318 research outputs found
Pulse counting circuit which simultaneously indicates the occurrence of the nth pulse Patent
RC transistor circuit to indicate each pulse of pulse train and occurrence of nth puls
Spatio-temporal dynamics of wormlike micelles under shear
Velocity profiles in a wormlike micelle solution (CTAB in D2O) are recorded
using ultrasound every 2 s after a step-like shear rate into the shear-banding
regime. The stress relaxation occurs over more than six hours and corresponds
to the very slow nucleation and growth of the high-shear band. Moreover,
oscillations of the interface position with a period of about 50 s are observed
during the growth process. Strong wall slip, metastable states and transient
nucleation of three-band flows are also reported and discussed in light of
previous experiments and theoretical models.Comment: 4 pages, 5 figures, submitted to Phys.Rev.Let
Shift in recreational fishing catches as a function of an extreme cold event
There is an increasing recognition that the influence of extreme climate events (ECE) can be more significant in structuring ecosystem dynamics than the gradual effects of climate change. Still, our understanding of the effects of climate extremes on ecosystem services such as marine fisheries lags behind those of effects of gradual change. The significance of ECEs depends on the severity of the disturbance event and the resilience of a fish community. South Florida experienced an extreme cold spell in 2010 that provided the opportunity to study recreational fisheries resilience to ECEs. Our main goal was to examine how recreational fishing catch structures responded to the cold spell, and illustrate any spatial-specific recovery trajectory dynamics after extreme ecological responses. To address this, we implemented multivariate and nonlinear statistics on fishing guide reports for 20 recreational species. A significant shift in the catch structure occurred after the event, suggesting a high sensitivity of fish populations and fisheries in the region to ECEs. All fishing regions considered were affected, but the trajectory of the response and recovery varied across study areas. While some fish species experienced an expected decline (due to mortality), other species manifested an increase in catch. Of the main seven species considered in nonlinear models, three experienced a decline (bonefish, snook, goliath grouper), two experienced an increase (red drum, gray snapper), and the two had various weak trends (tarpon, spotted seatrout). Three years after the event, the catch structure has not returned to the original state, indicating a possible state shift, whose stability needs to be determined in future tracking of affected populations. Future work should also address the extent to which harvest may interfere with resilience to ECEs. Our work highlights the need to account for rare environmental forcing induced by ECEs to ensure the ecological and economical sustainability of key services such as recreational fisheries
Oscillations of a solid sphere falling through a wormlike micellar fluid
We present an experimental study of the motion of a solid sphere falling
through a wormlike micellar fluid. While smaller or lighter spheres quickly
reach a terminal velocity, larger or heavier spheres are found to oscillate in
the direction of their falling motion. The onset of this instability correlates
with a critical value of the velocity gradient scale
s. We relate this condition to the known complex rheology of wormlike
micellar fluids, and suggest that the unsteady motion of the sphere is caused
by the formation and breaking of flow-induced structures.Comment: 4 pages, 4 figure
Velocity profiles in shear-banding wormlike micelles
Using Dynamic Light Scattering in heterodyne mode, we measure velocity
profiles in a much studied system of wormlike micelles (CPCl/NaSal) known to
exhibit both shear-banding and stress plateau behavior. Our data provide
evidence for the simplest shear-banding scenario, according to which the
effective viscosity drop in the system is due to the nucleation and growth of a
highly sheared band in the gap, whose thickness linearly increases with the
imposed shear rate. We discuss various details of the velocity profiles in all
the regions of the flow curve and emphasize on the complex, non-Newtonian
nature of the flow in the highly sheared band.Comment: 4 pages, 5 figures, submitted to Phys. Rev. Let
Thermodynamics of the glassy state: effective temperature as an additional system parameter
A system is glassy when the observation time is much smaller than the
equilibration time. A unifying thermodynamic picture of the glassy state is
presented. Slow configurational modes are in quasi-equilibrium at an effective
temperature. It enters thermodynamic relations with the configurational entropy
as conjugate variable. Slow fluctuations contribute to susceptibilities via
quasi-equilibrium relations, while there is also a configurational term.
Fluctuation-dissipation relations also involve the effective temperature.
