Bulk Liquid Region Research Articles

Structure, Dynamics, and the Free Energy of Solute Adsorption at Liquid−Vapor Interfaces of Simple Dipolar Systems: Molecular Dynamics Results for Pure and Mixed Stockmayer Fluids

We have performed molecular dynamics simulation studies of the structural, thermodynamic, and dynamical properties of liquid-vapor interfaces of pure and binary Stockmayer fluids of different polarity. The density profiles, the width of the liquid-vapor interface, and the orientational structure of the interfaces are calculated to characterize the structural aspects of the interfaces. Among the thermodynamic properties, we have computed the surface tension and also the free energy of transfer of a charged solute across the liquid-vapor interface for both pure and mixed fluids. Among the dynamical properties of the interfaces, we have calculated the time dependence of the velocity and angular velocity autocorrelation functions, continuous and intermittent survival probabilities, mean square displacements, diffusion coefficients, and also the dipole correlation functions and orientational relaxation times of interfacial solvent molecules. It is found that the width of the interfaces decreases with increase of concentration of the more polar component. The dipole vectors of the interfacial molecules tend to align parallel to the surfaces and this alignment is enhanced with increasing dipole moment of the fluid molecules. Also, the surface tension shows an increasing trend with increase of dipole moment of the molecules. The dynamical properties of the interfaces are found to be different from those of the corresponding bulk liquid phases. In general, the molecules at the interfaces are found to rotate and translate in the parallel direction at a somewhat faster rate than the bulk molecules. Also, on increase of concentration of the more polar component, the diffusion and orientational relaxation of interfacial molecules are found to show a weaker slowing down than those of the bulk molecules, which can be attributed to the preferential presence of the more polar component in the bulk liquid regions. The temporal behavior of the interfacial survival probabilities reveals a decrease of the survival times with increasing polarity, which can be attributed to a corresponding decrease in the interfacial thickness. Results are presented for both continuous and intermittent survival times and the origins of their differences are discussed. The free energy calculations reveal no minimum at the interfaces for adsorption of a charged solute, which shows that the ions would prefer to stay in the interior of the liquid phases, rather than at interfaces, for these model dipolar systems.

Molecular Dynamics Exploration of Thin Liquid Films on Solid Surfaces. 1. Monatomic Fluid Films

The thickness of the interfacial region dividing bulk liquid and vapor regions is typically on the order of a few molecular diameters. Furthermore, in systems where the characteristic length scale is the same order of magnitude as the thickness of the interface such as a thin liquid film on a solid surface, behavior may be different than that for larger systems. The small thickness of such films leads to difficult experimental observation of phenomena within various regions of the film: the wall-affected region, the bulk liquid, and the liquid–vapor interfacial region. A hybrid simulation methodology is applied that combines a deterministic molecular dynamics simulation of the liquid regions with a stochastic treatment of the far-field vapor region boundary. In this simulation scheme, the imposed far-field pressure may be held fixed or iterated as the simulation is advanced in time until the mass in the system stabilizes at the specified temperature, which establishes the equilibrium saturation vapor pressure for the specified temperature as dictated by the intermolecular force interaction models for the fluid and molecules near the solid surface. Simulation results are presented for an argon liquid film on a solid metallic surface.

Multicomponent surfactant mass transfer in GTA-welding

A mathematical model simulating multicomponent, surfactant mass transfer in the liquid region arising in the specimen directly beneath the electrode during GTA-welding is developed. The sorption controlled surfactant mass transfer is due to convection and diffusion in the surface and bulk liquid regions. Interaction coefficients and multicomponent Langmuir adsorption isotherms are used to consider the interaction between the several surfactants involved. The model is added to a mathematical formulation that is often used for GTA-welding, and is finally used to calculate the fluid and heat flow and surfactant distributions in the weld pool. In the simulations, GTA-welding on stainless steel plates for a ternary Fe-S-O system is considered for varying surfactant concentrations and for varying heat input conditions. A very good agreement between numerically obtained and corresponding experimental weld pool shapes is found. Thus, it is shown that previous shortcomings in modelling of GTA-welding can be overcome if multicomponent surfactant mass transfer is considered.

