外电场作用下水化聚全氟磺酸钾膜中水分子的电渗迁移

用分子动力学模拟方法研究外电场(简称电场)作用下水化聚全氟磺酸钾膜中水分子的电渗迁移运动,并分析探讨膜的结构与水分子的电渗迁移特性的关系.结果表明,无外加电场时水分子和K+的速度都服从麦克斯韦分布;施加适当电场时,水分子和K+在垂直电场方向上的速度分量仍服从麦克斯韦分布,但平行电场方向上的速度分量则服从峰值漂移的麦克斯韦分布.并且,峰值漂移速度可作为水分子和K+的平均迁移速度的近似值,从而计算得到水分子的电渗系数.结果还显示,K+第一配位层内平均含有约4.04个水分子,它们的平均迁移速度只有K+的57%.这部分水分子贡献的电渗迁移系数为总电渗迁移系数(2.97)的77%.     

The drag of the row of parallel chains of spherical particles in the Stokes flow

The drag of thin-layered porous deposit consisting of dendrites of identical spherical particles with respect to the flow of viscous incompressible liquid is calculated. The deposit is approximated by a model system, a row of parallel chains of particles oriented perpendicular to a flow direction. The expression is derived for the dimensionless drag force acting on the unit chain length as a function of the ratio of a particle radius to a half-distance between chain axes, a/h. It is shown that, at a/h < 0.5, the hydrodynamic equivalent of the chains is the smooth cylinder whose radius is 1.16 times smaller than the particle radius that agrees with the experiment. It is also shown that, at a/h = 1, the drag force of a particle contacting with four adjacent particles in the layer with square packing is equal to F = 44F _St, where F _St is the Stokes drag force of a spherical particle. The pressure drop in this single layer is by 3.5% higher than in the layer of spherical particles with cubic packing. At a/h = 2/√3, drag force F of the particle contacting with six particles in a single layer with hexagonal packing is equal to 340F _St.     

Drag on a metallic or nonmetallic particle exposed to a rarefied plasma flow

Drag force on a metallic or nonmetallic spherical particle exposed to a plasma flow is studied for the extreme case of a free-molecule regime. Analytical expressions are derived for the drag components due to, respectively, atoms, ions, and electrons and for the total drag on the whole sphere due to all the gas species. It has been shown that the drag is proportional to the square of the particle radius or the drag coefficient is independent of the particle radius. At low gas temperatures with a negligible degree of ionization, the drag is caused mainly by atoms and could be predicted by using the well-known drag expression given in ordinary-temperature rarefied gas dynamics. On the other hand, the drag is caused mainly by ions at high plasma temperatures with a great degree of ionization. The contribution of electrons to the total drag is always negligible. Ignoring gas ionization at high plasma temperatures would overestimate the particle drag. There is a little difference between metallic and nonmetallic spheres in their total drag forces, with a slightly higher value for a metallic sphere at high plasma temperatures, but usually such a small difference could be neglected in engineering calculations. The drag increases rapidly with increasing gas pressure or oncoming speed ratio. For a two-temperature plasma, the drag increases at low electron temperatures but decreases at high electron temperatures with the increase in the electron/heavy-particle temperature ratio.Nomenclature C _d Drag coefficient - e Elementary charge - f _D,F _D Local and total drag (N/m ² andN) - f ^– Velocity distribution function for incident gas particles - f ⁺ Velocity distribution function for reflected gas particles - k Boltzmann's constant - m Gas particle mass (kg) - n Number density of gas species (m ^–3) - P ^–,P ⁺ Surface pressure due to incident and reflected gas particles - R ₀ Sphere radius (m) - S Speed ratio,S _j=U/(2kT _j/m_j)^1/2 - T _e,T _h Electron and heavy-particle (atom, ion) temperature - T _w Wall temperature - U Oncoming plasma flow velocity - v _x, v_y, v_z Velocity components of gas particles in thex, y, andz directions (m/sec) - v Thermal motion speed of gas particles,v _j =(8kT _j /m _j )^1/2 - v _ze Smallestv _z of electrons which could reach the sphere surface,v _ze=(2e/m _e)^1/2 (m/sec) - v _zw Value ofv _z of ions or electrons as arriving at the sphere surface (m/sec) - Center angle - Gas density (kg/m³) - Shear stress (N/m²) - Absolute value of the floating potential (V) - , Local and total particle fluxes incident to the surface - a Atoms - e Electrons - h Heavy particles - i Ions - j jth gas species - m Metallic sphere - mn Nonmetallic sphere A preliminary version of this paper was presented at the Eighth International Symposium on Plasma Chemistry held in Tokyo, September 1987.     

