Pressure-driven Transport Research Articles

Nanochannel-confined charge repulsion of ions in a reduced graphene oxide membrane

Pressure-driven transport of hydrated ions will be highly restrained in two-dimensional nanochannels featuring steady confinement and pore-wall decoration.

Density Functional Hydrodynamics in Multiscale Pore Systems: Chemical Potential Drive

We use the method of density functional hydrodynamics (DFH) to model compositional multiphase flows in natural cores at the pore-scale. In previous publications the authors demonstrated that DFH covers many diverse pore-scale phenomena, starting from those inherent in RCA and SCAL measurements, and extending to much more complex EOR processes. We perform the pore-scale modelling of multiphase flow scenarios by means of the direct hydrodynamic (DHD) simulator, which is a numerical implementation of the DFH. In the present work, we consider the problem of numerical modelling of fluid transport in pore systems with voids and channels when the range of pore sizes exceed several orders of magnitude. Such situations are well known for carbonate reservoirs, where narrow pore channels of micrometer range can coexist and interconnect with vugs of millimeter or centimeter range. In such multiscale systems one cannot use the standard DFH approach for pore-scale modeling, primarily because the needed increase in scanning resolution that is required to resolve small pores adequately, leads to a field of view reduction that compromises the representation of large pores. In order to address this challenge, we suggest a novel approach, in which transport in small-size pores is described by an upscaled effective model, while the transport in large pores is still described by the DFH. The upscaled effective model is derived from the exact DFH equations using asymptotic expansion in respect to small-size characterization parameter. This effective model retains the properties of DFH like chemical and multiphase transport, thus making it applicable to the same range of phenomena as DFH itself. The model is based on the concept that the transport is driven by gradients of chemical potentials of the components present in the mixture. This is a significant generalization of the Darcy transport model since the proposed new model incorporates diffusion transport in addition to the usual pressure-driven transport. In the present work we provide several multiphase transport numerical examples including: a) upscaling to chemical potential drive (CPD) model, b) combined modeling of large pores by DFH and small pores by CPD.

Simultaneous pressure and electro-osmosis driven flow in charged porous media: Pore-scale effects on mixing and dispersion

Electrokinetic effects in porous media play a key role in a number of natural and industrial processes. Applications such as enhanced oil recovery, soil remediation and even drug delivery are affected by the Coulombic forces created by the solid-fluid interfacial interactions. These electrokinetic effects promote the development of non-homogenous slipping flow over charged surfaces at the pore scale, which can have a significant impact in the hydrodynamics of tight porous materials. For transport of ionic solutions in such systems (e.g. transport of low salinity water in tight oil reservoirs), combined effect of hydrodynamic transport and electrokinetic transport would be expected. While transport in pressure-driven transport will be pronounced in high permeability flow pathways, transport due to electric fields (e.g. electro-osmosis) will be more pronounced in tight pores were electrical diffuse layer is not negligible. In this work, we explored the pore-scale hydrodynamic characteristics of charged porous media using computational fluid dynamics. Different flow driving mechanisms were studied, e.g. conventional pressure driven flow, pure electro-osmosis as well as their superposition under different amounts of charged material. We then analyzed the effect of these distinct flow regimes on the transport of a passive tracer, finding how different driving mechanism result in distinct dispersion and mixing characteristics.

Water transport through graphene and MoS2 nanopores

In this work, pressure-driven water transport through graphene and molybdenum disulfide (MoS2) nanopores is investigated through molecular dynamics simulations. The dependence of water flow rate on the pore area and pressure drop is studied for various pore structures. Power-law relationships, Q∝Aα, with α ranging from 1.6 to 1.9 between the flow rate (Q) and the pore area (A), are found, while the flow rate increases linearly with increasing pressure drop. The transport results are explained by the diffusion coefficient and friction coefficient of water molecules through the nanopores, as well as the free energy barriers at the pores.

