Repulsive Barrier Research Articles

Unified Description of Dynamics of a Repulsive Two-Component Fermi Gas.

We study a binary spin mixture of a zero-temperature repulsively interacting ^{6}Li atoms using both the atomic-orbital and density-functional approaches. The gas is initially prepared in a configuration of two magnetic domains and we determine the frequency of the spin-dipole oscillations which are emerging after the repulsive barrier, initially separating the domains, is removed. We find, in agreement with recent experiment [G. Valtolina etal., Nat. Phys. 13, 704 (2017)NPAHAX1745-247310.1038/nphys4108], the occurrence of a ferromagnetic instability in an atomic gas while the interaction strength between different spin states is increased, after which the system becomes ferromagnetic. The ferromagnetic instability is preceded by the softening of the spin-dipole mode.

Influence of the decomposition of particle units on the coagulation kinetics of monodisperse silicon sol

Based on the results of studying the coagulation kinetics of monodisperse sol of SiO2 in NaCl solutions at various pH values, it is shown that the coagulation rate, flowing in the far potential minimum while maintaining a high repulsion barrier between the particles, depends on the formation and decomposition times of the aggregates. The boundary values of the depth and power of the far potential minimum—at the excess of which sol coagulates—are determined.

Analog of discontinuous shear thickening flows under confining pressure

We use 2D numerical simulations to study dense suspensions of non-Brownian hard particles using the Critical Load Model (CLM) under constant confining pressures. This simple model shows discontinuous shear thickening (DST) as the tangential forces get activated upon increased shear stresses. By parameterizing a simple binary system of frictional and non-frictional particles of different proportions we show that the jamming packing fraction, at which the viscosity diverges, is controlled by the fraction of frictional contacts. The viscosity of dense suspensions can thereby be expressed as a function of the fraction of frictional contacts as well as the packing fraction of solid particles. In addition, we show that there exists a simple relationship between the fraction of frictional contacts and the two control parameters (under confining pressure): the viscous number J and the ratio between the repulsive barrier force and confining pressure. Under confining pressures the viscosity curves are found to depend on the shear protocol, with the possibility of yielding negative dynamic compressibility.

Charge‐assisted ultrafiltration membranes for monovalent ions separation in electrodialysis

ABSTRACTSelectivity for monovalent ions is a very important feature of ion exchange membranes in view of a further expansion of the scope of electrodialysis (ED). An efficient one‐step chemical process to graft a thin polyaniline (PANI) layer on the surface and pores of an ultrafiltration (UF) membrane is reported. The existence of a PANI layer on such modified UF membrane was confirmed by Fourier transform infrared spectroscopy and scanning electron microscopy images. These membranes were extensively characterized by studying their properties such as water uptake, ion‐exchange capacity, contact angle, and performance in diffusion dialysis. ED experiments were conducted by comparing desalting of sodium chloride and magnesium chloride solutions. During the desalting procedure, the driving force has two contributions, the electrical field and the salinity‐gradient. Initially, transport of ions by the electrical field is dominant, while at the end of the experiment, diffusion dialysis by the salinity gradient plays a larger role in ion transport. The presence of PANI can hinder Mg2+ transport from the concentrate to the diluate compartment because of the electrostatic effect, while there is no obvious effect on Na+ ions. The ability of such a surface modification layer to improve the membrane selectivity for Na+/Mg2+ was confirmed by means of ED in a mixed solution. The flux of Na+ was slightly increased to 13.21 × 10−8 mol/L/min while the flux of Mg2+ was reduced to 3.32 × 10−8 mol/L/min, so that the permselectivity is almost doubled. The protonation reaction of the PANI layer creates positive charges, thus leading to an electrical repulsion barrier, which may reduce the penetration of divalent cations with respect to monovalent ions. Polyacrylonitrile‐based UF membranes with different molecular cutoff was used to determine the influence of the permselectivity. Results showed that the membrane permselectivity of smaller molecular cutoff UF membranes is higher. It can be concluded that this method is suitable for preparing membranes based on UF membranes for efficient separation of monovalent ions by electro‐driven separation techniques. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 45692.

