Weak Force Field Research Articles

Reactive molecular dynamics simulations of plastics pyrolysis with additives: Comparison of ReaxFF branches and experimental results

In this work, the efficiency of the combustion and aqueous branches of the reactive potential ReaxFF are compared through pyrolysis investigations from reactive molecular simulations of three polymers: polyethylene (PE), polystyrene (PS) and polypropylene (PP) with additive compounds. Thermogravimetric analyses were implemented, computing the weight loss percentage along with analyzes of the pyrolysis products. Three force fields were considered: the CHON-2019 and the CHOSFClN potentials coming from the combustion branch and the CHONCl-2022_weak potential coming from the aqueous branch. The results show that both CHON-2019 and CHONCl-2022_weak force fields give similar results in adequacy with the experimental results and are better than the CHOSFClN force field from the combustion branch. They can correctly predict the kinetic order of polymer degradation and identify the species formed and their quantities. As a result, the latest versions of the ReaxFF potential, belonging to the aqueous branch, are well suited to study the pyrolysis processes and may give equal or better results than force fields from the combustion branch. Furthermore, force fields from the aqueous branch seem to be more reliable for the investigation of the effect of the additives’ role on plastic pyrolysis. Finally, an important aspect presented in this article is the use of statistical simulations for the different investigations. An uncertainty estimation of 30 K was computed from ten thermogravimetric analyses simulated with ReaxFF on the PP pyrolysis.

Development of a novel ReaxFF reactive potential for organochloride molecules.

This article presents a new reactive potential in the ReaxFF formalism. It aims to include the chlorine element and opens up the fields of use of ReaxFF to the whole class of organochloride compounds including conjugated or aromatic groups. Numerous compounds in this family raise global awareness due to their environmental impact, and such a reactive potential will help investigate their degradation pathways. The new force field, named CHONCl-2022_weak, belongs to the aqueous branch. The force field parameters were fitted against high-level quantum chemistry calculations, including complete active space self-consistent field/NEVPT2 calculations and density functional theory calculations, and their accuracy was evaluated using a validation set. The root means square deviation against quantum mechanics energies is 0.38eV (8.91kcal mol-1). From a structural point of view, the root means square deviation is about 0.06 Å for the bond lengths, 11.86° for the angles, and 4.12° for the dihedral angles. With CHONCl-2022_weak new force field, we successfully investigated the regioselectivity for nucleophilic or electrophilic attacks on polychlorinated biphenyls, which are toxic and permanent pollutants. The rotation barriers along the bond linking the two benzene rings, which is crucial in the toxicity of these compounds, are well reproduced by CHONCl-2022_weak. Then, our new reactive potential is used to investigate the chlorobenzene reactivity in the presence of hydroxyl radicals in atmospheric condition or in aqueous solution. The reaction pathways computed with ReaxFF agree with the quantum mechanics results. We showed that, in the presence of dioxygen molecules, in atmospheric condition, the oxidation of chlorobenzene likely leads to the formation of highly oxygenated compounds after the abstraction of hydrogen radicals. In water, the addition of a hydroxyl radical leads to the formation of chlorophenol or phenol molecules, as already predicted from plasma-induced degradation experiments.

Thermophysical properties of water using reactive force fields

The widescale importance and rich phenomenology of water continue to motivate the development of computational models. ReaxFF force fields incorporate many characteristics desirable for modeling aqueous systems: molecular flexibility, polarization, and chemical reactivity (bond formation and breaking). However, their ability to model the general properties of water has not been evaluated in detail. We present comprehensive benchmarks of the thermophysical properties of water for two ReaxFF models, the water-2017 and CHON-2017_weak force fields. These include structural, electrostatic, vibrational, thermodynamic, coexistence, and transport properties at ambient conditions (300K and 0.997g cm-3) and along the standard pressure (1bar) isobar. Overall, CHON-2017_weak predicts more accurate thermophysical properties than the water-2017 force field. Based on our results, we recommend potential avenues for improvement: the dipole moment to quadrupole moment ratio, the self-diffusion coefficient, especially for water-2017, and the gas phase vibrational frequencies with the aim to improve the vibrational properties of liquid water.

Isotropically active colloids under uniform force fields: from forced to spontaneous motion

Abstract

Phase-coherent sensing of the center-of-mass motion of trapped-ion crystals.

