The Effects of Inter-particle Attractions on Colloidal Sedimentation

We use a mesoscopic simulation technique to study the effect of short-ranged inter-particle attraction on the steady-state sedimentation of colloidal suspensions. Attractions increase the average sedimentation velocity $v_s$ compared to the pure hard-sphere case, and for strong enough attractions, a non-monotonic dependence on the packing fraction $\phi$ with a maximum velocity at intermediate $\phi$ is observed. Attractions also strongly enhance hydrodynamic velocity fluctuations, which show a pronounced maximum size as a function of $\phi$. These results are linked to a complex interplay between hydrodynamics and the formation and break-up of transient many-particle clusters.Comment: 4 pages 4 figure

Effects of interparticle attractions on colloidal sedimentation

We use a mesoscopic simulation technique to study the effect of short-ranged interparticle attractions on the steady-state sedimentation of colloidal suspensions. Attractions increase the average sedimentation velocity vs compared to the pure hard-sphere case, and for strong enough attractions, a nonmonotonic dependence on the packing fraction f with a maximum velocity at intermediate f is observed. Attractions also strongly enhance hydrodynamic velocity fluctuations, which show a pronounced maximum size as a function of f. These phenomena arise from a complex interplay between nonequilibrium hydrodynamic effects and the thermodynamics of transient cluster formation. © 2010 The American Physical Society

Slope selection-driven Ostwald ripening in ZnO thin film growth

The morphology evolution of polycrystalline ZnO films grown by pulsed laser deposition was investigated by atomic force microscopy and compared with morphologies simulated in 2 + 1 dimensions from a mesoscopic continuum model of selection of surface slopes. The distinctive feature of such an evolution is that the competition between grains gives rise to a singular grain coarsening mechanism, which although it matches the fingerprints of the Ostwald ripening, it remains operative under atypical growth conditions (temperatures as low as 0.28T melting and grains with sizes ranged between 20-500 nm) and is driven by the faceting of the grain faces. The resulting pyramidal single-crystalline grains from such a coarsening mechanism have been correlated with the enhanced ultraviolet lasing activity at room temperature of nanostructured ZnOWork supported by the projects 200960I182 (CSIC) and CCG10-UAM/MAT-5537 (DGUI-Comunidad de Madrid and Universidad Autonoma de Madrid). A.G.G. acknowledges the financial support of the MICINN Spanish Ministry under the project ESP2006-14282-C02-02 and PEI201160E05

How Peclet number affects microstructure and transient cluster aggregation in sedimenting colloidal suspensions

We study how varying the P \'eclet number (Pe) affects the steady state sedimentation of colloidal particles that interact through short-ranged attractions. By employing a hybrid molecular dynamics simulation method we demonstrate that the average sedimentation velocity changes from a non- monotonic dependence on packing fraction {\phi} at low Pe numbers, to a monotonic decrease with {\phi} at higher Pe numbers. At low Pe number the pair correlation functions are close to their equilibrium values, but as the Pe number increases, important deviations from equilibrium forms are observed. Although the attractive forces we employ are not strong enough to form permanent clusters, they do induce transient clusters whose behaviour is also affected by Pe number. In particular, clusters are more likely to fragment and less likely to aggregate at larger Pe numbers, and the probability of finding larger clusters decreases with increasing Pe number. Interestingly, the life-time of the clusters is more or less independent of Pe number in the range we study. Instead, the change in cluster distribution occurs because larger clusters are less likely to form with increasing Pe number. These results illustrate some of the subtleties that occur in the crossover from equilibrium like to purely non-equilibrium behaviour as the balance between convective and thermal forces changes.Comment: 8 page

Postcoalescence evolution of growth stress in polycrystalline films

The growth stress generated once grains coalesce in Volmer-Weber-type thin films is investigated by time-multiscale simulations comprising complementary modules of (i) finite-element modeling to address the interactions between grains happening at atomic vibration time scales (∼0.1 ps), (ii) dynamic scaling to account for the surface stress relaxation via morphology changes at surface diffusion time scales (∼μs-ms), and (iii) the mesoscopic rate equation approach to simulate the bulk stress relaxation at deposition time scales (h). On the basis of addressing the main experimental evidence reported so far on the topic dealt with, the simulation results provide key findings concerning the interplay between anisotropic grain interactions at complementary space scales, deposition conditions (such as flux and mobility), and mechanisms of stress accommodation-relaxation, which underlies the origin, nature and spatial distribution, and the flux dependence of the postcoalescence growth stressWork supported by the projects 200960I182 (CSIC), CCG10-UAM/MAT-5537 (Comunidad de Madrid and Universidad Autónoma de Madrid), and AYA2010-22032. A. G.-G. acknowledges the financial support of CSIC under Project No. 201160E05

Barley Yellow Dwarf Virus (BYDV) y los áfidos en los arrozales levantinos: una prospección

Se ha llevado a cabo una prospección de las poblaciones emigrantes de áfidos alados en el área de La Albufera de Valencia, zona donde se da el «enrojat» del arroz, enfermedad causada por una raza del Barley Yellow Dwarf Virus, y se discute aquí el potencial presumible de las distintas especies como vectores de la enfermedad. Se muestrearon dos áreas con diferente incidencia de la enfermedad, obteniéndose 30 especies, de las que ocho están descritas como vectores de tales virus. Fueron mayores las capturas en las áreas donde se cultivaba el arroz por plantel y transplante con incidencia marcada de la enfermedad, que en otras de menor incidencia. La aparente homogeneidad de la zona hace difícil comprender la razón de tales diferencias. Entre los vectores conocidos de BYDV, sólo Rhopalosiphon padi L. y Hyaiopterus pruni (L) Geof. aparecieron al principio de la estación, cuando tiene lugar la infección, con poblaciones considerables. E1 primero es el vector conocido de la enfermedad. El segundo, es de las pocas especies que tiene niveles similares de captura en ambas áreas. Ninguno de los otros áfidos vectores capturados como alados parece verdaderamente importante para la transmisión a pleno campo por lo tardío de su llegada a los campos o por razones de su ciclo vital. Se ha intentado estudiar el potencial de Hyalopterus pruni, pulgón muy abundante en los carrizos como transmisor a corta distancia a partir de estas plantas. No se han obtenido resultados positivos ni a partir de la planta, ni a partir de áfidos alimentados en arroz o avena infectadas previamente con la enfermedad.A prospection of the álate aphid was made on the Spanish central Mediterranean area of paddy rice. Their potential for Barley Yellow Dwarf transmission was discused. Areas closeby with different incidence of the disease «enrojat» as it is called, were sampled for oncoming alate aphids. More than 30 species were caught, eight of them known as potential vectors for BYDV transmission. Aphid catches were markedly bigger on the traditional nursery planted area with high incidence than in the low incidence area. The apparent homogenity of the country and the crop does not permit to ascertain the reason for such differences. Between the BYDV known vectors, only Rhopalosiphon padi L. and Hyalopterus pruni (L.) Geof. ocurred early in the season with high populations. The former one is the known vector of the rice disease. The second, as exception, appearent at similar extent in both areas of sampling. No other potential vector seems to be important on the natural transmission of the BYDV to the rice because its late arrival or due to their cycle characteristics. Au study was developed to ascertain the potential of the Hyalopterus pruni as short disunce vector for BYDV from its hosts Phragmites comunis, where is prevalent on very high population all along the year. N o positive result was obtained from this weed material or was transmitted by such aphid