Lloyd L. Lee

Liquid water can slip on a hydrophilic surface

Understanding and predicting the behavior of water, especially in contact with various surfaces, is a scientific challenge. Molecular-level understanding of hydrophobic effects and their macroscopic consequences, in particular, is critical to many applications. Macroscopically, a surface is classified as hydrophilic or hydrophobic depending on the contact angle formed by a water droplet. Because hydrophobic surfaces tend to cause water slip whereas hydrophilic ones do not, the former surfaces can yield self-cleaning garments and ice-repellent materials whereas the latter cannot. The results presented herein suggest that this dichotomy might be purely coincidental. Our simulation results demonstrate that hydrophilic surfaces can show features typically associated with hydrophobicity, namely liquid water slip. Further analysis provides details on the molecular mechanism responsible for this surprising result.

Boundary slip and wetting properties of interfaces: Correlation of the contact angle with the slip length

Correlations between contact angle, a measure of the wetting of surfaces, and slip length are developed using nonequilibrium molecular dynamics for a Lennard-Jones fluid in Couette flow between graphitelike hexagonal-lattice walls. The fluid-wall interaction is varied by modulating the interfacial energy parameter epsilonr=epsilonsfepsilonff and the size parameter sigmar=sigmasfsigmaff, (s=solid, f=fluid) to achieve hydrophobicity (solvophobicity) or hydrophilicity (solvophilicity). The effects of surface chemistry, as well as the effects of temperature and shear rate on the slip length are determined. The contact angle increases from 25 degrees to 147 degrees on highly hydrophobic surfaces (as epsilonr decreases from 0.5 to 0.1), as expected. The slip length is functionally dependent on the affinity strength parameters epsilonr and sigmar: increasing logarithmically with decreasing surface energy epsilonr (i.e., more hydrophobic), while decreasing with power law with decreasing size sigmar. The mechanism for the latter is different from the energetic case. While weak wall forces (small epsilonr) produce hydrophobicity, larger sigmar smoothes out the surface roughness. Both tend to increase the slip. The slip length grows rapidly with a high shear rate, as wall velocity increases three decades from 100 to 10(5) ms. We demonstrate that fluid-solid interfaces with low epsilonr and high sigmar should be chosen to increase slip and are prime candidates for drag reduction.

Chemical potentials based on the molecular distribution functions. An exact diagrammatical representation and the star function

A closed form for the chemical potentials of a fluid is presented that involves only integrals of the molecular distribution functions at the given state, (e.g., temperature and density). Thus no Kirkwood charging or thermodynamic integration is needed. An exact formula from a previous study is reanalyzed and a diagrammatical representation of the correlation functions involved is given. This representation involves, in addition to the usual total correlations, direct correlations, and the bridge function, B(r), a new star function, S(r). Analysis shows that the integral of the star function is the primitive of the bridge function, i.e., its functional derivative yields B(r). It is also related to the free‐energy functional F[ρ] in density‐functional theories for nonuniform systems. Methods for estimating the star function are given. Tests on uniform hard‐sphere fluid are carried out to demonstrate the new formulas. We have examined several current closures: the Percus–Yevick, Martynov–Sarkisov, Ballone–P...

SELF-CONSISTENT EQUATIONS FOR CALCULATING THE IDEAL GAS HEAT CAPACITY, ENTHALPY, AND ENTROPY

Abstract Aly, F.A. and Lee, L.L., 1981. Self-consistent equations for calculating the ideal gas heat capacity, enthalpy and entropy. Fluid Phase Equilibria , 6: 169—179. Self-consistent equations for calculating the ideal gas heat capacity, enthalpy, and entropy are derived based on statistical mechanical formulae and with simplifications to facilitate engineering calculation. Some sixty compounds have been investigated. Comparison with existing heat capacity correlations, notably those of Passut and Danner and Duran et al., shows that the present equations are more accurate for most cases. The formulae are consistent in that they obey the basic thermodynamic relations among C p * , H * , and S * . Only five (for C p * ) or six (for H * and S * ) constants are used which are common to all properties.

An accurate integral equation theory for hard spheres: Role of the zero‐separation theorems in the closure relation

We evaluate a number of current closure relations used in the integral equations for hard sphere fluids, such as the Percus–Yevick, Martynov–Sarkisov, Ballone–Pastore–Galli–Gazillo, and Verlet modified (VM) closures with respect to their abilities of satisfying the zero‐separation theorems for hard spheres. Only the VM closure is acceptable at high densities (ρ∼0.7), while all fail at lower densities (lim 0<ρ<0.5). These shall have deleterious effects when used in perturbation theories, especially at low densities. To improve upon this, we propose a closure, ZSEP, that is flexible and suited to satisfying the known zero separation theorems [e.g., the ones for the cavity function y(0) and the indirect correlation γ(0), and others for their derivatives dy(0)/dr, etc.], plus the pressure consistency condition. This particular closure, after numerical solution with the Ornstein–Zernike equation, is shown to perform well at high densities (ρ∼0.9) as well as low densities (0.1<ρ<0.5) for the cavity function y(r...

