Bimodal Pore Size Distribution Research Articles

Improving Pore Characteristics, Mechanical Properties and Thermal Performances of Near-Net Shape Manufacturing Phenolic Resin Aerogels.

Thermally stable high-performance phenolic resin aerogels (PRAs) are of great interest for thermal insulation because of their light weight, fire retardancy and low thermal conductivity. However, the drawbacks of PRA synthesis, such as long processing time, inherent brittleness and significant shrinkage during drying, greatly restrict their wide applications. In this work, PRAs were synthesized at ambient pressure through a near-net shape manufacturing technique, where boron-containing thermosetting phenolic resin (BPR) was introduced into the conventional linear phenolic resin (LPR) to improve the pore characteristics, mechanical properties and thermal performances. Compared with the traditional LPR-synthesized aerogel, the processing time and the linear shrinkage rate during the drying of the PRAs could be significantly reduced, which was attributed to the enhanced rigidity and the unique bimodal pore size distribution. Furthermore, no catastrophic failure and almost no mechanical degradation were observed on the PRAs, even with a compressive strain of up to 60% at temperatures ranging from 25 to 200 °C, indicating low brittleness and excellent thermo-mechanical stability. The PRAs also showed outstanding fire retardancy. On the other hand, the PRAs with a density of 0.194 g/cm3 possessed a high Young's modulus of 12.85 MPa and a low thermal conductivity of 0.038 W/(m·K).

Facile one-step synthesis of in situ WO3@Gr nanorods as an efficient material for antimicrobial and decoloration applications

This work examined the synthesis, antibacterial activity, and decolourisation of WO3@Graphene nanorods (WO3@Gr NR). WO3@Gr NR nanocomposite was in situ produced via a facile one-step hydrothermal process employing sodium tungstate dihydrate and exfoliated graphene as precursors. The resulted NR exhibited an average diameter of 13 nm, a large specific surface area of 53.3 m2 g−1, and a bimodal pore size distribution with an average pore size of 5.5 nm. The optical bandgap is extrapolated to be 2.75 eV. Graphene was shown to be responsible for the sample’s elaborate visible-light absorption, which improved adsorption and the ability to harvest visible light. WO3@Gr NR are more efficient against E. coli than S. aureus, killing up to 52% and 39% of cells, respectively, after two hours of treatment. When used in conjunction with invisible light, the NR killed E. coli and S. aureus by 78 and 62%, respectively. The bactericidal activity of photoinduced WO3@Gr NR was evaluated against P. aerugunosa, E. faecalis, E. coli, and S. aureus. The photocatalytic constant rates of organic dye methylene blue (MB) were determined to be 0.01 min−1. An IC50 (50% cell growth inhibition) value of 97 (μg ml−1) was determined for the nanocomposite against human liver cancer cell lines (HepG2). Our findings suggest that this nanorod may be utilised to degrade bacteria and organic colours in wastewater simultaneously while posing no risk to human health.

Directional saturation of a strongly bimodal pore size distribution carbon interlock fabric: Measurement and multiphase flow modeling

This study focuses on the analysis of the directionality of saturation of materials with bimodal pore size distributions. The material studied is an anisotropic carbon interlock with highly contrasted dual-scale porosity, with heavy warp and light weft tows. Experimentally, 6 configurations are tested using injection combined with dielectric sensors to measure saturation times and unsaturated lengths. The experimental results show that this material has a high contrast in dual-scale porosity and the unsaturation is anisotropic. In parallel, a two-phase flow in porous media continuum model is developed within the OpenFOAM® toolbox of libraries to simulate the transient impregnation. The need to use a tensorial relative permeability and its alignment with the saturated permeability is analyzed by a numerical multiphase mesoscale flow model. The combined experimental and numerical analysis paves the way for a methodology to analyze the saturation of various bimodal pore size distribution materials.

