Showing papers on "Transport phenomena published in 2012"

A universal critical density underlying the physics of electrons at the LaAlO 3 /SrTiO 3 interface

Abstract: The two-dimensional electron system at the interface between the insulating oxides LaAlO3 and SrTiO3 exhibits ferromagnetism, superconductivity and a range of unique magnetotransport properties. An open experimental challenge is to identify, out of the multitudinous energy bands predicted to exist at the interface, the key ingredients underlying its emergent transport phenomena. Here we show, using magnetotransport measurements, that a universal Lifshitz transition between d orbitals of different symmetries lies at the core of the observed phenomena. We find that LaAlO3/SrTiO3 systems generically switch from one- to two-carrier transport at a universal carrier density, which is independent of the LaAlO3 thickness and electron mobility. Interestingly, the maximum superconducting critical temperature occurs also at the Lifshitz density, indicating a possible connection between the two phenomena. A simple band model, allowing for spin-orbit coupling at the atomic level, connects the observed transition to a variety of previously reported properties. Our results demonstrate that the fascinating behaviour observed so far in these oxides follows from a small but fundamental set of bands. When lanthanum aluminate and strontium titanate are brought together, a 2D electron gas with many interesting properties forms at the interface. Magnetotransport results obtained by Joshuaet al. suggest that the behaviour of this interface is governed by a small but fundamental set of electronic bands.

Thermodynamics and kinetics of atmospheric aerosol particle formation and growth.

Abstract: In this tutorial review we summarize the standard approaches to describe aerosol formation from atmospheric vapours and subsequent growth - with a particular emphasis on the interplay between equilibrium thermodynamics and non-equilibrium transport. We review the use of thermodynamics in describing phase equilibria and formation of aerosol particles from supersaturated vapour via nucleation. We also discuss the kinetics of cluster formation and transport phenomena, which are used to describe dynamic mass transport between the gaseous and condensed phases in a non-equilibrium system. Finally, we put these theories into the context of atmospheric observations of aerosol formation and growth.

Modelling and Applications of Transport Phenomena in Porous Media

Abstract: These books are written in very different styles. In a sense they are complementary, though many workers comfortable with the content and presentation of one might find the other relatively unattractive. The first is intended ‘to provide a user-friendly introduction to the topic of convection in porous media . . . (employing) only routine classical mathematics , . . as a review and as a tutorial work ’. It adopts familiar constitutive models for transport of mass momentum and energy in homogeneous saturated porous media, and uses them to solve boundary value problems by standard mathematical techniques. The book is tightly written, the applications are precisely defined and the approach, which relies heavily on dimensionless formulations, will be familiar to most engineering scientists. Within the range of topics chosen it is comprehensive, but it tends not to stress the real difficulties of reducing engineering or geomechanical problems to tractable mathematical form. The second is a collection of separately authored chapters of greatly varying length and content, which has the paradoxical advantage of showing that there are different ways of looking at any given process. Much of it is very formal, and is concerned with setting up the basic continuum models (constitutive relations) that others might use. Three relatively specialized examples (two of these from the nuclear industry) are considered, covering boiling and drying. There is little connection between the various chapters. It should be noted that it is volume 5 in a series on Theory and Applications of Transport in Porous Media. I found considerable overlap with parts of volume 4 : the first long chapter by Bear, which occupies nearly half of volume 5 , is to some extent a digest of volume 4. Neither is as balanced a text from the point of view of an engineering scientist as that reviewed earlier (Theory of Fluid Flows through Natural R O C ~ S , by Barenblatt, Entov & Ryzhik, volume 3 of the Kluwer series, see J . Fluid Mech. 223 (1991), p. 663). Though Nield & Bejan stands very well on its own, it is primarily an academic textbook (and a very good one too) ; I hope that it does not sound churlish to argue that the limitations that academics impose upon themselves to maintain elegance and simplicity detract somewhat from the merit of their work as sources for engineers : real problems arise because of the inadequacies of our existing models, and unless this is emphasized there is a danger that engineers trained by academics-will use inappropriate models. In the case of real porous media, the continuum assumption of an isotropic homogeneous undeforming medium with Darcy-like behaviour always has to be carefully examined : if the mathematical modeller does not carry out such an examination and assess the consequences of any departures from the assumption, the chances are that nobody will. To this extent the rather arid and formal development of continuum equations is an important discipline. An understanding of the relevance of averaging processes and of their consequences for averaged continuum equations should always be emphasized in texts for engineers. I can therefore commend chapter 6 by de Marsily 408 pp. DM128.

