Nanochannel Height Research Articles

Improving ion rejection by optimizing the structure and charge distribution of the Janus membrane

Due to the asymmetry of charge distribution and geometric structure, the Janus membrane can simultaneously intercept counter-ions and co-ions. This work constructed four Janus membrane configurations using numerical simulation methods. Due to the different potential within the membrane and the selectivity level, there is a significant deviation in the interception performance. The results found that the change in the height of the Janus membrane affects the distribution of anion and cation concentration inside the nanochannel, and the asymmetry of the nanochannel height is more conducive to interception than a uniform nanochannel. The effect of the nanochannel length of the first section on the rejection rate is higher than that of the second section. Considering the design of the membrane, we recommend that the pore length of the first section be set to about 50 nm. Since the flow potential balances the electroviscous force, the optimization of the flow rate for ion transport is often opposite to the interception effect. The Janus membrane has asymmetric pore size and charge distribution characteristics and has excellent retention ability for divalent salts under low-pressure intensity and high-concentration conditions.

Wet Etching of Silicon in Planar Nanochannels.

Silicon (Si) alkaline etching constitutes a fundamental process in the semiconductor industry. Although its etching kinetics on plain substrates have been thoroughly investigated, the kinetics of Si wet etching in nanoconfinements have yet to be fully explored despite its practical importance in three-dimensional (3-D) semiconductor manufacturing. Herein, we report the systematic study of potassium hydroxide (KOH) wet etching kinetics of amorphous silicon (a-Si)-filled two-dimensional (2-D) planar nanochannels. Our findings reveal that the etching rate would increase with the increase in nanochannel height before reaching a plateau, indicating a strong nonlinear confinement effect. Through investigation using etching solutions with different ionic strengths and/or different temperatures, we further find that both electrostatic interactions and the hydration layer inside the nanoconfinement contribute to the confinement-dependent etching kinetics. Our results offer fresh perspectives into the kinetic study of reactions in nanoconfinements and will shed light on the optimization of etching processes in the semiconductor industry.

Thermal conductivity and structural behavior of confined H2 from molecular dynamics simulation

In this work, we perform equilibrium molecular dynamics simulation to study the thermal conductivity of hydrogen molecules (H2) under extreme confinement within graphene nanochannel. We analyze the structural behavior of H2 molecules inside the nanochannel and also examine the effect of nanochannel height, the number of H2 molecules, and temperature of the system on the thermal conductivity. Our results reveal that H2 molecules exhibit a strong propensity for absorption onto the nanochannel wall, consequently forming a dense packed layer in close to the wall. This phenomenon significantly impacts the thermal conductivity of the confined system. We made a significant discovery, revealing a strong correlation between the mass density near the nanochannel wall and the thermal conductivity. This finding highlights the crucial role played by the density near the wall in determining the thermal conductivity behavior. Surprisingly, the average thermal conductivity for nanochannels with a height (h) less than 27 Å exhibited an astonishing increase of over 12 times when compared to the bulk. Moreover, we observe that increasing the nanochannel height, while the number of H2 molecules fixed, leads to a notable decrease in thermal conductivity. Furthermore, we investigate the influence of temperature on thermal conductivity. Our simulations demonstrate that higher temperature enhance the thermal conductivity due to increased phonon activity and energy states, facilitating more efficient heat transfer and higher thermal conductivity. To gain deeper insights into the factors affecting thermal conductivity, we explored the phonon density of states. Studying the behavior of hydrogen in confined environments can offer valuable insights into its transport properties and its potential for industrial applications.

Analytical solutions for viscoelectric effects in electrokinetic nanochannels.

Understanding electrokinetic transport in nanochannels and nanopores is essential for emerging biological and electrochemical applications. The viscoelectric effect is an important mechanism implicated in the increase of local viscosity due to the polarization of a solvent under a strong electric field. However, most analyses of the viscoelectric effect have been limited to numerical analyses. In this work, we present a set of analytical solutions applicable to the physical description of viscoelectric effects in nanochannel electrokinetic systems. To achieve such closed-form solutions, we employ the Debye-Hückel approximation of small diffuse charge layer potentials compared to the thermal potential. We analyze critical parameters, including electroosmotic flow profiles, electroosmotic mobility, flow rate, and channel conductance. We compare and benchmark our analytical solutions with published predictions from numerical models. Importantly, we leverage these analytical solutions to identify essential thermophysical and nondimensional parameters that govern the behavior of these systems. We identify scaling parameters and relations among surface charge density, ionic strength, and nanochannel height.

