Nanoporous Media Research Articles

System nanoporous media - non-wetting liquid, as a basis for the development of shock damper

In this work the method of system nanoporous medium - non-wetting liquid testing for damping devices presented. The method was tested for four systems nanoporous medium - non-wetting liquid. As a nanoporous medium was used commercial samples produced by Sigma-Aldrich hydrophobic silica Fluka 60 C8, Fluka 100 C8, Fluka 100 C18 and Fluka 90 C18. As a non-wetting liquid was used distilled water. These systems have been investigated in the temperature range 20 ÷ 60° C, the maximum energy of impact was 100 J. According to the method, time, power and energy characteristics of the process of non-wetting liquid intrusion - extrusion was determined. Also presented the dependences of impact processes characteristics on temperature.

The Distribution of Captured Non-Wetting Liquid Dispersed in Nanoporous Medium Recovery Method

In this paper the recovery method of a distribution function of non-wetting liquid captured in porous medium presented. Computation algorithm described. The recovered distribution functions for the system nanoporous medium (Libersorb 23) - non-wetting liquid (water) at the temperature of experiments 6°C presented. The used for calculation experimental data obtained in the time relaxation of metastable states of the non-wetting liquid in a porous medium experiment [Borman et al. 2016 Phys Rev E 93 022142].

Monitoring local configuration and anomalously slow relaxation of a nonergodic system of interacting liquid nanoclusters in a disordered confinement of a random porous medium

The relaxation of a confined nonwetting liquid dispersed in a disordered nanoporous medium has been experimentally studied in the system consisting of water and the L23 hydrophobized silica gel. Discovered that the relaxation of these states to study the system occurs as a result of the relaxation of local metastable configurations of filled and empty pores of the porous medium. Such relaxation occurs abnormally slowly back to the power law with the exponent α < 0.16. The observed anomalously slow relaxation of such a system and comparison with the time dependence of the volume of the confined liquid obtained in confirm the correctness of the description of disordered media on the basis of the notion of local metastable structures.

Evolution of the filling pressure of the porous medium by non-wetting liquid at pulse pressure changes

The paper presents the results of experimental studies of impact filling of nanoporous medium with non-wetting liquid. With increasing the impact energies nontrivial pattern of pressure changes in the system over time is observed. A physical explanation is proposed for the observed phenomena.

Scenario SOC abnormally slow relaxation of the system clusters the fluid in disordered nanoporous confinement environment

The relaxation of the system of the nanoporous medium with the nonwetting liquid is a self-organized criticality process characterized by waiting for fluctuation necessary for overcoming a barrier of local metastable state with the subsequent avalanche decay of local metastable configurations of pores. The dependence of the interaction between local configurations on the number of filled pores belonging to the infinite percolation cluster of filled pores serves as an internal feedback initiating the SOC process.

Applying a cellular automaton model to describing the anomalous relaxation of the metastable states in the disordered porous media

We propose a numerical model of outflow of the nonwetting liquid from previously completely filled nanoporous media based on cellular automata theory. It was shown that the relaxation of this system has an avalanche-like behaviour, which is inherent to phenomenon of self-organized criticality (SOC). Avalanche size and energy distributions were obtained. It was found that spectra have a typical for SOC theory power-law dependence N(s) ∼ s-β with β = 2.73, and N(E) ∼ E-βen with βen = 2.25.

Molecular Dynamics Study on Mechanism of Preformed Particle Gel Transporting Through Nanopores: Deformation and Dehydration

Understanding the translocation mechanism of preformed particle gel (PPG) through the nanoporous medium is crucial for gel treatment during enhanced oil recovery. On the basis of nonequilibrium molecular dynamics simulation, the translocation process of PPG in silica nanopores consisting of two different diameters was investigated. During the simulation, an external pulling force was applied to PPG representing the injection pressure. The simulation results suggest that a synergetic deformation and dehydration of PPG occurs during the translocation from the wide side into the narrow side. The energy barrier of the translocation process mainly result from the conformational energy change of PPG (mainly from the angle bend and dihedral torsion) and the dissociation energy barrier between PPG’s hydrophilic groups and water. Furthermore, the nanopore size has a crucial impact on the translocation mechanism of PPG, not only the degree of the deformation and dehydration near the entrance, but also the transloca...

