Nearly Perfect Fluidity: From Cold Atomic Gases to Hot Quark Gluon Plasmas

Shear viscosity is a measure of the amount of dissipation in a simple fluid. In kinetic theory shear viscosity is related to the rate of momentum transport by quasi-particles, and the uncertainty relation suggests that the ratio of shear viscosity eta to entropy density s in units of hbar/k_B is bounded by a constant. Here, hbar is Planck's constant and k_B is Boltzmann's constant. A specific bound has been proposed on the basis of string theory where, for a large class of theories, one can show that eta/s is greater or equal to hbar/(4 pi k_B). We will refer to a fluid that saturates the string theory bound as a perfect fluid. In this review we summarize theoretical and experimental information on the properties of the three main classes of quantum fluids that are known to have values of eta/s that are smaller than hbar/k_B. These fluids are strongly coupled Bose fluids, in particular liquid helium, strongly correlated ultracold Fermi gases, and the quark gluon plasma. We discuss the main theoretical approaches to transport properties of these fluids: kinetic theory, numerical simulations based on linear response theory, and holographic dualities. We also summarize the experimental situation, in particular with regard to the observation of hydrodynamic behavior in ultracold Fermi gases and the quark gluon plasma.Comment: 76 pages, 11 figures, review article, extensive revision

Impact of drops of surfactant solutions on small targets

International audienceThe collisions of drops of surfactant solutions (dioctyl sulfosuccinate sodium salt (DOS) and trisiloxane oxypropylene polyoxyethylene (Silwett L77)) with small disc-like targets were studied both experimentally and theoretically. Upon impact, the drops spread very fast beyond the target in the shape of a thin lamella surrounded by a thick rim. No significant difference between water and surfactant solutions was observed in the early stage of the impact. But the collapse stages were very different. In particular, the lamellas of solutions of Silwett L77 disintegrated owing to a spontaneous nucleation of holes, giving to the lamella a web-like structure prior to its break-up. In contrast, lamellas of DOS solutions collapsed like water lamellas, except that the maximum diameter and the lifetime of the lamella of the most concentrated DOS solution were significantly increased compared with pure water and other surfactant solutions. A theoretical analysis shows that the observed instability effects in the lamella and the increase in the size and lifetime of the lamella can be caused by the coupling between liquid inertia and Marangoni stresses. Keywords: drop impact; surfactant solutions; lamella instability; Marangoni stresse

Dynamics of a liquid lamella resulting from the impact of a water drop on a small target

International audienceThe dynamics of a circular liquid lamella resulting from the collision of a water drop with a small disc-like target was studied experimentally and theoretically. Such a type of collision also acts as a model of drop impacts on plane surfaces in the absence of liquid friction, and therefore for more widespread collisions of drops of inviscid liquid with solid surfaces. We propose a simple model to describe the dynamics of the lamella resulting from the drop impact and also predict the structure of the liquid flow in the lamella. It is based on the observations that during the drop collision with the target, the liquid is ejected at an approximately constant flow rate with a velocity that significantly decreases in time. The resulting distributions of velocities, local flow rates and film thickness in the lamella are calculated. Besides, we have measured the distribution of the local Weber numbers by generating Mach-like rupture waves (we have called them Mach-Taylor waves) in the lamella, which follows the Taylor theory of disintegration of fluid sheets. Unknown parameters of the model are obtained from the comparison between the theoretical expression for local Weber number and the experimental data. The time evolution of the lamella diameter was obtained by numerical integration of the model. It was found that during the lamella life, zones of metastability could be formed in the lamella. In these zones a propagating rupture hole cannot be transported away by the flow and it Yields to destabilization. One metastability zone expands from the target towards the external rim, and it is the opposite for the other one

