56,983 research outputs found
Dynamics of a Monolayer of Microspheres on an Elastic Substrate
We present a model for wave propagation in a monolayer of spheres on an
elastic substrate. The model, which considers sagittally polarized waves,
includes: horizontal, vertical, and rotational degrees of freedom; normal and
shear coupling between the spheres and substrate, as well as between adjacent
spheres; and the effects of wave propagation in the elastic substrate. For a
monolayer of interacting spheres, we find three contact resonances, whose
frequencies are given by simple closed-form expressions. For a monolayer of
isolated spheres, only two resonances are present. The contact resonances
couple to surface acoustic waves in the substrate, leading to mode
hybridization and "avoided crossing" phenomena. We present dispersion curves
for a monolayer of silica microspheres on a silica substrate, assuming
adhesive, Hertzian interactions, and compare calculations using an effective
medium approximation to a discrete model of a monolayer on a rigid substrate.
While the effective medium model does not account for discrete lattice effects
at short wavelengths, we find that it is well suited for describing the
interaction between the monolayer and substrate in the long wavelength limit.
We suggest that a complete picture of the dynamics of a discrete monolayer
adhered to an elastic substrate can be found using a combination of the results
presented for the discrete and effective medium descriptions. This model is
potentially scalable for use with both micro- and macroscale systems, and
offers the prospect of experimentally extracting contact stiffnesses from
measurements of acoustic dispersion
Coupling of Length Scales and Atomistic Simulation of MEMS Resonators
We present simulations of the dynamic and temperature dependent behavior of
Micro-Electro-Mechanical Systems (MEMS) by utilizing recently developed
parallel codes which enable a coupling of length scales. The novel techniques
used in this simulation accurately model the behavior of the mechanical
components of MEMS down to the atomic scale. We study the vibrational behavior
of one class of MEMS devices: micron-scale resonators made of silicon and
quartz. The algorithmic and computational avenue applied here represents a
significant departure from the usual finite element approach based on continuum
elastic theory. The approach is to use an atomistic simulation in regions of
significantly anharmonic forces and large surface area to volume ratios or
where internal friction due to defects is anticipated. Peripheral regions of
MEMS which are well-described by continuum elastic theory are simulated using
finite elements for efficiency. Thus, in central regions of the device, the
motion of millions of individual atoms is simulated, while the relatively large
peripheral regions are modeled with finite elements. The two techniques run
concurrently and mesh seamlessly, passing information back and forth. This
coupling of length scales gives a natural domain decomposition, so that the
code runs on multiprocessor workstations and supercomputers. We present novel
simulations of the vibrational behavior of micron-scale silicon and quartz
oscillators. Our results are contrasted with the predictions of continuum
elastic theory as a function of size, and the failure of the continuum
techniques is clear in the limit of small sizes. We also extract the Q value
for the resonators and study the corresponding dissipative processes.Comment: 10 pages, 10 figures, to be published in the proceedings of DTM '99;
LaTeX with spie.sty, bibtex with spiebib.bst and psfi
A Review on Mechanics and Mechanical Properties of 2D Materials - Graphene and Beyond
Since the first successful synthesis of graphene just over a decade ago, a
variety of two-dimensional (2D) materials (e.g., transition
metal-dichalcogenides, hexagonal boron-nitride, etc.) have been discovered.
