arXiv subjects

E. Altshuler

Publications and source records attributed to E. Altshuler.

At least 37 records · Page 2Linked to original sources

arXiv2016

Sink vs. tilt penetration into shaken dry granular matter: the role of foundation

We study the behavior of cylindrical objects as they sink into a dry granular bed fluidized due to lateral oscillations, in order to shed light on human constructions and other objects. Somewhat unexpectedly, we have found that, within a large range of lateral shaking powers, cylinders with flat bottoms sink vertically, while those with a "foundation" consisting in a shallow ring attached to their bottom, tilt besides sinking. The latter scenario seems to dominate independently from the nature of the foundation when strong enough lateral vibrations are applied. We are able to reproduce the observed behavior by quasi-2D numerical simulations, and the vertical sink dynamics with the help of a Newtonian equation of motion for the intruder.

cond-mat.soft↗

arXiv2015

Simple cellular automata to mimic foraging ants submitted to abduction

Many species of ants forage by building up two files: an outbound one moving from the nest to the foraging area, and a nestbound one, returning from it to the nest. Those files are eventually submitted to different threats. If the danger is concentrated at one point of the file, one might expect that ants returning to the nest will pass danger information to their nestmates moving in the opposite direction towards the danger area. In this paper, we construct simple cellular automata models for foraging ants submitted to localized abduction, were danger information is transmitted using different protocols, including the possibility of no transmission. The parameters we have used in the simulations have been estimated from actual experiments under natural conditions. So, it would be easy to test our information-transmission hypothese in real experiments. Preliminary experimental results published elsewhere suggest that the behavior of foraging ants of the species Atta insularis is best described using the hypothesis of no transmission of danger information.

nlin.AO↗

arXiv2015

Uninformed sacrifice: evidence against long-range alarm transmission in foraging ants exposed to a localized perturbation

It is well stablished that danger information can be transmitted by ants through relatively small distances, provoking either a state of alarm when they move away from potentially dangerous stimulus, or charge toward it aggressively. There is almost no knowledge if danger information can be transmitted along large distances. In this paper, we perturb leaf cutting ants of the species Atta insularis while they forage in their natural evioronment at a certain point of the foraging line, so ants make a "U" turn to escape from the danger zone and go back to the nest. Our results strongly suggest that those ants do not transmit "danger information" to other nestmates marching towards the danger area. The individualistic behavior of the ants returning from the danger zone results in a depression of the foraging activity due to the systematic sacrifice of non-informed individuals.

q-bio.PE↗

arXiv2015

In-plane transport anisotropy in BSCCO-Ag multi-filamentary tapes

Composite structures such as High-Tc multi-filamentary tapes display a complex anisotropy arising from the combination of the intrinsic anisotropy of the Bi-2223 grains, and that associated to the superconducting phase distribution in the superconductor-metal composite, as well as cracks and other defects. In this paper we characterize the in-plane anisotropy of BSCCO-Ag tapes, i.e., the difference between the transport properties along the longitudinal axis and those along the transverse direction also lying on the wide face of the tape. In particular, we demonstrate that the dissipation associated to transport along the transverse direction approaches that of the longitudinal direction as the temperature or the current increase, which may be relevant to transport applications in situations where the superconducting properties have significantly degraded.

cond-mat.supr-con↗

arXiv2013

Extraterrestrial sink dynamics in granular matter

A loosely packed bed of sand sits precariously on the fence between mechanically stable and flowing states. This has especially strong implications for animals or vehicles needing to navigate sandy environments, which can sink and become stuck in a "dry quicksand" if their weight exceeds the yield stress of this fragile matter. While it is known that the contact stresses in these systems are loaded by gravity, very little is known about the sinking dynamics of objects into loose granular systems under gravitational accelerations different from the Earth's (g). A fundamental understanding of how objects sink in different gravitational environments is not only necessary for successful planetary navigation and engineering, but it can also improve our understanding of celestial impact dynamics and crater geomorphology. Here we perform and explain the first systematic experiments of the sink dynamics of objects into granular media in different gravitational accelerations. By using an accelerating experimental apparatus, we explore gravitational conditions ranging from 0.4g to 1.2g. With the aid of discrete element modeling simulations, we reproduce these results and extend this range to include objects as small as asteroids and as large as Jupiter. Surprisingly, we find that the final sink depth is independent of the gravitational acceleration, an observation with immediate relevance to the design of future extraterrestrial structures land-roving spacecraft. Using a phenomenological equation of motion that includes a gravity-loaded frictional term, we are able to quantitatively explain the experimental and simulation results.