Impact of water drops on small targets

International audienceThe collision of water drops against small targets was studied experimentally by means of a high-speed photography technique. The drop impact velocity was about 3.5 m/s. Drop diameters were in the range of 2.8-4.0 mm. The target was a stainless steel disk of 3.9 mm diameter. The drop spread beyond the target like a central cap surrounded by a thin, slightly conical lamella bounded by a thicker rim. By mounting a small obstacle near the target, surface-tension driven Mach waves in the flowing lamella were generated, which are formally equivalent to the familiar compressibility driven Mach waves in gas dynamics. From the measurement of the Mach angle, the values of some flow parameters could be obtained as functions of time, which provided insight into the flow structure. The liquid flowed from the central cap to the liquid rim through the thin lamella at constant momentum flux. At a certain stage of the process, most of the liquid accumulated in the rim and the internal part of the lamella became metastable. In this situation, a rupture wave propagating through the metastable internal part of the lamella caused the rim to retract while forming outwardly directed secondary jets. The jets disintegrated into secondary droplets due to the Savart-Plateau-Rayleigh instability. Prior to the end of the retraction, an internal circular wave of rupture was formed. It originated at the target and then it propagated to meet the retracting rim. Their meeting resulted in a crown of tiny droplets. A theoretical analysis of the ejection process is proposed. (C) 2002 American Institute of Physics

Topology of slightly polydisperse real foams

International audienceThe topology of slightly polydisperse, (meta-)stable, real foams was investigated by means of optical tomography associated with a numerical reconstruction procedure. The values of the mean numbers of faces per hubble and edges per face were very close to Matzke's data (1946). The real foams were essentially disordered and possessed a noncentered symmetry, and ideal structures also could not be observed. The disorder was quantified by the second moment of the edge per face and face per bubble distributions. and also by a statistical correlation coefficient between the numbers of edges of adjacent faces. It was found that the edge distributions of the internal bubbles, and not of the external ones, were significantly anticorrelated even during foam aging, which provided a measure of the disorder in the foam. No obvious relationship could be deduced between the isoperimetric quotient and the face combination in an individual bubble. Eventually, it was shown that the physical boundaries of the foam sample had no influence on the foam topology beyond a single bubble layer

Impact of drops of polymer solutions on small targets

International audienceThe collision of drops of polymer solutions with small targets was studied experimentally. The tested liquids were aqueous solutions of polyethylene oxide (MW=4 000 000) at concentrations of 10, 100, 1000 wt ppm. The drop impact velocity was about 3.5 m/s, and the drop diameters were in the range of 2.6-3.8 mm. The target was a stainless steel disk of 3.9 mm diameter. The collision was monitored by means of high-speed photography technique. As in the case of pure water, a circular liquid lamella was formed, and then it retracted with formation of outwards-directed secondary jets. There was no significant difference between the values of the maximum diameter and the retraction velocity of the lamella in the cases of water and polymeric liquids. On the contrary, the polymeric additives drastically changed the character of the lamella retraction. The secondary jets were transformed into thinning filaments submitted to elastic stresses with an attached droplet. Then, depending on the polymer concentration, the filaments ruptured and the attached droplets escaped, or the liquid filaments pulled the attached droplets back and the whole liquid returned to the target. A splashing threshold has been derived for polymeric liquids based on the liquid relaxation time and the impact conditions. (C) 2003 American Institute of Physics

Lower extremity kinematics during running and hip abductor strength in iliotibial band syndrome: A systematic review and meta-analysis

Background Iliotibial band syndrome is a common overuse injury that is twice as likely to affect female runners compared to male runners. It is unclear if there is a consistent running pattern and strength profile exhibited by female and male runners with iliotibial band syndrome. Research question The purpose of this systematic review and meta-analysis was to determine if any differences existed in lower-extremity kinematics and hip strength between runners who retrospectively, currently, or prospectively had iliotibial band syndrome. Methods Papers included must have reported three-dimensional kinematic running data and/or hip strength data that were statistically analyzed between runners that never developed iliotibial band syndrome and runners with iliotibial band syndrome. Meta-analysis was performed for each kinematic or strength variable reported in at least three studies. Female and male runners were analyzed separately and grouped into three cohorts (retrospective, current, prospective). Results Seventeen articles were included in this systematic review. Data from 10 cross-sectional studies were included for meta-analysis. Female runners with current iliotibial band syndrome exhibited smaller peak hip internal rotation angles and lower isometric hip abductor strength compared to controls. Significance Although limited biomechanical evidence exists, risk factors for ITBS are different between female and male runners and may vary according to injury status. Specifically, transverse plane hip motion and hip abductor strength weakness may be biomechanical risk factors in female runners with current iliotibial band syndrome only