Polyelectrolyte thin films composed of alternating layers were spin-assembled by sequentially dropping cationic and anionic aqueous solutions onto a spinning substrate. In this work, we show the applicability of our technique to multiple systems and present two methods for producing linear film growth. The polycations used were PEI (poly(ethylenimine), PDDA (poly(diallyldimethylammonium chloride), PAH (poly(allylamine hydrochloride), and two poly(propylenimine) dendrimers (generations 3.0 and 4.0). The polyanions used were PAZO (poly[1-[4-(3-carboxy-4-hydroxy-phenylazo)benzene sulfonamido]-1,2-ethanediyl, sodium salt]), PSS (poly(styrenesulfonate)), and PAA (poly(acrylic acid)). Layer thicknesses for all systems were determined using single-wavelength ellipsometry. UV−vis spectroscopy was used to measure deposition amounts in films containing the chromophoric polyanions PAZO and PSS. We demonstrate the ability to spin-assemble multilayered thin films up to 50 bilayers with linear increases in deposition amount between bilayers. Additionally, we show that layers of a single polyelectrolyte species can be spin-assembled with multiple deposition cycles in which consistent amounts are deposited in each cycle. In a comparison of films built from two dendrimer generations, films incorporating generation 3.0 dendrimer and PAZO show signs of higher interpenetration between layers and a more collapsed film structure than films assembled from generation 4.0 dendrimer and PAZO. Our results also suggest that a substrate effect influences the packing density of the first few bilayers, eventually dissipating around a film thickness of 50−80 Å.

In this paper we propose an API to pause and resume task execution depending on external events. We leverage this generic API to improve the interoperability between MPI synchronous communication primitives and tasks. When an MPI operation blocks, the task running is paused so that the runtime system can schedule a new task on the core that became idle. Once the MPI operation is completed, the paused task is put again on the runtime system's ready queue. We expose our proposal through a new MPI threading level which we implement through two approaches.The first approach is an MPI wrapper library that works with any MPI implementation by intercepting MPI synchronous calls, implementing them on top of their asynchronous counterparts. In this case, the task-based runtime system is also extended to periodically check for pending MPI operations and resume the corresponding tasks once MPI operations complete. The second approach consists in directly modifying the MPICH runtime system, a well-known implementation of MPI, to directly call the pause/resume API when a synchronous MPI operation blocks and completes, respectively.Our experiments reveal that this proposal not only simplifies the development of hybrid MPI+OpenMP applications that naturally overlap computation and communication phases; it also improves application performance and scalability by removing artificial dependencies across communication tasks. Using the comm thread approach of the hybrid MPI+SMPSs programming model [9], authors allowed to exploit distant parallelism separated by taskified MPI calls. These tasks were also identified as communication tasks and were executed by an additional thread called communication thread. The runtime's task scheduler could reorder the execution of communication and computational tasks in such a way that communication can happen as soon as possible, increasing the parallelism within and across MPI processes. That proposal requires changes to the programming model to allow to identify ahead of time those tasks that have blocking-like behavior. In addition, only one thread can execute them, and it must do so in sequential order. Hence, this solution

No abstract

A well-established problem in surface structure determination is the experimental difficulty in distinguishing a uniformly incommensurate structure from a domain wall structure. Surface extended x-ray absorption fine structure (SEXAFS), as a local structural probe, should provide an unambiguous means of making the distinction. We have studied chlorine adsorbed on Ag{110} by SEXAFS at several coverages. The Cl–Ag nearest-neighbor distance, 2.56±0.04 Å, is found to be substantially shorter than on the {111} and {100} faces, and is independent of coverage. At θ=0.75 an incommensurate phase is formed. A multishell analysis (up to 5.6 Å) allowed a complete structural determination and has, for the first time, confirmed our expectation. The Cl–Cl spacing unambiguously favors a domain wall structure. Structural models have been tested by laser simulation experiments, and by kinematic analysis of the low-energy electron diffraction data.

Spray freezing offers a novel manufacturing route to fine powders with controlled crystalline structures.Here we simulate this process by freezing (using a cold dry air flow) suspended 2-mm diameter droplets of cocoa butter such that X-ray diffraction observation of the droplet's evolving crystalline structure is possible in situ. Initially the Form I polymorph is observed in the droplets: this transforms to Form II over a few minutes and then to Form III over a few hours, even at 0°C. If the droplet is then warmed to 24°C, further transformation to Form IV and then Form V occur over approximately 2 h. These phase transformations are similar to those which would be expected in a bulk cocoa butter sample, but occur significantly faster in the droplets. Small crystal sizes in the frozen droplet, resulting from the droplet's low Biot number (and thus even temperature distribution), is postulated as being the cause for the unexpectedly rapid evolution in the crystal habit.