Thick Porous Substrate Research Articles

Absorption and Spreading of a Liquid Droplet Over a Thick Porous Substrate.

Spreading over porous substrates occurs in several processes including printing, cleaning, coating, and manufacturing of ceramic structures. For small drops, viscous and capillary forces are ultimately the predominant forces. The process typically undergoes three phases: a first stage in which the droplet spreads, a second phase in which the area of contact with the solid substrate nearly remains constant, and a third stage in which the droplet retracts with its volume reaching zero finally. The objective of the investigation is to find the dynamics of spreading and absorption of the droplet using fundamentals while making relevant approximations to account for both radial and vertical dynamics. The proposed model requires minimal computational work. The results are compared with the published experimental data for the perfect wetting case, and are found to be in good agreement with detailed published experimental data for both droplet dynamics and dynamics of penetration in the porous substrate.

Kinetics of Spreading over Porous Substrates

The spreading of small liquid drops over thin and thick porous layers (dry or saturated with the same liquid) is discussed in the case of both complete wetting (silicone oils of different viscosities over nitrocellulose membranes and blood over a filter paper) and partial wetting (aqueous SDS (Sodium dodecyl sulfate) solutions of different concentrations and blood over partially wetted substrates). Filter paper and nitrocellulose membranes of different porosity and different average pore size were used as a model of thin porous layers, sponges, glass and metal filters were used as a model of thick porous substrates. Spreading of both Newtonian and non-Newtonian liquid are considered below. In the case of complete wetting, two spreading regimes were found (i) the fast spreading regime, when imbibition is not important and (ii) the second slow regime when imbibition dominates. As a result of these two competing processes, the radius of the drop goes through a maximum value over time. A system of two differential equations was derived in the case of complete wetting for both Newtonian and non-Newtonian liquids to describe the evolution with time of radii of both the drop base and the wetted region inside the porous layer. The deduced system of differential equations does not include any fitting parameter. Experiments were carried out by the spreading of silicone oil drops over various dry microfiltration membranes (permeable in both normal and tangential directions) and blood over dry filter paper. The time evolution of the radii of both the drop base and the wetted region inside the porous layer were monitored. All experimental data fell on two universal curves if appropriate scales are used with a plot of the dimensionless radii of the drop base and of the wetted region inside the porous layer on dimensionless time. The predicted theoretical relationships are two universal curves accounting quite satisfactorily for the experimental data. According to the theory prediction, (i) the dynamic contact angle dependence on the same dimensionless time as before should be a universal function and (ii) the dynamic contact angle should change rapidly over an initial short stage of spreading and should remain a constant value over the duration of the rest of the spreading process. The constancy of the contact angle on this stage has nothing to do with hysteresis of the contact angle: there is no hysteresis in the system under investigation in the case of complete wetting. These conclusions again are in good agreement with experimental observations in the case of complete wetting for both Newtonian and non-Newtonian liquids. Addition of surfactant to aqueous solutions, as expected, improve spreading over porous substrates and, in some cases, results in switching from partial to complete wetting. It was shown that for the spreading of surfactant solutions on thick porous substrates there is a minimum contact angle after which the droplet rapidly absorbs into the substrate. Unfortunately, a theory of spreading/imbibition over thick porous substrates is still to be developed. However, it was shown that the dimensionless time dependences of both contact angle and spreading radius of the droplet on thick porous material fall on to a universal curve in the case of complete wetting.

Diffusion Layers with Localized Hydrophilic Domains: Influence of Electron Energy on Spatial Resolution

