Nanofluidic Systems Research Articles

Hydrodynamic slip enhanced nanofluidic reverse electrodialysis for salinity gradient energy harvesting

Nanofluidic reverse electrodialysis offers an alternative way to harvest the widely-existing salinity gradient energy. In this study, we investigate the impacts of surface hydrodynamic slip modification on the ionic current rectification and salinity gradient energy conversion via a conical nanopore by thermodynamic analysis and numerical simulation. Results reveal that in the configuration where hydrodynamic slip modification is employed on the surface near the tip side, a small modification fraction contributes to the ionic current rectification due to significantly enhanced ion enrichment, while a larger hydrodynamic slip modification fraction goes against the ionic current rectification for deteriorated ion enhancement and induced counter-electric field concentration gradient near the base side. For energy conversion, at low concentration ratios, a large modification fraction brings an obvious augment on the electric power. However, it decreases energy conversion efficiency. At a concentration ratio of 100-fold, the electric power is increased by 60.7%, when half of the nanopore wall is modified into hydrodynamic slip. And the energy conversion efficiency is decreased by 6.41%. These findings shed light on the role of hydrodynamic slip modification on the ion transportation and salinity gradient energy conversion, and help developing high performance nanofluidic systems via hydrodynamic slip modifications.

Transition to a Superpermeation Phase of Confined Water Induced by a Terahertz Electromagnetic Wave.

Excellent permeation of one-dimensional (1-D) confined water across membrane channels is implicated in physiological processes and widely inspires the design of novel nanodevices and materials. Here, through molecular dynamics simulations, we proposed a phase transition to superpermeation (approximately 1 order of magnitude enhancement) of confined water across a 1-D water channel caused by a terahertz electromagnetic stimulus with a limited thermal effect. The underlying mechanism is revealed to be a combination of strength matching and frequency resonance between a relatively weak stimulus and the hydrogen bond network of 1-D confined water, rather than the bulk water outside. This combination causes an anomalously structural phase transition of only the confined water while efficiently limiting the thermal effect of bulk water. Our findings are promising for promoting the developments of advanced nanofluidic systems and terahertz technology and even physiological research.

Engineering Smart Nanofluidic Systems for Artificial Ion Channels and Ion Pumps: From Single-Pore to Multichannel Membranes.

Biological ion channels and ion pumps with intricate ion transport functions widely exist in living organisms and play irreplaceable roles in almost all physiological functions. Nanofluidics provides exciting opportunities to mimic these working processes, which not only helps understand ion transport in biological systems but also paves the way for the applications of artificial devices in many valuable areas. Recent progress in the engineering of smart nanofluidic systems for artificial ion channels and ion pumps is summarized. The artificial systems range from chemically and structurally diverse lipid-membrane-based nanopores to robust and scalable solid-state nanopores. A generic strategy of gate location design is proposed. The single-pore-based platform concept can be rationally extended into multichannel membrane systems and shows unprecedented potential in many application areas, such as single-molecule analysis, smart mass delivery, and energy conversion. Finally, some present underpinning issues that need to be addressed are discussed.

Magneto-mechanical stability of axially functionally graded supported nanotubes

In this paper, size-dependent vibration analysis of axially functionally graded (AFG) supported nanotubes conveying nanoflow under longitudinal magnetic fields are performed, aiming at performance improvement of fluid-interaction nanosystems. Either the density or the elastic modulus of the AFG nanotube varies linearly or exponentially along the axial direction. Based on the nonlocal continuum theory, the higher-order dynamical equation of motion of the system is derived considering no-slip boundary condition. Galerkin discretization technique and eigenvalue analysis are implemented to solve the modeled equation. The validity of the simplified model is justified by comparing the results with findings currently available in the literature. Influence of material gradient, magnetic strength, and nonlocal parameter on the system’s stability is illustrated. The results indicated that the elastic modulus gradient parameter can profoundly displace the instability threshold of the system and the effect of the density profile is negligible. Stability analysis showed that by fine-tuning of material gradation, the nonlocal effect can be significantly alleviated. Furthermore, it was shown that at high or low values of elastic gradient parameters, the stability borders are highly sensitive to magnetic field strength. Results of this paper can be applied as a benchmark in the optimal design of nanofluidic systems.