Influence of silver doping on the electrical and magnetic behavior of La0.7Ca0.3MnO3 manganites

A systematic investigation of structural, magnetic and magnetotransport behavior of La_0.7Ca_0.3?xAg_xMnO₃ manganites has been undertaken. The X-ray diffraction shows a structural transformation from orthorhombic to rhombohedral with increasing Ag concentration. The undoped and 10% Ag substituted samples exhibit double transition in M–T curves. The electrical resistivity in the entire temperature range is fitted to effective medium approximation and phase separation models. The sign of S changes from negative to positive with increase in Ag doping. The low temperature thermopower data has been fitted to an equation containing diffusion, magnon drag and phonon drag terms. The paramagnetic insulting part of the TEP data has been analyzed using small polaron hopping mechanism.     

Efficient water‐soluble drag reducing star polymers with improved mechanical stability

We describe here the first example of the synthesis of 4‐arm star poly(acrylic acid) for use as a water‐soluble drag reducing agent, by applying Cu(0)‐mediated polymerization technique. High molecular weight 4‐arm star poly(tert‐butyl acrylate) (M_n = 3.0–9.0 × 10⁵ g mol^?1) was first synthesized using 4,4′‐oxybis(3,3‐bis(2‐bromopropionate)butane as an initiator and a simple Cu(0)/TREN catalyst system. Then, 4‐arm star poly(tert‐butyl acrylate) were subjected to hydrolysis using trifluoroacetic acid resulting in water‐soluble 4‐arm star poly(acrylic acid). Drag reduction test rig analysis showed 4‐arm star poly(acrylic acid) to be effective as a drag reducing agent with drag reduction of 24.3%. Moreover, 4‐arm star poly(acrylic acid) exhibited superior mechanical stability when compared with a linear poly(acrylic acid) and commercially available drag reducing polymers; Praestol and poly(ethylene oxide). The linear poly(acrylic acid), Praestol, and poly(ethylene oxide) all showed a large decrease in drag reduction of 8–12% when cycled 30 times through the drag reduction test rig while, in contrast, 4‐arm star poly(acrylic acid) demonstrated much higher mechanical stability. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 335–344     

A new proposal for polymer dynamics in steady shearing flows

Beginning with a recently proposed expression for the drag force on a single macromolecule pulled with constant velocity through a fluid of long‐entangled molecules (V. R. Mhetar and L. A. Archer, Macromolecules 1998, 31, 6639), we investigate the effect of entanglement loss on polymer dynamics in steady shearing flows. At steady‐state, a balance between the elastic restoring force and viscous drag acting on entangled polymer segments reveals a critical molecular strain γ_m,c beyond which the drag force exerted on polymer molecules by their neighbors is insufficient to support arbitrarily small orientation angles. Specifically, we find that in fast steady shear flows τ < γ˙ < τ, polymer orientation in the shear plane approaches a limiting angle χ_c ≈ atau(1/(1 + γ_m,c)) beyond which flow becomes incapable of producing further molecular alignment. Shear flow experiments using a series of concentrated polystyrene/diethyl phthalate solutions with fixed entanglement spacing, but variable polymer molecular weight 0.94 × 10⁶ ≤ M_w ≤ 5.48 × 10⁶, reveal a limiting steady‐state orientation angle between 6° and 9° over a range of shear rates; confirming the theoretical result. Orientation angle undershoots observed during start‐up of fast steady shearing flows are also explained in terms of a transient imbalance of elastic restoring force and viscous drag on oriented polymer molecules. Our findings suggest that the Doi–Edwards affine orientation tensor (Q) is not universal, but rather depends on deformation type and deformation history through a balance of elastic force and viscous drag on polymer molecules. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 222–233, 2000     