The development of advanced water management strategies for polymer electrolyte fuel cells (PEFCs) could enable further improvements in power density, and therefore decrease the cost. A recently developed approach, consisting on electron radiation grafting using masks, has been utilized to fabricate gas diffusion layers with patterned wettability [1]. These novel materials feature hydrophilic, straightforward pathways for the transport of liquid water; and hydrophobic, low-resistance domains for the transport of reactant gases [2]. It has been recently demonstrated that PEFCs performance can be improved by employing the modified gas diffusion layers on the cathode side, due to the segregated distribution of liquid water that leads to reduced mass transport overpotentials [3]. However, further improvements in resolution are required to fabricate advanced porous materials with localized properties for various applications [4]. Pursuant to this goal, simulations can be used, in tandem with experiments, to elucidate the dominating parameters that influence the resolution (e.g. electron energy, mask-to-sample separation). Moreover, simulations can be utilized in a predictive fashion to calculate the electron beam parameters required to fabricate a given material. In this presentation, we will discuss the influence of electron beam energy on the spatial resolution of porous materials typically used in PEFCs. A combination of experiments and Monte-Carlo simulations was employed to advance the fundamental understanding of mask-assisted through-volume modifications of thick porous substrates. Interestingly, we found that an energy of, at least, 140 keV was required to modify throughout the complete thickness and that the use of energies lower than 300 keV results in a blurring in the adjacent regions. Additionally, we will discuss the influence of housing materials on the back-scattered radiation. Finally, and due to its practical relevance, we will discuss the possibility of selectively modifying one layer out of a bi-layered material (i.e. GDL-MPL arrangement). [1] Forner-Cuenca, A. et al., Adv. Mater. 27, 6317–6322 (2015). [2] Forner-Cuenca, A. et al., J. Electrochem. Soc. 163, F1038–F1048 (2016). [3] Forner-Cuenca, A. et al., J. Electrochem. Soc. 163, F1389–F1398 (2016). [4] Forner Cuenca, A. et al., Radiat. Phys. Chem. 135, 133–141 (2017). Acknowledgments AFC gratefully acknowledges the financial support of the Swiss National Science Foundation (P2EZP2_172183).

Foam drainage on thick porous substrate

The use of foam-based applications as a method of drug delivery represents a recent and promising area of research. The interaction of foam and porous substrates have been recently theoretically described using a mathematical model, which combines the equation of foam drainage with that of imbibition of liquid into the porous substrate. Below the drainage of foam placed on chalk experimentally investigated to verify the theory prediction. The surfactants sodium dodecyl sulfate (SDS) and Triton X-100 were used to form a foam. The initial liquid volume fractions of the foam were found to be ranging in between 14.12 and 16.46%. The porosity and permeability of the chalk substrate were experimentally obtained at 59.1% and 3.122.10 -11 m 2 respectively. The height of foam deposited onto the thick porous substrate (chalk) was 2.5 cm and 6 cm. The imbibition into the chalk, the height of foam, and the bubble size within the foam were monitored. The latter enabled the kinetics of the drainage/imbibition to be determined and compared with the predictions according to the theoretical model. The rate of decrease in foam height was initially high and decreased over time as predicted by the theoretical model. All the foam displayed an initial rapid imbibition through the porous substrate, which is again in the agreement with the theory predictions. It was found that solutions with lower surfactant concentrations could penetrate deeper into the chalk. The imbibition front was observed to be uniform: evenly distributed liquid throughout the cross-section of the porous substrate.

Optimization of anode structure for intermediate temperature solid oxide fuel cell via phase‐inversion cotape casting

AbstractA functional layer and a porous support that together constitute an anode for a solid oxide fuel cell were simultaneously formed by the phase‐inversion tape casting method. Two slurries, one composed of NiO and yttria‐stabilized zirconia (YSZ) powders and the other of NiO, YSZ, and graphite were cocasted and solidified by immersion in a water bath via the phase‐inversion mechanism. The as‐formed green tape consisted of a sponge‐like thin layer and a fingerlike thick porous layer, derived from the first slurry and the second slurry, respectively. The former acted as the anode functional layer (AFL), while the latter was used as the anode substrate. The AFL thickness was varied between 20 and 60 μm by adjusting the blade gap for the tape casting. Single cells based on such NiO‐YSZ anodes were prepared with thin YSZ electrolytes and YSZ‐(La0.8Sr0.2)0.95MnO3−δ (LSM) cathodes, and their electrochemical performance was measured using air as oxidant and hydrogen as fuel. The maximum power densities obtained at 750°C were 720, 821, and 988 mW cm−2 with the AFL thickness at 60, 40, and 20 μm, respectively. The satisfactory electrochemical performance was attributed to the dual‐layer structure of the anode, where the sponge‐like AFL layer provided plenty of triple‐phase boundaries for hydrogen oxidation, and the fingerlike thick porous substrate allowed for facile fuel transport. The phase‐inversion tape casting developed in this study is applicable to the preparation of other planar ceramic electrodes with dual‐layer asymmetric structure.

