Estimate of the Cutoff Errors in the Ewald Summation for Dipolar Systems

Theoretical estimates for the cutoff errors in the Ewald summation method for dipolar systems are derived. Absolute errors in the total energy, forces and torques, both for the real and reciprocal space parts, are considered. The applicability of the estimates is tested and confirmed in several numerical examples. We demonstrate that these estimates can be used easily in determining the optimal parameters of the dipolar Ewald summation in the sense that they minimize the computation time for a predefined, user set, accuracy.Comment: 22 pages, 6 figures, Revtex style, submitted to J. Chem. Phy

Entanglement dynamics at flat surfaces: investigations using multi-chain molecular dynamics and a single-chain slip-spring model

The dynamics of an entangled polymer melt confined in a channel by parallel plates is investigated by Molecular Dynamics (MD) simulations of a detailed, multi-chain model. A Primitive Path Analysis predicts that the density of entanglements remains approximately constant throughout the gap and drops to lower values only in the immediate vicinity of the surface. Based on these observations, we propose a coarse-grained, single-chain slip-spring model with a uniform density of slip-spring anchors and slip-links. The slip-spring model is compared to the Kremer-Grest MD bead-spring model via equilibrium correlation functions of chain orientations. Reasonably good agreement between the single-chain model and the detailed multi-chain model is obtained for chain relaxation dynamics, both away from the surface and for chains whose center of mass positions are at a distance from the surface that is less than the bulk chain radius of gyration, without introducing any additional model parameters. Our results suggest that there is no considerable drop in topological interactions for chains in the vicinity of a single flat surface. We infer from the slip-spring model that the experimental plateau modulus of a confined polymer melt may be different to a corresponding unconfined system even if there is no drop in topological interactions for the confined case

Media Distraction in College Students.

Recent development of media technology has greatly changed how students learn. Studying has become increasingly dependent on computer and the Internet, where students have easy access to a world of distractions. This dissertation consists of three studies that observed the amount of media usage during college students’ study activities (Study 1) and investigated the effect of media distraction on their memory (Study 2), reading and quantitative reasoning (Study 3). Results showed that college students from both China and the USA spent a sizable amount of their study time on media activities; lab experiments showed that media activities negatively affected students’ logical memory and reading comprehension, but did not affect performance on a quantitative reasoning task. In addition, the effect of media distraction on reading was negatively related to students’ daily social media usage, suggesting that heavy social media users might have developed adaptations to media distractions. Current college students have grown up with social media websites, and many of them are constantly connected to smart devices. By studying the impact of these technological experiences on their learning and cognition, the dissertation identifies problems of student learning in this digital era, which in turn has implications for educational practices. It also contributes to understanding of the interaction between technological development and changes in human cognition.PhDEducation and PsychologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/113320/1/zwwang_1.pd

Microscopic picture of constraint release effects in entangled star polymer melts

The constraint release (CR) effect in entangled branched polymers is generally described by the widely accepted Dynamic Tube Dilation (DTD) theory based on the tube model, which predicts the stress relaxation function reasonably well, but not the dielectric or arm end-to-end vector relaxation. The microscopic picture of entanglement dynamics even in the simple case of star polymers is still not fully resolved. In this work, we first perform molecular dynamics simulations of symmetric star polymer melts using the Kremer-Grest bead-spring model. The entanglement events are analysed microscopically using the persistent close-contacts between mean paths of neighboring polymer strands. The resulted survival probability function of these entanglements or close-contacts show reasonably good agreement with the stress relaxation function, which provides qualitative evidence for the binary picture of entanglements. Based on this understanding we further investigate the star arm retraction and CR effects using the coarse-grained single-chain slip-spring model originally developed by Likhtman and also a simplified single-chain stochastic model. Our simulations revealed that, for entanglements sitting on a target star arm, only those destroyed by the arm free end dominate the arm end-to-end vector relaxation, while the constraint release events produce an accelerated drift of the mean positions of these specific entanglements towards the arm-end, which is an essential mechanism for understanding the relaxation of star polymers in concentrated solutions or melts. Our findings call for an examination of the microscopic foundation of conventional DTD picture and inspire the development of quantitative theories with consideration of more microscopic details

The effect of pressure on high‐ and low‐working‐memory students: An elaboration of the choking under pressure hypothesis

