Surface tension-driven self-folding polyhedra

We discuss finite element simulations and experiments involving the surface tension-driven self-folding of patterned polyhedra. Two-dimensional (2D) photolithographically patterned templates folded spontaneously when solder hinges between adjacent faces were liquefied. Minimization of interfacial free energy of the molten solder with the surrounding fluidic medium caused the solder to ball up, resulting in a torque that rotated adjacent faces and drove folding. The simulations indicate that the folding process can be precisely controlled, has fault tolerance, and can be used to fold polyhedra composed of a variety of materials, ranging in size from the millimeter scale down to the nanometer scale. Experimentally, we have folded metallic, arbitrarily patterned polyhedra ranging in size from 2 mm to 15 microm.     

Synthesis and rearrangements of 2,3,4,5-tetrahydro-3-benzazocin-6(1H)ones

The preparation of 3-benzenesulfonyl- and 3-benzoyl-2,3,4,5-tetrahydro-3-benzazoein-6(1H)-ones by intramolecular acylation is described. Acid catalyzed rearrangements of these 3-benzazocin-6-ones occurs by 3,4-bond cleavage, which is followed by intramolecular or intermolecular alkylation, to produce indanones (i.e. XIV) or a 3,4-dihydroisoquinoline (II), respectively. In addition to spectral support for these structures, an independent synthesis of II is described from a known 3,4-dihydroisoquinoline. Two rearrangements of 3-benzazocin-6-ones under alkaline conditions are also described, and the products are again evidence for 3,4-bond cleavage to vinylphenones. These beuzazocine rearrangements have been observed only with the 6-keto compounds.     

Critical properties of non-spherical molecule fluids from the virial expansion

Critical constants of pure fluids (as important reference data in constructing vapour-liquid phase diagrams and basic input of various estimation methods) were determined for systems of non-spherical Kihara molecules; values of the critical temperature, density, compression factor and pressure of fluids composed of prolate and oblate molecules were evaluated from the fourth-order virial expansion. The second and third virial coefficients of the Kihara molecules were determined by applying the recently proposed method in which the effect of molecular core geometry and functional dependence of a pair interaction on the surface-surface distance are factorized and the former contribution determined from a formula for the corresponding hard convex body virial coefficient. The virial expansion for non-spherical Kihara molecules is applied to determine the critical constants of n-alkanes (methane to octane) and cyclic hydrocarbons (cyclopentane, cyclohexane, benzene and naphthalene); a fair agreement with experimental data was found.     

Critical properties of non-spherical molecule fluids from virial expansion

For systems of Kihara molecules with circular cores, the values of the reduced critical constants were determined from the fourth-order virial expansion as functions of the core diameter/thickness ratio. From expressions for the reduced functions both for the oblate and prolate shapes, the values of critical constants of four cyclic hydrocarbons and four branched alkanes were evaluated and compared with the experimental data and values obtained from the perturbation theory.     

A two-dimensional pseudospectral model for time reversal and nonlinear elastic wave spectroscopy

One way to characterize metallic materials in the presence of defects like dislocation networks is to measure their large dynamic nonlinear elastic response. In this numerical study, a new method combining the nonlinear elastic wave spectroscopy (NEWS) method with a time reversal (TR) process is proposed. This method, called NEWS-TR, uses nonlinear analysis as a pretreatment of time reversal and then consists of retrofocusing only nonlinear components on the defect position. A two-dimensional pseudospectral time domain algorithm is developed here to validate the NEWS-TR method as a potential technique for damage location. Hysteretic nonlinear behavior of the materials being studied is introduced using the Preisach-Mayergoyz model. Moreover, in order to extend this solver in two dimensions, the Kelvin notation is used to modify the elastic coefficient tensor. Simulations performed on a metallic sample show the feasibility and value of the NEWS-TR methodology for microdamage imaging. Retrofocusing quality depends on different parameters such as the filtering method used to keep only nonlinear components and the nonlinear effect measured. In harmonic generation, pulse inversion filtering seems to be a more appropriate filtering method than classical harmonic filtering for most defect positions, mainly because of its ability to filter all fundamental components.     

Global minimum structure searches via particle swarm optimization

Novel implementation of the evolutionary approach known as particle swarm optimization (PSO) capable of finding the global minimum of the potential energy surface of atomic assemblies is reported. This is the first time the PSO technique has been used to perform global optimization of minimum structure search for chemical systems. Significant improvements have been introduced to the original PSO algorithm to increase its efficiency and reliability and adapt it to chemical systems. The developed software has successfully found the lowest-energy structures of the LJ(26) Lennard-Jones cluster, anionic silicon hydride Si(2)H(5) (-), and triply hydrated hydroxide ion OH(-) (H(2)O)(3). It requires relatively small population sizes and demonstrates fast convergence. Efficiency of PSO has been compared with simulated annealing, and the gradient embedded genetic algorithm.     

Toward efficient photomodulation of conjugated polymer emission: optimizing differential energy transfer in azobenzene-substituted PPV derivatives

We present fluorescence studies of quenching behavior in photoaddressable azobenzene-substituted derivatives of the fluorescent conjugated polymer poly(p-phenylenevinylene) (PPV). The azobenzene side chains partially quench the PPV fluorescence, and we have shown previously that the quenching efficiency is greater when the azobenzene side chains are cis than when they are trans. This effect provides a photoaddressable means of modulating the fluorescence intensity of PPV derivatives. To optimize the efficiency of photoinduced intensity modulation, it is important to understand the molecular nature of quenching by both trans- and cis-azobenzene side chains. Here we investigate the photophysical origins of quenching by the two isomers using steady-state and time-resolved fluorescence spectroscopy. We present results from the azobenzene-modified PPV derivative poly(2-methoxy-5-((10-(4-(phenylazo)phenoxy)decyl)oxy)-1,4-phenylenevinylene) (MPA-10-PPV) and two new related polymers, a copolymer lacking half of the azobenzene side chains and an analogue of MPA-10-PPV with a tert-butyl-substituted azobenzene. These studies reveal that steric interactions influence the extent of PPV emission quenching by trans-azobenzene but do not affect the efficient quenching by cis-azobenzene. The difference in dynamic quenching efficiencies between trans- and cis-azobenzene isomers is consistent with fluorescence resonance energy transfer. These results show that it is possible to control the efficiency of photoswitchable fluorescence modulation through specific structural variations designed to encourage or block quenching by trans-azobenzene. This is a promising approach to providing useful general guidelines for designing photomodulated PPV derivatives.