When a large set of discrete bodies passes through a bottleneck, the flow may become intermittent due to the development of clogs that obstruct the constriction. Clogging is observed, for instance, in colloidal suspensions, granular materials and crowd swarming, where consequences may be dramatic. Despite its ubiquity, a general framework embracing research in such a wide variety of scenarios is still lacking. We show that in systems of very different nature and scale -including sheep herds, pedestrian crowds, assemblies of grains, and colloids- the probability distribution of time lapses between the passages of consecutive bodies exhibits a power-law tail with an exponent that depends on the system condition. Consequently, we identify the transition to clogging in terms of the divergence of the average time lapse. Such a unified description allows us to put forward a qualitative clogging state diagram whose most conspicuous feature is the presence of a length scale qualitatively related to the presence of a finite size orifice. This approach helps to understand paradoxical phenomena, such as the faster-is-slower effect predicted for pedestrians evacuating a room and might become a starting point for researchers working in a wide variety of situations where clogging represents a hindrance.
The "faster-is-slower" (FIS) effect was first predicted by computer simulations of the egress of pedestrians through a narrow exit [D. Helbing, I. J. Farkas, and T. Vicsek, Nature (London) 407, 487 (2000)]. FIS refers to the finding that, under certain conditions, an excess of the individuals' vigor in the attempt to exit causes a decrease in the flow rate. In general, this effect is identified by the appearance of a minimum when plotting the total evacuation time of a crowd as a function of the pedestrian desired velocity. Here, we experimentally show that the FIS effect indeed occurs in three different systems of discrete particles flowing through a constriction: (a) humans evacuating a room, (b) a herd of sheep entering a barn, and (c) grains flowing out a 2D hopper over a vibrated incline. This finding suggests that FIS is a universal phenomenon for active matter passing through a narrowing.
We present an experimental study of a flock passing through a narrow door. Video monitoring of daily routines in a farm has enabled us to collect a sizable amount of data. By measuring the time lapse between the passage of consecutive animals, some features of the flow regime can be assessed. A quantitative definition of clogging is demonstrated based on the passage time statistics. These display broad tails, which can be fitted by power laws with a relatively large exponent. On the other hand, the distribution of burst sizes robustly evidences exponential behavior. Finally, borrowing concepts from granular physics and statistical mechanics, we evaluate the effect of increasing the door size and the performance of an obstacle placed in front of it. The success of these techniques opens new possibilities regarding their eventual extension to the management of human crowds.
Rates of ammonia excretion, and respiration to excretion (atomic O:N) ratios were measured for three species of scleractinian coral from the Bahamas, during August 1986 and March 1987, to test the hypothesis that zooxanthellate reef species have lower rates of amino acid catabolism and higher dependence on lipid and carbohydrate catabolism than nonzooxanthellate species. Freshly collected individuals of two reef species, Montastrea annularis and Acropora cervicornis, have significantly lower mean ammonia excretion rates [51_+66 nmol (rag-at N) -1 h -1 and 192_+ 172 nmol (mg-at N) -1 h -1, respectively] than those of the tropical nonzooxanthellate species Tubastrea coccinea [257_+ 68 nmol (mg-at N ) -1 h -1]. The temperate nonzooxanthellate coral Astrangia poculata has mean excretion rates [632_+242 nmol (mg-at N ) -l h 1] which are much higher than those of all three tropical species. O : N ratios for the two reef species were generally greater than 300, while those of the nonzooxanthellate species ranged from 17 to 39 for T. coccinea and from 8 to 12 for A. poculata. The two reef species conserve nitrogen by having relatively low rates of amino acid catabolism, and support most of their metabolic needs by catabolizing the lipids and carbohydrates they receive from their zooxanthellae. The tropical nonzooxanthellate species has lower rates of ammonia excretion and respiration, and higher O : N ratios than the temperate nonzooxanthellate coral, which may be an indication that the former has less food available to it. The ammonia production rates of the reef species would support doubling times (growth rates) of the zooxanthellae of 13 to 22 d. These growth rates agree with observed rates obtained by mitotic index methods, and with suggestions that growth of zooxanthellae within many corals may be nitrogen-limited.
Toxoplasma gondii is an apicomplexan parasite that infects almost all warm-blooded animals. Little is known about how the parasite virulence in mice extrapolates to other relevant hosts. In the current study, in vitro phenotype and in vivo behavior in mice and sheep of a type II T. gondii isolate (TgShSp1) were compared with the reference type II T. gondii isolate (TgME49). The results of in vitro assays and the intraperitoneal inoculation of tachyzoites in mice indicated an enhanced virulence for the laboratory isolate, TgME49, compared to the recently obtained TgShSp1 isolate. TgShSp1 proliferated at a slower rate and had delayed lysis plaque formation compared to TgME49, but it formed more cyst-like structures in vitro. No mortality was observed in adult mice after infection with 1–105 tachyzoites intraperitoneally or with 25–2,000 oocysts orally of TgShSp1. In sheep orally challenged with oocysts, TgME49 infection resulted in sporadically higher rectal temperatures and higher parasite load in cotyledons from ewes that gave birth and brain tissues of the respective lambs, but no differences between these two isolates were found on fetal/lamb mortality or lesions and number of T. gondii-positive lambs. The congenital infection after challenge at mid-pregnancy with TgShSp1, measured as offspring mortality and vertical transmission, was different depending on the challenged host. In mice, mortality in 50% of the pups was observed when a dam was challenged with a high oocyst dose (500 TgShSp1 oocysts), whereas in sheep infected with the same dose of oocysts, mortality occurred in all fetuses. Likewise, mortality of 9 and 27% of the pups was observed in mice after infection with 100 and 25 TgShSp1 oocysts, respectively, while in sheep, infection with 50 and 10 TgShSp1 oocysts triggered mortality in 68 and 66% of the fetuses/lambs. Differences in vertical transmission in the surviving offspring were only found with the lower oocyst doses (100% after infection with 10 TgShSp1 oocysts in sheep and only 37% in mice after infection with 25 TgShSp1 oocysts). In conclusion, virulence in mice of T. gondii type II isolates may not be a good indicator to predict the outcome of infection in pregnant sheep.
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