关键词： Inverse dynamics control   Underactuated systems   Differential flatness  

摘要： Underactuated systems are featured by fewer control inputs than the degrees-of-freedom, m < n. The determination of an input control strategy that forces such a system to complete a set of m specified motion tasks is a challenging task, and the explicit solution existence is conditioned to differential flatness of the problem. The flatness-based solution denotes that all the 2n states and m control inputs can be algebraically expressed in terms of the m specified outputs and their time derivatives up to a certain order, which is in practice attainable only for simple systems. In this contribution the problem is posed in a more practical way as a set of index-three differential-algebraic equations, and the solution is obtained numerically. The formulation is then illustrated by a two-degree-of-freedom underactuated system composed of two rotating discs connected by a torsional spring, in which the pre-specified motion of one of the discs is actuated by the torque applied to the other disc, n = 2 and m = 1. Experimental verification of the inverse simulation control methodology is reported. (C) 2010 Elsevier B.V. All rights reserved.

关键词： Dispersion   Simulation   Distinct element method   Aggregates   Drag force  

摘要： The dispersion of bulk powders is important for a number of applications including particle characterisation, and the delivery of therapeutic drugs via the lung using dry powder inhalers (DPIs). In recent years the distinct element method (DEM) coupled with continuum models for a fluid phase to simulate fluid-solids interactions has received much attention. In this paper these computational techniques are used to investigate the aerodynamic dispersion in a uniform fluid flow. As intuitively expected, it is seen that with increasing surface energy it is progressively more difficult to disperse a loose aggregate. However, once the relative particle-fluid velocity goes beyond a certain threshold, dispersion occurs quickly and approaches a completely dispersed state asymptotically. In addition, as loose aggregate diameter is increased the necessary threshold velocity reduces. A relationship between the fluid drag forces acting upon a spherical aggregate and the adhesive force, given by the JKR model, leads to a dimensionless group termed the dispersion index, which includes the Weber number. It is shown that the effect of surface energy and loose aggregate diameter on dispersion behaviour can be described by this dimensionless group. (C) 2010 Published by Elsevier B.V. on behalf of The Institution of Chemical Engineers.

关键词： Molecular dynamics simulation   mechanical properties   Zn4Sb3   nanofilms  

摘要： This paper reports molecular dynamics simulations performed to study the mechanical properties of Zn4Sb3 nanofilms. In the simulations, interatomic interactions are represented by an enhanced atomic potential, and the crystal structure is based on the core structure of beta-Zn4Sb3. For tensile loading along the [0 1 0] direction, the stability of the crystal structure of the Zn4Sb3 nanofilms is analyzed by the radial distribution function method, and the stress-strain relation of the nanofilms is obtained at room temperature. Our present work indicates that the mechanical properties of Zn4Sb3 nanofilms are quite different from those of bulk Zn4Sb3 due to the impact of surface atoms of the nanostructure. From the atomic configuration, Zn4Sb3 nanofilms exhibit typical brittleness. The size effect and the strain-rate effect on the extension of Zn4Sb3 nanofilms are discussed in detail. Lastly, the mechanical properties of nanofilms based on different Zn4Sb3 crystal structure models are examined.

关键词： myocardial infarction   ventricular remodeling   heart transplantation   inflammation  

