Rectangular Nanochannels Research Articles

Defect Engineering in a Nanoporous Thulium-Organic Framework in Catalyzing Knoevenagel Condensation and Chemical CO2 Fixation.

Defect engineering is an extremely effective strategy for modifying metal-organic frameworks (MOFs), which can break through the application limitations of traditional MOFs and enhance their functionality. Herein, we report a highly robust nanoporous thulium(III)-organic framework, {[Tm2(BDCP)(H2O)5](NO3)·3DMF·2H2O}n (NUC-105), with [Tm(COO)2(H2O)]n chains and [Tm2(COO)4(H2O)8] dinuclears as metal nodes and 2,6-bis(2,4-dicarboxylphenyl)-4-(4-carboxylphenyl)pyridine (BDCP) linkers. In NUC-105, each of the four chains of [Tm(COO)2]n and the two rows of [Tm2(COO)4(H2O)8] units is unified by the organic skeleton, resulting in a rectangular nanochannel with dimensions of 15.35 Å × 11.29 Å, which leads to a void volume of 50%. It is worth mentioning that the [Tm2(COO)4(H2O)8] cluster is very rare in terms of its higher level of associated water molecules, implying that the activated host framework can serve as a strong Lewis acid. NUC-105a exhibited great heterogeneous catalytic performance for CO2 cycloaddition with epoxides under the reaction conditions (0.60 mol % NUC-105a, 5.0 mol % n-Bu4NBr, 65 °C, 5 h), ensuring exclusive selectivity and high conversion rates. In addition, NUC-105a's strong catalytic impact on the Knoevenagel condensation of aldehydes and malononitrile can be attributed to the collaboration between the drastically unsaturated Lewis acidic Tm3+ centers and Lewis basic pyridine groups.

Highly Robust {In2}-Organic Framework for Efficiently Catalyzing CO2 Cycloaddition and Knoevenagel Condensation.

To improve the catalytic performance of metal-organic frameworks (MOFs), creating higher defects is now considered as the most effective strategy, which can not only optimize the Lewis acidity of metal ions but also create more pore space to enhance diffusion and mass transfer in the channels. Herein, the exquisite combination of scarcely reported [In2(CO2)5(H2O)2(DMF)2] clusters and 2,6-bis(2,4-dicarboxylphenyl)-4-(4-carboxylphenyl)pyridine (H5BDCP) under solvothermal conditions generated a highly robust nanoporous framework of {[In2(BDCP)(DMF)2(H2O)2](NO3)}n (NUC-65) with nanocaged voids (14.1 Å) and rectangular nanochannels (15.94 Å × 11.77 Å) along the a axis. It is worth mentioning that an In(1) ion displays extremely low tetra-coordination modes after the thermal removal of its associated four solvent molecules of H2O and DMF. Activated {[In2(BDCP)](Br)}n (NUC-65Br), as a defective material because of its extremely unsaturated metal centers, could be generated by bromine ion exchange, solvent exchange, and vacuum drying. Catalytic experiments proved that the conversion of epichlorohydrin with 1 atm CO2 into 4-(chloromethyl)-1,3-dioxolan-2-one catalyzed by 0.11 mol % NUC-65Br could reach 99% at 65 °C within 24 h. Moreover, with the aid of 5 mol % cocatalyst n-Bu4NBr, heterogeneous NUC-65Br owns excellent universal catalytic performance in most epoxides under mild conditions. In addition, NUC-65Br, as a heterogeneous catalyst, exhibits higher activity and better selectivity for Knoevenagel condensation of aldehydes and malononitrile. Hence, this work offers a fresh insight into the design of structure defect cationic metal-organic frameworks, which can be better applied to various fields because of their promoted performance.

Investigation of surface structure-wettability coupling on heat transfer and flow characteristics in nanochannels

• The effect of surface wettability and roughness coupling is investigated by MD. • The progressively subdivided rough nanostructures are established. • The effect of wettability becomes slight when the scaling parameter exceeds 1.0. • The enhanced solid-fluid interaction energy reveals the microcosmic mechanism. The high integration of electronic devices has become an irresistible trend, what bottlenecks its development is the heat dissipation in micro/nanoscale. Surface roughness and wettability are the two main factors affecting convective heat transfer in nanochannels. However, the combined effects of wettability and roughness are still unclear. Consequently, by molecular dynamics simulation, the combined impact of wettability and roughness on heat transfer and flow characteristics are investigated in rectangular nanochannels with various wettability and progressively subdivided roughness. The results show, when the surface becomes more hydrophilic and subdivided, the temperature and Nu increase, whereas the temperature slip decreases, leading to an efficient improvement of the heat transfer process. Moreover, the velocity slip decreases, and the flow resistance increases, so the fluid flow process is relatively inhibited. Particularly, the effects of wettability and roughness are inconsonant. For the cases in this research, when the wettability scaling parameter exceeds 1.0, the hindrance of wettability to velocity development becomes slight. However, the improvement of wettability to temperature development is still significant. Microscopically, it’s found more atoms accumulate at the solid-liquid interface, and solid-liquid interaction energy enhances. This investigation provides theoretical guidance for the efficient cooling of micro/nano electronic devices.