Among the many unique and attractive properties of 2D materials, mechanical
properties play important roles in manufacturing, integration and performance
for their potential applications. Mechanics is indispensable in the study of
mechanical properties, both experimentally and theoretically. The coupling
between the mechanical and other physical properties (thermal, electronic,
optical) is also of great interest in exploring novel applications, where
mechanics has to be combined with condensed matter physics to establish a
scalable theoretical framework. Moreover, mechanical interactions between 2D
materials and various substrate materials are essential for integrated device
applications of 2D materials, for which the mechanics of interfaces (adhesion
and friction) has to be developed for the 2D materials. Here we review recent
theoretical and experimental works related to mechanics and mechanical
properties of 2D materials. While graphene is the most studied 2D material to
date, we expect continual growth of interest in the mechanics of other 2D
materials beyond graphene
Dangling and hydrolyzed ligand arms in [Mn3] and [Mn6] coordination assemblies: synthesis, characterization, and functional activity
Two flexible, branched, and sterically constrained di- and tripodal side arms around a phenol backbone were utilized in ligands H3L1 and H5L2 to isolate {Mn6} and {Mn3} coordination aggregates. 2,6-Bis{(1-hydroxy-2-methylpropan-2-ylimino)methyl}-4-methylphenol (H3L1) gave trinuclear complex [Mn3(μ-H2L1)2(μ1,3-O2CCH3)4(CH3OH)2](ClO4)2·4CH3OH (1), whereas 2,6-bis[{1-hydroxy-2-(hydroxymethyl)butan-2-ylimino}methyl]-4-methylphenol (H5L2) provided hexanuclear complex [Mn6(μ4-H2L2)2(μ-HL3)2(μ3-OH)2(μ1,3-O2CC2H5)4](ClO4)2·2H2O (2). Binding of acetates and coordination of {H2L1}− provided a linear MnIIIMnIIMnIII arrangement in 1. A MnIII6 fused diadamantane-type assembly was obtained in 2 from propionate bridges, coordination of {H2L2}3–, and in situ generated {HL3}2–. The magnetic characterization of 1 and 2 revealed the properties dominated by intramolecular anti-ferromagnetic exchange interactions, and this was confirmed using density functional theory calculations. Complex 1 exhibited field-induced slow magnetic relaxation at 2 K due to the axial anisotropy of MnIII centers. Both the complexes show effective solvent-dependent catechol oxidation toward 3,5-di-tert-butylcatechol in air. The catechol oxidation abilities are comparable from two complexes of different nuclearity and structure
Self-organization and Mechanical Properties of Active Filament Bundles
A phenomenological description for active bundles of polar filaments is
presented. The activity of the bundle results from crosslinks, that induce
relative displacements between the aligned filaments. Our generic description
is based on momentum conservation within the bundle. By specifying the internal
forces, a simple minimal model for the bundle dynamics is obtained, capturing
generic dynamic behaviors. In particular, contracted states as well as solitary
and oscillatory waves appear through dynamic instabilities. The introduction of
filament adhesion leads to self-organized persistent filament transport.
Furthermore, calculating the tension, homogeneous bundles are shown to be able
to actively contract and to perform work against external forces. Our
description is motivated by dynamic phenomena in the cytoskeleton and could
apply to stress-fibers and self-organization phenomena during cell-locomotion.Comment: 19 pages, 10 figure
CMOS design of chaotic oscillators using state variables: a monolithic Chua's circuit
This paper presents design considerations for monolithic implementation of piecewise-linear (PWL) dynamic systems in CMOS technology. Starting from a review of available CMOS circuit primitives and their respective merits and drawbacks, the paper proposes a synthesis approach for PWL dynamic systems, based on state-variable methods, and identifies the associated analog operators. The GmC approach, combining quasi-linear VCCS's, PWL VCCS's, and capacitors is then explored regarding the implementation of these operators. CMOS basic building blocks for the realization of the quasi-linear VCCS's and PWL VCCS's are presented and applied to design a Chua's circuit IC. The influence of GmC parasitics on the performance of dynamic PWL systems is illustrated through this example. Measured chaotic attractors from a Chua's circuit prototype are given. The prototype has been fabricated in a 2.4- mu m double-poly n-well CMOS technology, and occupies 0.35 mm/sup 2/, with a power consumption of 1.6 mW for a +or-2.5-V symmetric supply. Measurements show bifurcation toward a double-scroll Chua's attractor by changing a bias current
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