Effect of Carrier Gas Pressure on Condensation in a Supersonic Nozzle

Supersonic nozzle experiments were performed with a fixed water or ethanol vapor pressure and varying amounts of nitrogen to test the hypothesis that carrier gas pressure affects the onset of condensation. Such an effect might occur if nonisothermal nucleation were important under conditions of excess carrier gas in the atmospheric pressure range, as has been suggested by Ford and Clement [J. Phys. A 22, 4007 (1989)]. Although a small increase was observed in the condensation onset temperature as the stagnation pressure was reduced from 3 to 0.5 atm, these changes cannot be attributed to any nonisothermal effects. The pulsed nozzle experiments also exhibited two interesting anomalies: (1) the density profiles for the water and ethanol mixtures were shifted in opposite directions from the dry N2 profile; (2) a long transient period was required before the nozzle showed good pulse-to-pulse repeatability for condensible vapor mixtures. To theoretically simulate the observed onset behavior, calculations of nucleation and droplet growth in the nozzle were performed that took into account two principal effects of varying the carrier gas pressure: (1) the change in nozzle shape due to boundary layer effects and (2) the variation in the heat capacity of the flowing gas. Energy transfer limitations were neglected in calculating the nucleation rates. The trend of the calculated results matched that of the experimental results very well. Thus, heat capacity and boundary layer effects are sufficient to explain the experimental onset behavior without invoking energy transfer limited nucleation. The conclusions about the rate of nucleation are consistent with those obtained recently using an expansion cloud chamber, but are at odds with results from thermal diffusion cloud chamber measurements

Doppler Shift Anisotropy in Small Angle Neutron Scattering

The two-dimensional patterns in our small angle neutron scattering (SANS) experiments from rapidly moving aerosols are anisotropic. To test the kinematic theory of two-body scattering that describes the anisotropy, we conducted SANS experiments using a constant source of D2O aerosol with droplets moving at ~440 m/s, and varied the neutron velocity from 267 to 800 m/s. The theoretically predicted anisotropy of the laboratory scattering intensities agrees well with the experimental results. Based on an analysis of the scattering intensity in the Guinier region, we also determined the particle velocity. The results are in very good agreement with independent velocity estimates based on supersonic flow measurements

Trace element emissions. Semi-annual report, October 1994--February 1995

Many trace elements can exist in raw coal gas either in the form of metallic vapors or gaseous compounds which, besides their action on potentially ``very clean`` advanced power generating systems such as fuel cells and gas turbines, can also be detrimental to plant and animal life when released into the atmosphere. Therefore, volatile trace contaminants from coal which can also be toxic must be removed before they become detrimental to both power plant performance/endurance and the environment. Five trace elements were selected in this project based on: abundance in solid coal, volatility during gasification, effects on downstream systems and toxicity to plant and animal life. An understanding was sought in this investigation of the interactions of these five trace elements (and their high temperature species) with the different components in integrated cleanup and power generating systems, as well as the ultimate effects with respect to atmospheric emissions. Utilizing thermodynamic calculations and various experimental techniques, it was determined that a number of trace contaminants that exist in coal may be substantially removed by flyash, and after that by different sorbent systems. High temperature cleanup of contaminants by sorbents such as zinc titanate, primarily to remove sulfur, can also absorb some metallic contaminants such as cadmium and antimony. Further polishing will be required, however, to eliminate trace contaminant species incorporating the elements arsenic, selemium, lead, and mercury

Non-Markovian polymer reaction kinetics

Describing the kinetics of polymer reactions, such as the formation of loops and hairpins in nucleic acids or polypeptides, is complicated by the structural dynamics of their chains. Although both intramolecular reactions, such as cyclization, and intermolecular reactions have been studied extensively, both experimentally and theoretically, there is to date no exact explicit analytical treatment of transport-limited polymer reaction kinetics, even in the case of the simplest (Rouse) model of monomers connected by linear springs. We introduce a new analytical approach to calculate the mean reaction time of polymer reactions that encompasses the non-Markovian dynamics of monomer motion. This requires that the conformational statistics of the polymer at the very instant of reaction be determined, which provides, as a by-product, new information on the reaction path. We show that the typical reactive conformation of the polymer is more extended than the equilibrium conformation, which leads to reaction times significantly shorter than predicted by the existing classical Markovian theory.Comment: Main text (7 pages, 5 figures) + Supplemantary Information (13 pages, 2 figures