Molecular dynamics simulation of thermal behavior of nanofluid flow in a nanochannel with Cetryltrimethylammoniu Bromide surfactant molecules

The poor heat transfer properties of the base fluid are the first effective obstacle to improving heat exchangers' efficiency. The key point of using very high conductivity of solid particles (in nanometer dimension) is hundreds of times more than common fluids in heat transfer in the composition of these fluids. Therefore, adding nanoparticles (NPs) to base fluids has been a solution to increase the heat transfer (HT) properties of common energy-carrying fluids. On the other hand, adding surfactants prevents the sedimentation and instability of NPs and improves the thermal performance of nanofluids (NF). So, this study surveyed the thermal properties of water-copper NF with/without surfactant molecules in a rectangular nanochannel using molecular dynamics (MD) simulation. Cetryltrimethylammoniu Bromide (CTAB) molecules were used as surfactants, copper structure as NPs, and water molecules as a base fluid in this simulation. In the present study, the effect of nanochannel wall temperature, copper nanoparticle (NP) size, and force field type (DREIDING, Universal Force-Field (UFF), and CHARM) on thermal conductivity (Knf) of water-copper NF were studied. The results show that as the wall temperature decreased from 80 to 30 K, Knf decreased from 0.783 to 0.753 Wm-1K−1. On the other hand, by increasing the radius of NPs from 0.5 to 2 nm, Knf and heat flux (HF) increase from 0.77 to 0.798 Wm-1K−1 and 5734 to 6352 W.m−2, respectively. One of the important purposes of researchers and engineers in various industries is to increase efficiency and reduce the systems' size. Therefore, it is expected that the results of this study will be effective in many different thermal industries.

Heat transfer and entropy generation analysis of electroosmotic flows in curved rectangular nanochannels considering the influence of steric effects

The heat transfer characteristics and entropy generation of an electroosmotic flow in curved rectangular nanochannels under high zeta potential conditions are investigated numerically with consideration of the steric effects. The numerical solutions of the Nusselt number (Nu) and the local entropy generation rate (SG) are derived under constant wall heat flux assumption. The variation trends of Nu and SG with corresponding parameters, including the curvature ratio of the curved nanochannel (δ), the effective ionic size (a0) and the wall heat flux (qw) are investigated. Results reveal that, for small values of qw (qw < 10), the consideration of the steric effects (corresponding to a non zero assumption for a0) leads to a lower SG and an enhanced Nu simultaneously. In this case, to further reduce entropy generation, a curved nanochannel with a large curvature ratio δ is adopted. However, for relatively large values of qw, the steric effects will have a negative effect on entropy generation minimization, especially for a curved nanochannel with large curvature ratio. Our results may be useful in designing the efficient thermal nano-equipment.

Slip length measurement in rectangular graphene nanochannels with a 3D flow analysis

Although many molecular dynamics simulations have been conducted on slip flow on graphene, experimental efforts remain very limited and our understanding of the flow friction on graphene remains far from sufficient. Here, to accurately measure the slip length in rectangular nanochannels, we develop a 3D capillary flow model that fully considers the nonuniform cross-section velocity profile, slip boundary conditions, and the dynamic contact angle. We show that the 3D analysis is necessary even for a channel with a width/height ratio of 100. We fabricated graphene nanochannels with 45-nm depth and 5-μm width, and measured slip lengths of about 30–40 nm using this 3D flow model. We also reevaluated the slip-length data for graphene obtained from capillary filling experiments in the literature: 30 nm instead of originally claimed 45 nm for a 25-nm-deep channel, and 47 nm instead of 60 nm for an 8.5-nm-deep channel. We discover a smaller slip length than existing experimental measurements due to our full 3D flow analysis considered in our method. This work presents a rigorous analysis approach while also providing a better understanding of slip flow in graphene nanochannels, which will benefit further innovation in nanofluidic applications, including electronics cooling and biomedical chips.