Fluctuations in the energy are non-universal, however. The picture is supported
by analytically solving the dynamics of a toy model.Comment: 5 pages, REVTEX. Phys. Rev. Lett, to appea
Resilience of a tropical sport fish population to a severe cold event varies across five estuaries in southern Florida
For species that are closely managed, understanding population resilience to environmental and anthropogenic disturbances (i.e., recovery trajectories across broad spatial areas) can guide which suite of management actions are available to mitigate any impacts. During January 2010, an extreme cold event in south Florida caused widespread mortality of common snook, Centropomus undecimalis, a popular sport fish. Interpretation of trends using fishery-independent monitoring data in five south Florida estuaries showed that changes in catch rates of adult snook (\u3e500 mm standard length) varied between no effects postevent to large effects and 4-yr recoveries. The reasons for the variation across estuaries are unknown, but are likely related to differences in estuary geomorphology and habitat availability (e.g., extent of deep rivers and canals) and differences in the proportions of behavior contingents (i.e., segments of the population that use divergent movement tactics) that place snook in different areas of the estuary during winter. Emerging awareness of the presence of behavior contingents, identification of overwintering sites, and improvements of abundance indices in remote nursery habitats should provide a better understanding of population resilience to disturbance events for snook. Given that changes in the frequency of short-lived, severe cold events are currently unknown, the findings and management actions described here for a tropical species living at the edge of its distribution should be useful to scientists forecasting the effects of climate change
Knocking back invasions: variable resistance and resilience to multiple cold spells in native vs. nonnative fishes
Extreme climate events can interact synergistically with invasions to dramatically alter ecosystem structure, function, and services. Yet, the effects of extreme climate events on species invasions remain unresolved. Extreme climate events may increase resources and decrease biotic resistance by causing physiological stress and/or mortality of native taxa, resulting in invasion opportunities for nonnative species. Alternatively, extreme climate events may regulate nonnative populations, preventing them from achieving dominance. We examined whether a sequence of three cold spells had a negative or positive effect on fish invasions in the coastal Everglades. We compared resistance (initial effects) and resilience (rate of recovery) to the cold spells between native fishes and the dominant nonnative invader, the Mayan cichlid, across eight populations expanding two mangroves drainages in the southern Everglades. We tracked native fish and nonnative Mayan cichlid populations for 10 yr including 3 yr pre- and 4 yr post-cold spells. In both drainages, native fishes were more resistant to the cold spells than the nonnative species. While native fishes experienced declines at only one site, nonnative Mayan numbers were reduced by 90–100% across six sites where they were abundant pre-disturbances. Four years after the last cold spell, we saw limited resilience in the affected nonnative populations. Only one of the six affected sites fully recovered, whereas the other five sites showed no recovery in Mayan cichlid numbers. The recovered site was closest to a canal, known to act as thermal refuges for nonnative fishes. In summary, cold spells can reduce nonnative abundances, but whether cold spells can effectively knock back invasions (and range expansions) by tropical/subtropical nonnative species will depend on how the frequency and severity of cold spells are affected by climate change. We propose that these mortality-causing extreme events could provide rare management opportunities late in an invasion
Thermodynamic picture of the glassy state
A picture for thermodynamics of the glassy state is introduced. It assumes
that one extra parameter, the effective temperature, is needed to describe the
glassy state. This explains the classical paradoxes concerning the Ehrenfest
relations and the Prigogine-Defay ratio. As a second part, the approach
connects the response of macroscopic observables to a field change with their
temporal fluctuations, and with the fluctuation-dissipation relation, in a
generalized non-equilibrium way.Comment: Proceedings of the Conference "Unifying Concepts in Glass Physics",
ICTP, Trieste, 15 - 18 September 199
Coexistence and Phase Separation in Sheared Complex Fluids
We demonstrate how to construct dynamic phase diagrams for complex fluids
that undergo transitions under flow, in which the conserved composition
variable and the broken-symmetry order parameter (nematic, smectic,
crystalline, etc.) are coupled to shear rate. Our construction relies on a
selection criterion, the existence of a steady interface connecting two stable
homogeneous states. We use the (generalized) Doi model of lyotropic nematic
liquid crystals as a model system, but the method can be easily applied to
other systems, provided non-local effects are included.Comment: 4 pages REVTEX, 5 figures using epsf macros. To appear in Physical
Review E (Rapid Communications
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