High-pressure triggered quantum tunneling tuning through classical percolation in a single nanowire of a binary composite

In the era of miniaturization, the one-dimensional nanostructures presented numerous possibilities to realize operational nanosensors and devices by tuning their electrical transport properties. Upon size reduction, the physical properties of materials become extremely challenging to characterize and understand due to the complex interplay among structures, surface properties, strain effects, distribution of grains, and their internal coupling mechanism. In this report, we demonstrate the fabrication of a single metal-carbon composite nanowire inside a diamond-anvil-cell and examine the in situ pressure-driven electrical transport properties. The nanowire manifests a rapid and reversible pressure dependence of the strong nonlinear electrical conductivity with significant zero-bias differential conduction revealing a quantum tunneling dominant carrier transport mechanism. We fully rationalize our observations on the basis of a metal-carbon framework in a highly compressed nanowire corroborating a quantum-tunneling boundary, in addition to a classical percolation boundary that exists beyond the percolation threshold. The structural phase progressions were monitored to evidence the pressure-induced shape reconstruction of the metallic grains and modification of their intergrain interactions for successful explanation of the electrical transport behavior. The pronounced sensitivity of electrical conductivity to an external pressure stimulus provides a rationale to design low-dimensional advanced pressure sensing devices.

Surface to Groundwater Interactions beneath the City of Berlin: Results from 3D Models

Knowing the thermal and hydraulic conditions below major urban centers is of increasing importance in the context of energy and water supply. With this study, focusing on the major urban center of Berlin, Germany, we aim to gain insights on the coupling of surface water bodies to the subsurface thermal and hydraulic field investigating shallow water to deep groundwater interactions. Therefore, we use a 3D structural model of the subsurface, constrained by all available data and observations, as a base for simulations of the coupled transport of fluid and heat. This model resolves the 3D configuration of the main geological units and thus enables us to account for related heterogeneities in physical properties. Additionally, we resolve surface water body geometries with newly available data. To assess how surface water bodies interact with the deeper groundwater at different depths in the model domain, the influence of different hydraulic boundary conditions is quantified, which indicates that the coupling of surface water bodies and groundwater strongly modifies predicted groundwater circulation. Consequently, changes in subsurface temperatures are also predicted, where lakes may account for temperature differences up to ±5°C and rivers could account for up to ±1°C visible at depths ≤-500 m.a.s.l. These differences are mainly connected to changes in the advective component of heat transport caused by the modifications of the hydraulic boundary condition. Pressure-driven heat transport is most efficient where differences between hydraulic heads of aquifers and surface water bodies are highest. This study therefore illustrates the impact of surface to subsurface water interactions in an urban context.

Molecular streaming and its voltage control in ångström-scale channels.

Over the past decade, the ability to reduce the dimensions of fluidic devices to the nanometre scale (by using nanotubes1-5 or nanopores6-11, for example) has led to the discovery of unexpected water- and ion-transport phenomena12-14. More recently, van der Waals assembly of two-dimensional materials15 has allowed the creation of artificial channels with ångström-scale precision16. Such channels push fluid confinement to the molecular scale, wherein the limits of continuum transport equations17 are challenged. Water films on this scale can rearrange into one or two layers with strongly suppressed dielectric permittivity18,19 or form a room-temperature ice phase20. Ionic motion in such confined channels21 is affected by direct interactions between the channel walls and the hydration shells of the ions, and water transport becomes strongly dependent on the channel wall material22. We explore how water and ionic transport are coupled in such confinement. Here we report measurements of ionic fluid transport through molecular-sized slit-like channels. The transport, driven by pressure and by an applied electric field, reveals a transistor-like electrohydrodynamic effect. An applied bias of a fraction of a volt increases the measured pressure-driven ionic transport (characterized by streaming mobilities) by up to 20 times. This gating effect is observed in both graphite and hexagonal boron nitride channels but exhibits marked material-dependent differences. We use a modified continuum framework accounting for the material-dependent frictional interaction of water molecules, ions and the confining surfaces to explain the differences observed between channels made of graphene and hexagonal boron nitride. This highly nonlinear gating of fluid transport under molecular-scale confinement may offer new routes to control molecular and ion transport, and to explore electromechanical couplings that may have a role in recently discovered mechanosensitive ionic channels23.

Electrokinetic infusions into hydrogels and brain tissue: Control of direction and magnitude of solute delivery

