Andreas G. Boudouvis

A 2-D pore-network model of the drying of single-component liquids in porous media

Abstract The drying of liquid-saturated porous media is typically approached using macroscopic continuum models involving phenomenological coefficients. Insight on these coefficients can be obtained by a more fundamental study at the pore- and pore-network levels. In this paper, we present a model based on a pore-network representation of porous media that accounts for various processes at the pore-scale. These include mass transfer by advection and diffusion in the gas phase, viscous flow in liquid and gas phases and capillary effects at the gas–liquid menisci in the pore throats. We consider isothermal drying in a rectilinear horizontal geometry, with no-flow conditions in all but one boundary, at which a purge gas is injected at a constant rate. The problem is mainly characterized by two dimensionless parameters, a diffusion-based capillary number, Ca , and a Peclet number, Pe , in addition to the various geometrical parameters of the pore network. Results on the evolution of the liquid saturation, the trapped liquid islands and the drying rate are obtained as a function of time and the dimensionless parameters. The importance of trapped liquid islands on screening mass transfer to the continuous liquid cluster is emphasized. For fixed parameter values, the drying front does not in general obey invasion percolation rules. However, as drying progresses, and depending on the relative magnitude of the capillary and Peclet numbers, a transition to a percolation-controlled problem occurs. Effects of capillarity and mass transfer on saturation profiles and drying rates are discussed. The results are then used to discuss upscaling to continuum models.

On the connection between dielectric breakdown strength, trapping of charge, and contact angle saturation in electrowetting.

Electrowetting on dielectric (EWOD) is simulated by solving the equations of capillary electrohydrostatics, by the Galerkin/finite element method. Aiming to provide reliable predictions of the voltage dependence of the apparent contact angle, close to or beyond the saturation limit, special attention is given in the treatment of the dielectric properties of the solid dielectric where the liquid sits. It is proposed that in regions where the electric field strength locally exceeds the material breakdown strength, the dielectric locally switches to a conductor. Without using any fitting parameter, the implementation of the proposed phenomenological idea realized a surprising matching of published experimental data concerning materials ranging from SiO(2) to Parylene N and Teflon. Charge trapping is naturally connected to the field-induced transition, and its distribution as well as its dependence on the applied voltage is calculated.

Surface and plasma simulation of deposition processes: CH4 plasmas for the growth of diamondlike carbon

A surface model was developed for diamondlike‐carbon film deposition, and was connected in a self‐consistent way with a one‐dimensional plasma chemistry and physics model for a CH4 radio‐frequency (rf) discharge. The surface model considers the adsorption of multiple species (CH3, CH2, and H), and solves for the surface coverage of each species. Comparison is also done with a one‐adsorbed‐species model. Deposition is assumed to take place via direct ion incorporation, and ion‐induced stitching of adsorbed neutrals; film removal takes place via etching and sputtering. The effects of ion flux/energy and surface temperature are examined in detail: At high ion energies direct ion incorporation dominates, in spite of competition with sputtering; at intermediate energies stitching prevails, while for lower ion energies etching can become largest. Mass balances are written at the surface–gas interface, permitting the determination of the effective sticking coefficients of the reacting neutrals. The sticking coef...

Manifestation of the connection between dielectric breakdown strength and contact angle saturation in electrowetting

Limiting phenomena on the electrostatically assisted wetting of dielectric solids by conducting liquids are illuminated by means of computer-aided analysis. The importance of the electrostatic edge effects and their influence on the dielectric properties of the solid is raised to demonstrate that contact angle saturation sets in when the electric field strength locally exceeds the breakdown strength of the dielectric solid where the liquid sits. The proposed argument along with the computed predictions is tested against published experimental measurements showing remarkable agreement.