Ultracold bosonic scattering dynamics off a repulsive barrier: Coherence loss at the dimensional crossover

We explore the impact of dimensionality on the scattering of a small bosonic ensemble in an elongated harmonic trap off a centered repulsive barrier, thereby taking particle correlations into account. The loss of coherence as well as the oscillation of the center of mass are studied and we analyze the influence of both particle and spatial correlations. Two different mechanisms of coherence losses in dependence of the aspect ratio are found. For small aspect ratios, loss of coherence between the region close to the barrier and outer regions occurs, due to spatial correlations, and for large aspect ratios, incoherence between the two density fragments of the left and right side of the barrier arises, due to particle correlations. Apart form the decay of the center of mass motion induced by the reflection and transmission, further effects due to the particle and spatial correlations are explored. For tight transversal traps, the amplitude of the center of mass oscillation experiences a weaker damping, which can be traced back to the population of a second natural orbital, and for a weaker transversal confinement, we detect a strong decay, due to the possibility of transferring energy to transversal excited modes. These effects are enhanced if the aspect ratio is integer valued.

A detailed-balance model for thermionic emission from polyanions: The case of fullerene dianions.

A detailed-balance model for thermionic emission from polyanions has been developed and applied to fullerene dianions. The specificity of this delayed decay process is electron tunneling through the repulsive Coulomb barrier (RCB). An analytical expression of the RCB is derived from electrostatic modeling of the fullerene cage. The reverse process, namely, electron attachment to the singly charged anion, is described by a hard sphere cross section weighted by the Wentzel-Kramers-Brillouin tunneling probability. This simple expression leads to a very good agreement with a measured time-resolved kinetic energy distribution of C842-. Electron binding energy is reduced when the fullerene cage size decreases, leading to an almost zero one for C702- and a negative one for C602-. Extension of the model to these systems of interest is discussed, and model outputs are compared with the experimental data from the literature.

Supercharging Proteins: How Many Charges Can a Protein Carry?

Very highly charged proteins, so-called "supercharged" ions, can lose (excess) protons to background gases like N2 . It is remarkable that such extremely acidic species can be generated in electrospray ionization, in the presence of not just N2 but also much higher-basicity solvents. What mechanism(s) can explain such high charging, and what is the ultimate limit?

Direct Observation of Interactions between Nanoparticles and Nanoparticle Self-Assembly in Solution.

