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This Week at Physics

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Events on Monday, February 14th, 2011

Special Seminar
Microbial Interaction Networks in Soil and in Silico
Time: 9:00 am
Place: Forum room, Wisconsin Institutes of Discovery
Speaker: Kalin H. Vetsigian, Harvard University
Abstract: Soil harbors a huge number of microbial species interacting through secretion of antibiotics and other chemicals. What patterns of species interactions allow for this astonishing biodiversity to be sustained, and how do these interactions evolve? I used a combined experimental-theoretical approach to tackle these questions. Focusing on bacteria from the genus Streptomyces, known for their diverse secondary metabolism and production of antibiotics, I isolated 64 natural strains from several individual grains of soil and systematically measured all pairwise interactions among them. Quantitative measurements on such scale were never possible before. They were enabled by a novel experimental platform based on robotic handling, a unique self-built scanner array and automatic image analysis. This unique platform allowed the simultaneous capturing of ~15,000 time-lapse movies of growing colonies of each isolate on media conditioned by each of the other isolates. The data revealed a rich network of strong negative (inhibitory) and positive (stimulating) interactions. Analysis of this network and the phylogeny of the isolates, together with mathematical modeling of microbial communities, revealed that: 1) The network of interactions has three special properties: "balance", "bi-modality", and "reciprocity"; 2) The interaction network is fast evolving; 3) Mathematical modeling explains how rapid evolution gives rise to the three special properties through an interplay between ecology and evolution. These properties are not a result of stable co-existence, but rather of continuous evolutionary turnover of strains with different production and resistance capabilities.
Host: Pupa Gilbert
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Plasma Physics (Physics/ECE/NE 922) Seminar
“Turbulent Mixing: Problems, Concepts, Solutions”
Time: 12:05 pm
Place: 2241 Chamberlin Hall
Speaker: Snezhana Abarzhi, University of Chicago
Abstract: Turbulent mixing plays an important role in a broad variety of plasma systems, spanning astrophysical to atomistic scales and low to high energy densities. Examples include inertial confinement fusion, Z-pinches, core-collapse supernovae, thermonuclear stellar flashes, magneto-convection, ionospheric plasmas, and light-material interaction. Theoretical description of non-equilibrium mixing transports is a challenging problem due to singular aspects of the governing (Euler or Navier-Stokes) equations. Furthermore these processes are statistically unsteady and their fluctuating quantities are essentially time-dependent and non-Gaussian.

We developed a novel theoretical concept, the rate of momentum loss, and applied it to describe the transports of mass, momentum and energy in turbulent mixing flow and to capture its anisotropic and inhomogeneous character. It was shown that invariant, scaling and spectral properties of unsteady turbulent mixing differ substantially from those of isotropic and homogeneous turbulence. Time- and
scale-invariance of the rate of momentum loss leads to non-dissipative momentum transfer, to and power-law
scale-dependencies of the velocity and Reynolds number and to non-Kolmogorov spectra. Turbulent mixing exhibits more order compared to isotropic turbulence, and its viscous and dissipation scales are finite and set by flow acceleration. We suggested how to describe the random character of the statistically unsteady turbulent flow and showed that the rate of momentum loss is the statistic invariant and a robust
diagnostic parameter for either sustained or time-dependent acceleration. Some criteria are outlined for the estimate of the fidelity and information capacity of the experimental and numerical data sets.
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