Exploring the viscosity and structural behavior of confined hydrogen: A molecular dynamics approach

In this study, we employ equilibrium molecular dynamics (EMD) simulation to explore the viscosity of hydrogen molecules (H2) when subjected to extreme confinement within a nanochannel made by graphene sheets. We show that the viscosity of confined H2 and, therefore, its flow rate are deeply affected by layered structure of H2 when the nanochannel height (h) becomes less than 20Å. At these heights, we observe a substantial increase in viscosity, ranging from 8 to 55 µPa.s as the h decreases from 20 to 6 Å. To this end, we investigate the impact of structural behavior of H2, the height of nanochannel, the number of H2 molecules (N), and system temperature on the viscosity. Our findings reveal a notable tendency of H2 molecules to adhere to the nanochannel wall, resulting in the formation of a densely packed layer near the wall. Additionally, our observations reveal a robust correlation between the density near the wall and the viscosity. Also, increasing the h, while keeping the N constant, leads to a noticeable decrease in viscosity. Moreover, we investigate the influence of temperature on the viscosity and demonstrate that higher temperatures enhance viscosity making more collisions of molecules to the nanochannel wall and between molecules. Furthermore, we find that viscosity increases linearly with the N due to more collision between the molecules. Exploring the behavior of hydrogen in confined environments provides valuable insights into its transport properties and its potential for applications in the industry.

Repeated Microphase Separation and Heating-free Distillation-like Behavior of Miscible Binary Liquid Mixtures Using Nanoconfined Grafted Polymer Layers.

Nanoconfinement is known to drive phase separation (often denoted as microphase separation) of two highly miscible liquids by subjecting the two liquids to disparate influences. Here, we propose a paradigm shift to this problem: we introduce the idea of "repeatability" in nanoconfinement-driven microphase separation. A drop consisting of two highly miscible liquids (A and B) is made to pass through a nanochannel grafted with a collapsed layer of polymer that is philic to A but phobic to B. Subsequently, a significant number of molecules of liquid A get imbibed into the polymer layer and the polymer layer partially swells, while the molecules of liquid B mostly remain out of the polymeric layer and are carried away, emerging as a drop on the other side of the polymer bilayer. This passage of drop (of liquids A and B) is continued, and each time liquids A and B get separated with liquid A imbibing into the polymer layer and liquid B being carried away with the drop. This scenario, therefore, points to the repeated occurrence of the microphase separation of miscible binary liquid mixtures, enabling the processing of a much larger volume of liquid, given the fact that the presence of a grafted polymer layer continues to provide a dynamically increasing space where liquid A can get localized after being separated from liquid B. We quantify such repeated microphase separation by noting the extent of separation (of liquid A) and extent of recovery (of liquid B) as functions of nanochannel height and number of passes. Interestingly, we establish that this process also leads to a distillation-like behavior (without any heat addition), where the concentration of liquid B (equivalent to the "less volatile" liquid in a standard distillation process) progressively increases inside the drop after its passage through the nanochannel.

Electrokinetic ion enrichment in asymmetric charged nanochannels

Artificial bionic nanochannels have attracted wide attention and successfully used in various fields. In this work, a novel nanochannel with asymmetric surface charge is proposed to investigate the ion enrichment effect. The results show that the proposed nanochannel has excellent ion enrichment performance and the obtained ion enrichment ratio is up to 1500 when the ion concentration is 0.01 mM, which is much higher than precedent researches typically ranging from tens to hundreds. Besides, we found that the forward voltage bias will produce ions enrichment and the reverse voltage bias will produce ions depletion. The ion enrichment ratio is higher at the larger voltage bias, absolute surface charge density and smaller nanochannel height. In addition, the ion enrichment performance is more sensitive to the change of charged wall length and not sensitive to the change of uncharged wall length. The research report offers important information and instructions for the design and optimum on ion enrichment performance.

Prediction of electrodiffusio-osmotic transport of shear-thinning fluids in a nanochannel using artificial neural network