Dielectric Study of Carboxyl (–COOH) Functionalized Multi-Walled Carbon Nano Tube-Silicone Oil Nanofluids at Different Temperatures

In this work, dielectric properties of carboxyl (–COOH) functionalized MWCNTs-Silicone oil (polydimethylsiloxanes) nanofluids have been studied at static frequency. Purchased carboxyl (–COOH)-f-multi-walled carbon nanotubes (MWCNTs) are purchased and characterized using transmission electron microscopy (TEM) technique. Carboxyl (–COOH)-f-MWCNT-Silicone oil nanofluids are prepared by dispersing COOH-MWCNT in Silicone oil (base fluid) with the help of probe sonicator. At various concentrations and at different temperatures dielectric parameters like static dielectric constant (ɛ0), excess dielectric constant (ɛE), dipole moment (μ), excess dipole moment (μE) and molar polarization (P) have been determined for COOH-f-MWCNT-Silicone oil nanofluids. The changes in the dielectric parameters with respect to five different concentrations and various temperatures (303 K to 323 K) are analyzed in the presence of intermolecular interactions happening in the COOH-f-MWCNT-Silicone oil nanofluid system. Excess dielectric constant and excess dipole moment values are found to be negative over entire concentration range of COOH-MWCNTs at every different temperature.

Highly Efficient Ionic Photocurrent Generation through WS2 -Based 2D Nanofluidic Channels.

The unique feature of nacre-like 2D layered materials provides a facile, yet highly efficient way to modulate the transmembrane ion transport from two orthogonal transport directions, either vertical or horizontal. Recently, light-driven active transport of ionic species in synthetic nanofluidic systems attracts broad research interest. Herein, taking advantage of the photoelectric semiconducting properties of 2D transition metal dichalcogenides, the generation of a directional and greatly enhanced cationic flow through WS2 -based 2D nanofluidic membranes upon asymmetric visible light illumination is reported. Compared with graphene-based materials, the magnitude of the ionic photocurrent can be enhanced by tens of times, and its photo-responsiveness can be 2-3.5 times faster. This enhancement is explained by the coexistence of semiconducting and metallic WS2 nanosheets in the hybrid membrane that facilitates the asymmetric diffusion of photoexcited charge carriers on the channel wall, and the high ionic conductance due to the neat membrane structure. To further demonstrate its application, photonic ion switches, photonic ion diodes, and photonic ion transistors as the fundamental elements for light-controlled nanofluidic circuits are further developed. Exploring new possibilities in the family of liquid processable colloidal 2D materials provides a way toward high-performance light-harvesting nanofluidic systems for artificial photosynthesis and sunlight-driven desalination.

Investigations of a new combined application of nanofluids in heat recovery and air purification

This paper conducts a new investigation of air purification with total heat recovery based on nanofluids for the first time to realize the synergistic application of nanofluids to the heat and mass transfer as well as the photocatalysis and sterilization fields. To explore the feasibility of the new application, the stable TiO2 nanofluids were obtained by a post-treated optimized preparation method. Then the experimental device is built to investigate the formaldehyde and total volatile organic compounds (TVOC) purification as well as sterilization. At last, the heat recovery characteristics of nanofluids are indirectly investigated by testing the gas-liquid bubbling heat transfer behaviour. The results show the post-treated method can improve the dispersion stability of nanofluids. The nanofluid system has efficient effects on air purification and sterilization for both fungi and bacteria. The nanofluids have higher gas-liquid bubbling heat transfer performance than water which increased as the particle loading.

