52 research outputs found
Influence of friction on granular segregation
Vertical shaking of a mixture of small and large beads can lead to
segregation where the large beads either accumulate at the top of the sample,
the so called Brazil Nut effect (BNE), or at the bottom, the Reverse Brazil Nut
effect (RBNE). Here we demonstrate experimentally a sharp transition from the
RBNE to the BNE when the particle coefficient of friction increases due to
aging of the particles. This result can be explained by the two competing
mechanisms of buoyancy and sidewall-driven convection, where the latter is
assumed to grow in strength with increasing friction.Comment: 3 pages, 2 figure
Nucleation in sheared granular matter
We present an experiment on crystallization of packings of macroscopic
granular spheres. This system is often considered to be a model for thermally
driven atomic or colloidal systems. Cyclically shearing a packing of frictional
spheres, we observe a first order phase transition from a disordered to an
ordered state. The ordered state consists of crystallites of mixed FCC and HCP
symmetry that coexist with the amorphous bulk. The transition, initiated by
homogeneous nucleation, overcomes a barrier at 64.5% volume fraction.
Nucleation consists predominantly of the dissolving of small nuclei and the
growth of nuclei that have reached a critical size of about ten spheres
Stationary state volume fluctuations in a granular medium
A statistical description of static granular material requires ergodic
sampling of the phase space spanned by the different configurations of the
particles. We periodically fluidize a column of glass beads and find that the
sequence of volume fractions phi of post-fluidized states is history
independent and Gaussian distributed about a stationary state. The standard
deviation of phi exhibits, as a function of phi, a minimum corresponding to a
maximum in the number of statistically independent regions. Measurements of the
fluctuations enable us to determine the compactivity X, a temperature-like
state variable introduced in the statistical theory of Edwards and Oakeshott
[Physica A {\bf 157}, 1080 (1989)].Comment: published with minor change
Onset of mechanical stability in random packings of frictional spheres
Using sedimentation to obtain precisely controlled packings of noncohesive
spheres, we find that the volume fraction of the loosest
mechanically stable packing is in an operational sense well defined by a limit
process. This random loose packing volume fraction decreases with decreasing
pressure and increasing interparticle friction coefficient . Using
X-ray tomography to correct for a container boundary effect that depends on
particle size, we find for rough particles in the limit a new lower
bound, .Comment: significantly revised, published versio
Correlation between Voronoi volumes in disc packings
We measure the two-point correlation of free Voronoi volumes in binary disc
packings, where the packing fraction ranges from 0.8175 to
0.8380. We observe short-ranged correlations over the whole range of and anti-correlations for . The spatial extent of
the anti-correlation increases with while the position of the
maximum of the anti-correlation and the extent of the positive correlation
shrink with . We conjecture that the onset of anti-correlation
corresponds to dilatancy onset in this system
Phase transition in a static granular system
We find that a column of glass beads exhibits a well-defined transition
between two phases that differ in their resistance to shear. Pulses of
fluidization are used to prepare static states with well-defined particle
volume fractions in the range 0.57-0.63. The resistance to shear is
determined by slowly inserting a rod into the column of beads. The transition
occurs at for a range of speeds of the rod.Comment: 4 pages, 4 figures. The paper is significantly extended, including
new dat
An invariant distribution in static granular media
We have discovered an invariant distribution for local packing configurations
in static granular media. This distribution holds in experiments for packing
fractions covering most of the range from random loose packed to random close
packed, for beads packed both in air and in water. Assuming only that there
exist elementary cells in which the system volume is subdivided, we derive from
statistical mechanics a distribution that is in accord with the observations.
This universal distribution function for granular media is analogous to the
Maxwell-Boltzmann distribution for molecular gasses.Comment: 4 pages 3 figure
An environmental justice perspective on ecosystem services
Mainstreaming of ecosystem service approaches has been proposed as one path toward sustainable development. Meanwhile, critics of ecosystem services question if the approach can account for the multiple values of ecosystems to diverse groups of people, or for aspects of inter- and intra-generational justice. In particular, an ecosystem service approach often overlooks power dimensions and capabilities that are core to environmental justice. This article addresses the need for greater guidance on incorporating justice into ecosystem services research and practice. We point to the importance of deep engagement with stakeholders and rights holders to disentangle contextual factors that moderate justice outcomes on ecosystem service attribution and appropriation in socio-political interventions. Such a holistic perspective enables the integration of values and knowledge plurality for enhancing justice in ecosystem services research. This broadened perspective paves a way for transformative ecosystem service assessments, management, and research, which can help inform and design governance structures that nourish human agency to sustainably identify, manage, and enjoy ecosystem services for human wellbeing
Does ohmic heating influence the flow field in thin-layer electrodeposition?
In thin-layer electrodeposition the dissipated electrical energy leads to a
substantial heating of the ion solution. We measured the resulting temperature
field by means of an infrared camera. The properties of the temperature field
correspond closely with the development of the concentration field. In
particular we find, that the thermal gradients at the electrodes act like a
weak additional driving force to the convection rolls driven by concentration
gradients.Comment: minor changes: correct estimation of concentration at the anode,
added Journal-re
Experimental investigation of the initial regime in fingering electrodeposition: dispersion relation and velocity measurements
Recently a fingering morphology, resembling the hydrodynamic Saffman-Taylor
instability, was identified in the quasi-two-dimensional electrodeposition of
copper. We present here measurements of the dispersion relation of the growing
front. The instability is accompanied by gravity-driven convection rolls at the
electrodes, which are examined using particle image velocimetry. While at the
anode the theory presented by Chazalviel et al. describes the convection roll,
the flow field at the cathode is more complicated because of the growing
deposit. In particular, the analysis of the orientation of the velocity vectors
reveals some lag of the development of the convection roll compared to the
finger envelope.Comment: 11 pages, 15 figures, REVTEX 4; reference adde
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