Ali Zand

Sessile droplet spread into porous substrates—Determination of capillary pressure using a continuum approach

The problem of primary and secondary spread of sessile droplets into a porous substrate was formulated and solved numerically. A continuum approach for liquid- and gas-phases was utilized. The governing equations were discretized by finite difference method and solutions for both phases are obtained by marching in time using the fourth-order Runge-Kutta integration algorithm. This type of spread is a purely momentum-driven process that is caused by gradients both in capillary pressure and in saturation. A methodology was developed for finding the capillary pressure function for sessile droplets, which has not been described before. This approach was based on experimental data for a liquid/porous medium pair, and using universal, non-dimensional curves. Similar solutions were generated by the continuum approach and validated using experimental results. The model shows qualitative and quantitative agreement with experimental data. Although the focus of this work was to understand the interaction of chemical warfare agents with porous media, the approaches are universal and can be applied to determining the spread of any liquid into a porous material.

Infiltration time and imprint shape of a sessile droplet imbibing porous medium

The infiltration of a sessile droplet into a homogeneous porous medium for a constant droplet base radius case is solved numerically, where the porous medium is represented as a capillary network consisting of pores and throats. For a homogeneous medium, the network is built of the spherical pores of constant radius, and the cylindrical throats of constant radius and height. Having such defined network, the droplet imbibes porous medium in a single-phase flow for which the free interface in porous medium is smooth, and the liquid phase permeability and the capillary pressure are constant. Using the numerical solution we carry out the parametric study in which: (i) liquid viscosity and surface tension, (ii) droplet volume and base radius, and (iii) porous medium porosity and permeability are varied. The droplet infiltration time, and the imprint shape that is given with two spheroid half-axes are calculated. The dimensionless analysis is utilized to correlate the droplet infiltration parameters from which master curves for the droplet infiltration time and the droplet imprint shape are obtained. Using the infiltration time correlation, both numerical and experimental results show a linear behavior.

Antioxidant potential of Juglans nigra, black walnut, husks extracted using supercritical carbon dioxide with an ethanol modifier

Abstract The black walnut, Junglas nigra, is indigenous to eastern North America, and abscission of its fruit occurs around October. The fruit consists of a husk, a hard shell, and kernel. The husk is commonly discarded in processing, though it contains phenolic compounds that exhibit antioxidant and antimicrobial properties. For this study, black walnut husks were extracted using supercritical carbon dioxide with an ethanol modifier. The effects of temperature, ethanol concentration, and drying of walnut husks prior to extraction upon antioxidant potential were evaluated using a factorial design of experiments. The solvent density was held constant at 0.75 g/mL. The optimal extraction conditions were found to be 68°C and 20 wt‐% ethanol in supercritical carbon dioxide. At these conditions, the antioxidant potential as measured by the ferric reducing ability of plasma (FRAP) assay was 0.027 mmol trolox equivalent/g (mmol TE/g) for dried walnut husk and 0.054 mmol TE/g for walnut husks that were not dried. Antioxidant potential was also evaluated using the total phenolic content (TPC) and 1,1‐diphenyl‐2‐picryl‐hydrazyl (DPPH) assays and the FRAP assay was found to linearly correlate to the TPC assay.

Superheated liquid and supercritical denatured ethanol extraction of antioxidants from Crimson red grape stems

Abstract Grapes are widely known for health benefits due to their antioxidant content. In wine production, grape stems are often discarded, though they has a higher content of antioxidants than the juice. The effectiveness of using an environmentally friendly solvent, ethanol, as a superheated liquid and supercritical fluid to extract antioxidant compounds from grape stems of organically grown Crimson Seedless grapes was evaluated. The Ferric Reducing Ability of Plasma (FRAP) assay and the Total Phenolic Content (TPC), or Folin‐Ciocalteu assay, were used to quantify the antioxidant power of grape stem extracts. The extractions were performed at temperatures between 160°C and 300°C at constant density. It was found that the optimal extraction temperature was 204°C, at superheated liquid conditions, with a FRAP value of 0.670 mmol Trolox Equivalent/g of dry grape stem. The FRAP values were higher than other studies that extracted antioxidants from grape stems using single‐pass batch extraction.

Preparation of hydroxylated polyethylene surfaces

The surfaces of high-density or ultra-high-molecular-weight polyethylenes were hydroxylated using a two-step process. The wetting and wear properties of the untreated (virgin) and surface hydroxylated polyethylenes were compared. The introduction of hydroxyl groups provided an increase in surface hydrophilicity resulting in reduced wear. Hydrophilicity was analyzed by optical analysis of water contact angle. Wear was determined by weight loss under conditions of a reciprocating pin-on-plate apparatus with the panels immersed in water or calf serum. These results suggest that hydroxylation of polyethylene friction-bearing orthopedic surfaces may lead to a longer joint life.

