Flint Pierce

Interfacial properties of semifluorinated alkane diblock copolymers

The liquid-vapor interfacial properties of semifluorinated linear alkane diblock copolymers of the form F(3)C(CF(2))(n-1)(CH(2))(m-1)CH(3) are studied by fully atomistic molecular dynamics simulations. The chemical composition and the conformation of the molecules at the interface are identified and correlated with the interfacial energies. A modified form of the Optimized Parameter for Liquid Simulation All-Atom (OPLS-AA) force field of Jorgensen and co-workers [J. Am. Chem. Soc. 106, 6638 (1984); 118, 11225 (1996); J. Phys. Chem. A 105, 4118 (2001)], which includes specific dihedral terms for H-F blocks-and corrections to the H-F nonbonded interaction, is used together with a new version of the exp-6 force field developed in this work. Both force fields yield good agreement with the available experimental liquid density and surface tension data as well as each other over significant temperature ranges and for a variety of chain lengths and compositions. The interfacial regions of semifluorinated alkanes are found to be rich in fluorinated groups compared to hydrogenated groups, an effect that decreases with increasing temperature but is independent of the fractional length of the fluorinated segments. The proliferation of fluorine at the surface substantially lowers the surface tension of the diblock copolymers, yielding values near those of perfluorinated alkanes and distinct from those of protonated alkanes of the same chain length. With decreasing temperatures within the liquid state, chains are found to preferentially align perpendicular to the interface, as previously seen.

Performance of mesoscale modeling methods for predicting rheological properties of charged polystyrene/water suspensions

We examine the accuracy and performance of several leading discrete-element-modeling approaches to predicting equilibrium and dynamic rheological properties of an aqueous, polystyrene suspension. What distinguishes each approach presented is the methodology of handling the solvent hydrodynamics. Specifically, we compare stochastic rotation dynamics (SRD), fast lubrication dynamics (FLD), and dissipative particle dynamics (DPD) methods of including solvent hydrodynamics against each other and against experimental data. Quantities examined are equilibrium structure properties (e.g., pair-distribution function), equilibrium dynamic properties (e.g., long-time diffusivities), and dynamic response (e.g., steady shear). In all approaches, we deploy the Derjaguin-Landau-Verwey-Overbeek (DLVO) potential for colloid–colloid interactions. Comparisons are made over a range of volume fractions and salt concentrations. Long-time diffusivities are especially difficult to compute and exhibit clear discrepancies across m...

Dynamics of polymers across an interface

The migration of polymers across an interface as welding and self-healing of polymer layers takes place, is in the core of polymer-based devices. Using large-scale molecular-dynamics simulations the motion of polymer chains across an interface has been investigated. Unlike self-diffusion of polymers in melts where the early stages of diffusion are characterized by the reptation-like motion of chains in constraining tube formed by neighboring molecules, we find that interdiffusion across an interface is dominated by motion of the chain ends.

Liquid-liquid interfaces of semifluorinated alkane diblock copolymers with water, alkanes, and perfluorinated alkanes.

The liquid-liquid interface between semifluorinated alkane diblock copolymers of the form F3C(CF2)n-1-(CH2)m-1CH3 and water, protonated alkanes, and perfluorinated alkanes are studied by fully atomistic molecular dynamics simulations. A modified version of the OPLS-AA (Optimized Parameter for Liquid Simulation All-Atom) force field of Jorgensen et al. has been used to study the interfacial behavior of semifluorinated diblocks. Aqueous interfaces are found to be sharp, with correspondingly large values of the interfacial tension. Due to the reduced hydrophobicity of the protonated block compared to the fluorinated block, hydrogen enhancement is observed at the interface. Water dipoles in the interfacial region are found to be oriented nearly parallel to the liquid-liquid interface. A number of protonated alkanes and perfluorinated alkanes are found to be mutually miscible with the semifluorinated diblocks. For these liquids, interdiffusion follows the expected Fickian behavior, and concentration-dependent diffusivities are determined.

Spreading of liquid droplets on permeable polymeric surfaces

The spreading of liquid droplets on viscous liquid and glassy polymer films has been studied with molecular dynamics simulations. Unlike spreading on solid surfaces the liquid droplet simultaneously spreads and permeates the film, a process that takes place at a large variety of interfaces from biomembranes to thin coatings. The degree of interpenetration and amount of damping from the film depends on the viscosity of the film and the relative interaction of the droplet and film. For low viscosity liquid films, the interpenetration of the droplet follows a t1/2 Fickian power law and the droplet spreads without a precursor foot. For very viscous films a precursor foot spreads ahead of the film, and the droplet does not penetrate the film. The kinetic, hydrodynamic, and combined kinetic/hydrodynamic models of droplet spreading have been generalized to include the absorption of the droplet by the film and are used to analyze the droplet spreading.

Sensitivity Analysis and Verification of a 1-D Surface Solid Combustion Model for a Fire CFD Boundary Condition.

Predicting the behavior of solid fuels in response to a fire is a complex endeavor. Heterogeneity, charring, and intumescence are a few examples of the many challenges presented by some common materials. If one desires to employ a 3-dimensional computational fluid dynamics (CFD) model for fire, an accurate solid combustion model for materials at the domain boundary is often desirable. Methods for such modeling are not currently mature, and this is a current topic of research. For some practical problems, it may be acceptable to abstract the surface combustible material as a 1-dimensional reacting boundary condition. This approach has the advantage of being a relatively simple model, and may provide acceptably accurate predictions for problems of interest. Such a model has recently been implemented in Sandia’s low-Mach number CFD code for reacting flows, the SIERRA/FUEGO code. Theory for the implemented model is presented. The thermal transport component of the model is verified by approximating a 1-D conduction problem with a closed form solution. The code is further demonstrated by predicting the fire behavior of a block of burning plexiglas (PMMA). The predictions are compared to the reported data from a corresponding experimental program. The predictions are also used to evaluate the sensitivity of model parameters through a sensitivity study using the same test configuration.