Loading different sizes of titania nanoparticles into transformer oil: A study on the dielectric behavior

TiO2 (titania) nanoparticles (NPs) were successfully synthesized by the sol–gel route with different calcination temperature varied from 300 to 500 °C. The microstructure and morphology of the synthesized samples were characterized under the effect of changing the calcination temperature using X-ray diffraction (XRD) and high-resolution transmission electron microscope (HRTEM). The result obtained from such characteristic tools confirmed the formation of the anatase phase for all the synthesized titania NPs with pronounced dependence of size and agglomeration of particles on the used calcination temperatures. Various nanofluids systems from a commercially mineral oil and the synthesized TiO2 NPs were fabricated with different concentrations up to 5 g/L from each of the synthesized NPs. The dielectric properties of the fabricated nanofluids were examined by studying the effect of the concentration of titania NPs on the variation of ac breakdown voltage and dielectric constant (e′). All the prepared oil transformer-nanofluid based on the investigated concentrations of the synthesized TiO2 NPs exhibited enhancement in the ac breakdown voltage reaching 111.8% for nanofluid with concentration 0.7 g/L based on T400 sample more than that obtained for the pure oil transformer. The dependency of e′ on NPs concentration was studied under different applied frequency values from 30 Hz to 1 MHz. The nanoparticle features were reflected on the behavior of e′ with NPs concentration throughout the investigated range of the applied frequency.

Effect of cationic, anionic and non-ionic polymeric surfactants on the stability, photo-catalytic and antimicrobial activities of yttrium oxide nanofluids

This work describes the utilization of combining the properties of yttrium oxide as a photoactive metal oxide and the properties of different kinds of charged surfactants: cationic, anionic and non-ionic polymeric types. The surface charge of surfactants was found to have a great influence on the stabilization performance of yttrium oxide particles in aqueous surfactant media, known as nanofluids, as well as on their photocatalytic and antimicrobial activities. Ionic surfactants (cetyltrimethylammonium bromide (CTAB) as a cationic surfactant and sodium dodecyl sulfate (SDS) as an anionic one) reduce the particle aggregation while the non-ionic polymeric surfactant (Polysorbate 80, Polyoxyethylene (20) sorbitan monooleate or (x)-sorbitan mono-9-octadecenoate poly(oxy-1,2-ethanediyl) (Tween 80)) enhanced the particles aggregation and formed a polymeric layer on the aggregates surface. The prepared nanofluids morphological structure and stability were studied by means of TEM, DLS and Zeta potential measurements. The prepared nanofluid systems were found to have an enhanced photo-catalytic performance for the degradation of MG dye studied under both UV and visible light irradiations and an enhanced wide spectrum antimicrobial activity against Gram positive and Gram negative bacteria as well as fungal stains.

Direct investigation of methane nanofluidic microstructure and system dynamics in rough silicon nanochannel

The microstructure and system dynamics of methane nanofluidic are important to determine the flow properties. In this study, we use non-equilibrium multiscale molecular dynamics simulation (NEMSMD) to investigate the microstructure and system dynamics of methane nanofluidic confined by rough silicon atomic walls. To capture the detailed atomic microstructure of methane moving in rough nanochannel accurately, the modification of OPLS methane fully atomic model is employed. The average number density distributions of C (H) atom, average velocity profile, velocity autocorrelation function and projection radial distribution function (XOY-, XOZ- and YOZ planes) plots give a clear observation of the impacts of different conditions (roughness, body driving force, fluid–wall interaction strength and cutoff radius) on the methane Poiseuille flow in nanochannel. The projection radial distribution functions and diffusivity appear anisotropic, and seem to be affected more significantly by roughness, fluid–wall interaction strength and cutoff radius than body driving force in the whole channel region. Moreover, the body driving force and cutoff radius also play an important role in methane nanofluidic simulation using NEMSMD. The present simulation results are very meaningful for the design of energy-saving emission reduction nanofluidic devices.

Enhanced thermal conductivity and reduced viscosity of aegirine-based VR/VGO nanofluids for enhanced thermal oil recovery application