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/106943/1/bjep12027.pd

Multiple scattering of flexural waves on Mindlin plates with circular scatterers

The multiple scattering of flexural waves on an elastic plate with circular scatterers is analyzed in the frequency domain based on the Mindlin plate theory accounting for the rotary inertia and shear deformation of the plate. To this purpose, a semi-analytical numerical method is formulated as an extension of the previous study based on the Kirchhoff plate theory. It consists of expressing the flexural wave field in terms of the superposition of the wave function expansion, and determining the expansion coefficients by a collocation technique. As demonstrative examples, the transmission of a plane flexural wave across a square array of circular through-thickness holes or thin-plate inclusions is analyzed using the proposed method. The comparison between the results based on the Mindlin and Kirchhoff theories is shown for the case of multiple holes. The analysis shows that the transmission amplitude of the flexural wave is reduced at certain frequencies due to the Bragg reflection by the inclusions. In the case of thin-plate inclusions, the resonance of the inclusions also brings about a sharp decrease of the transmission amplitude

Creep and recovery of magnetorheological fluids: experiments and simulations

A direct comparative study on the creep-recovery behavior of conventional MR fluids is carried out using magnetorheometry and particle-level simulations. Two particle concentrations are investigated (ϕ=0.05 and 0.30) at two different magnetic field strengths (53 kA•m-1 and 173 kA•m-1) in order to match the yield stresses developed in both systems for easier comparison. Simulations are mostly started with random initial structures with some additional tests of using preassembled single chains in the low concentration case. Experimental and simulation data are in good qualitative agreement. The results demonstrate three regions in the creep curves: i) In the initial viscoelastic region, the chain-like (at ϕ=0.05) or percolated three-dimensional network (at ϕ=0.30) structures fill up the gap and the average cluster size remains constant; ii) Above a critical strain of 10 %, in the retardation region, these structures begin to break and rearrange under shear. At large enough imposed stress values, they transform into thin sheet-like or thick lamellar structures, depending on the particle concentration; iii) Finally in the case of larger strain values either the viscosity diverges (at low stress values) or reaches a constant low value (at high stress values), showing a clear bifurcation behavior. For stresses below the bifurcation point the MR fluid is capable to recover the strain by a certain fraction. However, no recovery is observed for large stress values

Arm retraction dynamics of entangled star polymers: a forward-flux sampling method study

The study of dynamics and rheology of well-entangled branched polymers remains a challenge for computer simulations due to the exponentially growing terminal relaxation times of these polymers with increasing molecular weights. We present an efficient simulation algorithm for studying the arm retraction dynamics of entangled star polymers by combining the coarse-grained slip-spring (SS) model with the forward-flux sampling (FFS) method. This algorithm is first applied to simulate symmetric star polymers in the absence of constraint release (CR). The reaction coordinate for the FFS method is determined by finding good agreement of the simulation results on the terminal relaxation times of mildly entangled stars with those obtained from direct shooting SS model simulations with the relative difference between them less than $5\%$. The FFS simulations are then carried out for strongly entangled stars with arm lengths up to $16$ entanglements that are far beyond the accessibility of brute force simulations in the non-CR condition. Apart from the terminal relaxation times, the same method can also be applied to generate the relaxation spectra of all entanglements along the arms which are desired for the development of quantitative theories of entangled branched polymers. Furthermore, we propose a numerical route to construct the experimentally measurable relaxation correlation functions by effectively linking the data stored at each interface during the FFS runs. The obtained star arm end-to-end vector relaxation functions $\Phi(t)$ and the stress relaxation function $G(t)$ are found to be in reasonably good agreement with standard SS simulation results in the terminal regime. Finally, we demonstrate that this simulation method can be conveniently extended to study arm-retraction problem in entangled star polymer melts with CR by modifying the definition of the reaction coordinate

Distinct photon-ALP propagation modes

The detection of ultra high energy gamma-rays provides an opportunity to explore the existence of ALPs at the multi-hundred TeV and PeV energy scales. We discover that we can employ analytic methods to investigate the propagation of photon-ALP beams in scenarios where the energy of photons $\omega \geq 100$ TeV. Our analytical calculations uncover the presence of two distinct modes of photon propagation resulting from the interplay between ALP-photon mixing and attenuation effects. Next, we analyze observable quantities such as the degree of polarization and survival probability in these two modes. We determine the conditions under which a significant polarization effect can be observed and identify the corresponding survival probability. Finally, we extend our analytic methods to cover the energy range of $10^{-3}$ to $10^4$ GeV and analyze the influence of ALPs on the experimental signals.Comment: 19 pages, 8 figure