Fabrication of high reflectivity nanoporous distributed Bragg reflectors by controlled electrochemical etching of GaN

The nanoporous medium is a valuable feature of optical devices because of its variable optical refractive index with porosity. One important application is in a GaN-based vertical cavity surface emitting laser having a distributed Bragg reflector (DBR) composed of alternating nanoporous and bulk GaNs. However, optimization of the fabrication process for high reflectivity DBRs having wellcontrolled high reflection bands has not been studied yet. We used electrochemical etching to study the fabrication process of a nanoporous GaN DBR and analyzed the relationship between the morphology and optical reflectivity. Several electrolytes were examined for the formation of the optimized nanoporous structure. A highly reflective DBRs having reflectivity of ~100% were obtained over a wide wavelength range of 450-750 nm. Porosification of semiconductors into nanoporous layers could provide a high reflectivity DBR due to controlled index-contrast, which would be advantages for the construction of a high-Q optical cavity.

Controlled Wetting in Nanoporous Membranes for Thin Film Evaporation

With the ever increasing cooling demands of advanced electronics, thin film evaporation has emerged as one of the most promising thermal management solutions. High heat transfer rates can be achieved in thin films of liquids due to a small conduction resistance through the film to the evaporating interface. In thin film evaporation, maintaining a stable liquid film to attain high evaporation rates is challenging. We investigated nanoporous anodic aluminum oxide (AAO) membranes to supply liquid to the evaporating surface via capillarity. In this work, we achieved enhanced experimental control via the creation of a hydrophobic section within the nanopore. By creating a non-wetting section, the liquid is confined within the membrane to a region of well-controlled geometry. This non-wetting section also prevents flooding, where the formation of a thick liquid film degrades device performance. When heat flux is applied to the membrane surface, the liquid wicks into the membrane from the bottom and becomes pinned at the onset of the hydrophobic layer. As a result, the wetting in the membrane is controlled, flooding is prevented, and a stable evaporating surface in achieved. With this approach, thin film evaporation from nanoporous media can now be studied for varying parameters such as pore size, porosity, and location of the meniscus within the pore.

Modeling of 1D Anomalous Diffusion in Fractured Nanoporous Media

Fractured nanoporous reservoirs include multi-scale and discontinuous fractures coupled with a complex nanoporous matrix. Such systems cannot be described by the conventional dual-porosity (or multi-porosity) idealizations due to the presence of different flow mechanisms at multiple scales. More detailed modeling approaches, such as Discrete Fracture Network (DFN) models, similarly suffer from the extensive data requirements dictated by the intricacy of the flow scales, which eventually deter the utility of these models. This paper discusses the utility and construction of 1D analytical and numerical anomalous diffusion models for heterogeneous, nanoporous media, which is commonly encountered in oil and gas production from tight, unconventional reservoirs with fractured horizontal wells. A fractional form of Darcy’s law, which incorporates the non-local and hereditary nature of flow, is coupled with the classical mass conservation equation to derive a fractional diffusion equation in space and time. Results show excellent agreement with established solutions under asymptotic conditions and are consistent with the physical intuitions.

Activated desorption at heterogeneous interfaces and long-time kinetics of hydrocarbon recovery from nanoporous media.

Hydrocarbon recovery from unconventional reservoirs (shale gas) is debated due to its environmental impact and uncertainties on its predictability. But a lack of scientific knowledge impedes the proposal of reliable alternatives. The requirement of hydrofracking, fast recovery decay and ultra-low permeability—inherent to their nanoporosity—are specificities of these reservoirs, which challenge existing frameworks. Here we use molecular simulation and statistical models to show that recovery is hampered by interfacial effects at the wet kerogen surface. Recovery is shown to be thermally activated with an energy barrier modelled from the interface wetting properties. We build a statistical model of the recovery kinetics with a two-regime decline that is consistent with published data: a short time decay, consistent with Darcy description, followed by a fast algebraic decay resulting from increasingly unreachable energy barriers. Replacing water by CO2 or propane eliminates the barriers, therefore raising hopes for clean/efficient recovery.

How theory and simulation can drive fuel cell electrocatalysis

Over the last decade, theory and modeling have become essential tools to navigate the parameter space that governs activity and stability of electrocatalyst systems for polymer electrolyte fuel cells. This perspective covers essential phenomena from atomic scale to nanoscale and discusses the impact of the key parameters at play. It is centered around the development of first-principles electrochemical methods as a foremost goal in the field. The general modeling framework entails at its core a self-consistency problem that must be solved to relate the metal phase potential to descriptors of catalyst activity and stability. Density functional theory has captured a central role in this rapidly evolving field. The article puts more than usual emphasis on aspects of the multifaceted challenges in fuel cell electrocatalysis that at present lie beyond the capabilities of density functional theory; they include metal charging and solvent effects. Following the general discussion of the theoretical-computational framework, an approach for “deciphering” the oxygen reduction reaction is demonstrated; it reconciles reaction pathways and free energy profiles obtained from density functional theory simulations with kinetic modeling of surface reactions and effective kinetic parameters. Another section dwells on the importance of metal charging phenomena that are especially important for the catalytic function of nanoporous media. The penultimate section exposes the ambivalent role of Pt oxide formation in modulating catalytic properties for the oxygen reduction reaction as well as for catalyst corrosion.

