299,596 research outputs found

    Collective chemotactic dynamics in the presence of self-generated fluid flows

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    In micro-swimmer suspensions locomotion necessarily generates fluid motion, and it is known that such flows can lead to collective behavior from unbiased swimming. We examine the complementary problem of how chemotaxis is affected by self-generated flows. A kinetic theory coupling run-and-tumble chemotaxis to the flows of collective swimming shows separate branches of chemotactic and hydrodynamic instabilities for isotropic suspensions, the first driving aggregation, the second producing increased orientational order in suspensions of "pushers" and maximal disorder in suspensions of "pullers". Nonlinear simulations show that hydrodynamic interactions can limit and modify chemotactically-driven aggregation dynamics. In puller suspensions the dynamics form aggregates that are mutually-repelling due to the non-trivial flows. In pusher suspensions chemotactic aggregation can lead to destabilizing flows that fragment the regions of aggregation.Comment: 4 page

    Magnetic suspension options for spacecraft inertia-wheel applications

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    Design criteria for spacecraft inertia-wheel suspensions are listed. The advantages of magnetic suspensions over other suspension types for spacecraft inertia-wheel applications are cited along with the functions performed by magnetic suspension. The common designs for magnetic suspensions are enumerated. Materials selection of permanent magnets and core materials is considered

    Poisson suspensions and infinite ergodic theory

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    We investigate ergodic theory of Poisson suspensions. In the process, we establish close connections between finite and infinite measure preserving ergodic theory. Poisson suspensions thus provide a new approach to infinite measure preserving ergodic theory. Fields investigated here are mixing properties, spectral theory, joinings. We also compare Poisson suspensions to the apparently similar looking Gaussian dynamical systems.Comment: 18 page

    Geometric Cone Surfaces and (2+1)- Gravity coupled to Particles

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    We introduce the (2+1)-spacetimes with compact space of genus g and with r gravitating particles which arise by ``Minkowskian suspensions of flat or hyperbolic cone surfaces'', by ``distinguished deformations'' of hyperbolic suspensions and by ``patchworking'' of suspensions. Similarly to the matter-free case, these spacetimes have nice properties with respect to the canonical Cosmological Time Function. When the values of the masses are sufficiently large and the cone points are suitably spaced, the distinguished deformations of hyperbolic suspensions determine a relevant open subset of the full parameter space; this open subset is homeomorphic to the product of an Euclidean space of dimension 6g-6+2r with an open subset of the Teichm\"uller Space of Riemann surfaces of genus g with r punctures. By patchworking of suspensions one can produce examples of spacetimes which are not distinguished deformations of any hyperbolic suspensions, although they have the same masses; in fact, we will guess that they belong to different connected components of the parameter space.Comment: 14 pages Late

    Motility induced changes in viscosity of suspensions of swimming microbes in extensional flows

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    Suspensions of motile cells are model systems for understanding the unique mechanical properties of living materials which often consist of ensembles of self-propelled particles. We present here a quantitative comparison of theory against experiment for the rheology of such suspensions. The influence of motility on viscosities of cell suspensions is studied using a novel acoustically-driven microfluidic capillary-breakup extensional rheometer. Motility increases the extensional viscosity of suspensions of algal pullers, but decreases it in the case of bacterial or sperm pushers. A recent model [Saintillan, Phys. Rev. E, 2010, 81:56307] for dilute active suspensions is extended to obtain predictions for higher concentrations, after independently obtaining parameters such as swimming speeds and diffusivities. We show that details of body and flagellar shape can significantly determine macroscale rheological behaviour.Comment: 12 pages, 1 appendix, 7 figures, submitted to Soft Matter - under revie

    Bulk and interfacial stresses in suspensions of soft and hard colloids

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    We explore the influence of particle softness and internal structure on both the bulk and interfacial rheological properties of colloidal suspensions. We probe bulk stresses by conventional rheology, by measuring the flow curves, shear stress vs strain rate, for suspensions of soft, deformable microgel particles and suspensions of near hard-sphere-like silica particles. A similar behavior is seen for both kind of particles in suspensions at concentrations up to the random close packing volume fraction, in agreement with recent theoretical predictions for sub-micron colloids. Transient interfacial stresses are measured by analyzing the patterns formed by the interface between the suspensions and their own solvent, due to a generalized Saffman-Taylor hydrodynamic instability. At odd with the bulk behavior, we find that microgels and hard particle suspensions exhibit vastly different interfacial stress properties. We propose that this surprising behavior results mainly from the difference in particle internal structure (polymeric network for microgels vs compact solid for the silica particles), rather than softness alone.Comment: 20 pages, 8 figure