Trapped ions are sensitive detectors of weak forces and electric fields that excite ion motion. Here measurements of the center-of-mass motion of a trapped-ion crystal that are phase coherent with an applied weak external force are reported. These experiments are conducted far from the trap motional frequency on a two-dimensional trapped-ion crystal of approximately 100 ions, and determine the fundamental measurement imprecision of our protocol free from noise associated with the center-of-mass mode. The driven sinusoidal displacement of the crystal is detected by coupling the ion crystal motion to the internal spin degree of freedom of the ions using an oscillating spin-dependent optical dipole force. The resulting induced spin precession is proportional to the displacement amplitude of the crystal, and is measured with near-projection-noise-limited resolution. A 49 pm displacement is detected with a signal-to-noise ratio of 1 in a single experimental determination, which is an order-of-magnitude improvement over prior phase-incoherent experiments. This displacement amplitude is 40 times smaller than the zero-point fluctuations. With our repetition rate, an displacement sensitivity is achieved, which implies and sensitivities to forces and electric fields, respectively. This displacement sensitivity, when applied on-resonance with the center-of-mass mode, indicates the possibility of weak force and electric field detection below 10-3 yN/ion and 1 nV/m, respectively.

Improvement of the ReaxFF Description for Functionalized Hydrocarbon/Water Weak Interactions in the Condensed Phase.

The ReaxFF protein reactive force field (protein-2013) has been successfully employed to simulate the biomolecules and membrane fuel cells, but it inaccurately describes the weak interaction of functionalized hydrocarbon/water molecules in condensed phase, especially for the density. In this article, the development of a ReaxFF force field (CHON-2017_weak) on the basis of protein-2013 is presented that improves the weak interaction description for atom pairs of carbon, hydrogen, oxygen, and nitrogen. To examine the quality of the force field, we performed a series of molecular dynamics simulations with model systems. These simulations, describing density trends for pure and mixture compound systems, demonstrate that CHON-2017_weak force field predictions are in good agreement with experimental data. Furthermore, ReaxFF can also describe the phase separation in hexane-water mixture and dissolution of ethanol or tetramethylammonium (TMA) in liquid water. To validate it in the application of membrane fuel cells, we studied structural property and degradation mechanism of TMA in alkaline aqueous solution, as well as some typical chemical reactions for small compounds. On the basis of our results, an additional reaction pathway is proposed for the degradation of TMA, which seems to be more energetically favorable compared to the main mechanism predicted from quantum mechanics calculations.

Evaluation of Reflexology by "BIOCERAMIC Resonance" Operation producing Weak Force Field during Simultaneous Acupoint Stimulation of Urinary Bladder Point on Subject's Ear Resulting in Electric Current Change on Urinary Bladder reflex Point on Subject's Hands, and Related New Research Finding.

It was postulated in our previous publications that the meridian channels as conceived in Traditional Chinese Medicine (TCM) are various standing waves arising from harmonic rhythmic sound frequencies originating from the human heart beat. BIOCERAMIC is an artificial material able to produce a weak force field causing different biophysical and systemic health benefits, with the key characteristics of hydrogen bonds weakening and microcirculation enhancement. Since discovering that the effects of a BIOCERAMIC field can be transmitted via sound waves propagation, we then also developed a BIOCERAMIC Resonance device to produce weak force field throughout the body, and achieve resonance with the body's meridian channels to reinforce microcirculation. Since our previous research proved BIOCERAMIC can produces changes in ectodermal current levels, the present evaluation on reflexology is done by the application of Electric Current Detection (ECD) to the palmar surface of the hands matching correlative organs and glands loci to reflex points according to standard reflexology. The procedure will compare changes in the electrical current observed before and after a session of BIOCERAMIC Resonance treatment on the soles of the subjects' feet. We also conducted a procedure using corona discharge (Kirlian) photography of the hands to examine whether the coronal intensities could be affected by application of the BIOCERAMIC patch. Intensities are shown on the screen of a computer using special software that categorizes intensities into five zones. Under the continuous treatment of BIOCERAMIC Resonance on soles of the feet and simultaneous stimulation on the specific point on the surface of the ear representing the urinary bladder. The electrical current (Aji ampere) on the areas in the hands are decreased from the beginning of the experiment, but only the specific area on the surface of the ear representing the urinary bladder was exhibited increased of the electrical current (Aji ampere), with statistically significant difference (p<0.05). To the other study we evaluated the validity of reflexology and corona discharge (Kirlian) photography by applying BIOCERAMIC Resonance and small adhesive patches made from the BIOCERAMIC material. Significant differences were evident on four out of five different zones of the computerized images. Our findings suggest the existence of presupposed virtual channels or reflex points on the skin surface of the feet, hands, and ears that connect or somehow reflect back to specific internal organs, as mapped out on standard charts found in reflexology. Furthermore, the depicted corona intensities from five zones shown on a computer screen of corona discharge photography seem to indicate that the volunteer subjects are affected by the BIOCERAMIC patches. This study demonstrates the operation of the BIOCERAMIC Resonance device is able to produce weak force field through the body, which is objectively measurable and thereby scientifically integrating the concepts of reflexology, meridian channels and biofield therapy.

