Gerald G. Pereira

Organization of polymer chains onto long, single-wall carbon nano-tubes: effect of tube diameter and cooling method.

We use molecular dynamics simulations to investigate the arrangement of polymer chains when absorbed onto a long, single-wall carbon nano-tube (SWCNT). We study the conformation and organization of the polymer chains on the SWCNT and their dependence on the tubes diameter and the rate of cooling. We use two types of cooling processes: direct quenching and gradual cooling. The radial density distribution function and bond orientational order parameter are used to characterize the polymer chain structure near the surface. In the direct cooling process, the beads of the polymer chain organize in lamella-like patterns on the surface of the SWCNT with the long axis of the lamella parallel to the axis of the SWCNT. In a stepwise, gradual cooling process, the polymer beads form a helical pattern on the surface of a relatively thick SWCNT, but form a lamella-like pattern on the surface of a very thin SWCNT. We develop a theoretical (free energy) model to explain this difference in pattern structures for the gradual cooling process and also provide a qualitative explanation for the pattern that forms from the direct cooling process.

Polymers encapsulated in short single wall carbon nanotubes: Pseudo-1D morphologies and induced chirality

Molecular dynamics simulations are performed to investigate the stable morphologies of semi-flexible polymer chains within a single wall carbon nanotube (CNT). We characterize these morphologies with a variety of measures. Due to the different curvature inside the CNT to outside, there are increased numbers of polymer-CNT bead contacts for polymers which reside inside the CNT. A sufficiently long polymer chain first adsorbs on the exterior of the nanotube and subsequently moves inside the cavity of the nanotube. At equilibrium, the polymer configuration consists of a central stem surrounded by helically wrapped layers. Sections of the polymer outside the CNT have helical conformations (for CNTs of small radius) or circular arrangements (for CNTs of larger radius). Polymers encapsulated within the CNT have an increased chirality due to packing of the beads and this chirality is further enhanced for moderately stiff chains.

A Monte Carlo study of wetting transitions in polymer blends confined to a capillary

We investigate the problem of wetting transitions in polymer blends confined to a slitlike adsorbing capillary of thickness H by Monte Carlo methods. Two paths for capillary wetting are considered, either along a path of increasing temperature, T, or increasing surface chemical potential, μ1. We find that H can be thought of as an additional thermodynamic parameter which controls the nature of the transitions. We find that there exists a capillary critical separation, Hc. For separations less than Hc the blend falls in the one phase region of the phase diagram. Above Hc there exists a separation Hbulk, where the polymer blend begins to show bulk behavior. Hbulk is shown to separate regions of first order transitions (H<Hbulk) from critical wetting transitions (H≥Hbulk). Along a path of constant T and increasing μ1 we find for 2ξ<H1<H2<Hbulk that the first‐order transitions between the two separations is shifted according to μc1(H2)−μc1(H1)∝1/H1−1/H2. We discuss the implications of these results for the ca...

Grayscale lattice Boltzmann model for multiphase heterogeneous flow through porous media.

The grayscale lattice Boltzmann (LB) model has been recently developed to model single-phase fluid flow through heterogeneous porous media. Flow is allowed in each voxel but the degree of flow depends on that voxels resistivity to fluid motion. Here we extend the grayscale LB model to multiphase, immiscible flow. The new model is outlined and then applied to a number of test cases, which show good agreement with theory. This method is subsequently used to model the important case where each voxel may have a different resistance to each particular fluid that is passing through it. Finally, the method is applied to model fluid flow through real porous media to demonstrate its capability. Both the capillary and viscous flow regimes are recovered in these simulations.

Effect of Uniformly Applied Force and Molecular Characteristics of a Polymer Chain on Its Adhesion to Graphene Substrates

The force-induced desorption of a polymer chain from a graphene substrate is studied with molecular dynamics (MD). A critical force needs to be exceeded before detachment of the polymer from the substrate. It is found that for a chain to exhibit good adhesive properties the chain configuration should consist of fibrils-elongated, aligned sections of polymers and cavities which dissipate the applied energy. A fibrillation index is defined to quantify the quality of fibrils. We focus on the molecular properties of the polymer chain, which can lead to large amounts of fibrillation, and find that both strong attraction between the polymer and substrate and good solvency conditions are important conditions for this. We also vary the stiffness of the chain and find that for less stiff chains a plateau in the stress-strain curve gives rise to good adhesion however for very stiff chains there is limited elongation of the chain but the chain can still exhibit good fibrillation by a lamella-like rearrangement. Finally, it is found that the detachment time, t, of a polymer from the adsorbed substrate is inversely proportional to force, F (i.e., t ∝ F(-γ)), where exponent γ depends on the solvent quality, polymer-substrate attraction, and chain stiffness.

Wetting transitions in polymer blends: Comparison between simulation and theory

We investigate the problem of wetting transitions in polymer blends near a hard surface or wall by using a Monte Carlo technique to study the wetting transition along a path of increasing surface chemical potential. We introduce a parameter es which describes the monomer–monomer interactions in the layer adjacent to the wall. This parameter is shown to behave similarly to the parameter g, used in mean field theory to describe the change in monomer–monomer interactions due to the wall. We identify a wetting tricritical point which may be defined either with respect to es or the bulk density. For bulk densities less than the tricritical bulk density we obtain first‐order transitions while for bulk densities greater than the tricritical bulk density we obtain critical wetting transitions, in accordance with mean‐field theory. We also show how the molecular weight of the polymer can be varied to obtain first‐order or critical wetting, as desired.

Effect of diffusional relaxation in random sequential adsorption of polymer chains

We study by Monte Carlo computer simulations random sequential adsorption with diffusional relaxation of polymer chains of size N onto a square lattice. The coverage θ(t) is found to grow to full saturation with a power-law behaviour i.e., θ(t) ∼ 1 − t−y, where the exponent y ≈ 1/(N − 1) in the fast diffusion limit. This result agrees with recent mean-field results for a corresponding chemical reaction.

Lattice Boltzmann heat transfer model for permeable voxels

We develop a gray-scale lattice Boltzmann (LB) model to study fluid flow combined with heat transfer for flow through porous media where voxels may be partially solid (or void). Heat transfer in rocks may lead to deformation, which in turn can modulate the fluid flow and so has significant contribution to rock permeability. The LB temperature field is compared to a finite difference solution of the continuum partial differential equations for fluid flow in a channel. Excellent quantitative agreement is found for both Poiseuille channel flow and Brinkman flow. The LB model is then applied to sample porous media such as packed beds and also more realistic sandstone rock sample, and both the convective and diffusive regimes are recovered when varying the thermal diffusivity. It is found that while the rock permeability can be comparatively small (order milli-Darcy), the temperature field can show significant variation depending on the thermal convection of the fluid. This LB method has significant advantages over other numerical methods such as finite and boundary element methods in dealing with coupled fluid flow and heat transfer in rocks which have irregular and nonsmooth pore spaces.

Theoretical study of the prewetting transition in polymer blends

Prewetting transitions in polymer blends near a hard surface which favors one of the phases in the blend are studied by both mean-field and Monte Carlo methods. The mean-field results predict for systems that have a first-order wetting transition at a bulk density of (ρ∞)W, there exists first-order prewetting transitions for (ρ∞)W⩽ρ∞ (ρ∞)PW there exist second-order transitions so that (ρ∞)PW may be identified as the prewetting critical point. Monte Carlo simulations of the bond fluctuation model on a simple cubic lattice between two hard walls H lattice spacings apart are performed and qualitative agreement is found with the mean-field predictions.