Hierarchically organized nanoparticles (NPs) possess unique properties and are relevant to various technological applications. An important "bottom-up" strategy for building such hierarchical nanostructures is to guide the individual NPs into ordered nanoarchitectures using intermolecular interactions and external forces. However, our current understanding of the nanoscale interactions that govern such self-assembly processes usually relies on post-synthesis/assembly or indirect characterization. Theoretical models that can derive these interactions are presently constrained to systems with only a few particles or on short time scales. Hence, except for a number of special cases, a description that captures the detailed mechanisms of NP self-assembly still eludes us. By imaging the assembly of NPs in solution with subnanometer resolution and in real-time, in situ liquid cell transmission electron microscopy (LC-TEM) can identify previously unknown intermediate stages and improve our understanding of such processes. Here, we review recent studies where we explored NP self-assembly at different organization length scales using LC-TEM: (1) we followed the transformation of atoms into crystalline NPs in solution, (2) we highlighted the role of solvation forces on interaction dynamics between NPs, and (3) we described the assembly dynamics of NPs in solution. In the case of nanocrystal nucleation, we identified the existence of three distinct steps that lead to the formation of crystalline nuclei in solution. These steps are spinodal decomposition of the precursor solution into solute-rich and solute-poor liquid phases, nucleation of amorphous clusters within the solute-rich liquid phase, followed by crystallization of these amorphous clusters into crystalline NPs. The next question we ask is how NPs interact in solution once they form. It turns out that the hydration layer surrounding each NP acts as a repulsive barrier that prevents NPs from readily attaching to each other due to attractive vdW forces. Consequently, two interacting NPs form a metastable pair separated by their one water molecule thick hydration shell and they undergo attachment only when this water between them is drained. Next, we explore the self-assembly of many NP systems where the formation of linear chains from spherical NPs or nanorods (NRs) is mediated by linker molecules. At low linker concentration, both spherical NPs and NRs tend to form linear chains because of the need to reduce electrostatic repulsion between NP building blocks. When the concentration of linkers is increased, the attachment of NPs is no longer linear. For example, we find that two NRs undergo side-to-side assembly due to decreased electrostatic repulsion and the anisotropic distribution of linkers on NR surfaces at high linker concentration. Lastly, we look at the formation of NP nanorings directed by ethylenediaminetetraacetic acid (EDTA) nanodroplets in water. Our study shows that nanoring assemblies form via sequential attachment of NPs to binding sites located along the circumference of the EDTA nanodroplet, followed by rearrangement and reorientation of the attached NPs. Our approach based on real-time visualization of nanoscale processes not only reveals all the intermediate steps of NP assembly, but also provides quantitative description on the interactions between nanoscale objects in solution.

Influence of long-range Coulomb interaction in velocity map imaging.

The standard velocity-map imaging (VMI) analysis relies on the simple approximation that the residual Coulomb field experienced by the photoelectron ejected from a neutral or ion system may be neglected. Under this almost universal approximation, the photoelectrons follow ballistic (parabolic) trajectories in the externally applied electric field, and the recorded image may be considered as a 2D projection of the initial photoelectron velocity distribution. There are, however, several circumstances where this approximation is not justified and the influence of long-range forces must absolutely be taken into account for the interpretation and analysis of the recorded images. The aim of this paper is to illustrate this influence by discussing two different situations involving isolated atoms or molecules where the analysis of experimental images cannot be performed without considering long-range Coulomb interactions. The first situation occurs when slow (meV) photoelectrons are photoionized from a neutral system and strongly interact with the attractive Coulomb potential of the residual ion. The result of this interaction is the formation of a more complex structure in the image, as well as the appearance of an intense glory at the center of the image. The second situation, observed also at low energy, occurs in the photodetachment from a multiply charged anion and it is characterized by the presence of a long-range repulsive potential. Then, while the standard VMI approximation is still valid, the very specific features exhibited by the recorded images can be explained only by taking into consideration tunnel detachment through the repulsive Coulomb barrier.

Motion of spin-half particles in the axially symmetric field of naked singularities of the static q-metric

Quantum-mechanical motion of a half-spin particle was examined in the axially symmetric field of static naked singularities formed by mass distribution with quadrupole moment (q-metric). The analysis was performed by means of the method of effective potentials of the Dirac equation generalized for the case when radial and angular variables are not separated. As $-1<q<q_{lim}, |q_{lim}|<<1 $ the naked singularities do not except the existence of stationary bound states of Dirac particles for a prolate mass distribution in the q-metric along the axial axis. For the oblate mass distribution, the naked singularities of the q-metric are separated from the Dirac particle by infinitely large repulsive barriers with the subsequent potential well deepening while moving along the angle from the equator (or from $\theta=\theta_{min}, \theta=\pi-\theta_{min}$) towards poles. The exception are the poles and, as $0<q<q*$, some points $\theta_{i}$ for the states of the particle with j>=3/2.