Improved oil recovery in nanopores: NanoIOR.

Fluid flow through minerals pores occurs in underground aquifers, oil and shale gas reservoirs. In this work, we explore water and oil flow through silica nanopores. Our objective is to model the displacement of water and oil through a nanopore to mimic the fluid infiltration on geological nanoporous media and the displacement of oil with and without previous contact with water by water flooding to emulate an improved oil recovery process at nanoscale (NanoIOR). We have observed a barrier-less infiltration of water and oil on the empty (vacuum) simulated 4 nm diameter nanopores. For the water displacement with oil, we have obtained a critical pressure of 600 atm for the oil infiltration, and after the flow was steady, a water layer was still adsorbed to the surface, thus, hindering the direct contact of the oil with the surface. In addition, oil displacement with water was assessed, with and without an adsorbed water layer (AWL). Without the AWL, the pressure needed for oil infiltration was 5000 atm, whereas, with the AWL the infiltration was observed for pressures as low as 10 atm. Hence, the infiltration is greatly affected by the AWL, significantly lowering the critical pressure for oil displacement.

Phase behavior of gas hydrates in nanoporous materials: Review

A precise understanding of phase behavior for a variety of both artificial and natural processes is essential to achieving scientific and technological goals. There has been growing research interest in gas hydrates confined in nanoporous media aiming to simulate and analyze the unique behavior of natural gas hydrates in sediments. Moreover, the appearance of peculiar properties due to the confinement effect stimulates research on gas hydrate technology for gas separation, such as CO2 capture from versatile pre/post combustion emissions. In spite of their importance, reliable phase equilibrium data on gas hydrates confined at a nanoscale are scattered throughout the literature, while those in bulk state are abundant. Accordingly, we surveyed the previous studies on the phase behavior of gas hydrates in various nanoporous materials to include and provide valuable information and knowledge for start-up researchers in various gas hydrate fields.

Dusty-gas model. Allowance for surface forces

The dusty-gas model has been extended to the case of nanoporous media, in which the action of surface forces must be taken into account. A basic set of transport equations underlying the model has been derived proceeding from a set of kinetic equations for an ordinary gas and dust particles. In the kinetic equations, the interaction between the gas and dust particles is represented as a sum of a long-range (analog of surface forces) and short-range components. The contribution of the long-range component has been taken into account in the self-consistent approximation, while the short-range component has been considered in the standard manner. Allowance for the surface forces has been shown to result in a substantial modification of the equation for gas transport through porous bodies, with this modification being most pronounced at nonuniform temperatures.

Accurate measurement of liquid transport through nanoscale conduits.

Nanoscale liquid transport governs the behaviour of a wide range of nanofluidic systems, yet remains poorly characterized and understood due to the enormous hydraulic resistance associated with the nanoconfinement and the resulting minuscule flow rates in such systems. To overcome this problem, here we present a new measurement technique based on capillary flow and a novel hybrid nanochannel design and use it to measure water transport through single 2-D hydrophilic silica nanochannels with heights down to 7 nm. Our results show that silica nanochannels exhibit increased mass flow resistance compared to the classical hydrodynamics prediction. This difference increases with decreasing channel height and reaches 45% in the case of 7 nm nanochannels. This resistance increase is attributed to the formation of a 7-angstrom-thick stagnant hydration layer on the hydrophilic surfaces. By avoiding use of any pressure and flow sensors or any theoretical estimations the hybrid nanochannel scheme enables facile and precise flow measurement through single nanochannels, nanotubes, or nanoporous media and opens the prospect for accurate characterization of both hydrophilic and hydrophobic nanofluidic systems.

Anomalously slow relaxation of the system of strongly interacting liquid clusters in a disordered nanoporous medium: Self-organized criticality

Abstract It has been shown that changes in the energy of a system of nonwetting liquid clusters confined in a random nanoporous medium in the process of relaxation can be written in the quasiparticle approximation in the form of the sum of the energies of local (metastable) configurations of liquid clusters interacting with clusters in the connected nearest pores. The energy spectrum and density of states of the local configuration have been calculated. It has been shown that the relaxation of the state of the system occurs through the scenario of self-organized criticality (SOC). The process is characterized by the expectation of a fluctuation necessary for overcoming a local energy barrier of the metastable state with the subsequent rapid hydrodynamic extrusion of the liquid under the action of the surface buoyancy forces of the nonwetting framework. In this case, the dependence of the interaction between local configurations on the number of filled pores belonging to the infinite percolation cluster of filled pores serves as an internal feedback initiating the SOC process. The calculations give a power-law time dependence of the relative volume of the confined liquid θ ∼ t − α ( α ∼ 0.1 ) . The developed model of the relaxation of the porous medium with the nonwetting liquid demonstrates possible mechanisms and scenarios of SOC for disordered atomic systems.

