The Experimental Investigation of the Effect of Unsteady Obstacle Wakes on Stirring and Mixing of Gamete Filaments

One of the most common methods of reproduction in the sea is broadcast spawning where marine invertebrates release eggs and sperm into the ambient flow and fertilization occurs externally. Gamete coalescence at large scales is dominated by fluid stirring, and may be influenced by the presence of flow obstructions (e.g., coral heads, bed topography). The effect of turbulent wake behind a round obstacle on the second-order reaction between two initially distant scalars has been investigated by series of planar laser-induced fluorescence experiments. The scalars are released continuously, and are separated from each other by a lateral distance that initially impedes the reaction. The direct effect of the wake on mixing enhancement is determined by comparing segregation coefficient for cases with and without the cylinder wake. We measured mixing statistics for variety of flow regimes, streamwise locations, and scalar release geometries. This study suggests that the presence of turbulent obstacle wakes in spawning regions may substantially raise the efficacy of external fertilization. In addition, the effect of viscosity and non-Newtonian (shear-thinning) rheology on mixing and reaction between two initially distant scalars has been investigated. In this case, the ambient flow is pure water, but the scalar solutions include Xanthan gum to alter their rheology. Results indicate that mixing and reaction rates in the low-Damkohler limit between the two scalars plumes increase with the increase in viscosity of the scalars. The results of this study have broad implications for biological and ecological mixing processes involving now-Newtonian fluids

Nonlinear amplification by active sensory hair bundles

The human sense of hearing is characterized by its exquisite sensitivity, sharp frequency selectivity, and wide dynamic range. These features depend on an active process that in the inner ear boosts vibrations evoked by auditory stimuli. Spontaneous otoacoustic emissions constitute a demonstrative manifestation of this physiologically vulnerable mechanism. In the cochlea, sensory hair bundles transduce sound-induced vibrations into neural signals. Hair bundles can power mechanical movements of their tip, oscillate spontaneously, and operate as tuned nonlinear amplifiers of weak periodic stimuli. Active hair-bundle motility constitutes a promising candidate with respect to the biophysical implementation of the active process underlying human hearing. The responsiveness of isolated hair bundles, however, is seriously hampered by intrinsic fluctuations. In this thesis, we present theoretical and experimental results concerning the noise-imposed limitations of nonlinear amplification by active sensory hair bundles. We analyze the effect of noise within the framework of a stochastic description of hair-bundle dynamics and relate our findings to generic aspects of the stochastic dynamics of oscillatory systems. Hair bundles in vivo are often elastically coupled by overlying gelatinous membranes. In addition to theoretical results concerning the dynamics of elastically coupled hair bundles, we report on an experimental study. We have interfaced dynamic force clamp performed on a hair bundle from the sacculus of the bullfrog with real-time stochastic simulations of hair-bundle dynamics. By means of this setup, we could couple a hair bundle to two virtual neighbors, called cyber clones. Our theoretical and experimental work shows that elastic coupling leads to an effective noise reduction. Coupled hair bundles exhibit an increased coherence of spontaneous oscillations and an enhanced amplification gain. We therefore argue that elastic coupling by overlying membranes constitutes a morphological specialization for reducing the detrimental effect of intrinsic fluctuations

QCD in heavy ion collisions

These lectures provide a modern introduction to selected topics in the physics of ultrarelativistic heavy ion collisions which shed light on the fundamental theory of strong interactions, the Quantum Chromodynamics. The emphasis is on the partonic forms of QCD matter which exist in the early and intermediate stages of a collision -- the colour glass condensate, the glasma, and the quark-gluon plasma -- and on the effective theories that are used for their description. These theories provide qualitative and even quantitative insight into a wealth of remarkable phenomena observed in nucleus-nucleus or deuteron-nucleus collisions at RHIC and/or the LHC, like the suppression of particle production and of azimuthal correlations at forward rapidities, the energy and centrality dependence of the multiplicities, the ridge effect, the limiting fragmentation, the jet quenching, or the dijet asymmetry.Comment: Based on lectures presented at the 2011 European School of High-Energy Physics, 7-20 September 2011, Cheile Gradistei, Romania. 73 pages, many figure