Hydrodynamics of Liquid–Liquid Dispersion in an Advanced-Flow Reactor

Abstract: Hydrodynamics and mass transfer of immiscible liquid–liquid flows are explored in an Advanced-Flow Reactor (AFR). These systems are emerging as one of the major commercial systems for small scale continuous flow chemistry, and characterization of the transport phenomena is critical for reaction implementation. With hexane/water as a model system, we use flow visualization techniques to determine drop size distribution, hexane holdup, and specific interfacial areas for a phase flow rate range of 10–80 mL/min. The complex geometry of the AFR with its continuously changing cross section along the flow path and strategically placed obstacles creates pressure changes that cause drop breakup and enhance mass transfer. Observations show that a wide range of average drop size (0.33–1.3 mm) can be achieved in the AFR depending upon the inlet flow rates and inlet composition. Pressure drop measurements are performed to estimate the power consumption and are used to compare the efficiency of AFR with conventional li...

The Airborne Microparticle: Its Physics, Chemistry, Optics, and Transport Phenomena

Abstract: Background.- Particle Levitation.- Elastic Light Scattering.- Basic Single Particle Measurements.- Continuum Transport Processes.- Non-Continuum Processes.- Thermodynamic and Transport Properties.- Inelastic Light Scattering.- Spectroscopies and Mass Spectrometry.- Particle Chemical Reactions.- Phoretic and Radiometric Phenomena.

Stability of quasi-Keplerian shear flow in a laboratory experiment

Abstract: Subcritical transition to turbulence has been proposed as a source of turbulent viscosity required for the associated angular momentum transport for fast accretion in Keplerian disks. Previously cited laboratory experiments in supporting this hypothesis were performed either in a di erent type of flow than Keplerian or without quantitative measurements of angular momentum transport and mean flow profile, and all of them appear to su er from Ekman e ects, secondary flows induced by nonoptimal axial boundary conditions. Such Ekman e ects are expected to be absent from astronomical disks, which probably have stress-free vertical boundaries unless strongly magnetized. Aims. To quantify angular momentum transport due to subcritical hydrodynamic turbulence, if exists, in a quasi-Keplerian flow with minimized Ekman e ects. Methods.We perform a local measurement of the azimuthal-radial component of the Reynolds stress tensor in a novel laboratory apparatus where Ekman e ects are minimized by flexible control of axial boundary conditions. Results.We find significant Ekman e ects on angular momentum transport due to nonoptimal axial boundary conditions in quasi-Keplerian flows. With the optimal control of Ekman e ects, no statistically meaningful angular momentum transport is detected in such flows at Reynolds number up to two millions. Conclusions. Eithermore » a subcritical transition does not occur, or, if a subcritical transition does occur, the associated radial transport of angular momentum in optimized quasi-Keplerian laboratory flows is too small to directly support the hypothesis that subcritical hydrodynamic turbulence is responsible for accretion in astrophysical disks. Possible limitations in applying laboratory results to astrophysical disks due to experimental geometry are discussed.« less

Nonequilibrium molecular dynamics simulation of water transport through carbon nanotube membranes at low pressurea)

Abstract: Nonequilibrium molecular dynamics (NEMD) simulations are used to investigate pressure-driven water flow passing through carbon nanotube (CNT) membranes at low pressures (5.0 MPa) typical of real nanofiltration (NF) systems. The CNT membrane is modeled as a simplified NF membrane with smooth surfaces, and uniform straight pores of typical NF pore sizes. A NEMD simulation system is constructed to study the effects of the membrane structure (pores size and membrane thickness) on the pure water transport properties. All simulations are run under operating conditions (temperature and pressure difference) similar to a real NF processes. Simulation results are analyzed to obtain water flux, density, and velocity distributions along both the flow and radial directions. Results show that water flow through a CNT membrane under a pressure difference has the unique transport properties of very fast flow and a non-parabolic radial distribution of velocities which cannot be represented by the Hagen-Poiseuille or Navier-Stokes equations. Density distributions along radial and flow directions show that water molecules in the CNT form layers with an oscillatory density profile, and have a lower average density than in the bulk flow. The NEMD simulations provide direct access to dynamic aspects of water flow through a CNT membrane and give a view of the pressure-driven transport phenomena on a molecular scale.

