Range Repulsion Research Articles

Unveiling the nature of atomic defects in graphene on a metal surface.

Low-energy argon ion bombardment of graphene on Ir(111) induces atomic-scale defects at the surface. Using a scanning tunneling microscope, the two smallest defects appear as a depression without discernible interior structure suggesting the presence of vacancy sites in the graphene lattice. With an atomic force microscope, however, only one kind can be identified as a vacancy defect with four missing carbon atoms, while the other kind reveals an intact graphene sheet. Spatially resolved spectroscopy of the differential conductance and the measurement of total-force variations as a function of the lateral and vertical probe-defect distance corroborate the different character of the defects. The tendency of the vacancy defect to form a chemical bond with the microscope probe is reflected by the strongest attraction at the vacancy center as well as by hysteresis effects in force traces recorded for tip approach to and retraction from the Pauli repulsion range of vertical distances.

Instability of membranes containing ionizable cationic lipids: Effects of the repulsive range of headgroups and tail structures

The stability of membranes formed by ionizable cationic lipids, which constitute the primary components in lipid nanoparticles capable of endosomal escape, is explored using coarse-grained dissipative particle dynamics. Three types of ionizable model lipids with different tail structures are considered. Endosome acidification causes the ionization of lipids, leading to an increased repulsive range between their headgroups. When electrostatic repulsion is modeled as a conservative force with a long-range cutoff distance (rc,HH), the membrane and vesicle experience a loss of structural integrity and develop holes as rc,HH is beyond a critical value, which varies with the tail structure. When Coulombic repulsion is explicitly incorporated and intensified, a fully ionized lipid membrane undergoes a loss of structural integrity, displaying a qualitative similarity to the effect observed with the increase in rc,HH on the membrane stability. Qualitatively similar results are obtained for partially ionized membranes as the fraction of charged lipids increases. The stability of a mixed lipid membrane containing both ionizable and conventional lipids is also investigated. The disruption of the bilayer structure occurs for a sufficiently high charged fraction. The membrane instability can be attributed to the decrease in the packing parameter, which significantly deviates from unity as the interaction range increases.

Local minimum enhanced artificial potential field method

Currently, the artificial potential field technique is commonly used in vehicle obstacle avoidance and route planning. However, this method may lead to local minima where the gravitational force and repulsive force are equal during the path planning process, and thus the target location cannot be reached. This paper suggests an approach to enhance the artificial potential field method in light of this. First, the calculation method of the attraction field in the traditional artificial potential field is modified for resolving the issue of unreachable targets. Second, for the local minimum, the path calculation is recovered by modifying the repulsive force range of obstacles and creating a virtual obstacle point, and applying an additional force to get rid of the gravitational force and repulsive force balance. The simulation results check the effectiveness of the designed method, which can solve the cases of unreachable targets and local minima and produce reasonable planned paths.

Two-bands Ising superconductivity from Coulomb interactions in monolayer

The nature of superconductivity in monolayer transition metal dichalcogenides is still under debate. It has already been argued that repulsive Coulomb interactions, combined with the disjoint Fermi surfaces around the K, Kʹ valleys and at the Γ point, can lead to superconducting instabilities in monolayer . Here, we demonstrate the two-bands nature of superconductivity in . It arises from the competition of repulsive long range intravalley and short range intervalley interactions together with Ising spin–orbit coupling. The two distinct superconducting gaps, one for each spin-orbit split band, consist of a mixture of s-wave and f-wave components. Their different amplitudes are due to the difference between the normal densities of states of the two bands at the Fermi level. Using a microscopic multiband BCS approach, we derive and self-consistently solve the gap equation, demonstrating the stability of nontrivial solutions in a realistic parameter range. We find a universal behavior of the temperature dependence of the gaps and of the critical in-plane field which is consistent with various sets of existing experimental data.

Effect of decoration route on the nanomechanical, adhesive, and force response of nanocelluloses-An in situ force spectroscopy study.