Water retention behavior and shear strength of artificially cemented granite residual soil subjected to free drying

Artificially cemented soils have been widely used as filling materials in highway and railway construction. The shear strength evolution of filling materials upon moist variation can determine the stability of subgrade and embankments. This study conducted water retention tests, MIP tests, and multi-stage triaxial shear tests on cement-treated granite residual soil (GRS) to determine its water retention curve (WRC) upon free drying, pore structure, and peak shear strength qf, respectively. The water retention behavior and shear strength evolution upon free drying were modeled based on the dual-porosity structure of cement-treated GRS and the effective stress principle, respectively. Results show that the drying-WRC is bimodal and higher cement dosage yields a more severe decrease in the water retention capacity within a specific suction range. For a given confining pressure, the peak shear strength qf increased with increasing cement dosage or suction value s. The peak shear strength qf also solely depends on the suction value in the peak stress state. In addition, the cement-treated GRS has a bimodal pore size distribution curve, and its macro- and micro-void ratios remain almost unchanged after free drying. The bimodal drying-WRC of the cement-treated GRS can be modeled by differentiating the water retention mechanisms in macro- and micro-pores. Moreover, using the macro-pore degree of saturation as the effective stress parameter χ = SrM, the qf–pf′ relationship (where pf′ is the effective mean pressure at failure) under various suction and stress conditions can be unified, and the qf–s relationships at various net confining pressures σ3,net can be well reproduced. These findings can help design subgrade and embankments constructed by artificially cemented GRS and assess their safe operation upon climate change.

Permeability of surface clay-bearing sediments in Shenhu Area of South China Sea

The migration of fluids, such as the dissociation of natural gas hydrates, can affect the stability of the seabed and even induce submarine geological disasters and change the marine environment. The migration of fluids in sediments depends on its permeability properties. Therefore, it is crucial to investigate the sediment permeability and its influencing factors. This paper investigates the particle size, porosity, pore size distribution, and permeability of surface clay-bearing sediments in the Shenhu Area of the South China Sea by physical property tests, computed tomography (CT) scanning tests, constant rate of strain (CRS) consolidation tests, and nuclear magnetic resonance (NMR) tests. The permeability of the surface sediments in the Shenhu Area ranges from 3.32 × 10−15–6.76 × 10−14 m2. The pore size mainly ranges between 10−5 and 10−1 mm, exhibiting a bimodal distribution with peaks from 10−4 and 10−2 mm. The increase in clay content leads to enhanced permeability due to the bimodal distribution of pore sizes, which will cause the enlargement of pores. As the pore size increases, the sediment permeability also increases. Sediment permeability linearly decreases with increasing effective axial stress and deformation due to compression of the pore space. Additionally, a new model for calculating permeability with different sediment properties is established, which improves the accuracy of permeability calculations for clay-bearing sediments compared to previous research. The findings of our study can provide a reference for exploring the influence of sediment parameters on fluid migration mechanisms in clay-bearing sediments.

Hydromechanical coupling in unsaturated clayey rocks with double porosity based on a multiscale homogenization procedure

Shales and clay rocks are porous media with multiscale microstructures used in many engineering applications. Intact clay rocks often exhibit a bimodal pore size distribution in which the nanopores are related to the interlayer spacing in the clay platelet and the micropores are related to the interparticle pores between clay particles. The double-porosity microstructure has significant implications for the hydromechanical behavior of these rocks especially in unsaturated conditions. In this study, we develop a double-porosity hydromechanical framework incorporating a homogenization scheme to simulate fluid flow and elastic deformation in anisotropic clay rocks. The homogenization scheme provides an enriched description of the elastic behavior of clay rocks during changes in the degree of saturation by bridging the nano, micro, and macroscale characterizations. We also model the nanopores and micropores as two independent pore networks with distinct fluid flow mechanisms and permeabilities. The proposed framework is cast into a three-field mixed finite element formulation for solving fully coupled flow and deformation problems. Numerical examples of free and confined swelling in Opalinus clay samples are presented to demonstrate the impacts of a multiscale microstructure on wetting processes in clay rocks.

