There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

다중혈관 관상동맥 환자에서 y-문합을 이용하여 양쪽 내흉동맥만을 사용한 우회술의 조기 성적

抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

Freedom of design, mass customisation, waste minimisation and the ability to manufacture complex structures, as well as fast prototyping, are the main benefits of additive manufacturing (AM) or 3D printing. A comprehensive review of the main 3D printing methods, materials and their development in trending applications was carried out. In particular, the revolutionary applications of AM in biomedical, aerospace, buildings and protective structures were discussed. The current state of materials development, including metal alloys, polymer composites, ceramics and concrete, was presented. In addition, this paper discussed the main processing challenges with void formation, anisotropic behaviour, the limitation of computer design and layer-by-layer appearance. Overall, this paper gives an overview of 3D printing, including a survey on its benefits and drawbacks as a benchmark for future research and development.

Additive manufacturing (3D printing): A review of materials, methods, applications and challenges

Phononic crystals and metamaterials constitute a broad class of artificially engineered materials able to manipulate the propagation of acoustic/elastic waves at different lengths scale. Filtering and directivity properties of these materials can be controlled and tuned by designing their fundamental building blocks, usually referred as unit cells. Phononic crystals exploit periodicity to induce total wave reflection at selected frequencies. The obtained filtering properties emerge at wavelengths comparable to the periodicity. On the other hand, phononic metamaterials exploit the coupling between propagating waves and local resonators in the fundamental unit cells. As a result, propagation of waves with frequencies around the resonance is inhibited. More recently the use of phononic crystals and metamaterials has been envisioned for large-scale applications in the field of civil engineering as a mean to shield buildings and infrastructures from natural or man-induced seismic waves. Among these, a solution based on metamaterial concepts and referred as “seismic metastructure” has been recently proposed by some of the authors. The proposed system is realized by an array of buried heavy-cylindrical steel units encased in cylindrical concrete pipes that constitute the metamaterial resonant units. The proper design and arrangement of these resonant units allows filtering longitudinal and shear waves in the typical frequency range of seismic waves (1-10Hz). In fact, analytical/numerical models as well as experimental evidences on a scaled setup proved the shielding capabilities of the proposed seismic isolation system with respect to bulk waves. Building on the initial results on bulk waves, here we design “seismic metastructure” able to mitigate Rayleigh surface waves. This is of special interest as they are considered the most harmful seismic threat. An analytical model to analyze the propagation of seismic surface waves through a soil engineered with seismic metastructures is developed. The analytical model allows predicting the frequency range within seismic Rayleigh waves are inhibited. A full 3D finite element model is used to numerically validate the prediction of the analytical model. Experimental evidences of the filtering properties of the designed metastructures are obtained on a scaled experimental setup. The excellent agreement of analytical/numerical and experimental results demonstrates the potential of this novel class of seismic isolation devices.

Shielding buildings from surface waves with “Seismic Metastructures”

Super-compressible foam-like carbon nanotube films have been reported to exhibit highly nonlinear viscoelastic behaviour in compression similar to soft tissue. Their unique combination of light weight and exceptional electrical, thermal and mechanical properties have helped identify them as viable building blocks for more complex nanosystems and as stand-alone structures for a variety of different applications. In the as-grown state, their mechanical performance is limited by the weak adhesion between the tubes, controlled by the van der Waals forces, and the substrate allowing the forests to split easily and to have low resistance in shear. Under axial compression loading carbon nanotubes have demonstrated bending, buckling8 and fracture9 (or a combination of the above) depending on the loading conditions and on the number of loading cycles. In this work, we partially anchor dense vertically aligned foam-like forests of carbon nanotubes on a thin, flexible polymer layer to provide structural stability, and report the mechanical response of such systems as a function of the strain rate. We test the sample under quasi-static indentation loading and under impact loading and report a variable nonlinear response and different elastic recovery with varying strain rates. A Bauschinger-like effect is observed at very low strain rates while buckling and the formation of permanent defects in the tube structure is reported at very high strain rates. Using high-resolution transmission microscopy

Strain rate effects in the mechanical response of polymer anchored carbon nanotube foams

We propose the realization of capacitive temperature sensors based on the concept of displacement amplification. Our design features two high coefficient of thermal expansion (CTE) metallic layers separated by a low-CTE dielectric layer; conductive and dielectric layers are then separated by a thin air gap and glued together at a few locations. As the temperature increases, the high-CTE layer tends to expand more than the low-CTE one. Owing to the constraint to planar expansion imposed by the low-CTE layer, this results in large out-of-plane displacements of the high-CTE layer – hence the displacement amplification term. In our case, the high-CTE layer buckles and causes a reduction of the gap between conductive and dielectric layers; in turn, this results in a large change of capacitance. First, we illustrate the concept via numerical simulations. Then, we realize a low-cost prototype of such sensor by using aluminum foil as conductor, paper as dielectric, and by gluing the layers together with cyanoacrylate. Our results demonstrate the potential of this simple design as a route towards efficient and low-cost temperature sensors.

https://authors.library.caltech.edu/114512/2/Capacitive_temperature_sensing_via_displacement_amplification.pdf

Capacitive Temperature Sensing via Displacement Amplification

It is well known that nonlinear system of interacting particles transport energy from coherent modes and macroscopic scales to internal, microscopic modes (thermalization), carrying localized traveling waves for generic nonlinear potentials V. Due to the zerotensile strength and the nonlinear contact interaction law between the particles, granular systems present a strongly nonlinear interaction potential that can force traveling solitary waves to exhibit compact support (compactons) [1,2]. The thermalization effect can be used for fast decomposition of an external impulse into trains of solitons or compactons, energy trapping and shock disintegration, within optimally designed granular protectors or containers. Optimally randomized systems, involving e.g. scattering of particle sizes, masses and materials, show special dynamic features, such as: lack of uniform steady states of velocity profiles, high kinetic vs potential energy ratios, decay of wave amplitude, impulse disintegration and/or anomalous reflections through interfaces separating heavy (or hard) and light (or soft) particles, dependence of anomalous effects on the initial pre-compression. The present study exploits the use of Evolutionary Algorithms for the optimal design of composite granular protectors through simultaneous material, geometry and topology optimization. Generations of candidate design solutions are evaluated in terms of a quantitative fitness, through numerical evaluation of the impulse absorption capability of the system. Several optimization problems are examined, dealing with size, material and shape optimization of granular systems subject to dynamic impulses and shock-waves. The fitness is measured through the inverse of the maximum force F out transmitted from the system to a given “wall”, which simulates a large body in contact with the protector.

Schock absorbtion optimization of composite granular protectors

We study helical acoustic metamaterials and demonstrate the ability to vary the materials' dispersion properties by controlling geometrical structure and mass distribution. By locally adding eccentric, higher density elements in the unit cells, we perturb the moment of inertia of the system and introduce centro-asymmetry. This allows controlling the degree of mode coupling and the width of subwavelength bandgaps in the dispersion relation, which are the product of enhanced local resonance hybridization. We characterize the distinct normal modes in our metamaterials using finite element simulations and analytically quantify the coupling between each mode. The evolution of acoustic bandgaps induced by the increasing level of centro-asymmetry is experimentally validated with 3D-printed structures.

Chiara Daraio

Papers

Shielding buildings from surface waves with “Seismic Metastructures”

Strain rate effects in the mechanical response of polymer anchored carbon nanotube foams

Capacitive Temperature Sensing via Displacement Amplification

Schock absorbtion optimization of composite granular protectors

Mode hybridization in DNA-inspired helical metamaterials with variable centro-asymmetry