摘要： Zhou X, Yun JL, Han ZQ, Gao F, Li H, Jiang TM, Li YM. Postinfarction healing dynamics in the mechanically unloaded rat left ventricle. Am J Physiol Heart Circ Physiol 300: H1863-H1874, 2011. First published March 11, 2011;doi:10.1152/ajpheart.00830.2010.-The healing process is a key determinant for postinfarction left ventricular (LV) remodeling and the development of heart failure, which could be influenced by mechanical (pressure and/or volume) load. So far, limited information exists regarding an indepth characterization of the postinfarct healing process in the mechanically unloaded state. In the present work, we performed isogenic Lewis-to-Lewis rat abdominal heterotopic heart transplantation, which is characterized by hemodynamic unloading in the left ventricle, and simultaneously ligated the left anterior descending coronary artery (T-infarct group). Pathological evolution was dynamically compared with that of in situ infarcted Lewis hearts (I-infarct group) on days 3, 7, 14, and 35. There was a remarkable myocardial salvage in the unloaded heart, as shown by the improvement in infarct size (T-infarct group: 25.47% +/- 4.31% vs. I-infarct group: 38.46% +/- 4.82%, P < 0.01) and the smaller fraction of fibrosis in infarct segments (T-infarct group: 42.12% +/- 8.40% vs. I-infarct group: 75.65% +/- 10.51%, P < 0.01). in addition, there was a progressive disorganization of the two-dimensional collagen :fiber alignment as well as retarded collagen fiber maturation in the T-infarct group. We also observed enhanced angiogenesis, lymphangiogenesis, and inflammatory cell retention in the infarct region during mechanical unloading. Moreover, capillary density and collagen deposition were significantly increased in the noninfarcted area of the unloaded heart compared with the same region in the in situ infarcted heart. In conclusion, ischemic insult in the mechanically unloaded heart elicits an altered inflammatory and healing response, which is characterized by myocardi

关键词： SULFUR dioxide   WATER -- Composition   MOLECULAR dynamics   QUANTUM theory   DISTRIBUTION (Probability theory)  

摘要： An ab initio Quantum Mechanical Charge Field Molecular Dynamics Simulation (QMCF MD) was performed to investigate structure and dynamics behavior of hydrated sulfur dioxide (SO2) at the Hartree-Fock level of theory employing Dunning DZP basis sets for solute and solvent molecules. The intramolecular structural characteristics of SO2, such as S = O bond lengths and O = S = O bond angle, are in good agreement with the data available from a number of different experiments. The structural features of the hydrated SO2 were primarily evaluated in the form of S-O-wat and O-SO2-H-wat radial distribution functions (RDFs) which gave mean distances of 2.9 and 2.2 angstrom, respectively. The dynamical behavior characterizes the solute molecule to have structure making properties in aqueous solution or water aerosols, where the hydrated SO2 can easily get oxidized to form a number of sulfur(VI) species, which are believed to play an important role in the atmospheric processes.

关键词： QMCF-MD   hydrophobic   hydophilic   cage-like structure   H-bond donor   H-bond acceptor  

摘要： An ab initio quantum mechanical charge field molecular dynamics simulation was carried out for one methanol molecule in water to analyze the structure and dynamics of hydrophobic and hydrophilic groups. It is found that water molecules around the methyl group form a cage-like structure whereas the hydroxyl group acts as both hydrogen bond donor and acceptor, thus forming several hydrogen bonds with water molecules. The dynamic analyses correlate well with the structural data, evaluated by means of radial distribution functions, angular distribution functions, and coordination number distributions. The overall ligand mean residence time, tau identifies the methanol molecule as structure maker. The relative dynamics data of hydrogen bonds between hydroxyl of methanol and water molecules prove the existence of both strong and weak hydrogen bonds. The results obtained from the simulation are in excellent agreement with the experimental results for dilute solution of CH(3)OH in water. The overall hydration shell of methanol consists in average of 18 water molecules out of which three are hydrogen bonded. (C) 2010 Wiley Periodicals, Inc. J Comput Chem 32: 886-892, 2011

关键词： uroporphyrinogen decarboxylase III   uroporphyrinogen III   porphyrin biosynthesis   quantum mechanics/molecular mechanics and density functional theory  

摘要： In humans, uroporphyrinogen decarboxylase is intimately involved in the synthesis of heme, where the decarboxylation of the uroporphyrinogen-III occurs in a single catalytic site. Several variants of the mechanistic proposal exist;however, the exact mechanism is still debated. Thus, using an ONIOM quantum mechanical/molecular mechanical approach, the mechanism by which uroporphyrinogen decarboxylase decarboxylates ring D of uroporphyrinogen-III has been investigated. From the study performed, it was found that both Arg37 and Arg50 are essential in the decarboxylation of ring D, where experimentally both have been shown to be critical to the catalytic behavior of the enzyme. Overall, the reaction was found to have a barrier of 10.3 kcal mol(-1) at 298.15 K. The rate-limiting step was found to be the initial proton transfer from Arg37 to the substrate before the decarboxylation. In addition, it has been found that several key interactions exist between the substrate carboxylate groups and backbone amides of various active site residues as well as several other functional groups. (C) 2010 Wiley Periodicals, Inc. J Comput Chem 32: 822-834, 2011