The depleting of the available conventional energy supplies together with an industrial shift towards unconventional resources like heavy oil/bitumen has become more pronounced. The steam-based heating methods are primarily used by the oil industry for the heavy oil/bitumen recovery. However, the thermal recovery methods are energy-intensive and have limited applications, especially for both thin and deep reservoirs. Therefore, there is a high priority need to investigate alternative approaches. To date, the most progressive alternative technique that has proven its potential during pilot-plant tests is nanocatalytic in-situ heavy oil/bitumen upgrading via hot-fluid injection. Hence, the continual improvement of this technique is of utmost importance. This study aims to propose a new injecting nanofluid system suitable for high-temperature injection into the reservoir with consecutive heavy oil/bitumen upgrading and recovery. Here we report a new type of copper-based nanofluid using a blend of vacuum gas oil (VGO) and vacuum residue (VR) as the mother solvent. The nanoparticles were prepared by low-temperature hydrothermal synthesis route. Their detailed surface, morphology and size characterizations were achieved by X-ray diffraction, dynamic light scattering and scanning electron microscopy. The stable nanofluids were prepared by dispersing copper-based nanoparticles in a mixture of VGO and VR, at different ratios and temperatures. A set of measurements to determine the thermal conductivity and viscosity of the nanofluid with different loading of nanoparticle were performed. The thermal conductivity values of nanofluid systems are substantially higher than that of the base fluids. The nanofluid for 2 wt% of copper-doped aegirine nanoparticles dispersed in VGO and VGO/VR mixture exhibits a maximum thermal conductivity of 20% and 24%, respectively. It was found that the thermal conductivity of nanofluids increases with decreasing the hydrodynamic particle size. Moreover, the presence of chemo-physical interactions between nanoparticles and base fluid further enhances the thermal conductivity. Also, the temperature augmentation in a range from 80 to 110 °C exhibited a positive effect on thermal conductivity enhancement of vacuum residue-based nanofluid system. This particular nanofluid may find potential applications in enhancing heavy oil upgrading and recovery.

What dominates heat transfer performance of hybrid nanofluid in single pass shell and tube heat exchanger?

Influence of nanoparticle volume concentration and proportion on heat transfer performance (HTP) of Al2O3 – Cu/water hybrid nanofluid in a single pass shell and tube heat exchanger is analyzed. Multiphase mixture model is adopted to model the flow. Three-dimensional governing equations and associated boundary conditions are solved using finite volume method. The numerical results are validated with the experimental results. Results indicate that optimized nanoparticle volume concentration and proportion dominate HTP of hybrid nanofluid. Heat transfer coefficient and Nusselt number are monotonic increase functions of nanoparticle volume concentration and proportion. The percentage increase in heat transfer coefficient of hybrid nanofluid is 139% than water and 25% than Cu/water nanofluid. At higher Reynolds number, the increment in Number of Transfer Units (NTU) between water and hybrid nanofluid is close to 75%. Maximum enhancement in Nusselt number for hybrid nanofluid exceeds 90% when compared to nanofluid (Al2O3/Water nanofluid). Consequently, highest heat transfer performance is attained for hybrid nanofluid systems. Effectiveness of heat exchanger increases almost to 124% when hybrid nanofluid is employed. We show that it is higher than water as well (conventional coolant). Results are expected to be helpful in further industrial-scale deployment of nanofluids, which is an area that is currently relevant for ongoing academia-industry partnership efforts worldwide.

Finite Integral Transform Based Solution of Second Grade Fluid Flow between Two Parallel Plates

The importance of the slip flow over the no-slip condition is widely accepted in microscopic scaled domains with the direct impact on microfluidic and nanofluidic systems. The popular Navier Stoke’s (N-S) flow model is largely utilized with the slip flow phenomenon. In the present study, the finite integral transform scheme along with the shift of variables is implemented to solve the equation of motion of second grade fluid having third-order mixed partial derivative term. The velocity over the flow regime is studied with both the slip and no-slip boundary conditions for Newtonian and non-Newtonian characteristics by considering the generalized Couette flow. The impact of the pressure gradient and flow time on the velocity is investigated analytically. The output of the present research reveals that due to the slip flow velocity randomly varies at the vicinity of wall surface and such nature hasn’t been found for the no-slip condition. The validation of the present work was done by comparison with the published work and the numerical values, and it shows well verified.

Giant Thermoelectric Response of Nanofluidic Systems Driven by Water Excess Enthalpy.

Nanofluidic systems could in principle be used to produce electricity from waste heat, but current theoretical descriptions predict a rather poor performance as compared to thermoelectric solid materials. Here we investigate the thermoelectric response of NaCl and NaI solutions confined between charged walls, using molecular dynamics simulations. We compute a giant thermoelectric response, 2 orders of magnitude larger than the predictions of standard models. We show that water excess enthalpy-neglected in the standard picture-plays a dominant role in combination with the electro-osmotic mobility of the liquid-solid interface. Accordingly, the thermoelectric response can be boosted using surfaces with large hydrodynamic slip. Overall, the heat harvesting performance of the model systems considered here is comparable to that of the best thermoelectric materials, and the fundamental insight provided by molecular dynamics suggests guidelines to further optimize the performance, opening the way to recycle waste heat using nanofluidic devices.