cond-mat.soft↗

arXiv2011

Upstream contamination in water pouring

We report the observation of upstream transport of floating particles when clear water is poured on the surface of a flat water surface on which mate or chalk particles are sprinkled. As a result, particles originally located only at the surface of the lower container can contaminate the upper water source by "riding" on vorticial water currents. We speculate that Marangoni forces in combination with geometry-induced vortices may explain the observed phenomenon.

physics.flu-dyn↗

arXiv2009

Two-stage dissipation in a superconducting microbridge: Experiment and modeling

Using fluorescent microthermal imaging we have investigated the origin of "two-step" behavior in I-V curves for a current-carrying YBa_2Cu_3O_x superconducting bridge. High resolution temperature maps reveal that as the applied current increases the first step in the voltage corresponds to local dissipation (hot spot), whereas the second step is associated with onset of global dissipation throughout the entire bridge. A quantitative explanation of the experimental results is provided by a simple model for an inhomogeneous superconductor, assuming that the hot spot nucleates at a location with slightly depressed superconducting properties.

cond-mat.supr-con↗

arXiv2008

Avalanche prediction in Self-organized systems

It is a common belief that power-law distributed avalanches are inherently unpredictable. This idea affects phenomena as diverse as evolution, earthquakes, superconducting vortices, stock markets, etc; from atomic to social scales. It mainly comes from the concept of ``Self-organized criticality" (SOC), where criticality is interpreted in the way that at any moment, any small avalanche can eventually cascade into a large event. Nevertheless, this work demonstrates experimentally the possibility of avalanche prediction in the classical paradigm of SOC: a sandpile. By knowing the position of every grain in a two-dimensional pile, avalanches of moving grains follow a distinct power-law distribution. Large avalanches, although uncorrelated, are preceded by continuous, detectable variations in the internal structure of the pile that are monitored in order to achieve prediction.

cond-mat.dis-nn↗

arXiv2007

Revolving rivers in sandpiles: from continuous to intermittent flows

In a previous paper [Phys. Rev. Lett. 91, 014501 (2003)], the mechanism of "revolving rivers" for sandpile formation is reported: as a steady stream of dry sand is poured onto a horizontal surface, a pile forms which has a river of sand on one side owing from the apex of the pile to the edge of the base. For small piles the river is steady, or continuous. For larger piles, it becomes intermittent. In this paper we establish experimentally the "dynamical phase diagram" of the continuous and intermittent regimes, and give further details of the piles topography, improving the previous kinematic model to describe it and shedding further light on the mechanisms of river formation. Based on experiments in Hele-Shaw cells, we also propose that a simple dimensionality reduction argument can explain the transition between the continuous and intermittent dynamics.

cond-mat.soft↗

arXiv2006

Uphill solitary waves in granular flows

We have experimentally observed a new phenomenon in the surface flow of a granular material. A heap is constructed by injecting sand between two vertical glass plates separated by a distance much larger than the average grain size, with an open boundary. As the heap reaches the open boundary, "soliton-like" fluctuations appear on the flowing layer, and move "up the hill" (i.e., against the direction of the flow). We explain the phenomenon in the context of stop-and-go traffic models, and show that soliton-like behavior is allowed within a Saint-Venant description for the granular flow.

cond-mat.soft↗

arXiv2005

Local transport measurements in laser patterned BSCCO-Ag tapes

The determination of inter- and intra-filament transport characteristics in superconducting composites such as BSCCO-Ag tapes is of great importance for material evaluation towards applications. Most attempts to separate the two contributions have relied on indirect methods based on magnetic measurements such as SQUID or magneto-optic imaging techniques. Here we show that laser patterning of superconducting BSCCO-Ag tapes constitutes a simple approach to measure local transport properties in a direct way, even able to separate inter- and intra-filament contributions to the overall transport behavior of the sample. Our technique is potentially useful for sub-millimeter devices based on superconducting tapes.