Transport in Anisotropic Superfluids: A Holographic Description

Abstract: We study transport phenomena in p-wave superfluids in the context of gauge/gravity duality. Due to the spacetime anisotropy of this system, the tensorial structure of the transport coefficients is non-trivial in contrast to the isotropic case. In particular, there is an additional shear mode which leads to a non-universal value of the shear viscosity even in an Einstein gravity setup. In this paper, we present a complete study of the helicity two and helicity one fluctuation modes. In addition to the non-universal shear viscosity, we also investigate the thermoelectric effect, i.e. the mixing of electric and heat current. Moreover, we also find an additional effect due to the anisotropy, the so-called flexoelectric effect.

Faster Time Response by the Use of Wire Electrodes in Capacitive Salinity Gradient Energy Systems

Abstract: Capacitive energy extraction based on Donnan potential (CDP) and capacitive energy extraction based on double layer expansion (CDLE) are novel electroctrochemical processes to convert the potential free energy of mixing sea and river water into electric work. This is done by the use of supercapacitor electrodes with and without ion exchange membranes. Currently, these techniques rely on improved mass transport in order to become more efficient and give higher power output. In this paper we evaluate the transport phenomena by diffusion and the electrode geometry when switching between sea and river water at open circuit potential (OCP). By changing the electrode geometry from a flat plate to a cylindrical one, experiments and analytical models in combination show that mass transport by diffusion is increased. This is demonstrated without any changes in the hydrodynamic conditions. Improving mass transport without changing the hydrodynamic conditions breaks with what has been the convention in the scientific community of salinity gradient power. Moreover, in sea water the transport phenomena appear to be controlled by diffusion, and the response time for building open circuit potential in CDP and CDLE under this condition is reduced by a factor of 2 when using wire electrodes instead of flat plate electrodes. In river water, the trend is similar though the response time is generally larger.

Field effect control of electrokinetic transport in micro/nanofluidics

Abstract: Electrokinetics has emerged as one of the most promising techniques to transport and manipulate ions, fluid and particles in micro/nanofluidic devices. Field effect permits flexible and rapid control of the surface charge property on the channel wall, which in turn offers a more sophisticated control of the electrokinetic transport phenomena in micro/nanofluidics. In the field effect control, a potential named as gate potential is applied to a gate electrode patterned on the outer surface of the dielectric channel wall in contact with an aqueous solution, and the imposed radial electric field can effectively modulate the surface potential at the channel/liquid interface, resulting in the redistribution of ions and accordingly the ionic conductance of a nanochannel. The modulation of the surface potential at the channel/liquid interface can also affect the electrokinetic transport of fluids and particles. This tutorial review elucidates the physical mechanism and discusses some typical results of the field effect control of ion, fluid and particle electrokinetic transport in micro/nanofluidics.

Peristaltic transport of rheological fluid: model for movement of food bolus through esophagus

Abstract: Fluid mechanical peristaltic transport through esophagus is studied in the paper. A mathematical model has been developed to study the peristaltic transport of a rheological fluid for arbitrary wave shapes and tube lengths. The Ostwald-de Waele power law of a viscous fluid is considered here to depict the non-Newtonian behaviour of the fluid. The model is formulated and analyzed specifically to explore some important information concerning the movement of food bolus through esophagus. The analysis is carried out by using the lubrication theory. The study is particularly suitable for the cases where the Reynolds number is small. The esophagus is treated as a circular tube through which the transport of food bolus takes place by periodic contraction of the esophageal wall. Variation of different variables concerned with the transport phenomena such as pressure, flow velocities, particle trajectory, and reflux is investigated for a single wave as well as a train of periodic peristaltic waves. The locally variable pressure is seen to be highly sensitive to the flow index “n”. The study clearly shows that continuous fluid transport for Newtonian/rheological fluids by wave train propagation is more effective than widely spaced single wave propagation in the case of peristaltic movement of food bolus in the esophagus.

Ab initio simulation of transport phenomena in rarefied gases

Abstract: Ab initio potentials are implemented into the direct simulation Monte Carlo (DSMC) method. Such an implementation allows us to model transport phenomena in rarefied gases without any fitting parameter of intermolecular collisions usually extracted from experimental data. Applying the method proposed by Sharipov and Strapasson [Phys. Fluids 24, 011703 (2012)], the use of ab initio potentials in the DSMC requires the same computational efforts as the widely used potentials such as hard spheres, variable hard sphere, variable soft spheres, etc. At the same time, the ab initio potentials provide more reliable results than any other one. As an example, the transport coefficients of a binary mixture He-Ar, viz., viscosity, thermal conductivity, and thermal diffusion factor, have been calculated for several values of the mole fraction.

