721 research outputs found
The fluid dynamics of swimming by jumping in copepods
Author Posting. © The Authors, 2010. This is the author's version of the work. It is posted here by permission of Royal Society for personal use, not for redistribution. The definitive version was published in Journal of the Royal Society Interface 8 (2011): 1090-1103, doi:10.1098/rsif.2010.0481.Copepods swim either continuously by vibrating their feeding appendages or erratically
by repeatedly beating their swimming legs resulting in a series of small jumps. The two
swimming modes generate different hydrodynamic disturbances and therefore expose the
swimmers differently to rheotactic predators. We developed an impulsive stresslet model to
quantify the jump-imposed flow disturbance. The predicted flow consists of two counterrotating
viscous vortex rings of similar intensity, one in the wake and one around the body of
the copepod. We showed that the entire jumping flow is spatially limited and temporally
ephemeral owing to jump-impulsiveness and viscous decay. In contrast, continuous steady
swimming generates two well-extended long-lasting momentum jets both in front of and
behind the swimmer, as suggested by the well-known steady stresslet model. Based on the
observed jump-swimming kinematics of a small copepod Oithona davisae, we further
showed that jump-swimming produces a hydrodynamic disturbance with much smaller
spatial extension and shorter temporal duration than that produced by a same-size copepod
cruising steadily at the same average translating velocity. Hence, small copepods in jumpswimming
are much less detectable by rheotactic predators. The present impulsive stresslet
model improves a previously published impulsive Stokeslet model that applies only to the
wake vortex.This work was supported by National Science Foundation grants NSF OCE-0352284 &
IOS-0718506 and an award from WHOI’s Ocean Life Institute to H.J and by grants from the
Danish Research Council for independent research and the Niels Bohr Foundation to T.K
Ion holes in the hydrodynamic regime in ultracold neutral plasmas
We describe the creation of localized density perturbations, or ion holes, in an ultracold neutral
plasma in the hydrodynamic regime, and show that the holes propagate at the local ion acoustic wave
speed. We also observe the process of hole splitting, which results from the formation of a density
depletion initially at rest in the plasma. One-dimensional, two-fluid hydrodynamic simulations
describe the results well. Measurements of the ion velocity distribution also show the effects of the
ion hole and confirm the hydrodynamic conditions in the plasma
Turbulent Friction in Rough Pipes and the Energy Spectrum of the Phenomenological Theory
The classical experiments on turbulent friction in rough pipes were performed
by J. Nikuradse in the 1930's. Seventy years later, they continue to defy
theory. Here we model Nikuradse's experiments using the phenomenological theory
of Kolmog\'orov, a theory that is widely thought to be applicable only to
highly idealized flows. Our results include both the empirical scalings of
Blasius and Strickler, and are otherwise in minute qualitative agreement with
the experiments; they suggest that the phenomenological theory may be relevant
to other flows of practical interest; and they unveil the existence of close
ties between two milestones of experimental and theoretical turbulence.Comment: Accepted for publication in PRL; 4 pages, 4 figures; revised versio
A cautionary tale of virus and disease
The recent identification of the gammaretrovirus XMRV and a second gammaretrovirus of a different subtype in chronic fatigue syndrome has aroused much interest, not least among sufferers. However, it remains highly controversial whether the detection of these viruses represents true infection or laboratory artifacts
X-ray Absorption Studies of Ceria with Trivalent Dopants
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65899/1/j.1151-2916.1991.tb04328.x.pd
Reanalysis and Correction of Bed-Load Relation of Meyer-Peter and MĂĽller Using Their Own Database
Dihaploid Coffea arabica genome sequencing and assembly.
Coffea arabica which accounts for 70% of world coffee production is an allotetraploid with a genome size of approximately 1.3 Gb and is derived from the hybridization of C. canephora (710 Mb) and C. eugenioides (670 Mb). To elucidate the evolutionary history of C. arabica, and generate critical information for breeding programs, a sequencing project is underway to finalize a reference genome using a dihaploid line and a set of Menu Abstract: Dihaploid Coffea arabica Genome Sequencing and Assembly (Plant and Animal Genome XXIII Conference) https://pag.confex.com/pag/xxiii/webprogram/Paper16983.html [25/02/2015 15:00:12] 30 C. arabica accessions
Algal Toxins Alter Copepod Feeding Behavior
Using digital holographic cinematography, we quantify and compare the feeding behavior of free-swimming copepods, Acartia tonsa, on nutritional prey (Storeatula major) to that occurring during exposure to toxic and non-toxic strains of Karenia brevis and Karlodinium veneficum. These two harmful algal species produce polyketide toxins with different modes of action and potency. We distinguish between two different beating modes of the copepod’s feeding appendages–a “sampling beating” that has short durations (<100 ms) and involves little fluid entrainment and a longer duration “grazing beating” that persists up to 1200 ms and generates feeding currents. The durations of both beating modes have log-normal distributions. Without prey, A. tonsa only samples the environment at low frequency. Upon introduction of non-toxic food, it increases its sampling time moderately and the grazing period substantially. On mono algal diets for either of the toxic dinoflagellates, sampling time fraction is high but the grazing is very limited. A. tonsa demonstrates aversion to both toxic algal species. In mixtures of S. major and the neurotoxin producing K. brevis, sampling and grazing diminish rapidly, presumably due to neurological effects of consuming brevetoxins while trying to feed on S. major. In contrast, on mixtures of cytotoxin producing K. veneficum, both behavioral modes persist, indicating that intake of karlotoxins does not immediately inhibit the copepod’s grazing behavior. These findings add critical insight into how these algal toxins may influence the copepod’s feeding behavior, and suggest how some harmful algal species may alter top-down control exerted by grazers like copepods
Risk Factors for Cervical Precancer and Cancer in HIV-Infected, HPV-Positive Rwandan Women
Although cervical cancer is an AIDS-defining condition, infection with human immunodeficiency virus (HIV) may only modestly increase the risk of cervical cancer. There is a paucity of information regarding factors that influence the natural history of human papillomavirus (HPV) in HIV-infected women. We examined factors associated with cervical intraepithelial neoplasia grade 3 or cancer (CIN3+) in Rwandan women infected with both HIV and HPV (HIV+/HPV+).In 2005, 710 HIV+ Rwandan women ≥25 years enrolled in an observational cohort study; 476 (67%) tested HPV+. Each woman provided sociodemographic data, CD4 count, a cervical cytology specimen and cervicovaginal lavage (CVL), which was tested for >40 HPV genotypes by MY09/MY11 PCR assay. Logistic regression models calculated odds ratios (OR) and 95% confidence intervals (CI) of associations of potential risk factors for CIN3+ among HIV+/HPV+ women.Of the 476 HIV+/HPV+ women 42 (8.8%) were diagnosed with CIN3+. Factors associated with CIN3+ included ≥7 (vs. 0-2) pregnancies, malarial infection in the previous six months (vs. never), and ≥7 (vs. 0-2) lifetime sexual partners. Compared to women infected by non-HPV16 carcinogenic HPV genotypes, HPV16 infection was positively associated and non-carcinogenic HPV infection was inversely associated with CIN3+. CD4 count was significantly associated with CIN3+ only in analyses of women with non-HPV16 carcinogenic HPV (OR = 0.62 per 100 cells/mm(3), CI = 0.40-0.97).In this HIV+/HPV+ population, lower CD4 was significantly associated with CIN3+ only in women infected with carcinogenic non-HPV16. We found a trend for higher risk of CIN3+ in HIV+ women reporting recent malarial infection; this association should be investigated in a larger group of HIV+/HPV+ women
- …