Etching of SiO2 and Si in fluorocarbon plasmas: A detailed surface model accounting for etching and deposition

A surface model is presented for the etching of silicon (Si) and silicon dioxide (SiO2) in fluorocarbon plasmas. Etching and deposition are accounted for using a generalized concept for the “polymer surface coverage,” which is found to be equivalent to a normalized fluorocarbon film thickness covering the etched surfaces. The model coefficients are obtained from fits to available beam experimental data, while the model results are successfully compared with high-density plasma etching data.

Normal field instability and patterns in pools of ferrofluid

Abstract The normal field instability of ferrofluids and the associated pattern formation have been studied both theoretically and experimentally. Nonlinear stability and bifurcation analyses predict the experimentally observed first-order excitation accompanied by hysteresis in peak height. The effect of imperfections in instability evolution and pattern regularity has also been examined.

Illuminating the connection between contact angle saturation and dielectric breakdown in electrowetting through leakage current measurementsa)

Recent findings on the connection between the dielectric breakdown strength and the contact angle saturation in electrowetting triggered further investigation of the underlying mechanisms towards reporting the consequences of the proposed relation. High sensitivity current measurements are conducted to monitor the dielectric leakage current during a standard electrowetting experiment by testing thin (15–500 nm) dielectric films of materials widely used in microelectronics industry (SiO2, tetra-ethoxy-silane, Si3N4). The measurements confirmed that the current is negligible as long as the applied, direct current, voltage is kept below a critical value at saturation onset. This current, however, exhibits a sharp increase at higher voltages. By exploiting the increased breakdown strength of stacked oxide-nitride-oxide dielectrics, the appearance of the contact angle saturation is inhibited, suggesting the use of such composites for the design of efficient electrowetting devices.

Radiative heat transfer in natural gas-fired furnaces

The performance of the discrete transfer and of the six-flux radiation models is assessed in a swirling natural gas diffusion flame confined in an axisymmetric furnace. The predictions are evaluated as part of a complete prediction procedure involving the modeling of the simultaneously occurring flow, combustion, convection and radiation phenomena. Computational results with and without radiation effects are compared with experimental data and the two radiation models are evaluated in terms of computational efficiency, ease of application and predictive accuracy. The results have demonstrated that the effect of thermal radiation is important even in light flames, and that the six-flux model can be applied in industrial gas furnaces with relative ease, yielding accurate predictions.

Pore-Network Modeling of Isothermal Drying in Porous Media

In this paper we present numerical results obtained with a pore-network model for the drying of porous media that accounts for various processes at the pore scale. These include mass transfer by advection and diffusion in the gas phase, viscous flow in the liquid and gas phases and capillary effects at the liquid-gas interface. We extend our work by studying the effect of capillarity-induced flow in macroscopic liquid films that form at the pore walls as the liquid-gas interface recedes. A mathematical model that accounts for the effect of films on the drying rates and phase distribution patterns is presented. It is shown that film flow is a major transport mechanism in the drying of porous materials, its effect being dominant when capillarity controls the process, which is the case in typical applications.

Radio‐frequency plasmas in CF4: Self‐consistent modeling of the plasma physics and chemistry

A self‐consistent, one‐dimensional simulator for the physics and chemistry of radio frequency plasmas is developed and applied for CF4. The simulator consists of a fluid model for the discharge, a commercial Boltzmann code for calculations of electron energy distribution function (EEDF), a generalized plasma chemistry code, and an interface among the three models. Chemistry calculations are fed back into the physics model and the procedure is repeated until a self‐consistent solution is obtained. The CF4 discharge shows an electronegative behavior with ten times more negative ions than electrons even at low pressures of 100 mTorr. The EEDF high energy tail lies between the Maxwell and Druyvensteyn distribution. The chemistry model predicts densities of 3.5×1012 cm−3 for CF3, 3×1012 cm−3 for CF2, 2.5×1013 cm−3 for F, and 0.7×1012 cm−3 for CF, in agreement with experimental data from a Japanese group. CF and to a lesser extent CF2, are consumed at the surface, and CF, CF2, and F densities and profiles are s...