Thermodynamic assessment of adsorptive fouling with the membranes modified via layer-by-layer self-assembly technique

In this study, polyvinylidene fluoride (PVDF) microfiltration membrane was coated by dipping the membrane alternatingly in solutions of the polyelectrolytes (poly-diallyldimethylammonium chloride (PDADMAC) and polystyrenesulfonate (PSS)) via layer-by-layer (LBL) self-assembly technique to improve the membrane antifouling ability. Filtration experiments showed that, sludge cake layer on the coated membrane could be more easily washed off, and moreover, the remained flux ratio (RFR) of the coated membrane was obviously improved as compared with the control membrane. Characterization of the membranes showed that a polyelectrolyte layer was successfully coated on the membrane surfaces, and the hydrophilicity, surface charge and surface morphology of the coated membrane were changed. Based on the extended Derjaguin–Landau–Verwey–Overbeek (XDLVO) approaches, quantification of interfacial interactions between foulants and membranes in three different scenarios was achieved. It was revealed that there existed a repulsive energy barrier when a particle foulant adhered to membrane surface, and the enhanced electrostatic double layer (EL) interaction and energy barrier should be responsible for the improved antifouling ability of the coated membrane. This study provided a combined solution to membrane modification and interaction energy evaluation related with membrane fouling simultaneously.

RF-assisted DC single beam plasma generation for multi-beam nuclear fusion

Abstract At UOIT, there is extensive research on plasma generation. This paper focuses on RF assisted DC plasma generation. The goal of the research is to increase electron and ion energy and production such that fusion can occur. The high energy ions would overcome the coulomb repulsion barrier energy for fusion. The energy from the ions can be in the form of thermal or kinetic energy. In this research, the solenoid in the plasma generator serving as an auxiliary power source operates both in DC and RF mode. The paper discusses the simulations and experimental work under this research. Under DC mode where the magnetic field is in opposite direction as the plasma flow, the electron temperature and density are 4.44 eV and 354.7 · 1018 m−3. Under 125 MHz radio-frequency, the electron temperature and density are 0.30 eV and 291.7 · 1018 m−3 respectively.

Novel GO/OMWCNTs mixed-matrix membrane with enhanced antifouling property for palm oil mill effluent treatment

Abstract Membrane process is considered an effective and economical treatment technology to palm oil mill effluent (POME) which is a major pollution source discharged from the palm oil industry. In this study, graphene oxide (GO) and oxidized multi-walled carbon nanotubes (OMWCNTs) were used as nano additive in coagulation bath to prepare polyvinylidene fluoride (PVDF) membrane via in-situ colloidal precipitation method. The successful synthesis of carbon nanomaterials GO and OMWCNTs were validated using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and observed using field emission scanning electron microscopy (FESEM). In general, incorporation of GO and OMWCNTs into membrane matrix via in-situ colloidal precipitation method have significant effect on membrane characterization including contact angle, surface charge, porosity and pore size of the membrane. As observed using FESEM images, GO nanosheets had blocked some areas of the membrane surface thus reducing the effective filtration area. The mixed-matrix membranes M1c, M3a, and M5b demonstrated improved water permeability of 43.99 L/m 2 ·h·bar, 52.62 L/m 2 ·h·bar, and 43.38 L/m 2 ·h·bar, respectively owing to thinner skin layer, bigger voids, and increased hydrophilicity. The rejection performance of fabricated membrane was evaluated using diluted aerobic POME on physical characteristics (color, turbidity, total suspended solids (TSS), total dissolved solids (TDS), pH) and chemical properties (chemical oxygen demand (COD), hardness, total chlorine, and phosphorus). As compared to pristine membrane, the mixed-matrix membrane, M1c had improved the rejection of TDS, phosphorus, hardness, COD, chlorine, turbidity, color, and TSS with maximum rejection percentage of 1.51%, 6.55%, 21.79%, 75.5%, 76%, 81.94%, 86.3%, and 100%, respectively. This research demonstrated that the deposition of carbon nanomaterials increased the membrane hydrophilicity and formed a repulsive boundary barrier that enhanced antifouling properties.

Melting upon cooling and freezing upon heating: fluid-solid phase diagram for Švejk-Hašek model of dimerizing hard spheres.