Although cellulose derivatives are widely applied in high-tech materials, the relation between their force responses and their surface chemical properties in a biological environment as a function of pH is unknown. Here, interaction forces of surface modified cellulose nanocrystals (CNCs), lignin residual cellulose nanocrystals (LCNCs), and 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-oxidized cellulose nanofibres (TCNFs) with OSO3-, COO- and lignin chemical groups were measured using in situ peak force quantitative nanomechanical mapping and force spectroscopy in salt solution at two pH values. We found that the forces acting between the tip and CNC or LCNC are steric dominated showing long range and slow decay as a result of their low surface charge density. High Mw lignin contributed to the increased repulsion range for LCNCs compared to CNCs. The repulsion measured for TCNFs at the very short range was electrostatic force dominating showing a steep decay attributed to its high surface charge density. In the case of TCNFs, electrostatic double layer force was also evidenced by the attraction measured at secondary minima. In all the three cases the electro steric interactions are pH dependent. Dissipation maps verified that the force behavior for each material was related to structural conformation restriction of the groups at compression. The slow decayed repulsion of CNCs or LCNCs is related to a weak restriction of conformational change due to small surface groups or high molecular weight bound polymers forming flat layers, whereas the steep repulsion of TCNFs is attributed to a strong conformation restriction of carboxylic groups occurred by forming extended structure. Our results suggest that the force responses of the materials were dominated by surface charges and structural differences. TCNFs showed superior nanomechanical and repulsion properties over CNCs or LCNCs at neutral pH.

APF-DPPO: An Automatic Driving Policy Learning Method Based on the Artificial Potential Field Method to Optimize the Reward Function

To address the difficulty of obtaining the optimal driving strategy under the condition of a complex environment and changeable tasks of vehicle autonomous driving, this paper proposes an end-to-end autonomous driving strategy learning method based on deep reinforcement learning. The ideas of target attraction and obstacle rejection of the artificial potential field method are introduced into the distributed proximal policy optimization algorithm, and the APF-DPPO learning model is established. To solve the range repulsion problem of the artificial potential field method, which affects the optimal driving strategy, this paper proposes a directional penalty function method that combines collision penalty and yaw penalty to convert the range penalty of obstacles into a single directional penalty, and establishes the vehicle motion collision model. Finally, the APF-DPPO learning model is selected to train the driving strategy for the virtual vehicle, and the transfer learning method is selected to verify the comparison experiment. The simulation results show that the completion rate of the virtual vehicle in the obstacle environment that generates penalty feedback is as high as 96.3%, which is 3.8% higher than the completion rate in the environment that does not generate penalty feedback. Under different reward functions, the method in this paper obtains the highest cumulative reward value within 500 s, which improves 69 points compared with the reward function method based on the artificial potential field method, and has higher adaptability and robustness in different environments. The experimental results show that this method can effectively improve the efficiency of autonomous driving strategy learning and control the virtual vehicle for autonomous driving behavior decisions, and provide reliable theoretical and technical support for real vehicles in autonomous driving decision-making.

On the dynamically arrested states of equilibrium and non-equilibrium gels: a comprehensive Brownian dynamics study

In this work a systematic study over a wide number of final thermodynamic states for two gel-forming liquids was performed. Such two kind of gel formers are distinguished by their specific interparticle interaction potential. We explored several thermodynamic states determining the thermodynamic, structural and dynamic properties of both liquids after a sudden temperature change. The thermodynamic analysis allows to identify that the liquid with short range attraction and long range repulsion lacks of a stable gas–liquid phase separation liquid, in contrast with the liquid with short range attractions. Thus, although for some thermodynamic states the structural behavior, measured by the static structure factor, is similar to and characteristic of the gel phase, for the short range attractive fluid the gel phase is a consequence of a spinodal decomposition process. In contrast, gelation in the short range attraction and long range repulsion liquid is not due to a phase separation. We also analyze the similarities and differences of the dynamic behavior of both systems through the analysis of the mean square displacement, the self part of the intermediate scattering function, the diffusion coefficient and the α relaxation time. Finally, using one of the main results of the non-equilibrium self-consistent generalized Langevin equation theory (NE-SCGLE), we determine the dynamic arrest phase diagram in the volume fraction and temperature (φ vs T) plane.

Density fluctuations in liquid potassium

The microscopic theory that describes space and time dependent correlated motion of the liquid in terms of inter atomic potential and correlation function has been used to explain experimentally accurate coherent dynamical structure factor of liquid potassium near its melting point for various linear momentum transfers as measured by inelastic X- ray scattering with high resolution. In particular the computed spectra for low momentum transfers lying between 1.75 nm−1 and 10.0 nm−1, evaluated with the derived analytical form of potential, which is a superposition of a repulsive and long range oscillatory part, are in good agreement with the corresponding experimental results. Interplay of different characteristic times in the collective dynamics has been brought out.

