driver for toxicity in plants exposed to nZnO. Our findings suggest that the toxicity of nAg and nZnO could be caused by both the particulates and ionic forms, as modified by media properties.

Electrostatically driven adsorption of silica nanoparticles on functionalized surfaces

Adsorption of nanoparticles on solid supports is a scientifically interesting and technologically important phenomenon that has been attracting ever-increasing attention. Formation of particle-based films onto surfaces from stable suspensions is at the center of the development of new devices that utilize the plethora of newly synthesized nanoparticles with exciting properties. In this study we exploit the attractive electrostatic interactions between silica (SiO2) nanoparticles and functionalized substrates that display an amine termination in order to devise a simple method for the fabrication of SiO2 nanoparticle films. Electrostatically controlled adsorption allows for uniform coverage of nanoparticles over large areas. The Stöber method (a sol–gel approach) was employed to prepare uniformly sized SiO2 nanoparticles with a diameter of 50–80nm. Native oxide-covered silicon wafer substrates were amino-functionalized utilizing the self-assembled monolayer of 3-aminopropyltrimethoxysilane (APS). The adsorption of SiO2 nanoparticle film onto the silicon wafer substrate was controlled by modulation of the electrostatic interaction between nanoparticles and the substrate. Modification of surface charge of either the SiO2 NP or the substrate is a crucial step in the process. Thus the effect of APS adsorption time on the surface energy of the substrate was investigated. Also, process parameters such as NP concentration and solvent composition were varied in order to investigate the extent of NP adsorption. Moreover, NaCl was introduced to the SiO2 suspension as a charge-screening agent to reduce the inter-particle repulsion in the suspension as well as interaction of the particles with the surface. This resulted in denser/thicker films.