We have numerically investigated the electrodiffusio-osmotic (EDO) transport of non-Newtonian electrolytic solution, governed by an externally applied electric field and concentration difference, in a charged nanochannel connected with two reservoirs. We have examined the EDO transport characteristics by varying electrical, chemical, and rheological parameters. The relative augmentation in net throughput due to EDO transport is compared to the pure electro-osmotic flow and is found to be greater than unity [reaches up to the order of ∼O(103)] for the considered range of concentration difference and flow-behavior index. As shown, the EDO throughput with concentration difference follows an increasing–decreasing trend at the smaller nanochannel height (<10 nm), while exhibiting an increasing trend at the higher nanochannel height (>10 nm). Notably, the net flow for shear-thinning fluid gets fully reversed at higher concentration differences and for a higher value of zeta potential. In the second part of the work, we discuss the use of an artificial neural network (ANN) essentially to predict the net EDO throughput from the nanochannel. The ANN model considered here is of a single-hidden-layer feedforward type. For activation, we used a sigmoid-purelinear transfer function between the layers. Additionally, the Levenberg–Marquardt algorithm is used to perform the backpropagation. To predict the volume flow rate per unit width, we have used four input features: concentration difference, flow-behavior index, nanochannel height, and zeta potential. We have established that an ANN model with eight neurons in the hidden layer accurately predicts the flow rate per unit width with a very small root mean squared error. The inferences of this analysis could be of huge practical importance in designing the state-of-the-art nanodevices/systems intended for offering finer control over the underlying transport.

Competition between electroosmotic and chemiosmotic flow in charged nanofluidics

In electrolyte solutions, charged nanoscale pores or channels with overlapping electrical double layers are charge selective, thereby benefiting a wide range of applications such as desalination, bio-sensing, membrane technology, and renewable energy. As an important forcing mechanism, a gradient of electrolyte concentration along a charged nano-confinement can drive flow without an external electrical field or applied pressure difference. In this paper, we numerically investigate such a diffusioosmotic nanoflow, particularly for dilute electrolyte concentrations (0.01 mM–1 mM), and calculate the corresponding electrical and concentration fields in a charged nanochannel connecting two reservoirs of different salt concentrations—a typical fluidic configuration for a variety of experimental applications. Under a wide range of parameters, the simulation results show that the flow speed inside the nanochannel is linearly dependent on the concentration difference between the two reservoir solutions, Δc, whereas the flow direction is primarily influenced by three key parameters: nanochannel length (l), height (h), and surface charge density (σ). Through a comparison of the chemiosmotic (due to ion-concentration difference) and electroosmotic (as a result of the induced electric field) components of this diffusioosmotic flow, a non-dimensional number (C=h/lλGC) has been identified to delineate different nanoscale flow directions in the charged nanochannel, where λGC is a characteristic (so-called Gouy–Chapman) length associated with surface charge and inversely proportional to σ. This critical dimensionless parameter, dependent on the above three key nanochannel parameters, can help in providing a feasible strategy for flow control in a charged nanochannel.

Enhanced electroosmotic flow of Herschel-Bulkley fluid in a channel patterned with periodically arranged slipping surfaces

In this paper, we consider the electroosmotic flow (EOF) of a viscoplastic fluid within a slit nanochannel modulated by periodically arranged uncharged slipping surfaces and no-slip charged surfaces embedded on the channel walls. The objective of the present study is to achieve an enhanced EOF of a non-Newtonian yield stress fluid. The Herschel-Bulkley model is adopted to describe the transport of the non-Newtonian electrolyte, which is coupled with the ion transport equations governed by the Nernst-Planck equations and the Poisson equation for electric field. A pressure-correction-based control volume approach is adopted for the numerical computation of the governing nonlinear equations. We have derived an analytic solution for the power-law fluid when the periodic length is much higher than channel height with uncharged free-slip patches. An agreement of our numerical results under limiting conditions with this analytic model is encouraging. A significant EOF enhancement and current density in this modulated channel are achieved when the Debye length is in the order of the nanochannel height. Flow enhancement in the modulated channel is higher for the yield stress fluid compared with the power-law fluid. Unyielded region develops adjacent to the uncharged slipping patches, and this region expands as slip length is increased. The impact of the boundary slip is significant for the shear thinning fluid. The results indicate that the channel can be cation selective and nonselective based on the Debye layer thickness, flow behavior index, yield stress, and planform length of the slip stripes.

Numerical simulation of renewable power generation using reverse electrodialysis

We numerically investigate renewable power generation using reverse electrodialysis (RED), by harnessing salinity gradient energy of two constant inflows of salt solutions of different concentrations via a charged nanochannel. We compute and elucidate coupled flow, ion flux, electrical potential, and resultant electric outputs in the nanofluidic RED battery. The results of power output density and ion transfer across the nanochannel strongly depend on nanochannel height, whereas open-circuit voltage, short circuit current, and nanochannel resistance on the length. Energy conversion efficiency, on the other hand, display a non-linear dependence on both dimensions. More importantly, the results of nanochannel resistance and power output density reveal a power-law dependence on the concentration difference, quantitatively shedding light on the optimal RED flow designs. For the first time, our simulations at different inflow velocities show that the output power and conversion efficiency remain nearly constant at low speed but increase by ≈2.3 times and 10%, respectively, at high rate when advection effect becomes significant comparing to diffusion. These results quantitatively show profound effects of nanochannel dimensions, concentration difference, and inflow velocity on the renewable electrical energy outputs, offering optimal designs for maximizing reverse electrodialysis power.