Strong Correlation Effects and Localization in Metallic Systems

We have studied strong correlation effects and localization in metallic systems. Strong correlation effects and localization occurred in metallic systems due to strong electron-electron interactions and strong electron phonon coupling. Strong electron-phonon interactions have been found in such materials as cuprates, fullerides and manganites. The interplay of electronelectron and electron-phonon interactions in these correlated systems leaded to coexistence of or competition between various phases such as super conductivity, charge-density-wave or spin-density wave phases or formation of novel non Fermi liquid phases, polarons, bipolarons and so on. We have considered Holstein-Hubbard model for our work. The Holstein-Hubbard model provides on over simplified description of both electron-electron and electron-phonon interactions, it retains to be the relevant ingredients of a system in which electrons experience simultaneously an instantaneous short range repulsion and a phononmediated retarded attraction. In spite of its formal simplicity, it is not exactly solvable even in one dimension. So quantum simulators of Fermi-Hubbard modes based on either fermionic atoms inoptical lattice or electrons in artificial quantum dot crystals have been used. In our work a classical analog simulator was theoretically proposed for the two site Holstein-Hubbard model based on light transport in engineered waveguide lattices, which is capable of reproducing the temporal dynamics of the quantum model in Fock space as spatial evolution in photonic lattice. We have found that in the strong correlation regime the periodic temporal dynamics exhibited by the Holstein-Hubbard Hamiltonian, related to the excitation Holstein polarons has been explained in terms of generalized Bloch oscillations of a single particle in a semi-infinite inhomogeneous tight binding lattice. Light transport in two dimensional photonic lattices simulated in Fock space the hopping dynamics of two correlated electrons on a one dimensional lattice and exploited to visualize such phenomena as correlated tunneling of bond electron-electron molecules and their coherent motion under the action of d.c. or a.c. fields. The obtained results were found in good agreement with previous results.

Effect of salinity on the characteristics, pH-triggered demulsification and rheology of crude oil/water nanoemulsions

The interest in utilizing nanoemulsions in a number of industrial applications is growing rapidly because of their superiority over conventional emulsions. Therefore, the formulation of highly stable crude oil-in-water nanoemulsions with different salinity (i.e., NaCl) levels is preseented in this study. Despite the observed extreme emulsion stability, zeta potential of the nanoemulsions decreased with increasing the salt concentration as a result of the charge screening effect induced by NaCl addition. Charge screening effect and, accordingly, the weakening of the electrostatic repulsion range and strength resulted in an increase in the average droplet size and also in a wider size distribution as the salinity level in the nanoemulsions increased. Additionally, the nanoemulsion viscosity also increased with increasing the salt concentration, however, the increase was marginal (except at 20 g/L NaCl). One interesting and unprecedented observation reported herein is the change in the nanoemulsion flow behavior from shear-thinning to Newtonian and then to shear-thickening as the applied shear rate increased. Nonetheless, at a salt concentration of 20 g/L NaCl, the shear-thickening behavior disappeared. Another important finding is that the presence of NaCl made the on-purpose destabilization of these stable nanoemulsions through the addition of NaOH or HCl (i.e., pH-alteration induced demulsification) more effective.

Interplay between adsorption, aggregation and diffusion in confined core-softened colloids

The behavior of colloidal particles with a hard core and a soft shell has attracted the attention for researchers in the physical-chemistry interface not only due to the large number of applications, but also owing to the unique properties of these systems, both in bulk and at interfaces. The adsorption at the two-phase boundary can provide information about the molecular arrangement. Thus, by Langevin Dynamics simulations, we have employed a recently obtained core-softened potential to analyze the relation between adsorption, structure and dynamic properties of the polymer-grafted nanoparticles near a solid repulsive surface. Two cases were considered: flat or structured walls. At low temperatures, a maximum is observed in the adsorption. It is related to a fluid to clusters transition and with a minimum in the contact layer diffusion - which is explained by the competition between the scales in the core-softened interaction. Due to the long range repulsion, the particles stay at the distance correspondent to this length scale at low densities, and overcome the repulsive barrier as the packing increases. However, by increasing the temperature, the gain in kinetic energy allows the colloids to overcome the long range repulsion barrier even at low densities. As a consequence, there is no competition and no maximum was observed in the adsorption.

