Networks of coupled dynamical systems have been used to model biological oscillators, Josephson junction arrays, excitable media, neural networks, spatial games, genetic control networks and many other self-organizing systems. Ordinarily, the connection topology is assumed to be either completely regular or completely random. But many biological, technological and social networks lie somewhere between these two extremes. Here we explore simple models of networks that can be tuned through this middle ground: regular networks 'rewired' to introduce increasing amounts of disorder. We find that these systems can be highly clustered, like regular lattices, yet have small characteristic path lengths, like random graphs. We call them 'small-world' networks, by analogy with the small-world phenomenon (popularly known as six degrees of separation. The neural network of the worm Caenorhabditis elegans, the power grid of the western United States, and the collaboration graph of film actors are shown to be small-world networks. Models of dynamical systems with small-world coupling display enhanced signal-propagation speed, computational power, and synchronizability. In particular, infectious diseases spread more easily in small-world networks than in regular lattices.

/pdf/collective-dynamics-of-small-world-networks-205zhem3gm.pdf

Collective dynamics of small-world networks

In this review the theory and application of Smoothed particle hydrodynamics (SPH) since its inception in 1977 are discussed. Emphasis is placed on the strengths and weaknesses, the analogy with particle dynamics and the numerous areas where SPH has been successfully applied.

https://iopscience.iop.org/article/10.1088/0034-4885/68/8/R01/pdf

Smoothed particle hydrodynamics

One of the outstanding challenges in the field of porous materials is the design and synthesis of chemical structures with exceptionally high surface areas Such materials are of critical importance to many applications involving catalysis, separation and gas storage The claim for the highest surface area of a disordered structure is for carbon, at 2,030 m2 g(-1) (ref 2) Until recently, the largest surface area of an ordered structure was that of zeolite Y, recorded at 904 m2 g(-1) (ref 3) But with the introduction of metal-organic framework materials, this has been exceeded, with values up to 3,000 m2 g(-1) (refs 4-7) Despite this, no method of determining the upper limit in surface area for a material has yet been found Here we present a general strategy that has allowed us to realize a structure having by far the highest surface area reported to date We report the design, synthesis and properties of crystalline Zn4O(1,3,5-benzenetribenzoate)2, a new metal-organic framework with a surface area estimated at 4,500 m2 g(-1) This framework, which we name MOF-177, combines this exceptional level of surface area with an ordered structure that has extra-large pores capable of binding polycyclic organic guest molecules--attributes not previously combined in one material

/pdf/a-route-to-high-surface-area-porosity-and-inclusion-of-large-6gq9e8txip.pdf

A route to high surface area, porosity and inclusion of large molecules in crystals

Monthly Notices is one of the three largest general primary astronomical research publications. It is an international journal, published by the Royal Astronomical Society. This article 1 describes its publication policy and practice.

Monthly Notices of the Royal Astronomical Society

Using molecules as electronic components is a powerful new direction in the science and technology of nanometre-scale systems1. Experiments to date have examined a multitude of molecules conducting in parallel2,3, or, in some cases, transport through single molecules. The latter includes molecules probed in a two-terminal geometry using mechanically controlled break junctions4,5 or scanning probes6,7 as well as three-terminal single-molecule transistors made from carbon nanotubes8, C60 molecules9, and conjugated molecules diluted in a less-conducting molecular layer10. The ultimate limit would be a device where electrons hop on to, and off from, a single atom between two contacts. Here we describe transistors incorporating a transition-metal complex designed so that electron transport occurs through well-defined charge states of a single atom. We examine two related molecules containing a Co ion bonded to polypyridyl ligands, attached to insulating tethers of different lengths. Changing the length of the insulating tether alters the coupling of the ion to the electrodes, enabling the fabrication of devices that exhibit either single-electron phenomena, such as Coulomb blockade, or the Kondo effect.

Coulomb blockade and the Kondo effect in single-atom transistors

We develop empirical relationships for the accretion and erosion of colliding gravity-dominated bodies of various compositions under conditions expected in late-stage solar system formation. These are fast, easily coded relationships based on a large database of smoothed particle hydrodynamics (SPH) simulations of collisions between bodies of different compositions, including those that are water rich. The accuracy of these relations is also comparable to the deviations of results between different SPH codes and initial thermal/rotational conditions. We illustrate the paucity of disruptive collisions between major bodies, as compared to collisions between less massive planetesimals in late-stage planet formation, and thus focus on more probable, low-velocity collisions, though our relations remain relevant to disruptive collisions as well. We also pay particular attention to the transition zone between merging collisions and those where the impactor does not merge with the target, but continues downrange, a "hit-and-run" collision. We find that hit-and-run collisions likely occur more often in density-stratified bodies and across a wider range of impact angles than suggested by the most commonly used analytic approximation. We also identify a possible transitional zone in gravity-dominated collisions where larger bodies may undergo more disruptive collisions when the impact velocity exceeds the sound speed, though understanding this transition warrants further study. Our results are contrary to the commonly assumed invariance of total mass (scale), density structure, and material composition on the largest remnants of giant impacts. We provide an algorithm for adopting our model into N-body planet formation simulations, so that the mass of growing planets and debris can be tracked.

