Kinetic Theory of a Dilute Gas System under Steady Heat Conduction

The velocity distribution function of the steady-state Boltzmann equation for hard-core molecules in the presence of a temperature gradient has been obtained explicitly to second order in density and the temperature gradient. Some thermodynamical quantities are calculated from the velocity distribution function for hard-core molecules and compared with those for Maxwell molecules and the steady-state Bhatnagar-Gross-Krook(BGK) equation. We have found qualitative differences between hard-core molecules and Maxwell molecules in the thermodynamical quantities, and also confirmed that the steady-state BGK equation belongs to the same universality class as Maxwell molecules.Comment: 36 pages, 4 figures, 5 table

AVIATR - Aerial Vehicle for In-situ and Airborne Titan Reconnaissance A Titan Airplane Mission Concept

We describe a mission concept for a stand-alone Titan airplane mission: Aerial Vehicle for In-situ and Airborne Titan Reconnaissance (AVIATR). With independent delivery and direct-to-Earth communications, AVIATR could contribute to Titan science either alone or as part of a sustained Titan Exploration Program. As a focused mission, AVIATR as we have envisioned it would concentrate on the science that an airplane can do best: exploration of Titan's global diversity. We focus on surface geology/hydrology and lower-atmospheric structure and dynamics. With a carefully chosen set of seven instruments-2 near-IR cameras, 1 near-IR spectrometer, a RADAR altimeter, an atmospheric structure suite, a haze sensor, and a raindrop detector-AVIATR could accomplish a significant subset of the scientific objectives of the aerial element of flagship studies. The AVIATR spacecraft stack is composed of a Space Vehicle (SV) for cruise, an Entry Vehicle (EV) for entry and descent, and the Air Vehicle (AV) to fly in Titan's atmosphere. Using an Earth-Jupiter gravity assist trajectory delivers the spacecraft to Titan in 7.5 years, after which the AVIATR AV would operate for a 1-Earth-year nominal mission. We propose a novel 'gravity battery' climb-then-glide strategy to store energy for optimal use during telecommunications sessions. We would optimize our science by using the flexibility of the airplane platform, generating context data and stereo pairs by flying and banking the AV instead of using gimbaled cameras. AVIATR would climb up to 14 km altitude and descend down to 3.5 km altitude once per Earth day, allowing for repeated atmospheric structure and wind measurements all over the globe. An initial Team-X run at JPL priced the AVIATR mission at FY10 $715M based on the rules stipulated in the recent Discovery announcement of opportunity. Hence we find that a standalone Titan airplane mission can achieve important science building on Cassini's discoveries and can likely do so within a New Frontiers budget

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

A Study of Impacting Droplets of an Emulsion or Surfactant Solution on Solid Substrates

International audienc

Retraction Phenomena of Surfactant Solution Drops upon Impact on a Solid Substrate of Low Surface Energy

International audienceThe impact of surfactant solutions drop on a low-surface-energy solid substrate is investigated using a high-speed photographic technique (one picture every 100 µs) which allows simultaneous top and side views. The influence of physicochemical properties is analyzed by varying the adsorption kinetics of the surfactants and the initial diameter and velocity of the drop before impact. Generally, the drop spreads and retracts under the action of inertia and capillarity, respectively. During spreading, the drop shape changes from a "truncated sphere" to a "flat pancake" and the drop surface is increased such that it is no longer at thermodynamic equilibrium. The relevant surface property is therefore the dynamic surface tension which is evaluated at the maximum diameter γd max , using the maximum bubble pressure apparatus. The dynamic surface tension has a critical influence on the drop behavior at the maximum diameter dmax and during the subsequent retraction. A simple relation combining γd max and the dynamic contact angle at dmax is derived to predict dmax. The results of this prediction agree well with the experimental measurements. Since γd max is large compared with the critical surface tension of the solid surface, a retraction of the drop is induced. The physical origin of this retraction is the apparent dynamic spreading coefficient Smax whose absolute value is correlated with the extent of the retraction. Two types of retraction are observed: a fast, destabilizing one which is described as an inertial peripheral dewetting and a slow, stabilizing one which relaxes exponentially. An empirical criterion is given on the basis of the difference between the thickness of the flattened drop at the maximum diameter and the critical thickness of metastability of a film in partial wetting conditions. It is demonstrated experimentally that this retraction proceeds on a clean solid surface and that the dewetted area is not modified by any surfactant adsorption which could have occurred during the contact time