Density and viscosity profiles governing nanochannel flow

The density and viscosity profiles across the nanochannel height were respectively compared between the flow factor approach model and molecular dynamics simulation for the Poiseuille flow in a nanochannel, when the agreements between these two approaches were good in calculating both the flow velocity profile across the channel height and the volume flow rate through the channel. It was found that except close to the confining wall, the comparisons in the two regards showed satisfactory agreements; Because of the solidification of the thin layer close to the wall, the errors of the flow factor approach model in calculating both the local density and local viscosity close to the wall cannot considerably affect its calculation accuracy of both the flow velocity profile across the channel height and the volume flow rate through the channel, both of which were actually governed by the density and viscosity profiles of the flowing media in the middle section of the channel height.

Role of Oxygen Functionalities in Graphene Oxide Architectural Laminate Subnanometer Spacing and Water Transport.

Active research in nanotechnology contemplates the use of nanomaterials for environmental engineering applications. However, a primary challenge is understanding the effects of nanomaterial properties on industrial device performance and translating unique nanoscale properties to the macroscale. One emerging example consists of graphene oxide (GO) membranes for separation processes. Thus, here we investigate how individual GO properties can impact GO membrane characteristics and water permeability. GO chemistry and morphology were controlled with easy-to-implement photoreduction and sonication techniques and were quantitatively correlated, offering a valuable tool for accelerating characterization. Chemical GO modification allows for fine control of GO oxidation state, allowing control of GO architectural laminate (GOAL) spacing and permeability. Water permeability was measured for eight GOALs characterized by different GOAL chemistry and morphology and indicates that GOAL nanochannel height dictates water transport. The experimental outputs were corroborated with mesoscale water transport simulations of relatively large domains (thousands of square nanometers) and indicate a no-slip Darcy-like behavior inside the GOAL nanochannels. The experimental and simulation evidence presented in this study helps create a clearer picture of water transport in GOAL and can be used to rationally design more effective and efficient GO membranes.

Structure and dynamics of water confined in a graphene nanochannel under gigapascal high pressure: dependence of friction on pressure and confinement.

Recently, water flow confined in nanochannels has become an interesting topic due to its unique properties and potential applications in nanofluidic devices. The trapped water is predicted to experience high pressure in the gigapascal regime. Theoretical and experimental studies have reported various novel structures of the confined water under high pressure. However, the role of this high pressure on the dynamic properties of water has not been elucidated to date. In the present study, the structure evolution and interfacial friction behavior of water constrained in a graphene nanochannel were investigated via molecular dynamics simulations. Transitions of the confined water to different ice phases at room temperature were observed in the presence of lateral pressure at the gigapascal level. The friction coefficient at the water/graphene interface was found to be dependent on the lateral pressure and nanochannel height. Further theoretical analyses indicate that the pressure dependence of friction is related to the pressure-induced change in the structure of water and the confinement dependence results from the variation in the water/graphene interaction energy barrier. These findings provide a basic understanding of the dynamics of the nanoconfined water, which is crucial in both fundamental and applied science.

PH-regulated ionic conductance in a nanochannel with overlapped electric double layers.

Accurately and rapidly analyzing the ionic current/conductance in a nanochannel, especially under the condition of overlapped electric double layers (EDLs), is of fundamental significance for the design and development of novel nanofluidic devices. To achieve this, an analytical model for the surface charge properties and ionic current/conductance in a pH-regulated nanochannel is developed for the first time. The developed model takes into account the effects of the EDL overlap, electroosmotic flow, Stern layer, multiple ionic species, and the site dissociation/association reactions on the channel walls. In addition to good agreement with the existing experimental data of nanochannel conductance available from the literature, our analytical model is also validated by the full model with the Poisson-Nernst-Planck and Navier-Stokes equations. The EDL overlap effect is significant at small nanochannel height, low salt concentration, and medium low pH. Neglecting the EDL overlap effect could result in a wrong estimation in the zeta potential and conductance of the nanochannel in a single measurement.