cond-mat.supr-con↗

arXiv2005

Quasiperiodic events in an earthquake model

We introduce a modification of the OFC earthquake model [Phys. Rev. Lett. 68, 1244 (1992)] in order to improve resemblance with the Burridge and Knopoff mechanical model and with possible laboratory experiments. A constant force continually drives the system, and thresholds are distributed randomly following a narrow distribution. We find quasiperiodic behavior in the avalanche time series with a period proportional to the degree of dissipation of the system. Periodicity is not as robust as criticality when the threshold force distribution widens; and foreshocks and aftershocks are connected to the observed periodicity.

cond-mat.stat-mech↗

arXiv2004

On the interpretation of "off the edge" avalanches

We establish both experimentally and theoretically the relation between off the edge and internal avalanches in a sandpile model, a central issue in the interpretation of most experiments in these systems. In BTW simulations and also in the experiments the size distributions of internal avalanches show power laws and critical exponents related with the dimension of the system. We show that, in a SOC scenario, the distributions of off the edge avalanches do not show power laws but follow scaling relations with critical exponents different from their analogous for the internal avalanche distributions.

cond-mat.stat-mech↗

arXiv2004

Experiments in vortex avalanches

Avalanche dynamics is found in many phenomena spanning from earthquakes to the evolution of species. It can be also found in vortex matter when a type II superconductor is externally driven, for example, by increasing the magnetic field. Vortex avalanches associated with thermal instabilities can be an undesirable effect for applications, but "dynamically driven" avalanches emerging from the competition between intervortex interactions and quenched disorder constitute an interesting scenario to test theoretical ideas related with non-equilibrium dynamics. However, differently from the equilibrium phases of vortex matter in type II superconductors, the study of the corresponding dynamical phases - in which avalanches can play a role - is still in its infancy. In this paper we critically review relevant experiments performed in the last decade or so, emphasizing the ability of different experimental techniques to establish the nature and statistical properties of the observed avalanche behavior.

cond-mat.supr-con↗

arXiv2004

Panic induced symmetry breaking in escaping ants

We report the phenomenon of symmetry breaking in ants escaping under panic. Ants confined into a cell with two symmetrically located exits use both exits in approximately equal proportions to abandon the cell in normal conditions, but prefer one of the exits if panic is created by adding a repellent fluid. This finding is consistent with the predictions of recent theoretical models for the escape of humans in panic conditions, indicating that some features of the collective behaviour of humans and ants can be similar when escaping under panic.

physics.bio-ph↗

arXiv2002

Vortex avalanches and self organized criticality in superconducting niobium

In 1993 Tang proposed [1] that vortex avalanches should produce a self organized critical state in superconductors, but conclusive evidence for this has heretofore been lacking. In the present paper, we report extensive micro-Hall probe data from the vortex dynamics in superconducting niobium, where a broad distribution of avalanche sizes scaling as a power-law for more than two decades is found. The measurements are combined with magneto-optical imaging, and show that over a widely varying magnetic landscape the scaling behaviour does not change, hence establishing that the dynamics of superconducting vortices is a SOC phenomenon.

cond-mat.supr-con↗

arXiv2002

Sandpile formation by revolving rivers

Experimental observation of a new mechanism of sandpile formation is reported. As a steady stream of dry sand is poured onto a horizontal surface, a pile forms which has a thin river of sand on one side flowing from the apex of the pile to the edge of its base. The river rotates about the pile, depositing a new layer of sand with each revolution, thereby growing the pile. For small piles the river is steady and the pile formed is smooth. For larger piles, the river becomes intermittent and the surface of the pile becomes undulating. The frequency of revolution of the river is measured as the pile grows and the results are explained with a simple scaling argument. The essential features of the system that produce the phenomena are discussed.

cond-mat.soft↗

arXiv2000

A possible scenario for thermally activated avalanches in type II superconductors

Using a simple cellular automaton with stochastic rules we show the possible emergence of thermally activated avalanches (power law distributed) in type-II superconductors. Scaling relations between the exponents characterizing these distributions and those obtained from field driven experiments are derived and proved through simulations. It is also shown that the conditions for the appearance of these avalanches are independent of the pinning mechanism. The relevance of our simulations for recently reported experimental results is also outlined.

cond-mat.supr-con↗