BackgroundDelivering solutes to a particular region of the brain is currently achieved by iontophoresis for very small volumes and by diffusion from a microdialysis probe for larger volumes. There is a need to deliver solutes to particular areas with more control than is possible with existing methods. New MethodElectrokinetic infusions of solutes were performed into hydrogels and organotypic hippocampal slice cultures. Application of an electrical current creates electroosmotic flow and electrophoresis of a dicationic fluorescent solute through organotypic hippocampal tissue cultures or larger hydrogels. Transport was recorded with fluorescence microscopy imaging in real-time. ResultsElectrokinetic transport in brain tissue slice cultures and hydrogels occurs along an electrical current path and allows for anisotropic delivery over distances from several hundred micrometers to millimeters. Directional transport may be controlled by altering the current path. The applied electrical current linearly affects the measured solute fluorescence in our model system following infusions. Comparison with Existing MethodsLocalized drug delivery involves iontophoresis, with diffusion primarily occurring beyond infusion capillaries under current protocols. Pressure-driven infusions for intraparenchymal targets have also been conducted. Superfusion across a tissue surface provides modest penetration, however is unable to impact deeper targets. In general, control over intraparenchymal drug delivery has been difficult to achieve. Electrokinetic transport provides an alternative to deliver solutes along an electrical current path in tissue. ConclusionsElectrokinetic transport may be applied to living systems for molecular transport. It may be used to improve upon the control of solute delivery over that of pressure-driven transport.

Water Flow through Interlayer Channels of Two-Dimensional Materials with Various Hydrophilicities

Water transport through laminated membranes composed of two-dimensional (2D) materials has gained considerable attention because of their great potential in filtration and separation applications. However, water transport between interlayers formed by 2D materials cannot be comprehensively described by traditional transport theory because different 2D materials have varying hydrophilicity, which strongly affects water transport. Herein, we build interlayer channels formed by 2D sheets with various hydrophilicities and investigate the pressure-driven water transport via nonequilibrium molecular dynamics simulations. The influence of channel hydrophilicity on water transport is dominant, especially at the nanoscale. To model the water-transport phenomena through channels with various hydrophilicities, we define a new slip length to derive the appropriate equation. By two methods of calculating the slip length based on the simulation results, we validate our derived equation, which predicts the water flux in...

Heat and mass transfer, shrinkage, and thermal protein denaturation of kuruma prawn (Marsupenaeus japonicas) during water bath treatment: A computational study with experimental validation

Excessive shrinkage of processed food lowers its perceived quality by consumers, and therefore should be avoided. This study aims to clarify the multiphysics involved in the shrinkage of prawn during heating. A model that describes changes in moisture content of prawn due to pressure-driven water transport was reported according to Darcy's law. The transport model for the heating process included a stress-strain analysis (a structural mechanics model) coupled to a virtual work principle, which is applicable to a body undergoing shrinkage in two dimensions. Simultaneous calculation of changes in internal pressure and thermal protein denaturation (TPD, using a non-isothermal kinetics method) was used to describe the physics behind shrinkage. Temperature, moisture, pressure, as well as TPD profiles and distributions were calculated, and were validated with measured results. Results indicated that shrinkage was delayed due to slow rate of water release, which promoted the increment in internal pressure. This phenomenon, in combination with actin denaturation, resulted in dramatic water release and volumetric shrinkage. The proposed model improves the understanding of the shrinkage phenomena of prawn during heating and has the potential to contribute in the reduction of food quality losses associated with shrinkage and water release.

Molecular simulation of permeation behaviour of ethanol/water molecules with single-layer graphene oxide membranes

Graphene oxide (GO)-based materials have shown promise as water-permeating membranes in pervaporation separation. However, the feed permeation and surface affinity of single-layer nanoporous GO sheet for liquid mixtures remain unresolved. Here, the pressure-driven molecular transport of pure ethanol and pure water, as well ethanol-water mixtures, crossing through single-layer nanoporous GO sheet was studied by non-equilibrium molecular dynamics simulations. We show that single-layer GO sheet with controlled pore sizes can effectively reject ethanol and allow water permeation with high permeability. This means that porous GO sheets could act as an effective dehydration membrane, therefore providing the initial barrier for ethanol passage in GO-based membrane. The pore size effect was considered as the separation mechanism. Both ethanol and water molecules in the mixture show comparable affinity with GO surfaces. The hydrogen-bonding coupling interaction between mixture and surface functional groups provide addition influence on the molecular transport through GO pores.

Dramatic pressure-sensitive ion conduction in conical nanopores

Ion transporters in Nature exhibit a wealth of complex transport properties such as voltage gating, activation, and mechanosensitive behavior. When combined, such processes result in advanced ionic machines achieving active ion transport, high selectivity, or signal processing. On the artificial side, there has been much recent progress in the design and study of transport in ionic channels, but mimicking the advanced functionalities of ion transporters remains as yet out of reach. A prerequisite is the development of ionic responses sensitive to external stimuli. In the present work, we report a counterintuitive and highly nonlinear coupling between electric and pressure-driven transport in a conical nanopore, manifesting as a strong pressure dependence of the ionic conductance. This result is at odds with standard linear response theory and is akin to a mechanical transistor functionality. We fully rationalize this behavior on the basis of the coupled electrohydrodynamics in the conical pore by extending the Poisson-Nernst-Planck-Stokes framework. The model is shown to capture the subtle mechanical balance occurring within an extended spatially charged zone in the nanopore. The pronounced sensitivity to mechanical forcing offers leads in tuning ion transport by mechanical stimuli. The results presented here provide a promising avenue for the design of tailored membrane functionalities.