Electro-osmotic flow through nanochannel with different surface charge configurations: A molecular dynamics simulation study

Electro-osmotic flow behavior through rectangular graphene nanochannels with different charge (negative in nature) configurations is discussed in detail using non-equilibrium molecular dynamics (MD) simulations. Alternate patterning of charged and neutral stripes on the surface of the nanochannel lowers the water permeance and electro-osmotic flow velocity through the nanochannel. For all of the charge configurations, water permeance and electro-osmotic velocity through the nanochannel increase as surface charge density (σ) increases from 0.005 to 0.025 C m−2. This can be attributed to the increase in the number of counterions (Na+ ions) near the surface of the nanochannel. However, with further increase in σ, water permeance and electro-osmotic velocity through the nanochannel gradually decrease despite the increase in the number of counterions near the surface of the nanochannel. This is because of the significant increase in electrostatic interaction between the water molecules and the surface of the nanochannel. At a lower value of σ (σ≤0.025 C m−2), the overall interaction between the water molecules and the surface of the nanochannel is significantly dominated by van der Waals (vdW) interactions (electrostatic/vdW ≤0.40). The slip velocity of water molecules in the charged stripe portion of the wall (SlipCharge) is higher as compared to the slip velocity of water molecules in the neutral stripe portion (SlipNeutral) except at σ=0.2 cm−2. This difference between SlipCharge and SlipNeutral is highest at σ=0.025 C m−2 with SlipCharge &amp;gt; SlipNeutral, for all of the charge configurations.

Role of wall-fluid interaction and rough morphology in heat and momentum exchange in nanochannel

As an efficient cooling method, the convective heat transfer in nanochannel has been widely utilized in thermal management of various energy systems. However, the effects of surface roughness on velocity slip and flow resistance in nanochannel are still unclear. The effects of rough morphology and wall-fluid interaction on the flow and thermal characteristics in rectangular nanochannel are probed with molecular dynamics method. The results show that nanostructure morphology and wall-fluid interaction induce distinct variations in temperature jump and velocity slip, which further determine the heat and momentum exchange between the channel wall and fluid. Specifically, rough morphology is responsible for the augments of heat transfer and flow resistance, which is derived from limitation on the motion of fluid atoms by the nanostructure grooves in channel. In comparison, the strong wall-fluid interaction brings about the improvement of heat transfer and the increase of flow resistance, which owes to the adsorption enhancement. As the nanostructure free shear ratio increases from 0.1875 to 0.75, the flow resistance increases and heat transfer performance weakens. Yet, the combination of rough morphology and wall-fluid interaction is significant for overall heat transfer performance. The overall heat transfer performance in rough channel with the weak wall-fluid interaction is superior. The Nusselt number in rough channel only decreases by 1.78% while the resistance coefficient reduces by 27.1%. The optimal overall performance is achieved in rough channel with the nanostructure free shear ratio of 0.1875.

The thermal properties of water-copper nanofluid in the presence of surfactant molecules using molecular dynamics simulation

In this research, using the molecular dynamics simulation, the thermal properties of water-copper nanofluid in a rectangular nanochannel in the presence of surfactant molecules are investigated. Water molecules are considered the base fluid, copper atoms as nanoparticles, and CTAB molecules are considered surfactants. In this paper, some parameters such as thermodynamic stability, thermal conductivity, and radial distribution performance are investigated. The effect of surfactant on thermal conductivity is also studied. The addition of surfactant molecules to the nanofluid improves the thermal behavior of atomic structures. The thermal conductivity of the nanofluid containing the surfactant molecules is higher than the simulated primary nanofluid. Generally, this study investigates the effect of interactions between the base fluid and the surfactant to improve the thermal efficiency of nanofluid. Also The effect of the amount of surfactant added to the base fluid on the thermal properties of the base fluid and nanofluid is investigated. Numerically, the addition of surfactant molecules to the nanofluid increases the thermal conductivity of the structure from 0.741 Wm−1 K−1 to 0.783 Wm−1 K−1.

Electrochemomechanical energy conversion efficiency in curved rectangular nanochannels

Nanofluidic devices based on electrokinetic phenomena can have a major impact on the investigations of electric energy harvesting due to the possibility of opening a new avenue in the exploration of new energy sources. Many efforts have been made by a lot of experts to improve the performance of these devices. However, these efforts are still focused mainly in straight channels. In present study, a curved nanochannel with rectangular cross section is introduced to enhance the electrokinetic energy conversion efficiency. In practice, curved channels are often encountered in lab-on-chip systems due to the advantage of increasing the effective channel length per unit chip length in the flow direction. However, despite its importance, no one have investigated the electric energy conversion in such channels before. We present a theoretical study based on the solution of the linearized Poisson-Boltzmann equation. Our model yields analytical expressions for electrostatic potentials, fluid velocity, streaming potential and the energy conversion efficiency. The results show that, under certain parameter ranges, the electric energy conversion efficiency in curved rectangular nanochannel is 1.17 times larger than that in a straight one. The present endeavor is useful in designing more effective electrochemomechanical energy conversion devices.