A simple model of dimerizing hard spheres with highly nontrivial fluid-solid phase behavior is proposed and studied using the recently proposed resummed thermodynamic perturbation theory for central force (RTPT-CF) associating potentials. The phase diagram has the fluid branch of the fluid-solid coexistence curve located at temperatures lower than those of the solid branch. This unusual behavior is related to the strong dependence of the system excluded volume on the temperature, which for the model at hand decreases with increasing temperature. This effect can be also seen for a wide family of fluid models with an effective interaction that combines short range attraction and repulsion at a larger distance. We expect that for sufficiently high repulsive barrier, such systems may show similar phase behavior.

Interactions and design rules for assembly of porous colloidal mesophases.

Porous mesophases, where well-defined particle-depleted 'void' spaces are present within a particle-rich background fluid, can be self-assembled from colloidal particles interacting via isotropic pair interactions with competing attractions and repulsions. While such structures could be of wide interest for technological applications (e.g., filtration, catalysis, absorption, etc.), relatively few studies have investigated the interactions that lead to these morphologies and how they compare to those that produce other micro-phase-separated structures, such as clusters. In this work, we use inverse methods of statistical mechanics to design model isotropic pair potentials that form porous mesophases. We characterize the resulting porous structures, correlating features of the pair potential with the targeted pore size and the particle packing fraction. The former is primarily encoded by the amplitude and range of the repulsive barrier of the designed pair potential and the latter by the attractive well depth. We observe a trade-off with respect to the packing fraction of the targeted morphology: greater values support more spherical and monodisperse pores that themselves organize into periodic structures, while lower values yield more mobile pores that do not assemble into ordered structures but remain stable over a larger range of packing fraction. We conclude by commenting on the limitations of targeting a specific pore diameter within the present inverse design approach as well as by describing future directions to overcome these limitations.

Performing Hong-Ou-Mandel-type numerical experiments with repulsive condensates: The case of dark and dark-bright solitons

The Hong-Ou-Mandel experiment leads indistinguishable photons simultaneously reaching a 50:50 beam splitter to emerge on the same port through two-photon interference. Motivated by this phenomenon, we consider numerical experiments of the same flavor for classical, {\it wave} objects in the setting of repulsive condensates. We examine dark solitons interacting with a repulsive barrier, a case in which we find no significant asymmetries in the emerging waves after the collision, presumably due to their topological nature. We also consider case examples of two-component systems, where the dark solitons trap a bright structure in the second-component (dark-bright solitary waves). For these, pronounced asymmetries upon collision {\it are} possible for the non-topological bright component. We also show an example of a similar phenomenology for ring dark-bright structures in two dimensions.

The suppression of droplet-droplet coalescence in a sheared yield stress fluid

Efforts to stabilize emulsions against coalescence in flow have often focused on modifying properties of the interface between the continuous and dispersed phases, to create a repulsive barrier against coalescence. But prior to experiencing any interaction force, the liquid film between two colliding drops has to drain, and if this drainage process is arrested, coalescence will be suppressed. In this work, scaling analyses and thin-film lubrication simulations are used to study the hydrodynamic drainage properties of thin films of a Bingham fluid (a yield stress fluid, which flows only when a critical stress is exceeded) created between two drops colliding under the action of a constant force. Our study shows that the hydrodynamic drainage process can be arrested completely when the film reaches a critical thickness, before attractive forces result in the rupture of the film, provided that the film shape is in the dimpled configuration. This critical thickness is hf=6τ02R3/γ2, where τ0 is the yield stress of the suspending medium, R is the drop radius and γ is the interfacial tension between the fluids. The yield stress can thus serve as an independent tuning parameter that sets an upper bound on the drop size beyond which coalescence is turned off in sheared emulsions.