Mask-assisted electron radiation grafting for localized through-volume modification of porous substrates: influence of electron energy on spatial resolution

The spatial resolution aspects of the local modification of porous materials by electron induced graft-polymerization were studied by a combination of experiments and numerical simulations. Using blocking masks, only selected regions of the material were exposed to radiation and subsequently grafted. The main focus of this study is the application to gas diffusion layers, a carbonaceous ~200µm thick porous substrate widely used in fuel cells, with the goal of improving water management by locally tuning the wettability. The comparison of experiments performed with different electron energies and corresponding simulations shows good agreement, identifying the energy threshold necessary to graft through the material to be approximately 150keV. The impact of electron energy on spatial resolution was studied, showing that the blurring effects due to electron scattering reach a maximum at around 200keV and are reduced at higher electron energies. Finally, the numerical simulations were used to define the conditions necessary to selectively graft only parts of bi-layer fuel cell materials.

Comparison of thermo-mechanical characteristics of non-doped and 3mol% B-site Zr-doped Ba0.5Sr0.5Co0.8Fe0.2O3−δ

A dense membrane layer supported by a thick porous substrate is a favorable design for advanced oxygen separation units used in Oxy-fuel processes and membrane reactors. Based on the highest permeation rates verified so far, Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3− δ appears to be the most promising membrane and substrate material. This material, however, suffers from partial decomposition of cubic structure to hexagonal polymorphs at ∼825 °C, which is associated with a significant oxygen permeation drop. The partial substitution of the B-site with 3 mol% zirconium has been suggested as one of solutions for suppressing this transformation. The current study concentrated on the comparison of the thermo-mechanical characteristics of doped and non-doped Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3− δ . Although the later material showed higher elastic modulus and fracture strength in absolute values, overall their characteristics mostly agree. It was shown that shortening of the membrane tube from 1 m to half its length should not influence fracture strength significantly. Fractographic analysis ruled out the cutting edge effect in O-ring bending.

Preparation of microtubular solid oxide fuel cells based on highly asymmetric structured electrolyte hollow fibers

A simple and cost-effective method has been developed for the fabrication of microtubular solid oxide fuel cells (MT-SOFCs). Highly asymmetric electrolyte hollow fibers composed of a thin dense skin layer and a thick porous substrate are first prepared by a modified phase inversion/sintering technique. The porous substrate is then formed into the anode by deposition of a Ni catalyst via an electroless plating method inside the pores while the thin dense skin layer serves directly as the electrolyte film of the fuel cells. A porous cathode layer is produced on the outer surface of the Ni-deposited hollow fibers by slurry coating and subsequent sintering to form a complete micro tubular fuel cell. The process has been employed to fabricate yttrium stabilized zirconia (YSZ) supported Ni-YSZ|YSZ|La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) microtubular fuel cells. The maximum output of the resulting cells is 159.6 mW cm−2 at 800 °C when using H2 as the fuel feed and air as the oxidant.

Spreading and sorption of a droplet on a porous substrate

In this paper, a model for spreading and sorption of a droplet on a thick porous substrate is derived in the frame of lubrication theory. Flow in the saturated part of the porous material is governed by Darcy's law, assuming a discontinuous wetting front separating the saturated from the unsaturated regions. Numerical results are presented for spreading and sorption of droplets in their dependence on the material and process parameters for axisymmetric and two-dimensional configurations.

Spreading of liquid drops over porous substrates

Spreading of liquid drops over porous substrates

Spreading of Liquid Drops over Thick Porous Layers: Complete Wetting Case

The spreading of small silicone oil drops (capillary regime of spreading) over various dry thick porous substrates (permeable in both normal and tangential directions) was experimentally investigated. The time evolution of the radii of both the drop base and the wetted region on the surface of the porous substrate was monitored. It was observed that the total duration of the spreading process can be divided into two stages: a first stage, when the drop base expands until its maximum value is reached, and a subsequent second stage, when the drop base shrinks. It was found that the dynamic contact angle remains constant during the second stage of spreading. The latter fact has nothing to do with the contact angle hysteresis, as there is no hysteresis in the system. Appropriate scales are used, with a dimensionless time, to plot the dimensionless radii of the drop base and of the wetted circle on the surface of the porous substrate, the relative dynamic contact angle, and the effective contact angle inside the porous substrate. All these experimental data fell onto universal curves when the spreading of different silicone oils is done on porous substrates of similar pore size and porosity.

The sound field within a rigid porous layer

The sound field within a layered porous granular medium may be explored by means of suitably protected probe microphones. The field within a system consisting of a granular surface layer above a thick porous substrate with considerably higher flow resistivity is modelled by assuming the granular media to be characterized by uniform slit-pores within a rigid solid matrix and by applying boundary conditions at each interface. The predicted relative amplitude spectrum within the surface layer is influenced by interferences between the first few wave components of the internally reflected field. Predictions are in qualitative agreement with data obtained in washed sand and gravel.