Nanofiltration performance of conical and hourglass nanopores

Pressure-driven ion transport through conical and hourglass-shape (double conical) nanopores is investigated theoretically by solving the Poisson, Nernst-Planck and Navier-Stokes equations numerically. Due to the electrostatic asymmetry in conical nanopores, these latter exhibit nanofiltration rectification properties, i.e. they reject salts differently depending if the solution enters the nanopore from its base or its tip. The filtration rectification properties of conical nanopores result from two different phenomena, (i) the co-ion exclusion at the pore mouth and (ii) the sign and the magnitude of the (pressure-induced) electric field arising through the nanopores. Hourglass-shape nanopores exhibit improved separation performance compared with cylindrical and conical nanopores with identical average diameter, length and surface charge density. Notably, they allow target salt rejections to be reached with a smaller driving force than the other pore geometries, which makes them attractive candidates for the design of advanced nanofiltration membranes allowing less energy intensive separations.

Accelerated Transport of Particles in Confined Channels with a High Roughness Amplitude.

We investigate the pressure-driven transport of particles 200 or 300 nm in diameter in shallow microfluidic channels ∼1 μm in height with a bottom wall characterized by a high roughness amplitude of ∼100 nm. This study starts with the description of an assay to generate cracks in hydrophilic thin polymer films together with a structural characterization of these corrugations. Microfluidic chips of variable height are then assembled on top of these rough surfaces, and the transport of particles is assessed by measuring the velocity distribution function for a set of pressure drops. We specifically detect anomalous transport properties for rough surfaces. The maximum particle velocity at the centerline of the channel is comparable to that obtained with smooth surfaces, but the average particle velocity increases nonlinearly with the flow rate. We suggest that the change in the boundary condition at the rough wall is not sufficient to account for our data and that the occurrence of contacts between the particle and the surface transports the particle away from the wall and speeds up its motion. We finally draw perspectives for the separation by field-flow fractionation.

Pressure-driven supercritical CO2 transport through a silica nanochannel.

A thorough understanding of supercritical CO2 (scCO2) transport through nanochannels is of prime significance for the effective exploitation of shale resources and the mitigation of greenhouse gas emission. In this work, we employed the non-equilibrium molecular dynamics simulations method to investigate the pressure-driven scCO2 transport behavior through silica nanochannels with different external forces and pore sizes. The simulations reveal that the capability of scCO2 diffusion enhances both in the bulk region and the surface adsorbed layer with the increasing of pressure gradient or nanochannel size, in addition, the slip length increases nonlinearly with the external acceleration or nanochannel width increases and finally reaches a maximum value. The negative slippage occurs at lower pressure gradient or within the narrower nanochannel. Overall, it is the combined effect of strong adsorption, surface diffusion and slippage that causes the nonlinear relation between flow rate and pressure gradient or nanochannel size. The present work would provide theoretical guidance for the scCO2 enhanced shale oil/gas recovery, CO2 storage, and mass transport in nanoporous materials.

Underwater Oil Droplet Splitting on a Patterned Template.

Underwater oil droplets stretched and pinned by dual-dot oleophilic patterns on a superoleophobic substrate have been split into two nearly equal-volume daughter droplets using an underwater superoleophobic blade at substantially lower cutting speeds than reported in previous studies. A "liquid exchange model" based on Laplace pressure-driven liquid transport has been proposed to explain the mechanism of the underwater droplet split process. The dependence of droplet geometrical shape (curvature) and liquid properties (surface tension, viscosity) on the critical cutting speed that allows equal-volume split was investigated. Results demonstrate that critical cutting speed increases with increased curvature and surface tension of the split droplet, and decreases with increased droplet viscosity, which agrees with the proposed model. The ability to reproducibly split a single bulk oil droplet into daughter droplets with nearly equal volume facilitates the development of new functions for underwater microreactors.