关键词： abrasion   chemical mechanical polishing   elemental semiconductors   molecular dynamics method   silicon   silicon compounds   surface finishing  

摘要： In an abrasive chemical mechanical polishing (CMP) process, materials were considered to be removed by abrasive sliding and rolling. Abrasive sliding has been investigated by many molecular dynamics (MD) studies;while abrasive rolling was usually considered to be negligible and therefore was rarely investigated. In this paper, an MD simulation was used to study the effects of abrasive rolling on material removal and surface finish in the CMP process. As the silica particle rolled across the silicon substrate, some atoms of the substrate were dragged out from their original positions and adhered to the silica particle, leaving some atomic vacancies on the substrate surface. Meanwhile, a high quality surface could be obtained. During the abrasive rolling process, the influencing factors of material removal, e. g., external down force and driving force, were also discussed. Finally, MD simulations were carried out to examine the effects of abrasive sliding on material removal under the same external down force as abrasive rolling. The results showed that the ability of abrasive rolling to remove material on the atomic scale was not notably inferior to that of abrasive sliding. Therefore, it can be proposed that both abrasive sliding and rolling play important roles in material removal in the abrasive CMP of the silicon substrate. (C) 2011 American Institute of Physics. [doi:10.1063/1.3575177]

关键词： Absorption   Monomers   Hydrolysis   Polymerization   Polymers  

摘要： The temperature dependence of photopolymerization kinetics of tetrakis[(methacryloyloxy)ethoxy]silane (TetMESi) was compared with an analogue, diethylene glycol dimethacrylate and their copolymers. In all cases, the kinetics are controlled by the temperature dependence of the propagation step, the effect of diffusion on radical termination, the effect of rising glass transition during cure, and topological restraint on complete conversion. The glass transition regions of these polymers were very broad, suggesting their use in high temperature applications. NMR of the TetMESi monomer, and FTIR and water sorption measurements during exposure to water of the TetMESi-based polymers showed that the Si-O-C bond was slowly hydrolyzed, resulting in water-swollen polymers containing poly(hydroxyethyl methacrylate) which reduced the glass transition temperature and narrowed the glass transition region. UV-vis spectroscopic studies showed that during exposure to water, SiO2 nanoparticles or nanostructures were formed from the condensation of the silanol groups formed during the hydrolysis of TetMESi.

关键词： dissociation   hydrogen bonds   organic compounds   photon echo  

摘要： We have investigated the dynamics of the hydrogen bonds that connect the components of a [2]rotaxane in solution. In this rotaxane, the amide groups in the benzylic-amide macrocycle and the succinamide thread are connected by four equivalent N-H center dot center dot center dot O=C hydrogen bonds. The fluctuations of these hydrogen bonds are mirrored by the frequency fluctuations of the NH-stretch modes, which are probed by means of three-pulse photon-echo peak shift spectroscopy. The hydrogen-bond fluctuations occur on three different time scales, with time constants of 0.1, 0.6, and >= 200 ps. Comparing these three time scales to the ones found in liquid formamide, which contains the same hydrogen-bonded amide motif but without mechanical constraints, we find that the faster two components, which are associated with small-amplitude fluctuations in the strength of the N-H center dot center dot center dot O=C hydrogen bonds, are very similar in the liquid and the rotaxane. However, the third component, which is associated with the breaking and subsequent reformation of hydrogen bonds, is found to be much slower in the rotaxane than in the liquid. It can be concluded that the mechanical bonding in a rotaxane does not influence the amplitude and time scale of the small-amplitude fluctuations of the hydrogen bonds, but strongly slows down the complete dissociation of these hydrogen bonds. This is probably because in a rotaxane breaking of the macrocycle-axle contacts is severely hindered by the mechanical constraints. The hydrogen-bond dynamics in rotaxane-based molecular machines can therefore be regarded as liquidlike on a time scale 1 ps and less, but structurally frozen on longer (up to at least 200 ps) time scales. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3569761]