Magneto-Mechanical Enhancement of Elastic Moduli in Magnetoactive Elastomers with Anisotropic Microstructures

Magnetoactive elastomers (MAEs) have gained significant attention in recent years due to their wide range of engineering applications. This paper investigates the important interplay between the particle microstructure and the sample shape of MAEs. A simple analytical expression is derived based on geometrical arguments to describe the particle distribution inside MAEs. In particular, smeared microstructures are considered instead of a discrete particle distribution. As a consequence of considering structured particle arrangements, the elastic free energy is anisotropic. It is formulated with the help of the rule of mixtures. We show that the enhancement of elastic moduli arises not only from the induced dipole–dipole interactions in the presence of an external magnetic field but also considerably from the change in the particle microstructure.

Direction dependent photon superbunching effect from an atomic array

The probability of correlated emission of fluorescent photons as a function of detection directions has been investigated. The model system comprises identical two-level atoms arranged in the form of a line. A weak laser field resonantly excites only one of the atoms in the line. Two interaction mechanisms, namely, the vacuum-induced dipole–dipole interaction and the collective spontaneous emission couple the system of atoms. The aim is to observe the emission of a set of photon twins synchronized in time. It is seen that strongly directional emission of pairs of photons can take place due to the interference between the emitters. These highly correlated pairs of photons can be observed in very precise geometric directions. The observation is made based on two different detection procedures. It is found that the superradiant photons always tend to be bunched along the same direction.

Spectral lineshapes of collision-induced absorption (CIA) using isotropic intermolecular potential for N2-CH4

Quantum mechanical lineshapes of collision-induced absorption (CIA) at different temperatures are computed for gaseous mixtures of molecular nitrogen and methane using theoretical values for the induced dipole moments and intermolecular potential as input. Comparison with theoretical absorption spectra shows satisfactory agreement. An empirical model of the dipole moment which reproduces the experimental spectra and the first three spectral moments more closely than the fundamental theory, is also presented. Good agreement between computed and experimental absorption lineshapes is obtained when a potential model which is constructed from the thermophysical and transport properties is used.

Bridging the 12-6-4 Model and the Fluctuating Charge Model

Metal ions play important roles in various biological systems. Molecular dynamics (MD) using classical force field has become a popular research tool to study biological systems at the atomic level. However, meaningful MD simulations require reliable models and parameters. Previously we showed that the 12-6 Lennard-Jones nonbonded model for ions could not reproduce the experimental hydration free energy (HFE) and ion-oxygen distance (IOD) values simultaneously when ion has a charge of +2 or higher. We discussed that this deficiency arises from the overlook of the ion-induced dipole interaction in the 12-6 model, and this term is proportional to 1/r4 based on theory. Hence, we developed the 12-6-4 model and showed it could solve this deficiency in a physically meaningful way. However, our previous research also found that the 12-6-4 model overestimated the coordination numbers (CNs) for some highly charged metal ions. And we attributed this artifact to that the current 12-6-4 scheme lacks a correction for the interactions among the first solvation shell water molecules. In the present study, we considered the ion-included dipole interaction by using the 12-6 model with adjusting the atomic charges of the first solvation shell water molecules. This strategy not only considers the ion-induced dipole interaction between ion and the first solvation shell water molecules but also well accounts for the increased repulsion among these water molecules compared to the bulk water molecules. We showed this strategy could well reproduce the experimental HFE and IOD values for Mg2+, Zn2+, Al3+, Fe3+, and In3+ and solve the CN overestimation issue of the 12-6-4 model for Fe3+ and In3+. Moreover, our simulation results showed good agreement with previous ab initio MD simulations. In addition, we derived the physical relationship between the C4 parameter and induced dipole moment, which agreed well with our simulation results. Finally, we discussed the implications of the present work for simulating metalloproteins. Due to the fluctuating charge model uses a similar concept to the 12-6 model with adjusting atomic charges, we believe the present study builds a bridge between the 12-6-4 model and the fluctuating charge model.

The effect of Europa's perturbed electromagnetic fields and induced dipole on energetic proton depletions in the Alfvén wings

<p>We investigate energetic proton depletions during Europa flybys E17 and E25A* by the Galileo mission. Energetic ion observations along trajectories like those of E17 & E25A are suitable for isolating the characteristics of the global configuration of the interaction region of Europa (or any Galilean moon) with the Jovian magnetosphere. Both of these flybys passed through Europa’s Alfvén wings further away from the moon, where ionospheric effects are small.</p> <p>We simulate the measured flux with a Monte Carlo particle tracing code and investigate the effect of the following factors: inhomogeneous electromagnetic fields, Europa's induced dipole, atmospheric charge exchange and plumes.</p> <p>We find that the homogeneous fields do not explain the Galileo data. We propose that the perturbed fields associated with the Alfvén wings affect the proton depletions. The inhomogeneous fields and induced dipole alter the pitch angle distribution of the depletion along the trajectory. The plumes that are investigated in this study have a minor effect on the proton depletions compared to the inhomogeneous fields and Alfvén wings. The contribution of atmospheric charge exchange to the depletion is negligible for these flybys. Finally, we compare the simulations to the measured proton flux and discuss the contribution of the effects we have considered.</p> <p>* E25A is a segment of the Io flyby I25</p>

Assessment of Molecular Mechanics-based Zn2+ Models in Mono- and Bimetallic Ligand Binding Sites

Zn2+ ions play an important role in biology, but accurate sampling of metalloproteins using Molecular Mechanics remains challenging. Several models have been proposed to describe Zn2+ in biomolecular simulations, ranging from nonbonded models, employing classical 12-6 Lennard-Jones (LJ) potentials or extended LJ-potentials, to dummy-atom models and bonded models. We evaluated the performance of a large variety of these Zn2+ models in two challenging environments for which little is known about the performance of these methods, namely in a monometallic (Carbonic Anhydrase II) and a bimetallic ligand binding site (metallo-β-lactamase VIM-2). We focused on properties which are important for a stable, correct binding site description during molecular dynamics (MD) simulations, because a proper treatment of the metal coordination and forces are here essential. We observed that the strongest difference in performance of these Zn2+ models can be found in the description of interactions between Zn2+ and non-charged ligating atoms, such as the imidazole nitrogen in histidine residues. We further show that the nonbonded (12-6 LJ) models struggle most in the description of Zn2+-biomolecule interactions, while the inclusion of ion-induced dipole effects strongly improves the description between Zn2+ and non-charged ligating atoms. The octahedral dummy-atom models result in highly stable simulations and correct Zn2+ coordination, and are therefore highly suitable for binding sites containing an octahedral coordinated Zn2+ ion. The results from this evaluation study in ligand binding sites can guide structural studies of Zn2+ containing proteins, such as MD-refinement of docked ligand poses and long-term MD simulations.