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

Ionic liquids are composed entirely of ions. Because of the wide range of possible binary and ternary ionic liquids, they offer a potentially wide range of solvent properties. In their Perspective,
 Rogers and Seddon
 review recent progress on developing new ionic liquid solvents for use in chemical synthesis, catalysis, fuel cells, and other applications. Ionic liquids are considered advantageous not only because of their versatility but also for their "green" credentials, although it is important to remember that not all ionic liquids are environmentally benign. One industrial process has been reported, and others may not be far behind. The authors conclude that in the next decade, ionic liquids are likely to replace conventional solvents in many applications.

http://science.sciencemag.org/content/sci/302/5646/792.full.pdf

Ionic Liquids--Solvents of the Future?

Review of Charged Particle Dynamics. Beam Optics and Focusing Systems Without Space Charge. Linear Beam Optics with Space Charge. Self-Consistent Theory of Beams. Emittance Variation. Beam Physics Research from 1993 to 2007. Appendices. List of Frequently Used Symbols. Bibliography. Index.

Theory and Design of Charged Particle Beams

We report a method to generate steady coaxial jets of immiscible liquids with diameters in the range of micrometer/nanometer size. This compound jet is generated by the action of electro-hydrodynamic (EHD) forces with a diameter that ranges from tens of nanometers to tens of micrometers. The eventual jet breakup results in an aerosol of monodisperse compound droplets with the outer liquid surrounding or encapsulating the inner one. Following this approach, we have produced monodisperse capsules with diameters varying between 10 and 0.15 micrometers, depending on the running parameters.

https://science.sciencemag.org/content/sci/295/5560/1695.full.pdf

Micro/Nano Encapsulation via Electrified Coaxial Liquid Jets

Physics of shock waves and high-temperature hydrodynamic phenomena - Ya.B. Zeldovich and Yu.P. Raizer (translated from the Russian and then edited by Wallace D. Hayes and Ronald F. Probstein); Dover Publications, New York, 2002, 944 pp., $34.

The full spray emitted by Taylor cones of the ionic liquid 1-ethyl-3-methyl imidazolium tetrafluoroborate (EMI+BF4−) held in a vacuum is investigated at room temperature by time of flight mass spectrometry. The current is composed mainly of ions under most conditions studied, but contains a small component of nanometer drops that tends to dominate the emitted mass flow. Exceptionally, drop ejection vanishes close to the smallest flow rate at which the Taylor cone is steady. The present discovery of a stable strictly ionic regime in Taylor cones of substances other than liquid metals owes much to earlier observations with sulfuric acid, where most but not all the current was ionic. Most striking is the fact that this purely ionic regime is obtained at an electrical conductivity K of only 1.3 S/m, much smaller than that of sulfuric acid, and smaller than that at which formamide electrolytes with K>2 S/m do still emit substantial drop currents. The ion emission includes primarily the dimer (EMI–BF4)EMI+, acc...

/pdf/source-of-heavy-molecular-ions-based-on-taylor-cones-of-14bcivz3vr.pdf

Source of heavy molecular ions based on Taylor cones of ionic liquids operating in the pure ion evaporation regime

The need for electric propulsion in the thrust range of tens of micro-Newtons has triggered therebirth of colloid thrustertechnology.Theabilitytodeliverthrustattheselevelsinacontrollablefashionwillenhancetheuseofsmall satellites(massessmallerthan some20kg )and theexecutionofspacemissionsinwhichvery accuratepositioning of spacecraftsisrequired.Wepresenthereanovelapproachtothee eldofcolloidthrusters.Likepreviouseffortsituses electrospray as the mean of producing charged colloids. However, it differs in the spraying regime employed. Electrosprays of highly conducting propellants in single cone-jet mode allow the generation of droplets of high specie c charge at operating voltages lower than in the earlier era of colloid thruster research. In this article we use energy analysis techniques to characterize electrospray beams and to measure their thrust and specie c impulse. We have studied three novel propellants: formamide, tributyl phosphate, and the ionic liquid 1-ethyl-3-methylimidazolium bis (trie uoromethylsulfonyl)imide. The thrust, specie c impulse, and efe ciency associated with a single spray of the most conducting formamide solution are typically 0.3 πN, 300 s, and 75% for an acceleration voltage of 1300 V. The electrospray phenomenology presented in this article is diverse. For example, by varying the electrospraying parametersweareabletostudyemissionmodesinwhich1 )solvatedionsaree eldevaporatedfromthejet’ ssurface, 2) both satellite and main droplets result from the jet’ s breakup, and 3 ) only main droplets are emitted.

Electrospray as a Source of Nanoparticles for Efficient Colloid Thrusters

When concentrated solutions of NaI in formamide with electrical conductivities K larger than 1.1 S/m are electrosprayed from a Taylor cone-jet in a vacuum, ions are evaporated at substantial rates from the surface of the meniscus and the drops. This constitutes a new source of ions and nanoparticles, where the relative importance of these two contributions is adjustable. The currents of ions are measured independently from those associated with drops by a combination of stopping voltage analysis and preferential scattering in a gas background. The magnitude E of the electric field at the surface of the drops and at the apex of the cone-jet is controlled through the electrical conductivity K of the liquid and its flow rate Q through the jet. E is related through available scaling laws for Taylor cone-jets to the ratios K/Q or I/Q, where I is the current of drops emitted by the jet. Ion currents are very small or null at typical K/Q values used in the past. A relatively small initial ion current is attribut...

/pdf/direct-measurement-of-ion-evaporation-kinetics-from-3d0i2nv4ww.pdf

Direct measurement of ion evaporation kinetics from electrified liquid surfaces

Mechanisms of electrospray ionization of singly and multiply charged salt clusters

We describe the performance of a torsional balance suitable for the study of microNewton thrusters. A design based on flexural pivots and electrostatic forces (for calibration and active damping) makes it possible to test thrusters with a mass of a few kilograms, and measure submicroNewton level forces with high accuracy and a resolution better than 0.03 μN. The usefulness of the balance is proved by measuring the performance of two different electric propulsion accelerators: a colloid thruster generating thrust in the 2–13 μN range, and a micropulsed plasma thruster yielding single impulse bits of 19 μN s. The low noise figure of the balance (below 0.1 μN/Hz in the 7 mHz–1 Hz bandwidth) will make it possible to measure the thrust noise requirements imposed in colloid thrusters by missions such as the Space Technology 7 and the Laser Interferometer Space Antenna.

/pdf/a-torsional-balance-for-the-characterization-of-micronewton-38ccdrba8r.pdf

Manuel Gamero-Castaño

Papers

Source of heavy molecular ions based on Taylor cones of ionic liquids operating in the pure ion evaporation regime

Electrospray as a Source of Nanoparticles for Efficient Colloid Thrusters

Direct measurement of ion evaporation kinetics from electrified liquid surfaces

Mechanisms of electrospray ionization of singly and multiply charged salt clusters

A torsional balance for the characterization of microNewton thrusters