2,925 research outputs found
Fixed points and limit cycles in the population dynamics of lysogenic viruses and their hosts
Starting with stochastic rate equations for the fundamental interactions
between microbes and their viruses, we derive a mean field theory for the
population dynamics of microbe-virus systems, including the effects of
lysogeny. In the absence of lysogeny, our model is a generalization of that
proposed phenomenologically by Weitz and Dushoff. In the presence of lysogeny,
we analyze the possible states of the system, identifying a novel limit cycle,
which we interpret physically. To test the robustness of our mean field
calculations to demographic fluctuations, we have compared our results with
stochastic simulations using the Gillespie algorithm. Finally, we estimate the
range of parameters that delineate the various steady states of our model.Comment: 20 pages, 16 figures, 4 table
Speckle visibility spectroscopy and variable granular fluidization
We introduce a dynamic light scattering technique capable of resolving motion
that changes systematically, and rapidly, with time. It is based on the
visibility of a speckle pattern for a given exposure duration. Applying this to
a vibrated layer of glass beads, we measure the granular temperature and its
variation with phase in the oscillation cycle. We observe several transitions
involving jammed states, where the grains are at rest during some portion of
the cycle. We also observe a two-step decay of the temperature on approach to
jamming.Comment: 4 pages, 4 figures, experimen
Binary Colloidal Alloy Test-3 and 4: Critical Point
Binary Colloidal Alloy Test - 3 and 4: Critical Point (BCAT-3-4-CP) will determine phase separation rates and add needed points to the phase diagram of a model critical fluid system. Crewmembers photograph samples of polymer and colloidal particles (tiny nanoscale spheres suspended in liquid) that model liquid/gas phase changes. Results will help scientists develop fundamental physics concepts previously cloaked by the effects of gravity
Microwave Dielectric Heating of Drops in Microfluidic Devices
We present a technique to locally and rapidly heat water drops in
microfluidic devices with microwave dielectric heating. Water absorbs microwave
power more efficiently than polymers, glass, and oils due to its permanent
molecular dipole moment that has a large dielectric loss at GHz frequencies.
The relevant heat capacity of the system is a single thermally isolated
picoliter drop of water and this enables very fast thermal cycling. We
demonstrate microwave dielectric heating in a microfluidic device that
integrates a flow-focusing drop maker, drop splitters, and metal electrodes to
locally deliver microwave power from an inexpensive, commercially available 3.0
GHz source and amplifier. The temperature of the drops is measured by observing
the temperature dependent fluorescence intensity of cadmium selenide
nanocrystals suspended in the water drops. We demonstrate characteristic
heating times as short as 15 ms to steady-state temperatures as large as 30
degrees C above the base temperature of the microfluidic device. Many common
biological and chemical applications require rapid and local control of
temperature, such as PCR amplification of DNA, and can benefit from this new
technique.Comment: 6 pages, 4 figure
Study of effects of fuel properties in turbine-powered business aircraft
Increased interest in research and technology concerning aviation turbine fuels and their properties was prompted by recent changes in the supply and demand situation of these fuels. The most obvious change is the rapid increase in fuel price. For commercial airplanes, fuel costs now approach 50 percent of the direct operating costs. In addition, there were occasional local supply disruptions and gradual shifts in delivered values of certain fuel properties. Dwindling petroleum reserves and the politically sensitive nature of the major world suppliers make the continuation of these trends likely. A summary of the principal findings, and conclusions are presented. Much of the material, especially the tables and graphs, is considered in greater detail later. The economic analysis and examination of operational considerations are described. Because some of the assumptions on which the economic analysis is founded are not easily verified, the sensitivity of the analysis to alternates for these assumptions is examined. The data base on which the analyses are founded is defined in a set of appendices
Heterogeneity in susceptibility dictates the order of epidemiological models
The fundamental models of epidemiology describe the progression of an
infectious disease through a population using compartmentalized differential
equations, but do not incorporate population-level heterogeneity in infection
susceptibility. We show that variation strongly influences the rate of
infection, while the infection process simultaneously sculpts the
susceptibility distribution. These joint dynamics influence the force of
infection and are, in turn, influenced by the shape of the initial variability.
Intriguingly, we find that certain susceptibility distributions (the
exponential and the gamma) are unchanged through the course of the outbreak,
and lead naturally to power-law behavior in the force of infection; other
distributions often tend towards these "eigen-distributions" through the
process of contagion. The power-law behavior fundamentally alters predictions
of the long-term infection rate, and suggests that first-order epidemic models
that are parameterized in the exponential-like phase may systematically and
significantly over-estimate the final severity of the outbreak
Scaling Behavior in Shaving Cream
The coarsening of a three-dimensional foam is studied with multiple light-scattering techniques. Scaling behavior is observed with the average bubble diameter growing in time as tz where z=0.45±0.05. Changes in the packing conditions during coarsening give rise to a dynamical process that also exhibits temporal scaling. Neighboring bubbles undergo sudden structural rearrangement events at a rate per unit volume that decays as t−y where y=2.0±0.2
Avalanche statistics and time-resolved grain dynamics for a driven heap
We probe the dynamics of intermittent avalanches caused by steady addition of
grains to a quasi-two dimensional heap. To characterize the time-dependent
average avalanche flow speed v(t), we image the top free surface. To
characterize the grain fluctuation speed dv(t), we use Speckle-Visibility
Spectroscopy. During an avalanche, we find that the fluctuation speed is
approximately one-tenth the average flow speed, and that these speeds are
largest near the beginning of an event. We also find that the distribution of
event durations is peaked, and that event sizes are correlated with the time
interval since the end of the previous event. At high rates of grain addition,
where successive avalanches merge into smooth continuous flow, the relationship
between average and fluctuation speeds changes to dv Sqrt[v]
Binary Colloidal Alloy Test-5: Phase Separation
The Binary Colloidal Alloy Test - 5: Phase Separation (BCAT-5-PhaseSep) experiment will photograph initially randomized colloidal samples onboard the ISS to determine their resulting structure over time. This allows the scientists to capture the kinetics (evolution) of their samples, as well as the final equilibrium state of each sample. BCAT-5-PhaseSep studies collapse (phase separation rates that impact product shelf-life); in microgravity the physics of collapse is not masked by being reduced to a simple top and bottom phase as it is on Earth
Low-emissivity impact craters on Venus
An analysis of 144 impact craters on Venus has shown that 11 of these have floors with average emissivities lower than 0.8. The remaining craters have emissivities between 0.8 and 0.9, independent of the specific backscatter cross section of the crater floors. These 144 impact craters were chosen from a possible 164 craters with diameters greater than 30 km as identified by researchers for 89 percent of the surface of Venus. We have only looked at craters below 6053.5 km altitude because a mineralogical change causes high reflectivity/low emissivity above the altitude. We have also excluded all craters with diameters smaller than 30 km because the emissivity footprint at periapsis is 16 x 24 km and becomes larger at the poles
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