Plasma engineered surfaces for orthopedic devices

Abstract Atmospheric pressure plasma was used to graft various biocompatible polymers to the surface of ultra-high molecular weight polyethylene (UHMWPE). Polymers used as grafts in this study were poly(2-hydroxyethylmethacrylate) (PHEMA) and polyethylene glycol (PEG). A significant decrease in contact angle was noted for grafted surfaces, indicating increased hydrophilicity. Surface functionalities were verified using Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. The wear properties of the coatings were determined by weight loss under conditions of a random motion pin-on-plate apparatus with the coated polyethylene plaques immersed in DI water. Based on these tests, the grafted surfaces exhibited an improved resistance to wear, compared to UHMWPE. Cell viability studies were used to confirm that the plasma treatment had no negative effects on the surface bio-toxicity. Based on the results, it is anticipated that the incorporation of these biocompatible polymer-grafted UHMWPE surfaces in metal-on-plastic orthopedic implants should improve their performance and longevity.

Fate of sessile droplet chemical agents in environmental substrates in the presence of physiochemical processes

A general-purpose multi-phase and multi-component computer model capable of solving the complex problems encountered in the agent substrate interaction is developed. The model solves the transient and time-accurate mass and momentum governing equations in a three dimensional space. The provisions for considering all the inter-phase activities (solidification, evaporation, condensation, etc.) are included in the model. The chemical reactions among all phases are allowed and the products of the existing chemical reactions in all three phases are possible. The impact of chemical reaction products on the transport properties in porous media such as porosity, capillary pressure, and permeability is considered. Numerous validations for simulants, agents, and pesticides with laboratory and open air data are presented. Results for chemical reactions in the presence of pre-existing water in porous materials such as moisture, or separated agent and water droplets on porous substrates are presented. The model will greatly enhance the capabilities in predicting the level of threat after any chemical such as Toxic Industrial Chemicals (TICs) and Toxic Industrial Materials (TIMs) release on environmental substrates. The model’s generality makes it suitable for both defense and pharmaceutical applications.

Simple and effective method to measure the diffusion coefficient of organic vapors in porous media

A quick and reliable method to measure a vapor concentration within porous substrate was developed. The technique consists of two steps, where a modified head space single drop microextraction (HS-SDME) sampling is used to entrap the vapor phase. In the second step, the entrapped vapor concentration is measured by gas chromatography (GC-FID). The technique is used to measure an effective diffusion coefficient of n-pentane in dry medium grain sand, with the sand partially saturated with water as an inert liquid. The measurements are carried out in a cylindrical sand holder on which the HS-SDME sampling ports are mounted. A linear vapor concentration profile along the bed thickness is found. From known concentration gradient and measuring the mass flux gravimetrically, the effective diffusion coefficient is determined. It turns out that the diffusion coefficient decreases from 8.49 × 10−6 for dry sand to 7.13 × 10−6 m2 s−1 as a function of water saturation. Additional hindrance to the vapor transport is observed from both the simple volumetric effect due to the porous medium void space reduction caused by the presence of water and increase of the tortuosity.

Synthesis and Characterization of a Composite Membrane for Polymer Electrolyte Fuel Cell

A new proton exchange membrane (PEM) has been fabricated using a novel patented polymer structure modification technology. It has been shown that the new membrane has a higher proton transfer rate and lower resistance as compared to Nafion®. This paper discusses issues related to membrane fabrication and testing procedures. (i) A brief fabrication procedure of PEM is outlined. The fabrication technique used here separates the proton exchange and structural requirements of the PEM allowing greater flexibility in membrane design. The proton exchange material developed herein is a terpolymer composed of various ratios of acrylic acid, styrene, and vinylsulfonic acid. Following a patented polymer structure modification technology, these materials were bound to an ethylene-tetrafluoroethylene copolymer mesh that had been rendered adhesive by hydroxylation in a two-step water-borne process. (ii) A previously developed theoretical model is used to calculate the relative resistance and proton transfer rate. According to the model, a simple second order differential equation describes the entire process and established a relationship between the membrane resistance and the total time taken for a specific amount of protons to pass through it. Finally, (iii) a simple two-cell experimental procedure is developed to calculate the relative membrane resistance and proton transfer capacity. The results show that theoretical predictions are in excellent agreement with the experimental observations. The new membrane could transfer protons approximately 80% faster than Nafion® per unit area under the test conditions utilized. Membrane resistance is also 71% lower compared to Nafion®. These results suggest that there is now a new route of fabricating cost effective proton exchange membranes for fuel cell applications wherein one may focus more on the proton exchange capacity of the membrane allowing the structural properties of the membrane to be considered separately.