Direct simulation of supercritical gas flow in complex nanoporous media and prediction of apparent permeability

Mudrocks and shales contain pores within the size range of 2–100nm. Flow of supercritical gas in these pores at reservoir pressure–temperature conditions falls within the slip-flow and early transition-flow regime (0.001<Kn<1.0). Currently, the description of supercritical gas flow in these flow regimes is mostly limited to simple tube and channel geometries that are of limited applicability to the sponge-like or platy nanoporous geometry in organic matter or clays. Here, we present a local-effective-viscosity multi-relaxation-time lattice Boltzmann model (LEV-LBM) designed to simulate gas flow in the slip- and early-transition-flow regimes in complex geometries. The LEV-LBM is informed with local effective viscosities at each node to capture the variance of the mean free path of gas molecules in a bounded system. The corrected mean free path for each lattice node is determined using a three-dimensional wall function adaptable to complex pore geometries. To enforce a non-zero slip velocity at solid boundaries, a combined diffusive bounce-back scheme is applied to the pore-walls. The LEV-LBM is first validated in simple tube geometries, where good agreement is found for Knudsen numbers up to 1.0. We then use the LEV-LBM to quantify the finite tube length effect and comment on the implications for pore-network models. We finally demonstrate the utility of the LEV-LBM by simulating pure methane flow in digital reconstructions of nanoporous organic matter at reservoir conditions, and compare the results to bundle of tubes models. We show that the bundle of tubes models overestimate apparent permeability by factors between 1.52 and 153, due to the non-trivial dependence of flow on pore space connectivity and shape.

Modeling the density profiles and adsorption of pure and mixture hydrocarbons in shales

The production from shale resources in the US has shifted from the gas window to the condensate and oil windows recently due to the low natural gas price. Liquid-rich shales, such as Barnett, Woodford and Eagle Ford Shales, etc., are brought more attentions than ever before. Therefore, it is critical to understand the fluid phase behavior and their impacts on production in the condensate systems.Fluid phase behavior in porous media is governed by not only fluid molecule–fluid molecule interactions but also fluid molecule-porous media wall interactions. In the shale formations, a large amount of hydrocarbons are stored within the organic matters where the pore sizes are in the order of nanometers. Inside these nanopores, the interactions between the fluid molecules and porous walls play such an important role that can change the fluid properties of the stored fluids. Our work focused on the predictions of fluid density distributions of both dry gas and liquid rich systems inside nanoporous media. Simplified Local-Density theory coupled with Modified Peng–Robinson Equation of State was used to predict the density profiles of pure and mixture hydrocarbons in confined pores. Adsorption isotherms were generated based on the density profiles calculated. The adsorption isotherms of pure methane and the methane/ethane binary mixture were calculated and compared to experimental data and molecular simulation results in the literature with excellent accuracy.Our results showed that due to the fluid-wall interactions, the fluid density is not uniformly distributed across the pore width. In general, the fluid density is higher near the porous media wall than that in the center of the pore. It also showed the fluid density profiles are temperature, pressure, pore size and fluid composition dependent. In general, the adsorbed amount increased by increasing pressure and decreased by increasing temperature. The pore size range of interest is from 2nm to 20nm. In order to present the condensate system, a binary mixture of 80% methane and 20% n-butane was used. It was found that fluid composition for the fluid mixture was not uniformly distributed across the pore. Heavier component (n-butane) tended to accumulate near the wall while lighter component (methane) would like to stay in the center region of the pore. For the methane/ethane binary mixture, the composition of methane in confined space was found much smaller than the bulk methane composition. For example, the methane composition in confined silicalite is 7% when the bulk methane composition is 50% at 355kPa and 300K.

Anomalous capillary filling and wettability reversal in nanochannels.

This work revisits capillary filling dynamics in the regime of nanometric to subnanometric channels. Using molecular dynamics simulations of water in carbon nanotubes, we show that for tube radii below one nanometer, both the filling velocity and the Jurin rise vary nonmonotonically with the tube radius. Strikingly, with fixed chemical surface properties, this leads to confinement-induced reversal of the tube wettability from hydrophilic to hydrophobic for specific values of the radius. By comparing with a model liquid metal, we show that these effects are not specific to water. Using complementary data from slit channels, we then show that they can be described using the disjoining pressure associated with the liquid structuring in confinement. This breakdown of the standard continuum framework is of main importance in the context of capillary effects in nanoporous media, with potential interests ranging from membrane selectivity to mechanical energy storage.