    Stability of extemporaneously prepared cinacalcet oral suspensions

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    Purpose The stability of extemporaneously prepared cinacalcet suspensions over 90 days was evaluated. Methods Cinacalcet 5-mg/mL suspension was prepared by triturating 30-mg cinacalcet tablets. Twelve 30-mL batches were prepared with a 1:1 mixture of Ora-Plus and either Ora-Sweet or Ora-Sweet SF (sugar free). Three suspensions of each kind were stored at both room temperature and refrigerated conditions. A 1-mL sample was taken from each bottle at 0, 7, 18, 32, 64, and 90 days. Each sample was assayed using high-performance liquid chromatography (HPLC). A new HPLC method for evaluating drug peaks of pure cinacalcet was developed. Stability was defined as retention of at least 90% of the initial drug concentration. Results The HPLC method established in this study serves as a novel assay for evaluating cinacalcet oral suspensions. For all suspensions tested at individual conditions, the concentration remained above 90% of the initial concentration for 90 days of storage with the exception of Ora-Plus and Ora-Sweet SF suspensions stored under refrigeration, which were stable for 64 days. Usual sedimentation of the suspensions occurred over time but resolved with agitation; there was no other change in visual appearance of the suspensions over the course of the 90-day study. The color and odor of the suspensions throughout the study remained unchanged with respect to the initial time point. Conclusion Extemporaneously compounded cinacalcet 5-mg/mL oral suspensions prepared with a 1:1 mixture of Ora-Plus and either Ora-Sweet or Ora-Sweet SF and stored in 2-oz amber polypropylene plastic bottles were stable for at least 64 days at room temperature and under refrigeration

    Education Interrupted: The Growing Use of Suspensions in New York City's Public Schools

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    The New York Civil Liberties Union analyzed 10 years of discipline data from New York City schools, and found that:*The total number of suspensions in New York City grew at an alarming rate over the last decade: One out of every 14 students was suspended in 2008-2009; in 1999-2000 it was one in 25. In 2008-2009, this added up to more than 73,000 suspensions.*Students with disabilities are four times more likely to be suspended than students without disabilities.*Black students, who comprise 33 percent of the student body, served 53 percent of suspensions over the past 10 years. *Black students with disabilities represent more than 50 percent of suspended students with disabilities.*Black students also served longer suspensions on average and were more likely to be suspended for subjective misconduct, like profanity and insubordination.*Suspensions are becoming longer: More than 20 percent of suspensions lasted more than one week in 2008-2009, compared to 14 percent in 1999-2000. The average length of a long-term suspension is five weeks (25 school days).*Between 2001 and 2010, the number of infractions listed in the schools' Discipline Code increased by 49 percent. During that same period, the number of zero tolerance infractions, which mandate a suspension regardless of the individual facts of the incident, increased by 200 percent.*Thirty percent of suspensions occur during March and June of each school year

    Hydrodynamic fluctuations and instabilities in ordered suspensions of self-propelled particles

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    We construct the hydrodynamic equations for {\em suspensions} of self-propelled particles (SPPs) with spontaneous orientational order, and make a number of striking, testable predictions:(i) SPP suspensions with the symmetry of a true {\em nematic} are {\em always} absolutely unstable at long wavelengths.(ii) SPP suspensions with {\em polar}, i.e., head-tail {\em asymmetric}, order support novel propagating modes at long wavelengths, coupling orientation, flow, and concentration. (iii) In a wavenumber regime accessible only in low Reynolds number systems such as bacteria, polar-ordered suspensions are invariably convectively unstable.(iv) The variance in the number N of particles, divided by the mean , diverges as 2/3^{2/3} in polar-ordered SPP suspensions.Comment: submitted to Phys Rev Let
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