Interfacial water on crystalline silica: a comparative molecular dynamics simulation study

Understanding the properties of interfacial water at solid–liquid interfaces is important in a wide range of applications. Molecular dynamics is becoming a widespread tool for this purpose. Unfortunately, however, the results of such studies are known to strongly depend on the selection of force fields. It is, therefore, of interest to assess the extent by which the implemented force fields can affect the predicted properties of interfacial water. Two silica surfaces, with low and high surface hydroxyl density, respectively, were simulated implementing four force fields. These force fields yield different orientation and flexibility of surface hydrogen atoms, and also different interaction potentials with water molecules. The properties for interfacial water were quantified by calculating contact angles, atomic density profiles, surface density distributions, hydrogen bond density profiles and residence times for water near the solid substrates. We found that at low surface density of hydroxyl groups, the force field strongly affects the predicted contact angle, while at high density of hydroxyl groups, water wets all surfaces considered. From a molecular-level point of view, our results show that the position and intensity of peaks observed from oxygen and hydrogen atomic density profiles are quite different when different force fields are implemented, even when the simulated contact angles are similar. Particularly, the surfaces simulated by the CLAYFF force field appear to attract water more strongly than those simulated by the Bródka and Zerda force field. It was found that the surface density distributions for water strongly depend on the orientation of surface hydrogen atoms. In all cases, we found an elevated number of hydrogen bonds formed between interfacial water molecules. The hydrogen bond density profile does not depend strongly on the force field implemented to simulate the substrate, suggesting that interfacial water assumes the necessary orientation to maximise the number of water–water hydrogen bonds irrespectively of surface properties. Conversely, the residence time for water molecules near the interface strongly depends on the force field and on the flexibility of surface hydroxyl groups. Specifically, water molecules reside for longer times at contact with rigid substrates with high density of hydroxyl groups. These results should be considered when comparisons between simulated and experimental data are attempted.

Molecular thermodynamics of electrolyte solutions

Review of Electrostatics Solution Thermodynamics of Electrolytes The Debye-Huckel Theory The Statistical Mechanics of Ionic Solutions Molecular Thermodynamics Mean Spherical Approximation: The Hard Sphere Ions The Integral Equations for Ion Distributions The McMillan-Mayer and Lewis-Randall Scales in Non-Aqueous Electrolyte Solutions Generalization to Mixed-Salt and Non-Aqueous Electrolyte Solutions Application: Amine Solutions in Acid Gas Removal Application: Ionic Refrigerants in Adsorption Engine Cycles.

Phase stability of binary non‐additive hard‐sphere mixtures: A self‐consistent integral equation study

We have tested the capabilities of a new self‐consistent integral equation, closely connected with Verlet’s modified closure, for the study of fluid‐fluid phase separation in symmetric non‐additive hard‐sphere mixtures. New expressions to evaluate the chemical potential of mixtures are presented and play a key role in the construction of the phase diagram. The new integral equation, which implements consistency between virial and fluctuation theorem routes to the isothermal compressibility, together with chemical potential and virial pressure consistency via the Gibbs‐Duhem relation, yields a phase diagram which especially at high densities agrees remarkably well with the new semi‐Grand Ensemble Monte Carlo simulation data also presented in this work. Deviations close to the critical point can be understood as a consequence of the inability to enforce virial‐fluctuation consistency in the neighborhood of the spinodal decomposition curve.

Random walks in nanotube composites: Improved algorithms and the role of thermal boundary resistance

Random walk simulations of thermal walkers are used to study the effect of interfacial resistance on heat flow in randomly dispersed carbon nanotube composites. The adopted algorithm effectively makes the thermal conductivity of the nanotubes themselves infinite. The probability that a walker colliding with a matrix-nanotube interface reflects back into the matrix phase or crosses into the carbon nanotube phase is determined by the thermal boundary (Kapitza) resistance. The use of “cold” and “hot” walkers produces a steady state temperature profile that allows accurate determination of the thermal conductivity. The effects of the carbon nanotube orientation, aspect ratio, volume fraction, and Kapitza resistance on the composite effective conductivity are quantified.

A molecular theory for the thermodynamic behavior of polar mixtures. I. The statistical-mechanical local-composition model

Abstract The statistical-mechanical basis for the local-composition model is presented. In the statistical-mechanical framework, local compositions are expressed in terms of the potential of mean force, so that the approximations required to obtain the expressions commonly utilized can be analyzed and new and improved expressions can be developed. The consistency relations required for the use of the excess free energy for the configurational energy in the local-composition model are investigated in general. As a test, the local-composition model is applied to the case of Lennard—Jones mixtures. It is also demonstrated that the statistical-mechanical expressions for the internal energy in the local-composition model can be related to experimentally measurable quantities. This will allow direct comparison of nonstatistical and statistical-mechanical formulations of the local-composition model using experimental data for actual fluid mixtures of interest.