A novel social distance model reveals the sidewall effect at bottlenecks

Intermittent and periodic outbreaks of infectious diseases have had profound and lasting effects on societies throughout human history. During the global spread of SARS-CoV-2 and the resulting coronavirus disease (COVID-19), social distance has been imposed worldwide to limit the spread of the virus. An additional deliberate intention of keeping a minimum safety distance from neighbors can fundamentally alter the “social force” between individuals. Here, we introduce a new “social distance” term inspired by gas molecular dynamics and integrate it into an existing agent-based social force model to describe the dynamics of crowds under social-distanced conditions. The advantage of this “social distance” term over the simple increasing of the repulsive range of other alternatives is that the fundamental crowd properties are precisely described by our model parameters. We compare the new model with the Helbing and Molnar’s classical model and experimental data, and show that this new model is superior in reproducing experimental data. We demonstrate the usability of this model with a bottleneck motion base case. The new model shows that the bottleneck effect can be significantly alleviated through small wall modifications. Lastly, we explain the mechanism of this improvement and conclude that this improvement is due to spatial asymmetry.

Few picosecond dynamics of intraband transitions in THz HgTe nanocrystals

Abstract Optoelectronic devices based on intraband or intersublevel transitions in semiconductors are important building blocks of the current THz technology. Large nanocrystals (NCs) of Mercury telluride (HgTe) are promising semiconductor candidates owing to their intraband absorption peak tunable from 60 THz to 4 THz. However, the physical nature of this THz absorption remains elusive as, in this spectral range, quantum confinement and Coulomb repulsion effects can coexist. Further, the carrier dynamics at low energy in HgTe NCs, which strongly impact the performances of THz optoelectronic devices, is still unexplored. Here, we demonstrate a broad THz absorption resonance centered at ≈4.5 THz and fully interpret its characteristics with a quantum model describing multiple intraband transitions of single carriers between quantized states. Our analysis reveals the absence of collective excitations in the THz optical response of these self-doped large NCs. Furthermore, using optical pump-THz probe experiments, we report on carrier dynamics at low energy as long as 6 ps in these self-doped THz HgTe NCs. We highlight evidence that Auger recombination is irrelevant in this system and attribute the main carrier recombination process to direct energy transfer from the electronic transition to the ligand vibrational modes and to nonradiative recombination assisted by surface traps. Our study opens interesting perspectives for the use of large HgTe NCs for the development of advanced THz optoelectronic devices such as emitters and detectors and for quantum engineering at THz frequencies.

Clusters of vortices induced by thermal gradient in mesoscopic superconductors

The structural and magnetic properties of the vortex matter in mesoscopic superconductors have been of great interest in the last decades. Recently, it has been found that vortex-vortex interaction, which is generally repulsive, can be non-monotonic under some special circumstances. Here, we have studied this issue by applying the three dimensional time dependent Ginzburg-Landau equations to a thin type-I mesoscopic superconducting film under a thermal gradient. We find that this favors the formation of small clusters of vortices in some special domains of the superconductor. We also find that this is intimately related to the non-monotonic vortex-vortex interaction by investigating the behavior of two constrained vortices in the presence of a thermal gradient. We show that the vortex-vortex interaction alternates between short range attraction and long range repulsion. This study shows that the thermal gradient has the potential to be an important tool to manipulate the vortex matter.

Behavioral change of Bemisia tabaci and Trialeurodes vaporariorum (Hemiptera: Aleyrodidae) infected by Lecanicillium muscarium (Hypocreales: Cordycipitaceae)

The effects of entomopathogenic fungus Lecanicillium muscarium on feeding and host preference were investigated on Bemisia tabaci (Gennadius) and Trialeurodes vaporariorum (Westwood) (Hemiptera: Aleyroidae) using 2aF43 and Mycotal® strains. Feeding activity was monitored up to day five post fungal inoculation using electric penetration graph while host preference was evaluated by choice-test bioassay for 72 h. Our results showed a reduction in the ability of the infected insects to ingest xylem and reach the phloem. Interestingly the two tested fungal strains interfered on different feeding activities. Only 2aF43 reduced cell puncturing in both insects. Pre-inoculation of plants by 2aF43 increased feeding difficulties and searching time of B. tabaci by 74.9% and 40.1%, respectively. Host choice bioassay showed that insects started aborting the fungal-treated leaf disks after 24 h. By the lapse of 72 h, 2aF43 had a repulsion range of 93.3–95.3% which was higher than that of Mycotal® that ranged between 80.0% and 80.7% in both whiteflies. Using choice-test bioassay and electric penetration graph recording, we revealed that L. muscarium effectively repelled whiteflies and deterred them from normal feeding that potentially reduce plant damage.

