Liquid films flowing over rough substrates are influenced by asperities whose sizes are comparable to the film thickness. We investigate flows driven by jet impact on a regular array of micron-size posts. We show that the topography modifies the size and the shape of the observed hydraulic jumps: in addition to circular jumps, we obtain a variety of stable polygonal shapes. We rationalize our results by considering a leakage flow through the texture and the free-surface flow above it, coupled by an effective slip boundary condition accounting for the symmetry of the texture. This model is in good agreement with our experiments and allows us to account for the interplay between flow properties and roughness parameters, which has applicability in many other thin-film configurations.
International audienceWe analyze the inertia dominated flow of thin liquid films on microtextured substrates, which here are assemblies of micron-size posts arranged on regular lattices. We focus on situations for which the thin-film thickness and the roughness characteristic length scale are of the same order of magnitude, i.e., a few hundred microns. We assume that the liquid flows isotropically through the roughness at a flow rate that depends on the geometrical features of the porous layer; above the texture, the flow is characterized by a larger Reynolds number and modeled using a boundary layer approach. The influence of the microtexture on the thin-film flow above the microposts is captured by a reduction of the flow rate due to the leakage flow through the texture and a slip boundary condition, which depends on the flow direction as well as on the lattice properties. In this way, the velocity field in the free surface flow adopts the symmetry of the microtexture underneath. The results of this model are in good agreement with experimental observations obtained for thin-film flows formed upon jet impact on microtextures. The characteristics of the polygonal hydraulic jumps that we obtain depend on both the jet parameters and the topographical features of the surface roughness. We use the measurements and the numerical predictions to estimate the flow rate through the shallow porous layer and the effective slip length for this inertia dominated flow regime. We also discuss the limitations of the model
The variation with illumination of the grain boundary (GB) barrier height EB and of the effective recombination velocity Seff is calculated by means of a self-consistent procedure which takes into account the bending of the minority carrier quasi-Fermi level in the GB space-charge region and in the GB quasi-neutral region. The GB interface states have been assumed to be uniformly distributed in a half-filled band whose width and position in the band gap can vary. Seff is nearly proportional to exp(EB/kT), only when EB is sufficiently low. For high EB, Seff is limited by the thermal velocity. The influence of the density of interface states and the grain doping concentration has been studied. The experimental results obtained with Silso–Wacker polycrystalline silicon show that the grain boundaries present different behaviors.
2014 Les photocourants, les longueurs de diffusion (L) et les vitesses de recombinaison aux joints de grains (s) ont été mesurés à l'aide d'un système de balayage optique sur des échantillons de silicium polycristallin de type N et P. Selon le type des grains, L varie entre 5 03BCm et 150 03BCm et s est supérieur à 104 cm.s-1. Les resultats concernant les mesures locales des longueurs de diffusion montrent qu'au voisinage des joints de grains s'exercent des fortes contraintes. Il semble que les potentiels de barrière aux joints de grains sont plus importants dans les échantillons de type N que dans ceux de type P pour expliquer la plus grande activité recombinante des joints de grains dans le silicium polycristallin de type N que dans celui de type P.
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