Transport Phenomena in Porous Media: Aspects of Micro/Macro Behaviour

Abstract: Overview.- Basic continuum mechanics.- Non-equilibrium thermodynamics.- The virtual work equation, the variational method, and the minimum energy principle.- Diffusion and seepage problems.- Consolidation for saturated porous media.- Homogenization theory.- Homogenization theory for seepage. Diffusion in saturated porous media.- Long-term consolidation of bentonite and homogenization analysis.

Bosonic and fermionic transport phenomena of ultracold atoms in one-dimensional optical lattices

Abstract: Using the microcanonical picture of transport—a framework ideally suited to describe the dynamics of closed quantum systems such as ultracold atom experiments—we show that the exact dynamics of noninteracting fermions and bosons exhibits very different transport properties when the system is set out of equilibrium by removing the particles from half of the lattice. We find that fermions rapidly develop a finite quasisteady-state current reminiscent of electronic transport in nanoscale systems. This result is robust—it occurs with or without a harmonic confining potential and at zero or finite temperature. The zero-temperature bosonic current instead exhibits strong oscillatory behavior that decays into a steady-state of zero current only in the thermodynamic limit. These differences appear most strikingly in the different particle number fluctuations on half of the lattice as a consequence of the spin statistics. These predictions can be readily verified experimentally.

Effective pressure interface law for transport phenomena between an unconfined fluid and a porous medium using homogenization

Abstract: We present modeling of the incompressible viscous flows in the domain containing unconfined fluid and a porous medium in the case when the flow in the unconfined domain dominates. For such a setting a rigorous derivation of the Beavers–Joseph–Saffman interface condition was undertaken by Jager and Mikelic [SIAM J. Appl. Math., 60 (2000), pp. 1111–1127] using the homogenization method. So far the interface law for the pressure was conceived and confirmed only numerically. In this article we derive the Beavers and Joseph law for a general body force by estimating the pressure field approximation. Different from the Poiseuille flow case, the velocity approximation is not divergence-free and the precise pressure estimation is essential. This new estimate allows us to rigorously justify the pressure jump condition using the Navier boundary layer, already used to calculate the constant in the law by Beavers and Joseph. Finally, our results confirm that the position of the interface influences the solution only ...

An Introduction to Transport Phenomena in Materials Engineering, Second Edition

Abstract: Provides theory and knowledge from present research on heat transfer and fluid behavior, with ample examples of practical applications to materials processing and engineering. This title includes: a chapter on boiling and condensation; and, revised chapters on heat transport, mass transport in solid state and mass transport in fluids.

Development of an Analytical Time-Dependent Matrix/Fracture Shape Factor for Countercurrent Imbibition in Simulation of Fractured Reservoirs

Abstract: In dual porosity modeling of naturally fractured reservoirs, fluids exchange between the high porous matrix blocks and high permeable fracture systems is governed by transfer function. Therefore, transfer function, and specially shape factor as the main part of it, control fluids flow behavior, which certainly have significant effects on development and management plan of naturally fractured reservoirs. Also several formulations have been proposed for shape factor by a number of researchers, nearly all of them derived for expansion mechanism. But, shape factor is a phase sensitive parameter that can greatly affect results of simulation. Moreover, several shortcomings are inherent in the derived expressions of shape factor for imbibition process. The main aim of this work is to develop a new time-dependent matrix–fracture shape factor specific to countercurrent imbibition. In this study, fluid saturation distribution within a matrix block is analytically derived by solving capillary–diffusion equation under different imposed boundary conditions for the process where countercurrent imbibition is the dominant oil drive mechanism. The validity of the solutions is checked against literature experimental data (Bourbiaux and Kalaydjian, SPERE 5, 361–368, SPE 18283, 1990) and also by performing single porosity fine grid simulations. Then, the concept of analogy between the transport phenomena is employed to propose a new expression for matrix–fracture transfer function that is used to derive transient shape factor. It is illustrated in this article that time variation of imbibtion rate and shape factor can be used to diagnose different states of imbibition process. Although, the displacement process and employed approaches are completely different in this and other studies (Chang, Technical report, 1993; Kazemi and Gilman (eds.) Flow and contaminant transport in fractured rock. Academic Press, San Dieg, 1993; Zimmerman et al., Water Resour Res, 29, 2127–2137, 1993; Lim and Aziz, J Pet Sci Eng 13, 169–178, 1995), but we arrived at the consistent values of shape factor under limiting condition of pseudo steady state flow. This means that after establishment of pseudo steady state, shape factor is only controlled by matrix geometry regardless of the displacement process, i.e., expansion or imbibition mechanism, However, shape factor is completely phase sensitive and process dependent during unsteady and late-transient states. Finally, boundary condition dependency of shape factor is investigated.