Deterioration of volcanic tuffs from rock dwellings in Brhlovce (Slovakia) induced by freeze-thaw cycling studied by non-destructive tests and µCT visualization

The aim of this paper is to evaluate the effect of freeze-thaw-induced weathering of volcanic tuffs from the rock dwellings in the village of Brhlovce (Slovakia) based on a non-destructive and experimental study of tuff pore space properties. Traditional techniques for characterization of the porosity and pore size distribution and new experimental techniques like spontaneous imbibition (pore interconnection) or indicative rock pore structure method (pore size distribution) were compared with pore network models acquired by X-ray computed microtomography before and after freeze-thaw cycling in a custom-built thermodilatometer VLAP04. The results offer insight into how frost weathering altered the pore network of volcanic tuffs. Brhlovce tuffs are highly susceptible to frost damage due to their high porosity, pore interconnectivity, and bimodal pore size distribution pattern, where nanometric-sized micropores predominate over larger micrometric-sized capillary pores. Poromechanical principles were applied to confirm that ice crystallization initiates in well-connected micropores with a corresponding crystallization pressure below 10 MPa. After undergoing 100 freeze-thaw cycles with temperature oscillations ranging from − 10 to 10 °C, there was a significant increase in total porosity, pore interconnectivity, and rock permeability. Additionally, the pore size distribution was altered. These parameters influence the transport of fluids and the moisture regime in rock masses, which can serve as a precursor to physical and chemical weathering processes

Nanocomposite Methacrylated Silk Fibroin-Based Scaffolds for Bone Tissue Engineering.

The treatment of bone defects is a clinical challenge. Bone tissue engineering is gaining interest as an alternative to current treatments, with the development of 3D porous structures (scaffolds) helpful in promoting bone regeneration by ensuring temporary functional support. In this work, methacrylated silk fibroin (SilMA) sponges were investigated as scaffolds for bone tissue engineering by exploiting the combination of physical (induced by NaCl salt during particulate leaching) and chemical crosslinking (induced by UV-light exposure) techniques. A biomimetic approach was adopted to better simulate the extracellular matrix of the bone by introducing either natural (mussel shell-derived) or synthetic-origin hydroxyapatite nanoparticles into the SilMA sponges. The obtained materials were characterized in terms of pore size, water absorption capability and mechanical properties to understand both the effect of the inclusion of the two different types of nanoparticles and the effect of the photocrosslinking. Moreover, the SilMA sponges were tested for their bioactivity and suitability for bone tissue engineering purposes by using osteosarcoma cells, studying their metabolism by an AlamarBlue assay and their morphology by scanning electron microscopy. Results indicate that photocrosslinking helps in obtaining more regular structures with bimodal pore size distributions and in enhancing the stability of the constructs in water. Moreover, the addition of naturally derived hydroxyapatite was observed to be more effective at activating osteosarcoma cell metabolism than synthetic hydroxyapatite, showing a statistically significant difference in the AlamarBlue measurement on day 7 after seeding. The methacrylated silk fibroin/hydroxyapatite nanocomposite sponges developed in this work were found to be promising tools for targeting bone regeneration with a sustainable approach.