Improved understanding of the effect of electrical double layer on pressure-driven flow in microchannels

Traditionally, the effects of electrical double layer on pressure-driven flow in microchannels were modeled by using the Poisson–Boltzmann equation and the fluid momentum equation with a flow-induced body force term. Such a model, however, usually underestimate the electrical double layer effects on the flow. In this study, a theoretical model of the electrical double layer field is developed to provide a better understanding of the electrical double layer effects. The electrical potential and ionic concentration distribution in dilute solutions in small microchannels are investigated by numerically solving this new model. This newly developed model predicted the deficit of counter-ions in the bulk liquid region due to the accumulation of counter-ions in the EDL region, and the surplus of co-ions in the bulk liquid region due to rejection of the co-ions in the EDL region. The presence of the net charges in the bulk liquid region is responsible for the strong electroviscous effects in dilute solutions in small microchannels.

Effect of particle addition on sonochemical reaction

Enhancement of chemical reaction with a photocatalyst of titanium dioxide (TiO 2) by ultrasonic irradiation is studied through the absorbance measurements for liberation of iodine from a KI aqueous solution as an index of oxidation reaction. It is well known as a synergetic effect that the addition of TiO 2 fine particles under UV has an ability to enhance the yield in chemical reaction with OH-radical from hot spot at violent collapse of cavitation bubbles with intense ultrasound. In this study, the absorbance is measured after simultaneous irradiation of ultrasound and UV with the addition of TiO 2 much less than the usual concentration by two orders of content. It is shown that, even in case of quite a little TiO 2 addition where the photocatalytic effect is less, the yield is enhanced obviously in comparison with the summation in yield of independent procedure of ultrasound without TiO 2 and UV with TiO 2. The absorbance-peak deviation to the shorter wavelength implies the generation of titanium peroxide (TiO 3). The effect of particle addition is due to the chemically activated particle surface on the TiO 2 and probably to the increase in the cavitation bubbles that results in promoting a transfer of OH-radical and other oxidants to bulk liquid region at the collapse.

K-1611 一方向凝固過程におけるプルームの発生 : 初期過熱度の影響(S19-1 温度・濃度場複合伝熱)(S19 温度・濃度場複合伝熱)

Directional solidification experiments were carried out in a Hele-Shaw cell using an aqueous ammonium chloride solution by cooling it from below with a constant temperature surface. The experiments were examined by changing initial fluid temperature. Onset and subsequent evolution of salt-finger and plumes convection were investigated. The dynamic structures of salt-finger and plumes convection in the bulk liquid region were visualized by sequential three color light sheet method which can identify upward and downward flows in the melt. Furthermore, development of channel in the mushy layer was examined.

Curvature effects on the one-dimensional fluid interface

We derive equations for the spontaneous curvature and rigidity constant for a two-dimensional liquid-vapor interface. These equations are analogous to expressions given in a previous paper on the three-dimensional liquid-vapor interface. Using an approximation for the pair density of a circular interface we give expressions for these quantities in terms of integral over the derivative of the interaction potential and the pair correlation function in the bulk liquid region. Close to the critical point, the same relation between the rigidity constant and the surface tension is found as in the three-dimensional system. The description of the interface using a position dependent pressure tensor is analyzed.

A Comprehensive Dynamic Model of Attached Growth Anaerobic Fermenters

The model described here involves a highly mechanistic view of anaerobic attached growth processes, considering a four-culture microbial population interacting with and transforming six metabolic substrates in two distinct reactor regions. These regions include 1) the biofilm zone, which is a thin layer consisting of highly concentrated bacterial mass embedded within a polysaccharide matrix; and 2) the bulk liquid region, corresponding to the interstitial volume surrounding the biomass support media.