Hydrophobicity and drag reduction properties of surfaces coated with silica aerogels and xerogels

Superhydrophobic surfaces have application in self-cleaning, anti-fouling and drag reduction. Most superhydrophobic surfaces are constructed using complex fabrication methods. An alternative method is to use sol–gel methods to make hydrophobic aerogel and xerogel surfaces. In this work, hydrophobic silica aerogels and xerogels were made from the silica precursors tetramethoxysilane (TMOS) and methyltrimethoxysilane (MTMS) in volume ratios MTMS/TMOS of 0–75 % using a base-catalyzed recipe. Overall hydrophobicity was assessed using contact angle measurements on surfaces prepared from crushed aerogel and xerogel powders. The surfaces made from aerogels were super-hydrophobic (with contact angles of 167°–170°) for all levels of MTMS (10–75 %). Of the xerogel-coated surfaces, those made with 50 % MTMS were hydrophobic and with 75 % MTMS were superhydrophobic. Chemical hydrophobicity was assessed using Fourier transform infrared spectroscopy, which showed evidence of Si–CH₃ and Si–C bonds in the aerogels and xerogels made with MTMS. Morphological hydrophobicity was assessed using SEM imaging and gas adsorption. The drag characteristics of the aerogel- and xerogel-coated surfaces were measured using a rotational viscometer. Under laminar flow conditions all of the hydrophobic aerogel-coated surfaces (10–75 % MTMS) were capable of capturing an air bubble, thereby reducing the drag on a horizontal rotating surface by 20–30 %. Of the xerogel-coated surfaces, only the one made from 75 % MTMS could capture a bubble, which led to 27 % drag reduction. These results imply that morphological differences between silica aerogels and xerogels, rather than any differences in their chemical hydrophobicity, give rise to the observed differences in hydrophobicity and drag reduction for the sol–gel-coated surfaces.     

Contrôle par jet pulsé du décollement d'une couche limite turbulente

Experiments are conducted in a wind tunnel on a cylinder at a subcritical Reynolds number of 133 000. Trip wires are used to force the transition of the cylinder's boundary layers. A zero-mass-flux jet controls the wall pressure distribution around the cylinder. For a momentum fluctuation coefficient c_μ about 0.025, the control generates lift up to C_L≈0.2 and increases the drag force by 17%.     

Polymerization Mechanism of α-Linear Olefin

The density functional theory on the level of B3LYP/6-31G was empolyed to study the chain growth mechanism in polymerization process of α-linear olefin in TiCl₃/AlEt₂Cl catalytic system to synthesize drag reduction agent. Full parameter optimization without symmetryrestrictions for reactants, products, the possible transition states, and intermediates wascalculated. Vibration frequency was analyzed for all of stagnation points on the potential energy surface at the same theoretical level. The internal reaction coordinate was calculated from the transition states to reactants and products respectively. The results showed as flloes:(i) Coordination compounds were formed on the optimum configuration of TiCl₃/AlEt₂Cl.(ii) The transition states were formed. The energy di?erence between transition states and the coordination compounds was 40.687 kJ/mol. (iii) Double bond opened and Ti-C(4) bond fractured, and the polymerization was completed. The calculation results also showedthat the chain growth mechanism did not essentially change with the increase of carbon atom number of α-linear olefin. From the relationship between polymerization activation energy and carbon atom number of the α-linear olefin, it can be seen that the α-linear olefin monomers with 6-10 carbon atoms had low activation energy and wide range. It was optimum to synthesize drag reduction agent by polymerization.     