Towards a better comprehension of reactive transport coupling experimental and numerical approaches

In this work we focus on further understanding reactive transport in carbonate rocks, in particular limestones characterized by a bimodal pore size distribution. To this end, we performed injection experiments with CO2-saturated water on a sample of Euville limestone and monitored the experiments with a medical CT scanner. Microscanner imaging was performed before and after alteration. Experiments showed that permeability increased by nearly two decades due to the alteration process. This increase could be attributed to the formation of a preferential dissolution path visualized on the CT images. Microscanner images show that preferential dissolution areas are characterized by the presence of numerous enlarged macropores. The preferential dissolution path created therefore retains a porous structure and does not correspond to a wormhole-type channel. To provide further knowledge of the small-scale physics of reactive transport, we performed Lattice-Boltzmann simulations of flow in a numerically generated model 2D porous medium having geometrical and topological features designed to approach Euville limestone. We showed that the fluid velocity increased in nearly percolating paths of macropores. Considering the experiments, this means that the CO2-saturated water starts to enter high-velocity zones earlier than low-velocity zones, inducing an earlier onset of the alteration process and a more pronounced local dissolution. However, numerical results showed that the alteration of non-connected macropores leads to an increase of permeability much smaller than the experimentally observed one. To explain this fact we used effective medium modelling that permits predicting the variation in permeability as a function of the fraction of macropores and consequently as a function of alteration. It proved that as long as there is no alteration-induced percolating path consisting of macropores, the increase in permeability is relatively low as shown by the Lattice-Boltzmann simulations. An increase in permeability of several orders of magnitude is only observed when the macroporosity is close to the percolation threshold. This fact is in accordance with the experimentally observed results.

INVESTIGATIONS ON OPEN-CELL METAL FOAM–FITTED TUBES FOR HEAT TRANSFER ENHANCEMENT – A DISCUSSION ON ENHANCEMENT MECHANISMS

Metal foams are drawing increasing attention due to their high surface area-to-volume ratio, high thermal conductivity, and low density. Heat exchanger manufacturers are constantly looking for innovative methods toward building highly efficient and compact heat exchangers. To enhance heat transfer, the effective thermal conductivity, fitment of foam on pipe, influence of working fluids, and effect of the bonding method are major areas of investigation. This paper aims to present a review of various investigations conducted on open-cell metal foam for enhancing heat transfer. The pore size distribution of metal foam directly influences the effective thermal conductivity. Recent progress toward bimodal pore size distribution has been reviewed and discussed. There exists a tradeoff between enhancing heat transfer and the corresponding pressure drop. Different configurations of fitting metal foam on pipes, such as fully filled, partially filled, and tubes wrapped with metal foam, are critically reviewed, and their performance is compared. The working fluid and its conditions used with metal foam has tremendous potential toward enhancing heat transfer. The influence of nonrefrigerants, refrigerants, and nanofluids has been presented in this regard. Different bonding methods and their influence on thermal resistance are also reviewed. To date, there is hardly any literature that addresses the performance of metal foam–fitted tubes in condensers and evaporators for vapor compression refrigeration systems. Metal foam-fitted tubes have shown promising results in terms of heat transfer enhancement. The outcome of this review provides insights into further research on the use of metal foam–fitted tubes for refrigeration applications.

Breaking Free - Unlocking the Potential of Dynamic Hydrogen Bubble Templating Materials