Integrating force and position: testing model predictions

In this study, we investigated the integration of force and position information in a task in which participants were asked to estimate the center of a weak force field. Two hypotheses, describing how participants solved this task, were tested: (1) by only using the position(s) where the force reaches the detection threshold, and (2) by extrapolating the force field based on perceived stiffness. Both hypotheses were also described formally, assuming a psychophysical function obeying a power law with an exponent smaller than one. The hypotheses were tested in two psychophysical experiments, in which 12 participants took part. In Experiment 1, an asymmetric force field was used and the presence of visual feedback about hand position was varied. In Experiment 2, a unilateral force field was used. For both experiments, hypothesis 1 predicts biases between (Experiment 1) or at (Experiment 2) the position(s) of the force detection threshold, while hypothesis 2 predicts smaller biases. The measured data show significant biases in both experiments that coincide with the biases predicted by using force detection thresholds from the literature. The average measured responses and their variabilities also fitted very well with the mathematical model of hypothesis 1. These results underline the validity of hypothesis 1. So, participants did not use a percept of the stiffness of the force field, but based their estimation of the center of the force field on the position(s) where the force reached the detection threshold. This shows that force and position information were not integrated in this task.

The modelling of microconvection in an infinite strip

A model of the microconvection of an isothermally incompressible fluid, which can be used to investigate convection in weak force fields and on microscopic scales and can be characterized by non-solenoidality of the velocity field, is considered. An invariant solution in an infinite vertical strip occupied by a fluid is studied in the case where the heat flux on the two opposite faces of the strip fluctuates in antiphase. The use of the model of microconvection to construct an invariant solution gives rise to several non-standard-value initial-boundary problems. Their solvability in classes of Holder functions is proved.

Quantum Brownian motion in ratchet potentials

We investigate the dynamics of quantum particles in a ratchet potential subject to an ac force field. We develop a perturbative approach for weak ratchet potentials and force fields. Within this approach, we obtain an analytic description of dc current rectification and current reversals. Transport characteristics for various limiting cases -- such as the classical limit, limit of high or low frequencies, and/or high temperatures -- are derived explicitly. To gain insight into the intricate dependence of the rectified current on the relevant parameters, we identify characteristic scales and obtain the response of the ratchet system in terms of scaling functions. We pay a special attention to inertial effects and show that they are often relevant, for example, at high temperatures. We find that the high temperature decay of the rectified current follows an algebraic law with a non-trivial exponent, $j\propto T^{-17/6}$.

Interaction sensing in dynamic force microscopy

A first-order perturbative theory of the motion of a harmonic oscillator interacting with a weak arbitrary force field is presented, as it pertains to dynamic force microscopy. In essence the theory corresponds to a Born approximation for the scattering of standing waves trapped in a perturbed parabolic potential. In particular, it is shown that the scattering amplitudes are related to corresponding moments, involving Chebyshev polynomials and associated metrics of the conservative interaction force, and of a generalized friction coefficient accounting for irreversible interactions. Implications for dynamic force microscopy are discussed.