Gravity-dominated Collisions: A Model for the Largest Remnant Masses with Treatment for "Hit and Run" and Density Stratification.

https://es.ucsc.edu/~lbruesch/BrueschAsphaug2004.pdf

Modeling global impact effects on middle-sized icy bodies: applications to Saturn's moons

We consider the hypothesis that frequent cratering produces size- or compositionally-sorted asteroid regolith, affecting the structure, texture, and in extreme cases the shape of asteroids. Additional information is contained in the original extended abstract.

Brazil Nuts on Eros: Size-Sorting of Asteroid Regolith

Introduction: Interest in the possible presence of water ice on the Moon has both scientific and opera tional foundations. It is thought that a variety o f processes could be contributing to the accumulation of water within polar cold traps, including impacts of comets and meteorites, the reaction of solar proton s with lunar soil to form water, and outgassing from the moon’s interior. One possible process involves the migration of water in the Moon’s exosphere though ballistic trajectories with eventual capture of the se molecules in persistently shadowed polar cold traps. Recent discoveries of a veneer of hydroxyl and adsorb ed water in sunlit regions of the moon may support thi s process [1,2,3]. The form and amount of water pre sumably associated with the hydrogen concentrations at the poles, as observed by Lunar Prospector (LP) and Lunar Reconnaissance Orbiter (LRO) may represent one end-state in the chain of processes that involv e water on the moon. The LCROSS mission’s goal was to identify the source of the elevated hydrogen at the poles and provide a ground truth with regards to th e total concentration. Verification of the form and amount of hydrogen can constrain models of the impa ct history of the lunar surface and the effects of met eori e gardening, photo-dissociation, and solar wind sputt ering. With the combined observations of LCROSS, LRO and others it is possible to evaluate the globa l water distribution and provide a quantitative basis for studies of the Moon’s history and a test of current theories on the form and distribution of lunar hydrogen [4]. The LCROSS Mission: The primary objective of the Lunar Crater Observation and Sensing Satellite Figure 2. Image of the LCROSS impact ejecta cloud as seen in the visible context camera. Inset shows the eje cta cloud expanding to fill the shadowed region targeted at t he bottom of the crater Cabeus.

Water and More: An Overview of LCROSS Impact Results

Access to stable, long-duration microgravity environments for asteroid regolith experiments, rubble pile asteroid research, small body lander, sampler studies, basic examinations of planetesimal accretion and evolution need not be expensive. On the one hand, billion-dollar concepts of returning entire ~3-5 m asteroids [1] are being considered that would ‘nab’ an asteroid to cislunar space [2], where detailed investigations could be conducted at a relaxed pace by astronauts onboard a space station. A much cheaper and more immediate alternative is to pack a few kg of fine fragments of common meteorite into a cubesat, using it to build a ‘patch of asteroid’ inside two experimental chambers onboard a spun-up 3U cubesat. We show that our design (AOSAT, Fig. 1) can mimic the asteroid surface environment in sufficient detail, provideng us with a low cost orbiting laboratory for asteroid and primary accretion research. Design: Two 1U chambers containing meteorite fragments and dust are on either side of a central chamber containing the spacecraft itself (Figure 1). For zero-gravity research (primary accretion) the spaceraft is stabilized to zero rotation by a magnetorquer (see below). For microgravity research, the spacecraft is spun on its minor axis using a flywheel, until the rocks are accelerated to the walls in simulated microgravity. We show that this approach is suitable, in terms of being a good approximation, and further can be achieved at very low cost, well under $1M. Scientific Motivations. Zero-G experiments are common, using drop towers and parabolic flights, but in addition to any operational obstacles they are intrinsically limited to durations that are short compared

Erik Asphaug

Papers

Gravity-dominated Collisions: A Model for the Largest Remnant Masses with Treatment for "Hit and Run" and Density Stratification.

Modeling global impact effects on middle-sized icy bodies: applications to Saturn's moons

Brazil Nuts on Eros: Size-Sorting of Asteroid Regolith

Water and More: An Overview of LCROSS Impact Results

Asteroid Regolith Mechanics and Primary Accretion Experiments in a Cubesat