195 research outputs found
How Dilute are Dilute Solutions in Extensional Flows?
Submitted to J. Rheol.We investigate the concentration-dependence of the characteristic relaxation time of
dilute polymer solutions in transient uniaxial elongational flow. A series of monodisperse polystyrene solutions of five different molecular weights (1.8×10^6 ≤ M ≤ 8.3×10^6 g/mol) with concentrations spanning five orders of magnitude were dissolved in two solvents of differing solvent quality (diethyl phthalate and oligomeric styrene). Optical measurements of the rate of filament thinning and the time to break-up in each fluid are used to determine the characteristic relaxation time. A lower sensitivity limit for the measurements was determined experimentally and confirmed by comparison to numerical calculations.
Above this sensitivity limit we show that the effective relaxation time of moderately
dilute solutions (0.01 ≤ c/c* ≤ 1) in transient extensional flow rises substantially above the fitted value of the relaxation time extracted from small amplitude oscillatory shear flow and above the Zimm relaxation time computed from kinetic theory and intrinsic viscosity
measurements. This effective relaxation time exhibits a power-law scaling with the reduced
concentration (c/c*) and the magnitude of the exponent varies with the thermodynamic quality of the solvent. This scaling appears to be roughly consistent to that predicted when the dynamics of the partially elongated and overlapping polymer chains are described within the framework of blob theories for semi-dilute solutions.NASA Microgravity Fluid Dynamic
Chaotic flow and efficient mixing in a micro-channel with a polymer solution
Microscopic flows are almost universally linear, laminar and stationary
because Reynolds number, , is usually very small. That impedes mixing in
micro-fluidic devices, which sometimes limits their performance. Here we show
that truly chaotic flow can be generated in a smooth micro-channel of a uniform
width at arbitrarily low , if a small amount of flexible polymers is added
to the working liquid. The chaotic flow regime is characterized by randomly
fluctuating three-dimensional velocity field and significant growth of the flow
resistance. Although the size of the polymer molecules extended in the flow may
become comparable with the micro-channel width, the flow behavior is fully
compatible with that in a table-top channel in the regime of elastic
turbulence. The chaotic flow leads to quite efficient mixing, which is almost
diffusion independent. For macromolecules, mixing time in this microscopic flow
can be three to four orders of magnitude shorter than due to molecular
diffusion.Comment: 8 pages,7 figure
An N-acetylglucosamine transporter required for arbuscular mycorrhizal symbioses in rice and maize
Most terrestrial plants, including crops, engage in beneficial interactions with arbuscular mycorrhizal fungi. Vital to the association is mutual recognition involving the release of diffusible signals into the rhizosphere. Previously, we identified the maize () mutant to be defective in early signalling. Here, we report cloning of on the basis of synteny with rice. encodes a functional homologue of the -acetylglucosamine (GlcNAc) transporter , and represents the first plasma membrane GlcNAc transporter identified from plants. In , exposure to GlcNAc activates cell signalling and virulence. Similarly, in treatment with rice wild-type but not root exudates induced transcriptome changes associated with signalling function, suggesting a requirement of NOPE1 function for presymbiotic fungal reprogramming.Research in the U.P. laboratories was supported by the Swiss National Science Foundation grants 3100A0- 104132, PP00A-110874, PP00P3-130704 and by the Gatsby Charitable Foundation grant RG60824. S.N. and J.B.K. were supported by a grant from the National Institutes of Health (R01GM116048)
Wages in high-tech start-ups - do academic spin-offs pay a wage premium?
Due to their origin from universities, academic spin‐offs operate at the forefront of the
technological development. Therefore, spin‐offs exhibit a skill‐biased labour demand, i.e. spin‐offs
have a high demand for employees with cutting edge knowledge and technical skills. In order to accommodate
this demand, spin‐offs may have to pay a relative wage premium compared to other
high‐tech start‐ups. However, neither a comprehensive theoretical assessment nor the empirical
literature on wages in start‐ups unambiguously predicts the existence and the direction of wage differentials
between spin‐offs and non‐spin‐offs. This paper addresses this research gap and examines
empirically whether or not spin‐offs pay their employees a wage premium. Using a unique linked
employer‐employee data set of German high‐tech start‐ups, we estimate Mincer‐type wage regressions
applying the Hausman‐Taylor panel estimator. Our results show that spin‐offs do not pay a
wage premium in general. However, a notable exception from this general result is that spin‐offs that
commercialise new scientific results or methods provide higher wages to employees with linkages to
the university sector – either as university graduates or as student workers
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