Microconvection in weak force fields. a numerical comparison of two models

Microconvection in weak force fields. a numerical comparison of two models

Investigation of the mechanism of the activation of GaAs negative electron affinity photocathodes

This paper has discussed the mechanism of the formation of negative electron affinity (NEA) GaAs photocathodes which are treated with cesium and oxygen. The formation is mainly due to the drop of the electron affinity when the GaAs surface is covered with cesium atoms. This drop is not due to the formation of surface dipoles as usually supposed. The essence is that in cesium processing the surface potential barrier of GaAs is caused by the considerably weak nuclear force field of the surface cesium layer rather than by that of GaAs. The further decrease of the electron affinity by oxygen is due to the formation of O−2 ions below the cesium layer. These O−2 ions would let the excited photoelectrons in the conduction band pass easily into the weak nuclear force field of the cesium layer and get into the vacuum. A surface structure model and the energy diagram are suggested.

Characteristic stability parameter of the axisymmetric equilibrium surface of a capillary liquid

In [1] the question of stability of the equilibrium state of a capillary liquid in weak force fields was reduced to determination of conditions such that the smallest eigenvalue λ* of a certain boundary problem would be positive. In [2] it was shown that λ* is a monotonic function of the parameter χ, dependent on the form of the vessel. The basic properties of the function λ*(χ) were also described. In the present study, these properties are used to study the general problem of stability of an axisymmetric liquid surface. A method for calculation of the critical values of the parameter χ and construction of the maximum stability region is given. Special attention is given to the cases of complete weightlessness, and action of gravitational and centrifugal forces. Critical values of the parameter χ are presented for these cases either graphically or analytically, which, given the shape of the vessel, permits evaluation of the stability of any of the family of axisymmetric equilibrium surfaces. We note that in the case of action by gravitational forces χ values for certain equilibrium surfaces were obtained by Barnyak.

Orbits in the Solar Wind—a Mini-Research Problem

A remarkable motion occurs when a classical inverse-square-law orbit is perturbed by a weak uniform force field in the orbital plane. The bizarre nature of the motion, together with the fact that most physicists and physics teachers have never seen or been aware of it, make it an ideal topic for independent research at the undergraduate level. Several modes of attack on the problem are suggested. They proceed from experimentation on a computer simulation, through analytic approximations, to closed-form solutions. A quantum mechanical treatment is also suggested.

Hydrostatics in weak force fields. On annular figures of equilibrium of a rotating liquid and their stability

We consider the problem of annular equilibrium figures of a rotating weightless liquid, having surface tension, and their stability. This question has been studied by Charreaux (for a discussion of his results see [1]), who examined the evolution of the forms of annular equilibrium figures and showed that there exists a family of stable equilibrium shapes. However, these studies of Charreaux are incomplete, and the conclusion on the existence of stable forms is valid only for axisymmetric disturbances.

Hydrostatics in weak force fields. Equilibrium shapes of a rotating weightless liquid

We examine the problem of axisymmetric equilibrium forms of the surface of a rotating liquid which has surface tension in the absence of an external force field. We do not assume a priori closure of the equilibrium surface or that it intersects the axis of rotation [1, 2]. We study the properties of the two-parameter family of solutions of the equilibrium equation. Typical forms of the integral curves are constructed from the results of numerical computation on a computer. The methods of boundary layer theory are used to find the approximate expression for the form of the equilibrium surface for large angular rates of rotation.

Natural frequencies and forms of small oscillations of an ideal liquid in a spherical vessel in a weak force field

Natural frequencies and forms of small oscillations of an ideal liquid in a spherical vessel in a weak force field

Hydrodynamics in weak force fields onset of steady thermocapillary convection in a spherical fluid layer under zero-g conditions

In the study of cellular convection in an infinite plane fluid layer with a free surface, both the Archimedes and thermocapillary forces [1–3] have been cited as reasons for the onset of convection. This has also been confirmed experimentally [4], When mass forces are absent or negligibly small it is natural to pose the question of the onset of “pure” thermocapillary convection or convection caused only by the surface tension gradients (see [2–3]). In the present paper, this problem is examined for a spherical fluid layer under zero-g conditions.

Hydrodynamics in weak force fields. Small oscillations of an ideal liquid

Several problems concerned with small oscillations of an ideal liquid, taking account of the surface-tension forces, have been considered in [1–3] (as a rule, these are cases when the equilibrium liquid surface is spherical, plane, or differs only slightly from plane). Below we formulate the problem of the natural frequencies of small oscillations of a liquid for the general case of an equilibrium liquid surface in a weak potential mass force field. It is shown that the natural frequencies and the corresponding eigenfunctions of this problem may be found by the Ritz method. We note that analogous results in a somewhat different formulation have been obtained in the recently published [3].