42,518 research outputs found
Spreading Dynamics of Nanodrops: A Lattice Boltzmann Study
Spreading of nano-droplets is an interesting and technologically relevant
phenomenon where thermal fluctuations lead to unexpected deviations from
well-known deterministic laws. Here, we apply the newly developed fluctuating
non-ideal lattice Boltzmann method [Gross et al., J. Stat. Mech., P03030
(2011)] for the study of this issue. Confirming the predictions of Davidovich
and coworkers [PRL 95, 244905 (2005)], we provide the first independent
evidence for the existence of an asymptotic, self-similar noise-driven
spreading regime in both two- and three-dimensional geometry. The cross over
from the deterministic Tanner's law, where the drop's base radius grows (in
3D) with time as and the noise dominated regime where is also observed by tuning the strength of thermal noise.Comment: 5 page
Intermittency and roughening in the failure of brittle heterogeneous materials
Stress enhancement in the vicinity of brittle cracks makes the macro-scale
failure properties extremely sensitive to the micro-scale material disorder.
Therefore: (i) Fracturing systems often display a jerky dynamics, so-called
crackling noise, with seemingly random sudden energy release spanning over a
broad range of scales, reminiscent of earthquakes; (ii) Fracture surfaces
exhibit roughness at scales much larger than that of material micro-structure.
Here, I provide a critical review of experiments and simulations performed in
this context, highlighting the existence of universal scaling features,
independent of both the material and the loading conditions, reminiscent of
critical phenomena. I finally discuss recent stochastic descriptions of crack
growth in brittle disordered media that seem to capture qualitatively - and
sometimes quantitatively - these scaling features.Comment: 38 pages, invited review for J. Phys. D cluster issue on "Fracture:
from the Atomic to the Geophysics Scale
Form Perception
National Science Foundation (SBE-0354378); Office of Naval Research (N00014-01-1-0624
Laminar Cortical Dynamics of 3D Surface Perception: Stratification, transparency, and Neon Color Spreading
How does the laminar organization of cortical circuitry in areas VI and V2 give rise to 3D percepts of stratification, transparency, and neon color spreading in response to 2D pictures and 3D scenes? Psychophysical experiments have shown that such 3D percepts are sensitive to whether contiguous image regions have the same relative contrast polarity (dark-light or lightdark), yet long-range perceptual grouping is known to pool over opposite contrast polarities. The ocularity of contiguous regions is also critical for neon color spreading: Having different ocularity despite the contrast relationship that favors neon spreading blocks the spread. In addition, half visible points in a stereogram can induce near-depth transparency if the contrast relationship favors transparency in the half visible areas. It thus seems critical to have the whole contrast relationship in a monocular configuration, since splitting it between two stereogram images cancels the effect. What adaptive functions of perceptual grouping enable it to both preserve sensitivity to monocular contrast and also to pool over opposite contrasts? Aspects of cortical development, grouping, attention, perceptual learning, stereopsis and 3D planar surface perception have previously been analyzed using a 3D LAMINART model of cortical areas VI, V2, and V4. The present work consistently extends this model to show how like-polarity competition between VI simple cells in layer 4 may be combined with other LAMINART grouping mechanisms, such as cooperative pooling of opposite polarities at layer 2/3 complex cells. The model also explains how the Metelli Rules can lead to transparent percepts, how bistable transparency percepts can arise in which either surface can be perceived as transparent, and how such a transparency reversal can be facilitated by an attention shift. The like-polarity inhibition prediction is consistent with lateral masking experiments in which two f1anking Gabor patches with the same contrast polarity as the target increase the target detection threshold when they approach the target. It is also consistent with LAMINART simulations of cortical development. Other model explanations and testable predictions will also be presented.Air Force Office of Naval Research (F49620-01-1-0397); Office of Naval Research (N00014-01-1-0624
Friction law for dense granular flows: application to the motion of a mass down a rough inclined plane
The problem of the spreading of a granular mass released at the top of a
rough inclined plane was investigated. We experimentally measure the evolution
of the avalanche from the initiation up to the deposit using a Moir\'e image
processing technique. The results are quantitatively compared with the
prediction of an hydrodynamic model based on depth averaged equations. In the
model, the interaction between the flowing layer and the rough bottom is
described by a non trivial friction force whose expression is derived from
measurements on steady uniform flows. We show that the spreading of the mass is
quantitatively predicted by the model when the mass is released on a plane free
of particles. When an avalanche is triggered on an initially static layer, the
model fails in quantitatively predicting the propagation but qualitatively
captures the evolution.Comment: 19 pages, 10 figures, to be published in J. Fluid Mec
Solidification of liquid metal drops during impact
Hot liquid metal drops impacting onto a cold substrate solidify during their
subsequent spreading. Here we experimentally study the influence of
solidification on the outcome of an impact event. Liquid tin drops are impacted
onto sapphire substrates of varying temperature. The impact is visualised both
from the side and from below, which provides a unique view on the
solidification process. During spreading an intriguing pattern of radial
ligaments rapidly solidifies from the centre of the drop. This pattern
determines the late-time morphology of the splat. A quantitative analysis of
the drop spreading and ligament formation is supported by scaling arguments.
Finally, a phase diagram for drop bouncing, deposition and splashing as a
function of substrate temperature and impact velocity is provided
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