Mean first-passage times of non-Markovian random walkers in confinement

The first-passage time (FPT), defined as the time a random walker takes to reach a target point in a confining domain, is a key quantity in the theory of stochastic processes. Its importance comes from its crucial role to quantify the efficiency of processes as varied as diffusion-limited reactions, target search processes or spreading of diseases. Most methods to determine the FPT properties in confined domains have been limited to Markovian (memoryless) processes. However, as soon as the random walker interacts with its environment, memory effects can not be neglected. Examples of non Markovian dynamics include single-file diffusion in narrow channels or the motion of a tracer particle either attached to a polymeric chain or diffusing in simple or complex fluids such as nematics \cite{turiv2013effect}, dense soft colloids or viscoelastic solution. Here, we introduce an analytical approach to calculate, in the limit of a large confining volume, the mean FPT of a Gaussian non-Markovian random walker to a target point. The non-Markovian features of the dynamics are encompassed by determining the statistical properties of the trajectory of the random walker in the future of the first-passage event, which are shown to govern the FPT kinetics.This analysis is applicable to a broad range of stochastic processes, possibly correlated at long-times. Our theoretical predictions are confirmed by numerical simulations for several examples of non-Markovian processes including the emblematic case of the Fractional Brownian Motion in one or higher dimensions. These results show, on the basis of Gaussian processes, the importance of memory effects in first-passage statistics of non-Markovian random walkers in confinement.Comment: Submitted version. Supplementary Information can be found on the Nature website : http://www.nature.com/nature/journal/v534/n7607/full/nature18272.htm

Phase space reduction of the one-dimensional Fokker-Planck (Kramers) equation

A pointlike particle of finite mass m, moving in a one-dimensional viscous environment and biased by a spatially dependent force, is considered. We present a rigorous mapping of the Fokker-Planck equation, which determines evolution of the particle density in phase space, onto the spatial coordinate x. The result is the Smoluchowski equation, valid in the overdamped limit, m->0, with a series of corrections expanded in powers of m. They are determined unambiguously within the recurrence mapping procedure. The method and the results are interpreted on the simplest model with no field and on the damped harmonic oscillator.Comment: 13 pages, 1 figur

Diffusion in Stationary Flow from Mesoscopic Non-equilibrium Thermodynamics

We analyze the diffusion of a Brownian particle in a fluid under stationary flow. By using the scheme of non-equilibrium thermodynamics in phase space, we obtain the Fokker-Planck equation which is compared with others derived from kinetic theory and projector operator techniques. That equation exhibits violation of the fluctuation dissipation-theorem. By implementing the hydrodynamic regime described by the first moments of the non-equilibrium distribution, we find relaxation equations for the diffusion current and pressure tensor, allowing us to arrive at a complete description of the system in the inertial and diffusion regimes. The simplicity and generality of the method we propose, makes it applicable to more complex situations, often encountered in problems of soft condensed matter, in which not only one but more degrees of freedom are coupled to a non-equilibrium bath.Comment: 10 pages, accepted in Phys. Rev.

Diffusion in Stationary Flow from Mesoscopic Non-equilibrium Thermodynamics

We analyze the diffusion of a Brownian particle in a fluid under stationary flow. By using the scheme of non-equilibrium thermodynamics in phase space, we obtain the Fokker-Planck equation which is compared with others derived from kinetic theory and projector operator techniques. That equation exhibits violation of the fluctuation dissipation-theorem. By implementing the hydrodynamic regime described by the first moments of the non-equilibrium distribution, we find relaxation equations for the diffusion current and pressure tensor, allowing us to arrive at a complete description of the system in the inertial and diffusion regimes. The simplicity and generality of the method we propose, makes it applicable to more complex situations, often encountered in problems of soft condensed matter, in which not only one but more degrees of freedom are coupled to a non-equilibrium bath.Comment: 10 pages, accepted in Phys. Rev.

Perturbations of Noise: The origins of Isothermal Flows

We make a detailed analysis of both phenomenological and analytic background for the "Brownian recoil principle" hypothesis (Phys. Rev. A 46, (1992), 4634). A corresponding theory of the isothermal Brownian motion of particle ensembles (Smoluchowski diffusion process approximation), gives account of the environmental recoil effects due to locally induced tiny heat flows. By means of local expectation values we elevate the individually negligible phenomena to a non-negligible (accumulated) recoil effect on the ensemble average. The main technical input is a consequent exploitation of the Hamilton-Jacobi equation as a natural substitute for the local momentum conservation law. Together with the continuity equation (alternatively, Fokker-Planck), it forms a closed system of partial differential equations which uniquely determines an associated Markovian diffusion process. The third Newton law in the mean is utilised to generate diffusion-type processes which are either anomalous (enhanced), or generically non-dispersive.Comment: Latex fil