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

Microfabricated integrated circuits revolutionized computation by vastly reducing the space, labor, and time required for calculations. Microfluidic systems hold similar promise for the large-scale automation of chemistry and biology, suggesting the possibility of numerous experiments performed rapidly and in parallel, while consuming little reagent. While it is too early to tell whether such a vision will be realized, significant progress has been achieved, and various applications of significant scientific and practical interest have been developed. Here a review of the physics of small volumes (nanoliters) of fluids is presented, as parametrized by a series of dimensionless numbers expressing the relative importance of various physical phenomena. Specifically, this review explores the Reynolds number Re, addressing inertial effects; the Peclet number Pe, which concerns convective and diffusive transport; the capillary number Ca expressing the importance of interfacial tension; the Deborah, Weissenberg, and elasticity numbers De, Wi, and El, describing elastic effects due to deformable microstructural elements like polymers; the Grashof and Rayleigh numbers Gr and Ra, describing density-driven flows; and the Knudsen number, describing the importance of noncontinuum molecular effects. Furthermore, the long-range nature of viscous flows and the small device dimensions inherent in microfluidics mean that the influence of boundaries is typically significant. A variety of strategies have been developed to manipulate fluids by exploiting boundary effects; among these are electrokinetic effects, acoustic streaming, and fluid-structure interactions. The goal is to describe the physics behind the rich variety of fluid phenomena occurring on the nanoliter scale using simple scaling arguments, with the hopes of developing an intuitive sense for this occasionally counterintuitive world.

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Microfluidics: Fluid physics at the nanoliter scale

There is a special reason for reviewing this book at this time: it is the 50th edition of a compendium that is known and used frequently in most chemical and physical laboratories in many parts of the world. Surely, a publication that has been published for 56 years, withstanding the vagaries of science in this century, must have had something to offer. There is another reason: while the book is a standard fixture in most chemical and physical laboratories, including those in medical centers, it is not as frequently seen in the laboratories of physician's offices (those either in solo or group practice). I believe that the Handbook can be useful in those laboratories. One of the reasons, among others, is that the various basic items of information it offers may be helpful in new tests, either physical or chemical, which are continuously being published. The basic information may relate

Handbook of Chemistry and Physics

Global energy consumption due to friction in passenger cars

Capillarity and Wetting Phenomena: Drops, Bubbles, Pearls, Waves

This paper reviews research into the mechanisms of action of the lubricating oil additive, zinc dialkyldithophosphate (ZDDP). The development of the use and research into ZDDP is first charted historically, starting with the additive's first introduction in engine oils in the late 1930s. Then our current state of knowledge of each of the main facets of ZDDP behaviour both in solution and at metal surfaces is identified and discussed. It is concluded that we now know a great deal about the properties and morphology of ZDDP antiwear films but still relatively little about the reaction pathways that lead to ZDDP film formation or about the kinetics of ZDDP film generation and removal.

The History and Mechanisms of ZDDP

Optical interferometry is now a widely used technique for measuring the separating film thickness in model rolling and sliding elastohydrodynamic contacts. There are two limitations of the method as conventionally employed: first, it cannot easily be used to accurately measure films less than one quarter the wavelength of visible light, i.e. less than about 100 nm. Secondly, only certain, discrete thicknesses, spaced at least 50 nm apart can be determined. This paper describes work aimed at overcoming these limitations so as to make optical interferometry applicable to the study of boundary or very thin film elastohydrodynamic lubrication in rolling contacts. A combination of a solid spacer layer with spectrometric analysis of reflected light from the contact enables very thin lubricant films to be accurately measured. The approach is applied to the study of thin films formed in rolling contacts by low viscosity lubricants. Some anomalies in the relationship between film thickness and speed are found with...

The Measurement and Study of Very Thin Lubricant Films in Concentrated Contacts

The need for energy efficiency is leading to the growing use of additives that reduce friction in thin film boundary and mixed lubrication conditions. Several classes of such friction modifier additive exist, the main ones being organic friction modifiers, functionalised polymers, soluble organo-molybdenum additives and dispersed nanoparticles. All work in different ways. This paper reviews these four main types of lubricant friction modifier additive and outlines their history, research and the mechanisms by which they are currently believed to function. Aspects of their behaviour that are still not yet fully understood are highlighted.

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Friction Modifier Additives

Zinc dialkyldithiophosphate additives are used to control wear and inhibit oxidation in almost all engine oils as well as many other types of lubricant. They limit wear primarily by forming a thick, protective, phosphate glass-based tribofilm on rubbing surfaces. This film formation can occur at low temperatures and is relatively indifferent to the chemical nature of the substrate. There has been considerable debate as to what drives ZDDP tribofilm formation, why it occurs only on surfaces that experience sliding and whether film formation is controlled primarily by temperature, pressure, triboemission or some other factor. This paper describes a novel approach to the problem by studying the formation of ZDDP films in full film EHD conditions from two lubricants having very different EHD friction properties. This shows that ZDDP film formation does not require solid–solid rubbing contact but is driven simply by applied shear stress, in accord with a stress-promoted thermal activation model. The shear stress present in a high-pressure contact can reduce the thermal activation energy for ZDDP by at least half, greatly increasing the reaction rate. This mechanism explains the origins of many practically important features of ZDDP films; their topography, their thickness and the conditions under which they form. The insights that this study provides should prove valuable both in optimising ZDDP structure and in modelling ZDDP antiwear behaviour. The findings also highlight the importance of mechanochemistry to the behaviour of lubricant additives in general.

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On the Mechanism of ZDDP Antiwear Film Formation

Modern engine lubricant specifications include compositional constraints in terms of the permitted level of sulphated ash, phosphorus and sulphur (SAPS). This necessitates a reduction in the concentration of the additive zinc dialkyldithiophosphate (ZDDP) used in engine oils, and there is currently great interest in identifying anti-wear additives that contain low or zero SAPS to partially or wholly replace ZDDP.

This paper reviews the main chemical classes that are reported in the literature as potential low- or zero-SAPS anti-wear agents. There are many such possible additives, although none appears to be quite as versatile as ZDDP. Instead, combinations of anti-wear and extreme pressure additives are likely to be employed in the future. The literature reveals a strong imbalance in the amount and depth of research carried out on the various additive types, with a great deal of research on a few classes of additives and very little recent work on most others. Copyright © 2007 John Wiley & Sons, Ltd.

Hugh Spikes

Papers

The History and Mechanisms of ZDDP

The Measurement and Study of Very Thin Lubricant Films in Concentrated Contacts

Friction Modifier Additives

On the Mechanism of ZDDP Antiwear Film Formation

Low- and zero-sulphated ash, phosphorus and sulphur anti-wear additives for engine oils