Preparation of ultra high molecular weight amorphous poly(1‐hexene) by a Ziegler–Natta catalyst

High molecular weight polymers such as poly (α‐olefin)s play a key role as drag‐reducing agents which are commonly used in pipeline industry. Heterogeneous Ziegler–Natta catalyst system of MgCl₂.nEtOH/TiCl₄/donor was prepared using a spherical MgCl₂ support and utilized in synthesis of poly(1‐hexene)s with a viscosity average molecular weight (M_v) up to 3.5 × 10³ kDa. The influence of effective parameters including Al/Ti ratio, polymerization temperature, monomer concentration, effect of alkylaluminus type on the productivity, and molecular weight of the products was evaluated. It was suggested that the reactivity of the Al‐R group and the bulkiness of the cocatalyst were correlated to the performance of the Ziegler–Natta catalyst at different polymerization time and temperatures, affecting the catalyst activity and M_v of polymers. Moreover, bulk polymerization method leads to higher viscosity average molecular weights, revealing the remarkable effect of polymerization method on the chain microstructure. Fourier transform infrared, ¹³C Nuclear magnetic resonance spectra, and DSC thermogram of the prepared polymers confirmed the formation of poly(1‐hexene). The properties of the polymers measured by vortex test showed that these polymers could be used as a drag‐reducing agent. Drag‐reducing behaviors of the polymers exhibited a dependence on the M_v of the obtained polymers that was changed by variation in polymerization parameters. Copyright © 2016 John Wiley & Sons, Ltd.     

Short-wavelength fluorescence caused by sequential two-photon excitation of some cyanine dyes: Effect of solvent viscosity on the quantum yields

Short-wavelength (SW) fluorescence of some cyanine dyes caused by sequential two-photon excitation was studied. The fluorescence quantum yield of the SW fluorescence shows a significant dependence on the solvent viscosity, but only a small dependence on temperature. This reveals the dynamic character of the emitting state: much lower intramolecular barrier and larger solute-solvent viscous drag compared to the S₁ state in the molecular conformational change which is important as a radiationless decay channel.     

Effects of molar mass, concentration and thermodynamic conditions on polymer-induced flow drag reduction

Drag reduction in Taylor flow of polystyrene solutions is investigated using a commercial rheometer equipped with a standard double-gap sample holder with axial symmetry. The dependence of drag reduction on various factors, including polymer molar mass, polymer concentration, and thermodynamic conditions is studied. Drag reduction induced by polystyrene in toluene is found to increase with increasing polymer concentration in the dilute concentration regime. It is also seen that molecules with high molar mass of the polymer promote drag reduction. In terms of hydrodynamic volume fraction normalisation, it is found that most of the drag reduction effect occurs at volume fractions below 0.2. It is observed that drag reduction is favoured by good thermodynamic conditions of the polymer-solvent system. Both the flow induced extension of the polymer chains and the hydrodynamic volume fraction occupied by the polymer molecules seem to play an important role for the drag reduction effect.     

Drag on two coaxial, nonuniformly structured flocs in a uniform flow field

The drag on two coaxial flocs of nonuniform structure in a Newtonian fluid is evaluated for Reynolds number ranges from 0.1 to 40. A two-layer model is adopted to simulate various possible structures of a floc. The influences of the key parameters of the problem under consideration, including the separation distance between two flocs, the Reynolds number, and the ratios (inner radius/outer radius) and (permeability of outer layer/permeability of inner layer), on the drag acting on two flocs are investigated. We show that if Reynolds number is small, the drag on the leading floc is about the same as that on the rear floc. However, if Reynolds number is sufficiently large, because wakes are formed in the rear region of the leading floc, the drag on it is greater than that on the rear floc. For a fixed mean permeability, the more nonuniform the floc structure is, the more important is its influence on the drag, and the more appreciable the deviation of the drag coefficient-Reynolds number curve from a Stokes'-law-like relation.     

