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

The review deals with drop impacts on thin liquid layers and dry surfaces. The impacts resulting in crown formation are referred to as splashing. Crowns and their propagation are discussed in detail, as well as some additional kindred, albeit nonsplashing, phenomena like drop spreading and deposition, receding (recoil), jetting, fingering, and rebound. The review begins with an explanation of various practical motivations feeding the interest in the fascinating phenomena of drop impact, and the above-mentioned topics are then considered in their experimental, theoretical, and computational aspects.

Drop Impact Dynamics: Splashing, Spreading, Receding, Bouncing ...

Inkjet printing is viewed as a versatile manufacturing tool for applications in materials fabrication in addition to its traditional role in graphics output and marking. The unifying feature in all these applications is the dispensing and precise positioning of very small volumes of fluid (1–100 picoliters) on a substrate before transformation to a solid. The application of inkjet printing to the fabrication of structures for structural or functional materials applications requires an understanding as to how the physical processes that operate during inkjet printing interact with the properties of the fluid precursors used. Here we review the current state of understanding of the mechanisms of drop formation and how this defines the fluid properties that are required for a given liquid to be printable. The interactions between individual drops and the substrate as well as between adjacent drops are important in defining the resolution and accuracy of printed objects. Pattern resolution is limited by the extent to which a liquid drop spreads on a substrate and how spreading changes with the overlap of adjacent drops to form continuous features. There are clearly defined upper and lower bounds to the width of a printed continuous line, which can be defined in terms of materials and process variables. Finer-resolution features can be achieved through appropriate patterning and structuring of the substrate prior to printing, which is essential if polymeric semiconducting devices are to be fabricated. Low advancing and receding contact angles promote printed line stability but are also more prone to solute segregation or “coffee staining” on drying.

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Inkjet Printing of Functional and Structural Materials: Fluid Property Requirements, Feature Stability, and Resolution

Thermophysical Properties Research Literature Retrieval Guide Edited by Y. S. Touloukian, J. K. Gerritsen and N. Y. Moore Second edition, revised and expanded. Book 1: Pp. xxi + 819. Book 2: Pp.621. Book 3: Pp. ix + 1315. (New York: Plenum Press, 1967.) n.p.

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Heat and Mass Transfer

Preface 1.General Considerations 2.Basic Processes in Atomization 3.Drop Size Distributions of Sprays 4.Atomizers 5.Flow in Atomizers 6.Atomizer Performance 7.External Spray Charcteristics 8.Drop Evaporation 9.Drop Sizing Methods Index

Atomization and Sprays

The interaction between drops and porous matter has important applications in many fields such as painting, paper coating, design of textiles, filtration and therapeutic delivery, the latter can include also reconstructive surgery processes. Since the phenomenon of droplet impact onto a porous surface is particularly complex, a first step consists in analysing impacts on 2D structures, such as metallic porous layers. The present paper shows the case of drop impacts onto metallic meshes attached to a solid substrate. The pores are squared and not planar, due to the woven structure of the meshes: the dynamics of the flow is particulary complex, but it resembles more realistic cases. In analysing the impact of droplets of water, acetone and a mixture of glycerol and water on meshes with different pore sizes, three main outcomes were observed for both test cases: deposition, partial imbibition and penetration. Higher velocity impacts lead to droplet splashing followed by deposition, partial imbibition and penetration. A higher amount of liquid penetration is linked to a higher velocity impact, lower viscosity and a larger dimension of the pore size. A map of the regimes is proposed introducing two dimensionless numbers M and γ, that are functions of the Weber and Reynolds numbers and pore and wire sizes. Previous papers have not considered the role of the wire diameter. The two numbers allow a clear separation of the outcomes and a practical use of the results.

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Drop impact onto attached metallic meshes: liquid penetration and spreading

Wettability effect on pool boiling: a review

The paper reports an experimental analysis of impact of single drops on a solid heated surface at different temperatures, so as to consider different heat exchange regimes from nucleate boiling to film boiling. Secondary drops produced after impact were characterised by measuring size and velocity both using PDA and the analysis of high resolution images (IAT). Two impacting walls with different surface roughness were used to show the effect of this parameter on different atomisation regimes. Image analysis allowed also to define the details of the morphology of drop spreading and break-up.

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Secondary droplet atomization from single drop impact on heated surfaces

Using large scale molecular dynamics simulations, we study in detail the impact of nanometer droplets of low viscosity on flat substrates versus the wettability of the solid plate. The comparison between the molecular dynamics simulations and different macroscopic models reveals that most of these models do not correspond to the simulation results at the nanoscale, in particular for the maximal contact diameter during the nanodroplet impact (${D}_{\mathrm{max}}$). We have developed a new model for ${D}_{\mathrm{max}}$ that is in agreement with the simulation data and also takes into account the effects of the liquid-solid wettability. We also propose a new scaling for the time required to reach the maximal contact diameter ${t}_{\mathrm{max}}$ with respect to the impact velocity, which is also in agreement with the observations. With the new model for ${D}_{\mathrm{max}}$ plus the scaling found for ${t}_{\mathrm{max}}$, we present a master curve collapsing the evolution of the nanometer drop contact diameter during impact for different wettabilities and different impact velocities. We believe our results may help in designing better nanoprinters since they provide an estimation of the maximum impact velocities required to obtain a smooth and homogenous coverage of the surfaces without dry spots.

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How Wettability Controls Nanoprinting.

The paper reports on experimental investigation of multiple drop impacts on heated surfaces. A line array madeby three drops of millimetric size impacts on high effusivity surfaces at moderate Weber numbers (110-660) anddifferent surface temperatures to encompass various boiling regimes. A CCD camera and a Phase DopplerAnemometer were used to measure the size of the secondary droplets formed after impact, in a range from 5.5 mmto few mm. The effect of adjacent drop interaction was studied by comparison with single drop impact under thesame experimental conditions. The effect of drop spacing was studied by varying the array configuration. Theeffect of surface roughness is also addressed. Quantitative results are reported together with a qualitative analysisof the impact morphology. A strong effect of the surface temperature on the mean drop size was observed andrelated to the boiling regimes. The drop spacing was found to influence the drop size of secondary droplets, but theeffect is strongly related to the impact velocity.

Marco Marengo

Papers

Drop impact onto attached metallic meshes: liquid penetration and spreading

Wettability effect on pool boiling: a review

Secondary droplet atomization from single drop impact on heated surfaces

How Wettability Controls Nanoprinting.

Drop array impacts on heated surfaces: secondary atomization characteristics