Single-Molecule DNA Extension in Rectangular and Square Profile Nanochannels in the Extended de Gennes Regime

Experiments measuring the extension of moderately confined double-stranded DNA within nanochannels have consistently shown a stronger scaling relationship between extension length and nanochannel c...

Steric-effect-induced enhancement of electrokinetic energy conversion efficiency in curved nanochannels with rectangular sections at high zeta potentials

In this work, we investigate the electrokinetic energy conversion in curved rectangular nanochannels at high zeta potentials, intending to increase the efficiency of electrokinetic generation. As we demonstrate, the geometry effect can have an important role in the investigations of the conversion efficiency. In addition, we consider the influences of steric effect (the effect of finite ion size) on the energy conversion efficiency, which is significant and can’t be ignored, especially at high surface potentials. The finite difference method is used to obtain numerical solutions for the corresponding efficiency. Our results show that, within optimized parameters ranges, the conversion efficiency can be increased by up to 50 percent by considering the influences of the steric effect. Additionally, the conversion efficiency η shows a increasing trend with the curvature ratio δ and a reversed trend with the aspect ratio AR of the rectangular cross section. These results are useful for estimating and designing more effective electrokinetic energy conversion devices.

A novel design of nanochannel structure in a micro–nanofluidic preconcentrator for electrokinetic ion enrichment

Since the concentration of target molecules contained in most reagents in analytical chemistry experiments is lower than the minimum concentration required for subsequent detection, it is necessary for sample enrichment. Electrokinetic trapping that utilizes the principle of ion concentration polarization to achieve biomacromolecular enrichment is the most efficient. In this paper, based on the Poisson–Nernst–Plank equation, a novel design of nanochannel structure in a micro–nanofluidic preconcentrator for electrokinetic ion enrichment is carried out by numerical simulation. The results show that the enrichment process can be divided into three stages: enrichment generation, enrichment promotion and enrichment breakdown when the applied voltage is increased. Importantly, by comparing the six different structure nanochannels (straight line type, square wave type, rectangular type, circle type, zigzag type and multi-wave type), we found that the enrichment ratio produced by the rectangular nanochannel is the highest of the six nanochannels, which is 20.7 times. Rectangular nanochannel requires the lowest applied voltage to achieve the highest enrichment ratio. This work provides a novel design method and theoretical basis for the design of detection equipment in ultra-low concentration molecular detection tasks.

Effect of nanochannel geometry on electrokinetic ion transport in a micro-nanofluidic system

In this paper, the electrokinetic ion transportation in a micro-nanofluidic system is demonstrated through employing Poisson-Nernst-Planck equations coupled with Navier-Stokes equations. The effects of nanochannel geometry, volumic surface charge density, nanochannel length and nanochannel radius on the electric field, ion transport field, and fluid field are investigated. Theoretical research shows that the potential drop relies mainly on the nanochannel’s volume instead of nanochannel geometry. Two acute-angle structures in a trapezoid nanochannel relative to a right-angle structure in a semicircular or rectangular nanochannel can intensify the electric double layer field. For a cross sectional shape of the trapezoid, a higher ion concentration is obtained through enhancing the volumic surface electric charge, increasing the nanochannel length, and decreasing the radius. The results enabled the provision of a beneficial method for better understanding the design principle of the nanochannel structure.

Electroosmotic flow and ionic conductance in a pH-regulated rectangular nanochannel

Infinite series solutions are obtained for electrical potential, electroosmotic velocity, ionic conductance, and surface physicochemical properties of long pH-regulated rectangular nanochannels of low surface potential utilizing the double finite Fourier transform method. Closed form expressions are also obtained for channels of large height to width ratio for which the depthwise variations vanish. Neglecting the Stern layer impact, the effects of EDL (Electric Double Layer) overlap, multiple ionic species, and association/dissociation reactions on the surface are all taken into account. Moreover, finite-element-based numerical simulations are conducted to account for the end effects as well as to validate the analytical solutions. We show that, with the exception of the migratory ionic conductivity, all the physicochemical parameters are strong functions of the channel aspect ratio. Accordingly, a slit geometry is not a good representative of a rectangular channel when the width is comparable to the height. It is also observed that the distribution of the electrical potential is not uniform over the surface of a charge-regulated channel. In addition, unlike ordinary channels for which an increase in the background salt concentration is always accompanied by higher flow rates, quite the opposite may be true for a pH-regulated duct at higher salt concentrations.