Interaction between nearly hard colloidal spheres at an oil-water interface

We show that the interaction potential between sterically stabilized, nearly hard-sphere [poly(methylmethacrylate)-poly(lauryl methacrylate) (PMMA-PLMA)] colloids at a water-oil interface has a negligible unscreened-dipole contribution, suggesting that models previously developed for charged particles at liquid interfaces are not necessarily applicable to sterically stabilized particles. Interparticle potentials, $U(r)$, are extracted from radial distribution functions [$g(r)$, measured by fluorescence microscopy] via Ornstein-Zernike inversion and via a reverse Monte Carlo scheme. The results are then validated by particle tracking in a blinking optical trap. Using a Bayesian model comparison, we find that our PMMA-PLMA data is better described by a screened monopole only rather than a functional form having a screened monopole plus an unscreened dipole term. We postulate that the long range repulsion we observe arises mainly through interactions between neutral holes on a charged interface, i.e., the charge of the liquid interface cannot, in general, be ignored. In agreement with this interpretation, we find that the interaction can be tuned by varying salt concentration in the aqueous phase. Inspired by recent theoretical work on point charges at dielectric interfaces, which we explain is relevant here, we show that a screened $\frac{1}{r^2}$ term can also be used to fit our data. Finally, we present measurements for poly(methyl methacrylate)-poly(12-hydroxystearic acid) (PMMA-PHSA) particles at a water-oil interface. These suggest that, for PMMA-PHSA particles, there is an additional contribution to the interaction potential. This is in line with our optical-tweezer measurements for PMMA-PHSA colloids in bulk oil, which indicate that they are slightly charged.

Multiparticle collision dynamics for fluid interfaces with near-contact interactions.

We present an extension of the multiparticle collision dynamics method for flows with complex interfaces, including supramolecular near-contact interactions mimicking the effect of surfactants. The new method is demonstrated for the case of (i) short range repulsion of droplets in close contact, (ii) arrested phase separation, and (iii) different pattern formation during spinodal decomposition of binary mixtures.

Wake Induced Long Range Repulsion of Aqueous Dunes.

Sand dunes rarely occur in isolation, but usually form vast dune fields. The large scale dynamics of these fields is hitherto poorly understood, not least due to the lack of longtime observations. Theoretical models usually abstract dunes in a field as self-propelled autonomous agents, exchanging mass, either remotely or as a consequence of collisions. In contrast to the spirit of these models, here we present experimental evidence that aqueous dunes interact over large distances without the necessity of exchanging mass. Interactions are mediated by turbulent structures forming in the wake of a dune, and lead to dune-dune repulsion, which can prevent collisions. We conjecture that a similar mechanism may be present in wind driven dunes, potentially explaining the observed robust stability of dune fields in different environments.

Monte Carlo simulations and perturbation theory for highly correlated fluids: The Lennard-Jones core softened potential case

The growing interest in anomalous thermodynamic properties of isotropic fluids has been followed by several attempts to implement an accurate and simple theoretical description of these phenomena. Here, we have performed an extensive Monte Carlo exploration of the thermodynamic properties and vapor-liquid equilibrium of the Lennard-Jones core softened potential with variable Gaussian tail strengths λ spanning fluids with short range attraction and long range repulsion to those with short range repulsion and long range attraction. The results have been compared with the theoretical framework provided by the Barker and Henderson perturbation theory evaluated at longer ranges than those usually employed in the literature. The correlation in close shells particles is also incorporated in an empirical way. The agreement between the theoretical approach and the simulation results are in quantitative agreement in most cases, but worsens when the Gaussian tail is deep enough to provoke substantial modifications in the fluid structure. These structured radial distribution functions differ from those of typical atomic fluids in the liquid state and are mainly responsible of the inaccuracy of the perturbative approach for quantitative purposes. Despite this drawback, we have successfully applied the theoretical treatment for predicting anomalies in fluid models that are in agreement with those reported in the literature. These results open the door for a wide use of perturbative treatments for predicting fluid anomalies if care is taken of the range of the perturbative approach and of the particle correlation within neighboring shells.

Probing relaxations of atomic-scale junctions in the Pauli repulsion range

Clean metal as well as C60-terminated tips of an atomic force microscope probe the interaction with C60 molecules adsorbed on Cu(111) and Pb(111). The force measurements unveil a monotonic shift of the point of maximum attraction with the bias voltage. The conventional superposition of long-range van der Waals and electrostatic forces with short-range Pauli repulsion does not reproduce the shift. By phenomenologically including bias-dependent relaxations of the electrode geometry in the analytical expression for the short-range force the experimental data can qualitatively be described.