Spreading of a liquid film on a substrate by the evaporation—adsorption process

The importance of evaporation followed by multilayer adsorption in comparison to liquid flow at the leading edge of a volatile spreading film is analyzed. Presuming that both flows are functions of the same chemical potential gradient, a dimensionless group ( N) which delineates the relative importance of vapor diffusion flow to viscous flow on the surface is obtained: N = ρ i Dνx (− Aπ) . The relative importance of vapor flow increases with the vapor-pressure dependent partial density, ρ i, and diffusivity, D, of the diffusing vapor, the kinematic viscosity at the liquid, ν, and the distance downstream from the bulk liquid region, x, and decreses with the Hamaker constant, 6π A . Using physical properties the modifiers “volatile” and “nonvolatile” can thereby be put in perspective. Changes in the interfacial force field are a function of A . The spreading velocity due to the vapor diffusion process is obtained and is found to decrease with a decrease in the interfacial force field and the bulk vapor pressure. The infinite stress at the contact line can be easily relieved by evaporation-adsorption in many systems.

GRAVITY- AND SOLIDIFICATION-SHRINKAGE-INDUCED LIQUID FLOW IN A HORIZONTALLY SOLIDIFIED ALLOY INGOT

A basic-variable, explicit finite difference scheme is used to solve the unsteady and strongly pressure-coupled velocity problems of liquid flows induced both by thermosolutal buoyancies and solidification shrinkage in a binary alloy solidification process. A sample calculation, performed on an IBM personal computer, for a horizontally solidified Al-4.5%Cu alloy in a high-H/L ratio cavity with a top riser shows that the shrinkage established pressure gradient in the mushy region can be several hundred, even several thousand, times larger than that in the bulk liquid region.

Numerical simulation of binary solidification in a vertical channel with thermal and solutal mixed convection

Abstract A continuum model is used to investigate the solidification of an aqueous ammonium chloride solution in the presence of an imposed forced flow. Calculations are performed to determine the effect of flow rate, chill wall temperature, and entry liquid composition on momentum, energy and species transfer within both the multiphase and bulk liquid regions. Small mass flow rates and small solidification rates are shown to enhance the free flow of chilled and water-enriched interdendritic fluids across the permeable liquidus front and hence to enhance species redistribution.

Effect of the solid support on the chromatographic properties of a liquid crystal stationary phase

The gas—liquid chromatographic (GLC) retention characteristics of the liquid crystal stationary phase N,N′-bis( p-methoxybenzylidene)-α,α′-bi p-toluidine (BMBT) are significantly affected by the type of solid support used and the amount of liquid crystal deposited on the support. The solid examined were Chromosorb W HP, porous silica galss beads, Porasil C and F. The per cent loading above which the influence of the support is no longer observed varies with the type of support material, being highest for the silica beads of the Porasil C type. The retention volumes of the solute probes, phenanthrene and anthracene, were measured at four temperatures in the nematic range of BMBT coated on Chromosorb W HP and Porasil C. With Chromosorb W HP, the specific retention volume, V o g of a given probe at a given temperature decreases as the loading increases up to 2.5%, beyond which which V o g is practically constant. With Porasil C, V o g continuously decreases as the loading increases up to 30%. Differential scanning calorimetry in combination with GLC revealed the presence of two types of configuration of BMBT molecules on coated solid supports; namely, a liquid crystall film on the surface and a bulk liquid region in the capillaries of the support. With low per cent loadings only surface films are observed, while at higher loadings the support capillaries are also filled with bulk liquid. With surface-active support material the GLC working temperature range is extended to lower temperatures, however this advantage is offset by the drastically lower selectivity offered by thin films as compared to the bulk selectivity achieved with high loadings on inert solid supports.

Diffusion across an Interface

Ideal diffusion in the sense of Fick's law in a heterogeneous system composed of two liquids is discussed by means of a three-region model where the two bulk liquid regions adjoin a third region which characterizes the liquid-liquid interface. When diffusional resistance terms are defined in the usual manner, three classes of interfacial diffusion may be conveniently described by our model. It is found that extraordinary resistance to diffusion at the interface gives rise to perturbations in the observed concentration profile of the bulk regions; these perturbations diminish with time and disappear in the limit of vanishing resistance.