In recent years, porous electrodes for electrochemical conversion devices, such as low-temperature fuel cells or water electrolyzers, have been engineered to address issues related to mass transport to and within the catalyst. For example, the addition of a layer with reduced pore size facing the catalyst layer (i.e., microporous layer) results in an increased performance[1–3]. Furthermore, chemical[4], as well as mechanical[5,6]. methods of creating dedicated liquid and gas pathways in the porous electrodes, have been investigated with promising results, highlighting the need for further optimization of state-of-the-art materials with regards to multi-phase transport. The range of possible modifications is intrinsically limited by the microstructure of the material they are applied to and add cost and complexity to the already costly electrode production procedure, motivating the development of novel materials which contain these desired features, offer a large morphological design space, and are cost competitive.To tackle these stringent targets, we investigate the dynamic hydrogen bubble templating (DHBT) method to generate hierarchical structures containing both a pore size gradient as well as dedicated water and gas pathways in the form of bimodal pore size distributions. It leverages simultaneous deposition of a metal and gas evolution on the same surface to form complex structures from solution using electricity. It results in a hierarchical structure of primary pores (~5-40 µm) separated by porous walls containing secondary pores (10-1000 nm) . This type of material has been used successfully to improve boiling heat transfer[7], another process limited by the counterflow of liquid and gas. They have also been investigated as high surface area material for microbial anodes[8] and have been postulated to be appealing in other electrochemical devices such as batteries or fuel cells[9]. A significant hurdle to unlock the full potential of DHBT foams is the attachment of the foam to the solid electrode surface it is generated on. The presence of the electrode blocks the transport of media in the through-plane direction of the foam and thereby prevents its use in many electrochemical applications. Due to its fragile nature, separation of the foam from this surface easily damages the structure, which is why previous research was done exclusively in the presence of the electrode. Overcoming this limitation would enable new applications for DHBT foams and has the potential to address a wide range of problems in electrochemical systems related to porous media.In this talk, I will discuss our work towards creating free-standing, hierarchically structured metal electrodes through DHBT and the developments needed to adapt this material for the use in electrochemical systems. Owing to its well-studied synthetic parameters and facile electrodeposition, we chose copper foams to advance DHBT materials in terms of characterization and mechanical properties. As a key step of the synthesis route, we realized a method to manufacture free-standing sheets from this material from copper metal while preserving its delicate microstructure. This was supported by an optimization of synthesis parameters to improve and ensure adequate mechanical stability of the freestanding foams after isolation. This development enables the application of a wide range of characterization methods such as XTM, BET, SEM and interferometry and I will discuss how the synthesis parameters link to the resulting material microstructure. Building on this initial success, current work focuses on the transferring this knowledge to the synthesis of foams made from other metals which are more relevant to electrochemical applications such as nickel for the use in alkaline electrolyzers and paves the way to develop DHBT material into the next generation transport layer for electrochemical systems.

Characteristics and performance analysis of activated carbons derived from different precursors and activators for waste water adsorption

Domestic waste such as methylene blue from the dyeing process and detergent from laundry activities are sources of water pollution. Before being dumped into the water, this waste must be minimized. One approach that can be utilized is the adsorption method with activated carbon. To generate activated carbon with the properties required for this purpose, an appropriate precursor and manufacturing conditions must be chosen. The objective of this research is to investigate the characteristics and performance of activated carbon derived from various precursors (Petung and Santong bamboos) and activators (Ar, N2, and CO2) for detergent and methylene blue dye adsorption. The difficulty to be solved is determining the best precursor and activator for maximum methyl blue and detergent adsorption. Characterizations included FTIR, XRD, SEM, TGA, and adsorption isotherm testing. The study's findings show that activated carbons have functional groups O-H (hydroxyl), C-H (aldehydes and alkenes), C=C aromatic rings, C-H aromatic groups, and an amorphous structure. The presence of a porous and amorphous structure, as well as the C=C aromatic ring, makes activated carbons capable of absorbing methyl blue and detergent. Santong bamboo-activated carbon activated with argon has the best characteristics and adsorption capability. This activated carbon has a fixed carbon content of 76.34 %, pore volumes of 0.362 cc/g, average pore widths of 1.967 nm, pore surface area of 291.487 m2/g, bimodal pore size distribution, and methylene blue and detergent adsorption capabilities of 19.733 cc/g and 1.689 cc/g, respectively. The results obtained are on a laboratory scale, which necessitates additional research, particularly on how to create reservoirs and regenerate activated carbon simply and affordably

Cibacron Blue F3GA ligand dye-based magnetic silica particles for the albumin purification.

