Simulation of capillary infiltration into packing structures by the Lattice-Boltzmann method for the optimization of ceramic materials

In this work we want to simulate with the Lattice-Boltzmann method in 2D the capillary infiltration into porous structures obtained from the packing of particles. The experimental problem motivating our work is the densification of carbon preforms by reactive melt infiltration. The aim is to determine optimization principles for the manufacturing of high-performance ceramics. Simulations are performed for packings with varying structural properties. Our analysis suggests that the observed slow infiltrations can be ascribed to interface dynamics. Pinning represents the primary factor retarding fluid penetration. The mechanism responsible for this phenomenon is analyzed in detail. When surface growth is allowed, it is found that the phenomenon of pinning becomes stronger. Systems trying to reproduce typical experimental conditions are also investigated. It turns out that the standard for accurate simulations is challenging. The primary obstacle to overcome for enhanced accuracy seems to be the over-occurrence of pinning

Coarse-graining MARTINI model for molecular-dynamics simulations of the wetting properties of graphitic surfaces with non-ionic, long-chain and T-shaped surfactants

We report on a molecular dynamics investigation of the wetting properties of graphitic surfaces by various solutions at concentrations 1-8 wt% of commercially available non-ionic surfactants with long hydrophilic chains, linear or T-shaped. These are surfactants of length up to 160 [\AA]. It turns out that molecular dynamics simulations of such systems ask for a number of solvent particles that can be reached without seriously compromising computational efficiency only by employing a coarse-grained model. The MARTINI force field with polarizable water offers a framework particularly suited for the parameterization of our systems. In general, its advantages over other coarse-grained models are the possibility to explore faster long time scales and the wider range of applicability. Although the accuracy is sometimes put under question, the results for the wetting properties by pure water are in good agreement with those for the corresponding atomistic systems and theoretical predictions. On the other hand, the bulk properties of various aqueous surfactant solutions indicate that the micellar formation process is too strong. For this reason, a typical experimental configuration is better approached by preparing the droplets with the surfactants arranged in the initial state in the vicinity of contact line. Cross-comparisons are possible and illuminating, but equilibrium contanct angles as obtained from simulations overestimate the experimental results. Nevertheless, our findings can provide guidelines for the preliminary assessment and screening of surfactants. [See pdf file for full abstract]Comment: Revised version. Publication: http://dx.doi.org/10.1063/1.4747827. Material: https://sites.google.com/site/material4sim

Surface growth for molten silicon infiltration into carbon millimeter-sized channels: Lattice-Boltzmann simulations, experiments and models

The process of liquid silicon infiltration is investigated for channels with radii from $0.25$ to $0.75$ [mm] drilled in compact carbon preforms. The advantage of this setup is that the study of the phenomenon results to be simplified. For comparison purposes, attempts are made in order to work out a framework for evaluating the accuracy of simulations. The approach relies on dimensionless numbers involving the properties of the surface reaction. It turns out that complex hydrodynamic behavior derived from second Newton law can be made consistent with Lattice-Boltzmann simulations. The experiments give clear evidence that the growth of silicon carbide proceeds in two different stages and basic mechanisms are highlighted. Lattice-Boltzmann simulations prove to be an effective tool for the description of the growing phase. Namely, essential experimental constraints can be implemented. As a result, the existing models are useful to gain more insight on the process of reactive infiltration into porous media in the first stage of penetration, i.e. up to pore closure because of surface growth. A way allowing to implement the resistance from chemical reaction in Darcy law is also proposed

Wetting and contact-line effects for spherical and cylindrical droplets on graphene layers: A comparative molecular-dynamics investigation

In Molecular Dynamics (MD) simulations, interactions between water molecules and graphitic surfaces are often modeled as a simple Lennard-Jones potential between oxygen and carbon atoms. A possible method for tuning this parameter consists of simulating a water nanodroplet on a flat graphitic surface, measuring the equilibrium contact angle, extrapolating it to the limit of a macroscopic droplet and finally matching this quantity to experimental results. Considering recent evidence demonstrating that the contact angle of water on a graphitic plane is much higher than what was previously reported, we estimate the oxygen-carbon interaction for the recent SPC/Fwwater model. Results indicate a value of about 0.2 kJ/mol, much lower than previous estimations. We then perform simulations of cylindrical water filaments on graphitic surfaces, in order to compare and correlate contact angles resulting from these two different systems. Results suggest that modified Young's equation does not describe the relation between contact angle and drop size in the case of extremely small systems and that contributions different from the one deriving from contact line tension should be taken into account.Comment: To be published on Physical Review E (http://pre.aps.org/

Chapter Wetting and Navier-Stokes Equation — The Manufacture of Composite Materials

Current landfill regulations provide for the responsible management of solid waste and a safer alternative to the outdated practices of open or illegal dumping. Aside from imparting aesthetic value, natural or planted vegetation on landfill sites has an important role in erosion control and removal of contaminants, and may also be used in leaching treatment. The use of leachate for the irrigation of landfill vegetation reduces its harmful effects, and the reuse of water aids in water conservation. The aim of this study was to search for ways to use leachate water from solid waste landfill sites for irrigation of plant species that normally grow in the wild. The study focuses on the plant species Alcea rosea (hollyhock), Cynodon dactylon (Bermuda grass) and Melilotus officinalis (yellow melilot). Over the 2-year study period, plants were irrigated with tap and leachate water under drought conditions. Wild plant diversity was identified, and the landfill was rehabilitated with various plant species. After the experiment, populations of Escherichia coli, total coliforms and fecal coliform bacteria in soil samples were analyzed. We observed that the use of leachate water for cultivation of different kinds of plants affected the density of total and fecal coliforms in the soil

Wetting and Navier-Stokes Equation — The Manufacture of Composite Materials

It is well known that there are several processes to manufacture composite materials, a large part of which consist in the infiltration of a liquid (matrix) through a porous medium (reinforcement). To perform these processes, both thermodynamics (wetting) and kinetics (Navier-Stokes) must be considered if a good quality composite material is sought. Although wetting and the laws that govern it have been well known for over 200 years, dating back to the original works of Young and Laplace, this is not the case with the Navier-Stokes equation, which remains so far unsolved. Although the Navier-Stokes equation, which describes the motion of a fluid, has been solved for many particular cases, such as the motion of a fluid through a pipe, which has resulted in the well-known Poiseuille equation, or the motion of a fluid through a porous media, described by the Darcy’s law (empirical law obtained by Darcy), its general solution remains one of the greatest challenges of mathematicians today. Therefore, the objective of this chapter is to present the resolution of the Navier-Stokes equation with the laws of wetting for different cases of interest in the manufacture of composite materials