Avalanche dynamics are found in many phenomena, from earthquakes to the evolution of species. They can also be found in vortex matter when a type-II superconductor is externally driven, for example, by an increasing 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 may provide an interesting test scenario for nonequilibrium dynamics theory. In contrast to the equilibrium phases of vortex matter in type-II superconductors, the corresponding dynamical phases-in which avalanches can play a role-are only beginning to be studied. This article reviews relevant experiments performed in the last decade or so, emphasizing the ability of different experimental techniques to establish the nature and statistical properties of avalanche behavior. CONTENTS

The phenomenon of herding is a very general feature of the collective behavior of many species in panic conditions, including humans. It has been predicted theoretically that panic-induced herding in individuals confined to a room can produce a nonsymmetrical use of two identical exit doors. Here we demonstrate the existence of that phenomenon in experiments, using ants as a model of pedestrians. We show that ants confined to a cell with two symmetrically located exits use both exits in approximately equal proportions to abandon it in normal conditions but prefer one of the exits if panic is created by adding a repellent fluid. In addition, we are able to reproduce the observed escape dynamics in detail using a modification of a previous theoretical model that includes herding associated with a panic parameter as a central ingredient. Our experimental results, combined with theoretical models, suggest that some features of the collective behavior of humans and ants can be quite similar when escaping under panic.

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 pile of grains. 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 on average preceded by continuous, detectable variations in the internal structure of the pile that are monitored in order to achieve prediction.

We quantitatively study the transport of E. coli near the walls of confined microfluidic channels, and in more detail along the edges formed by the interception of two perpendicular walls. Our experiments establish the connection between bacteria motion at the flat surface and at the edges and demonstrate the robustness of the upstream motion at the edges. Upstream migration of E. coli at the edges is possible at much larger flow rates compared to motion at the flat surfaces. Interestingly, the bacteria speed at the edges mainly results from collisions between bacteria moving along this single line. We show that upstream motion not only takes place at the edge but also in an "edge boundary layer" whose size varies with the applied flow rate. We quantify the bacteria fluxes along the bottom walls and the edges and show that they result from both the transport velocity of bacteria and the decrease of surface concentration with increasing flow rate due to erosion processes. We rationalize our findings as a function of the local variations of the shear rate in the rectangular channels and hydrodynamic attractive forces between bacteria and walls.

The dendritic patterns of magnetic flux motion formed during field penetration into an MgB 2 film were observed using magneto-optic imaging. To investigate the origin of the dendrites, experiments were performed where the sample was partially covered with an thermally conducting foil serving as an efficient heat sink. We observed that the dendrites are formed only in areas lacking the thermal conductor. When dendrites develop in the uncovered part they never invade into the covered region. The results strongly suggest that the dendritic instability is thermal in origin. Ó 2001 Published by Elsevier Science B.V.

An object falling in a fluid reaches a terminal velocity when the drag force and its weight are balanced. Contrastingly, an object impacting into a granular medium rapidly dissipates all its energy and comes to rest always at a shallow depth. Here we study, experimentally and theoretically, the penetration dynamics of a projectile in a very long silo filled with expanded polystyrene particles. We discovered that, above a critical mass, the projectile reaches a terminal velocity and, therefore, an endless penetration.

Dispersion and migration of bacteria under flow in tortuous and confined structures such as porous or fractured materials is related to a large spectrum of practical interest, but is still poorly understood.Here, we address the question of transport and dispersion of an E. coli suspension flowing through a micro-fluidic channel with a funnel-like constriction in its center. We show a counter-intuitive symmetry breaking of the bacterial concentration, which increases significantly past the funnel. This concentration

Key Points:• The settling depth in granular media is independent of gravity • The settling time scales like g −1∕2• Layering driven by granular sedimentation should be similar Abstract While the penetration of objects into granular media is well-studied, there is little understanding of how objects settle in gravities, g eff , different from that of Earth-a scenario potentially relevant to the geomorphology of planets and asteroids and also to their exploration using man-made devices. By conducting experiments in an accelerating frame, we explore g eff ranging from 0.4 g to 1.2 g. Surprisingly, we find that the rest depth is independent of g eff and also that the time required for the object to come to rest scales like g. With discrete element modeling simulations, we reproduce the experimental results and extend the range of g eff to objects as small as asteroids and as large as Jupiter. Our results shed light on the initial stage of sedimentation into dry granular media across a range of celestial bodies and also have implications for the design of man-made, extraterrestrial vehicles and structures.A loosely packed bed of sand sits precariously on the fence between mechanically stable and flowing states. This has especially strong implications not only for the geomorphology of the Earth but for that of extraterrestrial bodies where the surface is predominantly granular [Shinbrot et al., 2004;Almeida et al., 2008;Thomas and Robinson, 2005;Asphaug, 2007;Miyamoto et al., 2007]. Beyond surface morphology, extraterrestrial exploration and development requires navigation in and on loose granular media, but little is known regarding how objects settle in granular systems with gravitational conditions different from Earth's. Such understanding may have helped prevent the difficulties encountered by the Mars rover, Spirit, as it sank into and tried to escape from a sand dune in 2009 (see, for example, http://marsrover.nasa.gov/spotlight/ 20091019a.html). Other endeavors, such as asteroid or lunar mining [Elvis, 2012], will certainly involve both navigation and construction on granular surfaces.During the last decade, our understanding of the resistance to objects penetrating into granular media under Earth-like conditions has advanced quickly [Uehara et al., 2003;Walsh et al., 2003;Boudet et al., 2006;de Vet and de Bruyn, 2007;Katsuragi and Durian, 2007;Pacheco-Vázquez et al., 2011;Katsuragi, 2012;Kondic et al., 2012;Ruiz-Suárez, 2013]. A handful of attempts have mimicked low-gravity conditions [Goldman and Umbanhowar, 2008;Brzinski and Durian, 2010;Chen et al., 2009;Constantino et al., 2011;Dorbolo et al., 2013;Brzinski et al., 2013], mainly by using air-fluidized granular beds or grains immersed in a liquid, but the main focus has typically been on the role of intruder velocity or grain friction. Here we focus exclusively on the role of gravity as an object settles into granular media. By conducting experiments in a freely falling reference frame, we are able to create true low-and high-gravity conditions as a sphere...