Turbulent Drag Reduction with an Ultra-High-Molecular-Weight Water-Soluble Polymer in Slick-Water Hydrofracking

Water-soluble polymers as drag reducers have been widely utilized in slick-water for fracturing shale oil and gas reservoirs. However, the low viscosity characteristics, high operating costs, and freshwater consumption of conventional friction reducers limit their practical use in deeper oil and gas reservoirs. Therefore, a high viscosity water-soluble friction reducer (HVFR), poly-(acrylamide-co-acrylic acid-co-2-acrylamido-2-methylpropanesulphonic acid), was synthesized via free radical polymerization in aqueous solution. The molecular weight, solubility, rheological behavior, and drag reduction performance of HVFR were thoroughly investigated. The results showed that the viscosity-average molecular weight of HVFR is 23.2 × 10⁶ g⋅mol⁻¹. The HVFR powder could be quickly dissolved in water within 240 s under 700 rpm. The storage modulus (G′) and loss modulus (G″) as well as viscosity of the solutions increased with an increase in polymer concentration. At a concentration of 1700 mg⋅L⁻¹, HVFR solution shows 67% viscosity retention rate after heating from 30 to 90 °C, and the viscosity retention rate of HVFR solution when increasing C_NaCl to 21,000 mg⋅L⁻¹ is 66%. HVFR exhibits significant drag reduction performance for both low viscosity and high viscosity. A maximum drag reduction of 80.2% is attained from HVFR at 400 mg⋅L⁻¹ with 5.0 mPa⋅s, and drag reduction of HVFR is 75.1% at 1700 mg⋅L⁻¹ with 30.2 mPa⋅s. These findings not only indicate the prospective use of HVFR in slick-water hydrofracking, but also shed light on the design of novel friction reducers utilized in the oil and gas industry.     

An experimental study of the drag force on a sphere exposed to an argon thermal plasma flow

Experimental data are presented concerning the drag force on a stationary phere exposed to an argon plasma flow with temperatures about 10⁴ K and velocities about 10² m/s. A novel probe construction has been employed in the drag measurements in order to exclude the effect of the supporting wire on the sphere drag data. By using the new probe construction with a compensating wire, drag forces on an individual steel sphere in the plasma flow have been measured and compared with those obtained by using the probe construction ernployed by a few previous authors. Experimental results show that the measured drag forces are always less than their counterparts obtained from the standard sphere-drag curve under isothermal flow conditions with the same Reynolds numbers based on the oncoming plasma properties. The drag force on a sphere increases only slightly with the increasing surface temperature of the sphere before it melts. Appreciable diference was found between the experimental data and the predicted results of the available expressions for drag on a sphere exposed to a thermal plasma flow. Further research effort is required to build a more suitable drag correlation.     

Fabrication of TiO2 Grating with Composites of Azobenzene Polymer and TiO2 Nanoparticles

We have used the formation of surface relief gratings (SRG) on azobenzene polymers to manipulate TiO₂ nanoparticles and to fabricate TiO₂ nanoparticle gratings. Suspensions of an azobenzene polymer (PDO3) and TiO₂ were used to spin coat thin films on glass slide substrates. By interfering coherent light from an Argon laser on the surface of the PDO3‐TiO₂ composite films, SRGs were fabricated. Atomic force microscopic images of the SRGs show TiO₂ nanoparticles dispersed throughout the sample, and in particular, at the peaks of the SRG after oxygen plasma treatment. The lateral forces acting on the azobenzene polymer during the SRG fabrication drag the TiO₂ nanoparticles. These results indicated that it is feasible to create TiO₂ nanoparticle gratings with the composites.     