Dye-ligand affinity chromatography is among the increasingly popular affinity chromatography based on molecular recognition for the purification of albumin. This study focuses on the binding of Cibacron Blue F3GA ligand dye with magnetic silica particles and purification by separation. Mono-disperse silica particles with bimodal pore size distribution were employed as a high-performance adsorbent for human serum albumin (HSA) protein purification under equilibrium conditions. The synthesized ligand-dye affinity based magnetic silica particles were characterized by electron spin resonance, Fourier-transform infrared spectroscopy, scanning electron microscopy, vibrating sample magnetometer, elemental analysis, and dispersive X-ray analysis. The HSA purification performance of the proposed material in the presence of a magnetic field was relatively investigated using magnetic-based particles with similar morphologies. The maximum adsorption capacity for HSA in an artificial plasma medium was defined as 48.6 mg/g magnetic silica particle. By using the designed magnetic silica particles, 1.0 M NaCl solution was successfully utilized for obtaining quantitative desorption with HSA. However, continued HSA purification performances of magnetic-based particles were significantly lower concerning the ligand-dye magnetic silica particles. The purity of the removed albumin was about 97%. The magnetic silica particles could be utilized many times without decreasing their protein adsorption capacities remarkably.

Investigating the coupled influence of flow fields and porous electrodes on redox flow battery performance

At the core of redox flow reactors, the design of the flow field geometry –which distributes the liquid electrolyte through the porous electrodes– and the porous electrode microstructure –which provides surfaces for electrochemical reactions– determines the performance of the system. To date, these two components have been engineered in isolation and their interdependence, although critical, is largely overlooked. Here, we systematically investigate the interaction between state-of-the-art electrode microstructures (a paper and a cloth) and prevailing flow field geometries (flow through, serpentine and four variations of interdigitated). We employ a suite of microscopic, fluid dynamics, and electrochemical diagnostics to elucidate structure-property-performance relationships. We find that interdigitated flow fields in combination with paper electrodes –which features a uniform microstructure with unimodal pore size distribution– and flow-through configurations combined with cloth electrodes –which have a hierarchical microstructure with bimodal pore size distribution– provide the most favorable trade-off between hydraulic and electrochemical performance. Our analysis evidences the importance of carrying out the co-design of flow fields and electrode microstructures in tandem.

Influence of boron doping level and calcination temperature on hydrogen evolution reaction in acid medium of metal-free graphene aerogels

In this work, metal-free boron-doped graphene-based aerogels were successfully synthesized via a one-step autoclave assembly followed by freeze-drying and used as electrocatalysts for the hydrogen evolution reaction (HER) in acidic media. The synthesized reduced graphene oxide aerogels (rGOA) showed improved electrocatalytic activity by introducing boron and structural defects. The amount of boric acid used both as a dopant and reducing agent in the synthesis was optimized (boric acid/GO mass ratio = 17.5) to practically reach the crystallization limit of boric acid (boric acid/GO mass ratio = 20). It was observed that the higher the amount of boric acid added, the more boron was incorporated into the carbonaceous structure, improving the electrocatalytic activity of the final aerogel. Furthermore, calcination of the boron-doped electrocatalyst at 600 °C resulted in final aerogels with low oxygen content, moderate surface area, bimodal pore size distribution, and a high electrochemical active surface area. The final 3D graphene aerogel developed in this work, showed such outstanding electrocatalytic activity in HER as to replace noble metal-based electrocatalysts in the future.

Starting from the Bottom: Coupling a Genetic Algorithm and a Pore Network Model for Porous Electrode Design