Measurements and Characterization of Turbulence in the Tip Region of an Axial Compressor Rotor

Modeling of turbulent flows in axial turbomachines is challenging due to the high spatial and temporal variability in the distribution of the strain rate components, especially in the tip region of rotor blades. High-resolution stereo-particle image velocimetry (SPIV) measurements performed in a refractive index-matched facility in a series of closely spaced planes provide a comprehensive database for determining all the terms in the Reynolds stress and strain rate tensors. Results are also used for calculating the turbulent kinetic energy (TKE) production rate and transport terms by mean flow and turbulence. They elucidate some but not all of the observed phenomena, such as the high anisotropy, high turbulence levels in the vicinity of the tip leakage vortex (TLV) center, and in the shear layer connecting it to the blade suction side (SS) tip corner. The applicability of popular Reynolds stress models based on eddy viscosity is also evaluated by calculating it from the ratio between stress and strain rate components. Results vary substantially, depending on which components are involved, ranging from very large positive to negative values. In some areas, e.g., in the tip gap and around the TLV, the local stresses and strain rates do not appear to be correlated at all. In terms of effect on the mean flow, for most of the tip region, the mean advection terms are much higher than the Reynolds stress spatial gradients, i.e., the flow dynamics is dominated by pressure-driven transport. However, they are of similar magnitude in the shear layer, where modeling would be particularly challenging.

Magnetic-field-driven alteration in capillary filling dynamics in a narrow fluidic channel.

We investigated pressure-driven transport of an immiscible binary system, constituted by two electrically conducting liquids, in a narrow fluidic channel under the influence of an externally applied magnetic field. The surface wettability was taken into account in the analysis considering that the walls of the channel are chemically treated to obtain various predefined contact angles as required for the study. Alterations in the capillary filling and wetting dynamics in the channel stemming from a complex interplay among different forces acting over the interface were investigated. It was shown that an alteration in the strength of the magnetic field leads to an alteration in the dynamics of the interface, which in turn, alters the filling and wetting dynamics nontrivially upon interaction with the surface tension force due to the wetted walls of the channel. It is emphasized that a contrast in properties of constituents of the binary system gives rise to an alteration in the forces being applied across the interface, leading to an intricate control over the filling and wetting dynamics for a given flow configuration and an applied field strength. We believe that the results obtained from this analysis may aid the design of microfluidic devices used for multiphase transport.

Transport of salty water through graphene bilayer in an electric field: A molecular dynamics study

Molecular dynamics simulations are carried out to study the pressure-driven transport of salty (NaCl) water through the nanochannels formed by a graphene (GE) bilayer with and without a vertical electric field. The simulation results show that the channel thickness influences not only the velocity and numbers of water molecules and ions inside the channel, but also their responses to varied pressure and electric field intensity. The salt rejection rate is characterized by considering both the numbers of ions and water molecules inside the channel and their velocities. When the electric field is not imposed, the salt rejection rate varies slightly in the high-pressure range (50–300MPa), and is mainly affected by the channel thickness. When an electric field with high intensity (1–5V/Å) is imposed vertically under high driving pressure (300MPa), ions can be trapped inside the nanochannel. Simulations under lower electric field (0.01–0.1V/Å and 0.05–0.2V/Å) and pressure (5MPa and 10MPa) are also conducted. Under these circumstances, GE nanochannels can reject ions from entering the channel, indicating improved salt rejection rate and reduced ion channel accumulation.

Buffer anions can enormously enhance the electrokinetic energy conversion in nanofluidics with highly overlapped double layers

Electrokinetic energy, where the pressure-driven transport of ions through nanofluidics yields streaming potential/current, is one of the next generation, sustainable, clean energies by converting hydraulic to electrical power. In this study, we develop an analytical model taking into account many practical effects, such as the Stern layer, buffer anions (e.g., HEPES, ACES, and lactic acid), electric double layers (EDLs) overlap, and surface equilibrium reactions, to investigate the buffer effect on the electrokinetic energy conversion in a long, pH-regulated nanochannel. Taking the nanochannel made of silica as an example, we for the first time show that introducing buffer anions into the working fluid can significantly enhance not only the maximum electrokinetic power output but also the corresponding conversion efficiency in a nanochannel under the condition of highly overlapped EDLs (e.g., low background salt concentration and/or small channel height). With buffer anions, the performance of electrokinetic energy, depending on the salt concentration, pH, and nanoscale channel height, can be enhanced at a degree as high as 1.5–26 times, as compared to the case without buffer anions. This work provides a useful receipt for estimating the electrokinetic energy in the nanochannel in the presence of buffer anions and the finding is of crucial importance for renewable energy applications.