Effect of electrostatic, hydrodynamic, and Brownian forces on particle trajectories and sieving in normal flow filtration

Particle deposition and fouling are critical factors governing the performance of microfiltration and ultrafiltration systems. Particle trajectories were evaluated by numerical integration of the Langevin equation, accounting for the combined effects of electrostatic repulsion, enhanced hydrodynamic drag, and Brownian diffusion. In the absence of Brownian forces, particles are unable to enter the membrane pores unless the drag associated with the filtration velocity can overcome the electrostatic repulsion. Brownian forces significantly alter this behavior, allowing some particles to enter the pore even at low filtration velocities. The average particle transmission, evaluated from the probability of having a particle enter the pore, increases with increasing filtration velocity due to the greater hydrodynamic drag force on the particle. These results provide important insights into particle behavior in membrane systems.     

On the development of proton conducting polymer membranes for hydrogen and methanol fuel cells

The transport properties and the swelling behaviour of NAFION and different sulfonated polyetherketones are explained in terms of distinct differences on the microstructures and in the pK_a of the acidic functional groups. The less pronounced hydrophobic/hydrophilic separation of sulfonated polyetherketones compared to NAFION corresponds to narrower, less connected hydrophilic channels and to larger separations between less acidic sulfonic acid functional groups. At high water contents, this is shown to significantly reduce electroosmotic drag and water permeation whilst maintaining high proton conductivity. Blending of sulfonated polyetherketones with other polyaryls even further reduces the solvent permeation (a factor of 20 compared to NAFION), increases the membrane flexibility in the dry state and leads to an improved swelling behaviour. Therefore, polymers based on sulfonated polyetherketones are not only interesting low-cost alternative membrane material for hydrogen fuel cell applications, they may also help to reduce the problems associated with high water drag and high methanol cross-over in direct liquid methanol fuel cells (DMFC). The relatively high conductivities observed for oligomers containing imidazole as functional groups may be exploited in fully polymeric proton conducting systems with no volatile proton solvent operating at temperatures significantly beyond 100°C, where methanol vapour may be used as a fuel in DMFCs.     

Rheologically interesting polysaccharides from yeasts

We have examined the relationships between primary, secondary, and tertiary structures of polysaccharides exhibiting the rheological property of friction (drag) reduction in turbulent flows. We found an example of an exopolysaccharide from the yeastCryptococcus laurentii that possessed high molecular weight but exhibited lower than expected drag reducing activity. Earlier correlations by Hoyt (8,10) showing that β1 → 3, β→4, and αl → 3 linkages in polysaccharides favored drag reduction were expanded to include correlations to secondary structure. The effect of sidechains in a series of gellan gums was shown to be related to sidechain length and position. Disruption of secondary structure in drag reducing polysaccharides reduced drag reducing activity for some but not all exopolysaccharides. The polymer fromC. laurentii was shown to be more stable than xanthan gum and other exopolysaccharides under the most vigorous of denaturing conditions. We also showed a direct relationship between extensional viscosity measurements and the drag reducing coefficient for four exopolysaccharides.

     

Freeing the Polarons to Facilitate Charge Transport in BiVO4 from Oxygen Vacancies with an Oxidative 2D Precursor

The BiVO₄ photoelectrochemical (PEC) electrode in tandem with a photovoltaic (PV) cell has shown great potential to become a compact and cost‐efficient device for solar hydrogen generation. However, the PEC part is still facing problems such as the poor charge transport efficiency owing to the drag of oxygen vacancy bound polarons. In the present work, to effectively suppress oxygen vacancy formation, a new route has been developed to synthesize BiVO₄ photoanodes by using a highly oxidative two‐dimensional (2D) precursor, bismuth oxyiodate (BiOIO₃), as an internal oxidant. With the reduced defects, namely the oxygen vacancies, the bound polarons were released, enabling a fast charge transport inside BiVO₄ and doubling the performance in tandem devices based on the oxygen vacancy eliminated BiVO₄. This work is a new avenue for elaborately designing the precursor and breaking the limitation of charge transport for highly efficient PEC‐PV solar fuel devices.