Porous electrodes are performance- and cost-defining components in modern electrochemical systems as they provide surfaces for electrochemical reactions, facilitate mass transport, conduct electrons and heat, and determine the hydraulic resistance [1]. Hence, electrode engineering is a valid approach to increase power density and improve const competitiveness. In convection-enhanced technologies, currently used porous materials are fibrous carbonaceous electrodes developed for low temperature fuel cells; yet their microstructure and surface chemistry limits the performance of emerging electrochemical systems such as redox flow batteries. Moreover, the empirical design of these electrodes is time- and resource-intensive which limits exploration of the wider design space. Microstructure-informed multiphysics simulations can be leveraged to aid the theoretical understanding and design of advanced electrode architectures [2]. However, while these simulations have improved our understanding, they have only recently [3] been deployed to realize the bottom-up design of electrode microstructures. The combination of microstructure-informed multiphysics with evolutionary algorithms could accelerate progress in the optimization of porous electrodes for specific applications. In this work, we deploy three-dimensional simulations in combination with a genetic algorithm for the bottom-up design of porous electrodes for redox flow batteries.In the first part of the talk, the coupling of a pore network modeling framework [4] with an evolutionary algorithm [5] is described. The pore network model is a microstructure-informed, electrolyte-agnostic simulation framework for flow batteries utilizing a network-in-series approach to account for species depletion over the entire length of the electrode, developed using an open-access platform (OpenPNM) [6]. The electrochemical model is solved for the electrolyte fluid transport and couples both half-cells by iteratively solving the species and charge transport at a low computational cost. In the genetic algorithm, the electrode microstructure evolves driven by a fitness function that minimizes pumping power requirements and maximizes electrochemical power output, where the optimization only relies on the electrolyte chemistry and initial electrode morphology and flow field geometries as inputs. The analyzed proof-of-concept employs a flow-through cubic lattice structure with fixed pore positions and shows significant improvement of the fitness function over 1000 generations, where the fitness improved by 75% predominantly driven by reducing the pumping requirements by 73%. The evolutionary design resulted in a bimodal pore size distribution containing longitudinal electrolyte flow pathways of larges pores. Additionally, we found an increase in surface area at the membrane-electrode interface resulting in a 42% enhancement of the electrochemical performance.In the second part of the talk, I will discuss our latest progress in the development of the topology optimization by implementing commercial fibrous electrodes as offspring networks, integrated flow field geometries, and extended evolutionary freedom during the optimization. Coupling the genetic optimization to the desired flow field geometry affects the evolution of the fitness function, shifting the balance between electrochemical and hydraulic performance, highlighting the importance of the coupled optimization of flow fields and electrodes. By including more evolutionary freedom in the algorithm (i.e., by allowing merging and splitting of pores outside fixed coordinates), existing fibrous electrodes can be enhanced by for example reducing their pumping losses. The presented genetic algorithm offers great potential for the predictive design of electrode microstructures tailored for specific reactor architectures and redox chemistries. Hence, this framework can be used to accelerate and broaden the design and fabrication process of advanced electrode structures. Although applied to flow batteries in this study, the methodology can be leveraged to advance electrode microstructures in other electrochemical systems by adapting the relevant physics.

A Non-Isothermal Pore Network Model of Primary Freeze Drying.

In this work, a non-isothermal pore network (PN) model with quasi-steady vapor transport and transient heat transfer is presented for the first time for the application of primary freeze drying. The pore-scale resolved model is physically based and allows for the investigation of correlations between spatially distributed structure and transport conditions. The studied examples were regular PN lattices with a significantly different structure, namely a spatially homogeneous PN, also denoted as monomodal PN, and a PN with significant structure variation, referred to as bimodal PN because of its bimodal pore size distribution. The material properties selected for the solid skeleton in this study are equivalent to those of maltodextrin. The temperature ranges applied here were -28 °C to -18 °C in the PN and -42 °C in the surrounding environment. The environmental vapor pressure was 10 Pa. The PNs were dried with constant temperature boundary conditions, and heat was transferred at the top side by the vapor leaving the PN. It is shown how the structural peculiarities affect the local heat and mass transfer conditions and result in a significant widening of the sublimation front in the case of the bimodal PN. The possibility of spatially and temporally resolved front structures is a unique feature of the PN model and allows the study of situations that are not yet described by classical continuum approaches, namely heterogeneous frozen porous materials. As demonstrated by the thin layers studied here, the pore-scale simulations are of particular interest for such situations, such as in lyomicroscopes or collagen scaffolds, where a length-scale separation between dry and ice-saturated regions is not possible.