The effect of like-charge attraction on aerosol growth in the atmosphere of Titan

The formation of aerosols in the atmosphere of Titan is based extensively on ion-neutral chemistry and physical condensation processes. Herein it is shown that the formation of aerosols may also occur through an alternative pathway that involves the physical aggregation of negatively charged particles, which are known to be abundant in the satellite’s atmosphere. It is shown that, given the right circumstances, like-charged particles with a dielectric constant characteristic of nitrated hydrocarbons have sufficient kinetic energy to overcome any repulsive electrostatic barrier that separates them and can subsequently experience an attractive interaction at very short separation. Aerosol growth can then unfold through a charge scavenging process, whereby nitrated aggregates preferentially grow by assimilating smaller like-charged particles. Since hydrocarbon aerosols have much lower dielectric constants, it is shown that a similar mechanism involving hydrocarbon particles will not be as efficient. As a consequence of this proposed growth mechanism, it is suggested that the lower atmosphere of Titan will be enriched in nitrogen-containing aerosols.

Reactions of doubly deprotonated 2,6-naphthalenedicarboxylic acid with alcohols: Proton transfer versus solvation

Electrospray ionization of 2,6-naphthalenedicarboxylic acid readily affords its doubly deprotonated dicarboxylate dianion (12−). This species clusters with background water and added alcohols in an ion trap at ∼10−3Torr. Sequential solvation is observed to afford mono and dicoordinated ions. Surprisingly, the latter cluster (12−• 2TFE) is protonated by 2,2,2-trifluoroethanol (TFE) whereas 12−and 12−• TFE are not even though ΔH°acid(TFE)=361.7±2.5kcalmol−1 (as given in the NIST website at http://webbook.nist.gov) and the B3LYP/6-31+G(d,p) proton affinities are 384.7 (12−), 377.6 (12−• TFE), and 362.7 (12−• 2TFE) kcal mol−1. That is, only the weakest base in this series, and the dianion with an equal number of solvent molecules and charged sites, undergoes proton transfer. In a FTMS instrument at lower pressures (∼10−8Torr) inefficient proton abstraction is observed with the monosolvated dianion. This difference, and the observed reactivities of 12−, 12−• TFE and 12−• 2TFE are rationalized with the aid of computed potential energy surfaces. The chemical structures of these cluster ions were also probed via collision-induced dissociations, infrared photodissociation from 2700 to 3200cm−1, and extensive calculations. All of the TFE species are found to be solvated dianions, but incipient proton transfer to afford electrostatically defying anion-anion clusters is noted in two cases. In proton transfer reactions, formation of the conjugate acid as a solvated ion lowers the energy of the system and reduces the Coulomb repulsion barrier facilitating the overall process.

Size-dependent interactions of silica nanoparticles with a flat silica surface

We have investigated the surface chemistry of SiO2 nanoparticles (NPs) with different sizes and their corresponding interactions with a flat substrate of surface curvature ∼0. As the size of the NPs increases, the SiO2 surface is increasingly covered with H-bonded silanol groups, thereby increasing the ζ-potential and shifting the isoelectric point higher in pH. Interactions between the SiO2 NPs and the flat SiO2 surface were analyzed in situ using quartz crystal microbalance with dissipation (QCM-D) method, and the results were interpreted based on an extended Derjaguin–Landau–Verwey–Overbeek theory. At very low ionic strength (1mM NaCl), there was no particle adsorption onto the surface due to the highly repulsive energy barriers to this interaction. On the other hand, QCM-D results showed that the significant adsorption of SiO2 NPs onto a flat SiO2 surface occurred under conditions of high ionic strength (100mM NaCl). Interestingly, the adsorption behaviors of three different-sized SiO2 NPs on the surface varied considerably with size. SiO2 NPs with small size have high adsorption affinity with the flat SiO2 surface due to an extremely low energy barrier for the interactions, whereas relatively large SiO2 NPs have very weak adsorption affinity with the flat surface due to the repulsive energy barrier formed by the increase in the electrostatic and hydration repulsion energy.