We describe the kinematics of escape jumps in three species of 0.3 -3.0 mm-sized planktonic copepods. We find similar kinematics between species with periodically alternating power strokes and passive coasting and a resulting highly fluctuating escape velocity. By direct numerical simulations, we estimate the force and power output needed to accelerate and overcome drag. Both are very high compared with those of other organisms, as are the escape velocities in comparison to startle velocities of other aquatic animals. Thus, the maximum weight-specific force, which for muscle motors of other animals has been found to be near constant at 57 N (kg muscle) 21, is more than an order of magnitude higher for the escaping copepods. We argue that this is feasible because most copepods have different systems for steady propulsion (feeding appendages) and intensive escapes (swimming legs), with the muscular arrangement of the latter probably adapted for high force production during short-lasting bursts. The resulting escape velocities scale with body length to power 0.65, different from the size-scaling of both similar sized and larger animals moving at constant velocity, but similar to that found for startle velocities in other aquatic organisms. The relative duration of the pauses between power strokes was observed to increase with organism size. We demonstrate that this is an inherent property of swimming by alternating power strokes and pauses. We finally show that the Strouhal number is in the range of peak propulsion efficiency, again suggesting that copepods are optimally designed for rapid escape jumps.

The soft disk model previously developed and applied by Durian [Phys. Rev. Lett., 75:4780-4783 (1995)] is brought to bear on problems of foam rheology of longstanding and current interest, using two-dimensional (2D) systems. The questions at issue include the origin of the Herschel-Bulkley relation, normal stress effects (dilatancy), and localisation in the presence of wall drag. We show that even a model that incorporates only linear viscous effects at the local level gives rise to nonlinear (power-law) dependence of the limit stress on strain rate. With wall drag, shear localisation is found.Its non-exponential form and the variation of localisation length with boundary velocity are well described by a continuum model in the spirit of Janiaud et al.

Many marine zooplankters, particularly among copepods, are ''ambush feeders'' that passively wait for their prey and capture them by fast surprise attacks. This strategy must be very demanding in terms of muscle power and sensing capabilities, but the detailed mechanisms of the attacks are unknown. Using high-speed video we describe how copepods perform spectacular attacks by precision maneuvering during a rapid jump. We show that the flow created by the attacking copepod is so small that the prey is not pushed away, and that the attacks are feasible because of their high velocity (Ϸ100 mm⅐s ؊1 ) and short duration (few ms), which leaves the prey no time for escape. Simulations and analytical estimates show that the viscous boundary layer that develops around the attacking copepod is thin at the time of prey capture and that the flow around the prey is small and remains potential flow. Although ambush feeding is highly successful as a feeding strategy in the plankton, we argue that power requirements for acceleration and the hydrodynamic constraints restrict the strategy to larger (> 0.25 mm), muscular forms with well-developed prey perception capabilities. The smallest of the examined species is close to this size limit and, in contrast to the larger species, uses its largest possible jump velocity for such attacks. The special requirements to ambush feeders with such attacks may explain why this strategy has evolved to perfection only a few times among planktonic suspension feeders (few copepod families and chaetognaths).biological fluid dynamics ͉ boundary layer ͉ copepod ͉ potential flow

We investigate the formation and dynamics of sand ripples under a turbulent water flow. Our experiments were conducted in an open flume with spherical glass beads between 100 and 500 microm in diameter. The flow Reynolds number is of the order of 10,000 and the particle Reynolds number of the order of 1 to 10. We study the development of ripples by measuring their wavelength and amplitude in course of time and investigate the influence of the grain size and the flow properties. In particular, we demonstrate two different regimes according to the grain size. For fine grains, a slow coarsening process (i.e., a logarithmic increase of the wavelength and amplitude) takes place, while for coarser grains, this process occurs at a much faster rate (i.e., with a linear growth) and stops after a finite time. In the later case, a stable pattern is eventually observed. Besides, we carefully analyze the wavelength of ripples in the first stages of the instability as a function of the grain size and the shear velocity of the flow, and compare our results with other available experimental data and with theoretical predictions based on linear stability analyses.

International audienceWe investigate numerically the failure, collapse, and flow of a two-dimensional brittlegranular column over a horizontal surface. In our discrete element simulations, we consider a verticalmonolayer of spherical particles that are initially held together by tensile bonds, which can be irreversiblybroken during the collapse. This leads to dynamic fragmentation within the material during the flow.Compared to what happens in the case of a noncohesive granular column, the deposit is much rougher,and the internal stratigraphic structure of the column is not preserved during the collapse. As has beenobserved in natural rockslides, we find that the deposit consists of large blocks laying on a lower layer offine fragments. The influence of the aspect ratio of the column on the runout distance is the same as inthe noncohesive case. Finally, we show that for a given aspect ratio of the column, the runout distance ishigher when the deposit is highly fragmented, which confirms previous hypotheses proposed by Davieset al. (1999)

We give an exact formula for the velocity profile of shear localisation in a 2D foam, represented by a continuum model that incorporates a Herschel-Bulkley constitutive relation and wall drag. A more approximate treatment provides a relation between the localisation length and the boundary velocity as a power law whose exponent is explicitly determined by the input parameters of the model. This is corroborated, and its conditions for validity are clarified, by the analysis of various expansions of the exact solution. The general consequences are consistent with the recent findings of Katgert and co-workers (G. Katgert, M. Mo¨bius, and M. van Hecke, available from http://arxiv.org/abs/0711.4024 [cond-mat.soft].

Jammed soft matter systems are often modelled as dense packings of overlapping soft spheres, thus ignoring particle deformation. For 2D (and 3D) soft disks packings, close to the critical packing fraction φ c , this results in an increase of the average contact number Z with a square root in φ − φ c . Using the program PLAT, we find that in the case of idealised two-dimensional foams, close to the wet limit, Z increases linearly with φ − φ c , where φ is the gas fraction. This result is consistent with the different distributions of separations for soft disks and foams at the critical packing fraction. Thus, 2D foams close to the wet limit are not well described as random packings of soft disks, since bubbles in a foam are deformable and adjust their shape. This is not captured by overlapping circular disks.

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