A simple water retention model of dual-porosity soils based on self-similarity of bimodal pore size distribution

The bimodal characteristic of water retention curves of dual-porosity soils has been fully recognized, however, the excessive number of fitted parameters in the existing models limits their application. This study develops a simple water retention model for dual-porosity soils that considers the self-similar characteristics of the pore size distribution (PSD) between macropore and micropore regions. In the model, we simulate the macropore PSD by differential VG model, which is then shifted and scaled to capture micropore PSD. It is hypothesized that the bimodal PSD can be obtained by superimposing the PSDs of macropore and micropore regions to derive the bimodal water retention curve (BWRC). The mercury intrusion porosimetry (MIP) tests and water retention tests are conducted on Nanyang clay with three different dry densities to evaluate the proposed BWRC. The results show that the new parameter can be determined by the MIP dataset and then used to predict the water retention behavior of dual-porosity soils well. Finally, we also explore the application of the proposed BWRC in predicting the hydraulic conductivity of dual-porosity soils without adding any extra parameters.

Stress-dependent water retention behaviour of two intact aeolian soils with multi-modal pore size distributions

The water retention curve (WRC) of unsaturated soils plays a significant role in evaluating rainfall-induced geohazards in geological and geotechnical engineering, such as ground subsidence and slope failure. Stress can affect WRC because loading/unloading alters not only density but also pore size distribution (PSD). Previous studies of stress-dependent WRC focused on soils with mono- and bi-modal PSDs only. In this study, the WRCs of intact paleosol and intact loess with quadri- and tri-modal PSDs were measured using suction- and stress-controlled pressure plate tests. The full-range PSDs of tested specimens were determined by combining X-ray computed tomography (CT) and mercury intrusion porosimetry (MIP). Experimental results show that the multi-modal PSDs and hence WRCs of both soils are strongly affected by stress in a similar approach. For instance, the relationship between degree of hysteresis and suction showed a double-humped pattern, in which the Peak-I and Peak-II were identified at the relatively low (0.1–20 kPa) and high (20–400 kPa) suctions, respectively. The application of stress resulted in a decrease of hysteresis in the Peak-I (about 30%) but an increase in the Peak-II (about 31%). The increase of hysteresis in the Peak-II differs from the behaviour of soils with mono- and bi-modal PSDs, mainly because the reduction of mega- and macro-pore diameters upon loading increases pore non-uniformity in the range corresponding to the Peak-II.

Evolution of pore structure in nanoparticle deposits from unimodal to bimodal pore size distributions: Focus on structural features of dendritic structures

Nanoparticle deposits can exhibit various structures, including compact and dendritic structures, depending on the deposition conditions. While previous studies have characterized deposit structures using factors such as porosity and spatial autocorrelation index, the pore size distribution (PSD), which is an essential structural property, has only been explored in compact structures. In this study, we investigate the PSDs of the entire nanoparticle deposits, ranging from compact to dendritic structures. Deposits are formed using an off-lattice Monte Carlo simulation, and deposition conditions are represented by two dimensionless numbers (KnD, χF). The diffusive Knudsen number (KnD) ranges from 10−1.5 to 101.5, while the dimensionless translational energy of a particle (χF) ranges from 10−3 to 101.5. We use the Delaunay triangulation algorithm for fast PSD calculation, and validate the calculation method using theoretical values of ideal structures and experimental literature data. Our results show that bimodal PSDs appear under thermally-dominated conditions with high values of KnD, while unimodal PSDs with a constant mode radius (∼2.5 rp) are observed under highly advective conditions with high values of χF. We present the geometric mean radii and mode radii of pores as color-filled contours to show their variation trends with KnD and χF. Additionally, we propose a simple model that predicts dendrite width by combining the overall porosity and mode radii of pores. Our model's predictions reasonably match the dendrite width data obtained using a density-based clustering algorithm.