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We have studied experimentally transport properties in a slowly driven granular system which recently was shown to display self-organized criticality [Frette et al., Nature 379, 49 (1996)]. Tracer particles were added to a pile and their transit times measured. The distribution of transit times is a constant with a crossover to a decaying power law. The average transport velocity decreases with system size. This is due to an increase in the active zone depth with system size. The relaxation processes generate coherently moving regions of grains mixed with convection. This picture is supported by considering transport in a 1D cellular automaton modeling the experiment. The avalanches that occur when grains are dropped onto a pile illustrate the spontaneous generation of complexity in simple dynamical systems [1]. When grains are dropped onto a finite base, a pile builds up. However, it cannot become infinitely high, and, eventually, the system settles in a stationary state where the outflux over the edge of the base on average equals the influx. Intermittent flow of grains down the slope of the pile (small and large avalanches) maintain the system in this state. Bak, Tang, and Wiesenfeld constructed a 2D cellular automaton of a slowly driven dynamical system. They showed, that the "pile" spontaneously evolves, or self-organizes, into a state with avalanches of all sizes distributed according to a power law, that is, there is no internal system-specific scale. Because of the lack of any characteristic avalanche size, the system is referred to as critical [1].It has been a longstanding question whether real granular systems display self-organized criticality (SOC) when slowly driven. Recently, however, an experiment on a quasi one-dimensional pile of rice has shown that the occurrence of SOC depends on details in the grain-level dissipation mechanisms [2]. With nearly spherical grains, a characteristic avalanche size appeared, inconsistent with SOC. Only with sufficiently elongated grains, avalanches with a power-law distribution occurred. We focus in this Letter on the transport that results from avalanches in the system displaying SOC. The elongated rice grains could pack in a variety of ways, and each avalanche replaced, locally or globally, one surface configuration with another. Thus a dynamically varying medium disorder (coupled to the relaxation processes) was generated. This is conceptually different from transport in media with a quenched disorder, see e.g. Refs. [3,4]. Furthermore, in SOC systems, a small perturbation may lead to arbitrarily large avalanches, and it is not clear at all, how this affects the transport properties. Thus it is quite surprising, that there are no experiments and only a few theoretical and numerical studies on transport in systems displaying SOC [5][6][7].We have measured the transit times of colored tracer particles in the rice pile. Experimentally, we find that the distribution of transit times is essentially constant for small transit times T and decays as a power law for l...

We have studied the pearling instability induced on hollow tubular lipid vesicles by hydrophilic polymers with hydrophobic side groups along the backbone. The results show that the polymer concentration is coupled to local membrane curvature. The relaxation of a pearled tube is characterized by two different well-separated time scales, indicating two physical mechanisms. We present a model, which explains the observed phenomena and predicts polymer segregation according to local membrane curvature at late stages.

We study experimentally a coiling instability of cylindrical multilamellar stacks of phospholipid membranes, induced by polymers with hydrophobic anchors grafted along their hydrophilic backbone. We interpret our experimental results in terms of a model, in which local membrane curvature and polymer concentration are coupled. The model predicts the occurence of maximally tight coils above a threshold anchor occupancy. Indeed, only maximally tight coils are observed experimentally. Our system is unique in that coils form in the absence of twist.Comment: 3 Postscipt figure