Utilization of hybrid nanofluids in solar energy applications: A review

Hybrid nanofluids have several advantages compared with the conventional types due to their modified properties. Their enhanced thermophysical and rheological properties make them more appropriate for solar energy systems. In this review paper, an overview of solar energy systems is represented, and afterwards, applications of hybrid nanofluids in various solar technologies, especially solar thermal, are reviewed. Comparison between the nanofluidic systems, and the conventional ones is performed in order to gain a deeper insight into the advantages of using nanofluids. According to the results of the reviewed studies, the most important reason for performance enhancement of nanofluidic solar energy systems can be attributed to the improved thermal properties of the convective fluid. In addition, it can be concluded that the nanofluids’ specifications such as concentration of the nanostructures, type of the solid phase, etc., have significant impact on the behavior of the considered systems.

Voltage-Gated Ion Transport in Two-Dimensional Sub-1 nm Nanofluidic Channels.

Voltage-gated nanofluidic systems have shown a wide range of potential applications in biosensors, energy harvest, and separation. Two-dimensional (2D) nanofluidic membranes fabricated with electrically conductive nanosheets have high ion conductivity and voltage-gated ion transport behaviors. However, the voltage-gating effect of the sub-nanometer-sized 2D channel membranes has not been well-investigated. In this work, a high-performance voltage-gated 2D nanofluidic device is constructed by assembling MXene nanosheets into a laminar membrane with sub-1 nm interlayer channels. By applying external voltage to the membrane, the ion conductivity of the device is enhanced by positive voltages and reduced by negative voltages, exhibiting a high voltage-gating on-off ratio of ∼10. The on-off ratio is found to be dependent on ion concentration and ion species. This work demonstrates that 2D membranes with interlayer spacings comparable to those of hydrated ion diameters can achieve high and tunable voltage-gating function, which provides a strategy to construct devices for highly efficient on-demand ion transport.

A High Efficiency Osmotic Energy Harvester with Vertically Aligned Boron-Nitride-Nanopore Membrane

Application of the abundant salinity-gradients in the environment, motivated by the Gibbs free energy of mixing fresh and salt water, has been under intensive investigation for clean-energy harvesting. Globally, there is a potential of at least 2.6 TW of power that could be generated at coastal estuaries from the ion-concentration gradient. Recent investigation on an energy conversion efficiency of single boron nitride nanotube (BNNT) has revealed the huge potential of an efficient exploitation of electrokinetic phenomena in nanofluidic systems for energy harvesting. Here, for the first time, we demonstrate rationally designed nanostructured vertically aligned boron-nitride-nanopore membranes (VA-BNNP) which can efficiently harness osmotic power from salinity gradient. A thin hexagonal boron nitride (hBN) layer was uniformly deposited within the pores of anodized alumina substrates by low-pressure chemical vapor deposition which produced first-ever macroscopic VA-BNNP membrane with high nanopore density, up to ~108 pores/cm2. Cross-sectional scanning-electron-microscope (SEM) images show the hBN layer (~35 nm) was uniformly deposited along the pores without excessive hBN on the top surface, ensuring that most pores remained open. In addition, the results of scanning confocal Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) showed the high quality of the hBN layers in the AAO pores. We investigated the power generation of the macroscopic VA-BNNP membranes at different pH and salinity concentrations. The power generation per unit pore area increased as the salt concentration and pH increased. The highest power density of the membrane was up to ~100 W/m2 which is two orders of magnitude higher than that of other macro-scale, salinity gradient driven, power generation system reported so far. In addition, we also elucidate the fundamental ion transport mechanism in BN nanopore using the molecular dynamic simulations to support to the experimental power density values. These findings indicate the great potential of large-area VA-BNNP membranes as next-generation nanostructured membranes for renewable energy harvesting.