Dynamics of DNA Squeezed Inside a Nanochannel via a Sliding Gasket.

We use Brownian dynamics (BD) simulation of a coarse-grained (CG) bead-spring model of DNA to study the nonequilibrim dynamics of a single DNA molecule confined inside a rectangular nanochannel being squeezed with a sliding gasket piston or “nanodozer”. From our simulations we extract the nonequilibrim density profile of the squeezed molecule along the channel axis (x-coordinate) and then analyze the non-equilibrium profile using a recently introduced phenomenological Nonlinear Partial Differential Equation (NPDE) model. Since the NPDE approach also fits the experimental results well and is numerically efficient to implement, the combined BD + NPDE methods can be a powerful approach to analyze details of the confined molecular dynamics. In particular, the overall excellent agreement between the two complementary sets of data provides a strategy for carrying out large scale simulation on semi-flexible biopolymers in confinement at biologically relevant length scales.

Molecular Dynamics Study of Gas–Surface Interactions in a Force-Driven Flow of Argon through a Rectangular Nanochannel

ABSTRACTIn recent times, flows through micro- and nanochannels have gained prominence due to their applicability to the fast growing fields of micro- and nanotechnology among others. Understanding gas–surface interactions in such flows is crucial, because the size of the micro/nanoscale devices is typically comparable to boundary layer thickness near a wall and the surface starts playing a significant role. An attempt is made to understand these interactions by modeling simple force-driven argon gas flow between two parallel platinum plates by the molecular dynamics method. One of the most important parameters that describes gas–surface interactions—that is, the tangential momentum accommodation coefficient, along with flow properties such as velocity and density—is calculated for a range of Knudsen numbers in the early transitional flow regime. A deeper insight into the flow physics is obtained by considering various case studies for the variation of aforesaid properties with respect to external driving forces and gas–wall interaction strengths.

The Backfolded Odijk Regime for Wormlike Chains Confined in Rectangular Nanochannels.

We confirm Odijk’s scaling laws for (i) the average chain extension; (ii) the variance about the average extension; and (iii) the confinement free energy of a wormlike chain confined in a rectangular nanochannel smaller than its chain persistence length through pruned-enriched Rosenbluth method (PERM) simulations of asymptotically long, discrete wormlike chains. In the course of this analysis, we also computed the global persistence length of ideal wormlike chains for the modestly rectangular channels that are used in many experimental systems. The results are relevant to genomic mapping systems that confine DNA in channel sizes around 50 nm, since fabrication constraints generally lead to rectangular cross-sections.

Temperature Fluctuations in a Rectangular Nanochannel

We consider an incompressible fluid in a rectangular nanochannel. We solve numerically the three dimensional Fourier heat equation to get the steady solution for the temperature. Then we set and solve the Langevin equation for the temperature. We have developed equations in order to determine relaxation time of the temperature fluctuations, τT = 4.62 × 10−10 s. We have performed a spectral analysis of the thermal fluctuations, with the result that temporal correlations are in the one-digit ps range, and the thermal noise excites the thermal modes in the two-digit GHz range. Also we observe long-range spatial correlation up to more than half the size of the cell, 600 nm; the wave number, q, is in the 10 6 m −1 range. We have also determined two thermal relaxation lengths in the z direction: l1 = 1.18 nm and l2 = 9.86 nm.

Amplified stretch of bottlebrush-coated DNA in nanofluidic channels.

The effect of a cationic-neutral diblock polypeptide on the conformation of single DNA molecules confined in rectangular nanochannels is investigated with fluorescence microscopy. An enhanced stretch along the channel is observed with increased binding of the cationic block of the polypeptide to DNA. A maximum stretch of 85% of the contour length can be achieved inside a channel with a cross-sectional diameter of 200 nm and at a 2-fold excess of polypeptide with respect to DNA charge. With site-specific fluorescence labelling, it is demonstrated that this maximum stretch is sufficient to map large-scale genomic organization. Monte Carlo computer simulation shows that the amplification of the stretch inside the nanochannels is owing to an increase in bending rigidity and thickness of bottlebrush-coated DNA. The persistence lengths and widths deduced from the nanochannel data agree with what has been estimated from the analysis of atomic force microscopy images of dried complexes on silica.