Transient streaming current measurements in nanochannels for molecular detection

Streaming currents were measured in silica slit-like nanochannels with integrated electrodes using a pulsed flow of deionized water. The analysis of the transient streaming current measurements in nanochannels shows that streaming currents are proportional to the surface charge density σ and that the effective reduction of nanochannel height by immobilized biomolecules needs to be taken into account. The σ of silica nanochannels was measured after performing sequential chemical surface modification steps. The nanochannel surface was first altered with positively charged polylysine and then with non-charged polyethylene glycol showing that nanochannels can sense the charge and size of surface-immobilized molecules.

Nanofluidic devices for dielectrophoretic mobility shift assays by soft lithography

We report development and application of 3D structured nano-microfluidic devices that were produced via soft lithography with poly(dimethylsiloxane). The procedure does not rely on hazardous or time-consuming production steps. Here, the nanochannels were created by channel-spanning ridges that reduce the flow height of the microchannel. Several realizations of the ridge layout and nanochannel height are demonstrated, depicting the high potential of this technique. The nanochannels proved to be stable even for width-to-height aspect ratios of 873:1. Additionally, an application of these submicrometer structures is presented with a new technique of a dielectrophoretic mobility shift assay (DEMSA). The DEMSA was used to detect different DNA variants, e.g. protein–DNA-complexes, via a shift in (dielectrophoretically retarded) migration velocities within an array of nanoslits.

What makes a nano-channel? A limiting-current criterion

It is shown theoretically that the occurrence of limiting currents at micro-/nano-interfaces critically depends on the ratio of nano-channel height (nano-pore size) and the Debye screening length. If this ratio is sufficiently large (>ca.6 for slit-like nano-channels, for example), the limiting-current phenomenon is suppressed due to the electroosmotic transfer of salt toward the polarized micro-/nano-interface. Therefore, not too narrow nano-channels effectively behave like micro-channels in the limiting-current terms. It is suggested that this qualitative change in the behavior can be used as a signature of “genuine” nano-channels. It is also shown that some recent experimental findings on the concentration polarization of joined micro-/nano-fluidic systems can be explained by using this criterion.

Transport properties of long straight nano-channels in electrolyte solutions: A systematic approach

The principle of local thermodynamic equilibrium is systematically employed for obtaining various transport properties of long straight nano-channels. The concept of virtual solution is used to describe situations of non-negligible overlap of diffuse parts of electric double layers (EDLs) in nano-channels. Generic expressions for a variety of transport properties of long straight nano-channels are obtained in terms of quasi-equilibrium distribution coefficients of ions and functionals of quasi-equilibrium distribution of electrostatic potential. Further, the Poisson-Boltzmann approach is used to specify these expressions for long straight slit-like nano-channels. In the approximation of non-overlapped diffuse parts of double electric layers in nano-channels, simple analytical expressions are obtained for the apparent electrophoretic mobilities of (trace) analytes of arbitrary charge as well as for the salt reflection coefficient (osmotic pressure), salt diffusion permeability and electro-viscosity (electrokinetic energy conversion). The approximate solutions are compared with the results of rigorous solution of non-linearized Poisson-Boltzmann equation, and the accuracy of approximation is shown to be typically excellent when the nano-channel half-height exceeds ca.3 Debye screening lengths. Due to non-negligible electrostatic adsorption of ions by nano-channels, the apparent electrophoretic mobilities of counter-ionic analytes in nano-channels are smaller than in micro-channels whereas those of co-ionic analytes are larger. This dependence on the charge is useful for the separation of analytes of close electrophoretic mobilities. The osmotic pressure is shown to be positive, negative or pass through maxima as a function of applied salt-concentration difference within a fairly narrow range of ratios of nano-channel height to the Debye screening length. The diffusion permeability of charged nano-channels to single salts is demonstrated (for the first time) to be typically larger than that of neutral nano-channels of the same dimensions due to electrical facilitation of salt diffusion.

Scalable attoliter monodisperse droplet formation using multiphase nano-microfluidics

We demonstrate a robust method to produce monodisperse femtoliter to attoliter droplets by using a nano-microfluidic device. Two immiscible liquids are forced through a nanochannel where a steady nanoscopic liquid filament forms, thinning close to the nanochannel exit to a microchannel due to the capillary focusing. When the nanoscopic filament enters the microchannel, monodisperse droplets are formed by capillary instability. In a certain range of physical parameters and geometrical configurations, the droplet size is only determined by the nanochannel height and independent of liquid flow rates and ratios, surfactants, and continuous phase viscosity. By using nanochannels with a height of 100–900 nm, 0.4–3.5 μm diameter droplets (volume down to 30 aL) have been produced. The generated droplets are stable for at least weeks.