Static and dynamic behavior of CO2 enhanced oil recovery in shale reservoirs: Experimental nanofluidics and theoretical models with dual-scale nanopores

Understanding and controlling confined CO2 fluids in nanopores is at the heart of the CO2 enhanced oil recovery in shale reservoirs. Here, for the first time, qualitative and quantitative static and dynamic behavior of complex confined CO2 fluids in dual-scale nanopores are experimentally performed in nanofluidics, which combines with the theoretical model, the statistical mechanics coupled with the thermodynamic equation of state, to investigate the CO2 utilization in shale reservoirs. In experiment, a series of phase behavior and fluid flow laboratory tests are conducted through a self-manufactured nanofluidic system at different conditions; in theory, a generalized equation of state including the confinements and pore-size distributions and five dynamic models are developed and applied to calculate the vapor–liquid equilibrium and fluid dynamics. Results from this study show that the static behavior has drastic changes in the dual-scale nanopores that the measured saturation pressure of the confined CO2–C10 fluids reduces by 10.19% at T = 25.0 °C and 7.26% at T = 53.0 °C from bulk phase to nanometer scale. Furthermore, under the strong confinements in the dual-scale nanopores, the calculated phase properties including the pore-size distribution effects are more accurate. In addition, effects of the temperature and feed gas to liquid ratio on the confined fluids in nanopores share similar manners with the bulk phase cases. The proposed theoretical models are capable of calculating the static and dynamic behavior of the confined fluids and all calculations have been validated by the experimentally measured results. This study supports the foundation of more general applications pertaining to producing shale fluids and sequestrating CO2 in shale reservoir characterization and exploration.

Passive microscopic fluidic diodes using asymmetric channels

In this paper, we propose a passive microscopic fluidic diodes with no moving parts for simple fluids using asymmetric channel structures. Finite element simulations demonstrate that the fluidic diode conducts water flows preferentially in one forward direction while blocks flows in the reverse direction in a wide pressure range. The exceptional rectification performance is owing to the anisotropic direction-dependent activation pressures. In the forward direction, the activation pressure is small, which is controlled by the infiltration pressure of the small channel. In the backward direction, the activation pressure is large due to the high release pressure at the channel exit. The effective working pressure range for the fluidic diode can be flexibly adjusted by modifying the channel size or the surface property. Furthermore, we create a microfluidic diode fabricated on silicon membranes using laser direct writing. The diode achieves flow rectifications in a certain pressure range, which confirms the underlying rectification mechanisms. This work provides a novel strategy for flow control or logic computations in integrated micro- and nanofluidic systems.

Universal amplification-free molecular diagnostics by billion-fold hierarchical nanofluidic concentration

Rapid and reliable detection of ultralow-abundance nucleic acids and proteins in complex biological media may greatly advance clinical diagnostics and biotechnology development. Currently, nucleic acid tests rely on enzymatic processes for target amplification (e.g., PCR), which have many inherent issues restricting their implementation in diagnostics. On the other hand, there exist no protein amplification techniques, greatly limiting the development of protein-based diagnosis. We report a universal biomolecule enrichment technique termed hierarchical nanofluidic molecular enrichment system (HOLMES) for amplification-free molecular diagnostics using massively paralleled and hierarchically cascaded nanofluidic concentrators. HOLMES achieves billion-fold enrichment of both nucleic acids and proteins within 30 min, which not only overcomes many inherent issues of nucleic acid amplification but also provides unprecedented enrichment performance for protein analysis. HOLMES features the ability to selectively enrich target biomolecules and simultaneously deplete nontargets directly in complex crude samples, thereby enormously enhancing the signal-to-noise ratio of detection. We demonstrate the direct detection of attomolar nucleic acids in urine and serum within 35 min and HIV p24 protein in serum within 60 min. The performance of HOLMES is comparable to that of nucleic acid amplification tests and near million-fold improvement over standard enzyme-linked immunosorbent assay (ELISA) for protein detection, being much simpler and faster in both applications. We additionally measured human cardiac troponin I protein in 9 human plasma samples, and showed excellent agreement with ELISA and detection below the limit of ELISA. HOLMES is in an unparalleled position to unleash the potential of protein-based diagnosis.