Showing papers on "Nanofluidics published in 2018"

Molecular Dynamics Simulations of Water Structure and Diffusion in a 1 nm Diameter Silica Nanopore as a Function of Surface Charge and Alkali Metal Counterion Identity

Abstract: Water confined in nanopores—particularly in pores narrower than 2 nm—displays distinct physicochemical properties that remain incompletely examined despite their importance in nanofluidics, molecular biology, geology, and materials sciences. Here, we use molecular dynamics simulations to investigate the coordination structure and mobility of water and alkali metals (Li, Na, K, Cs) inside a 1 nm diameter cylindrical silica nanopore as a function of surface charge density, a model system particularly relevant to the alteration kinetics of silicate glasses and minerals in geologic formations. We find that the presence of a negative surface charge and adsorbed counterions within the pore strongly impacts water structure and dynamics. In particular, it significantly orients water O–H bonds toward the surface and slows water diffusion by almost 1 order of magnitude. Ion crowding in the charged nanopore enhances the tendency of counterions to coordinate closely with the silica surface, which moderates the impact...

Metamaterials-Enhanced Infrared Spectroscopic Study of Nanoconfined Molecules by Plasmonics–Nanofluidics Hydrid Device

Abstract: The behavior of molecules under nanoconfinement is crucial for understanding the chemical processes in biological and nanomaterial systems. We demonstrated here an infrared spectroscopic method to characterize the molecular structures of molecules confined in several tens of nanometer cavities by employing the plasmonics–nanofluidics hybrid device. This device consists of an array of metal nanostructures and a metal mirror separated by a nanofluidic cavity. Its configuration enables the confinement of both molecules and light energy as localized surface plasmons inside the physicochemically well-defined nanocavity. Exploiting the plasmons–molecular coupling, the vibrational modes of the nanoconfined molecules are selectively detected with a prominent sensitivity. Applying water as a proof-of-concept sample, we have successfully measured the infrared absorption characteristic and elucidated the molecular structures of water confined in a 10 nm cavity. They unveiled the presence of a strong H-bond network w...

Fast water flow through graphene nanocapillaries: A continuum model approach involving the microscopic structure of confined water

Abstract: Water inside a nanocapillary becomes ordered, resulting in unconventional behavior. A profound enhancement of water flow inside nanometer thin capillaries made of graphene has been observed [Radha et al., Nature (London) 538, 222 (2016)]. Here, we explain this enhancement as due to the large density and the extraordinary viscosity of water inside the graphene nanocapillaries. Using the Hagen-Poiseuille theory with slippage-boundary condition and incorporating disjoining pressure term in combination with results from molecular dynamics simulations, we present an analytical theory that elucidates the origin of the enhancement of water flow inside hydrophobic nanocapillaries. Our work reveals a distinctive dependence of water flow in a nanocapillary on the structural properties of nanoconfined water in agreement with experiment, which opens a new avenue in nanofluidics.

Ion transport by gating voltage to nanopores produced via metal-assisted chemical etching method.

Abstract: In this work, we report a simple and low-cost way to create nanopores that can be employed for various applications in nanofluidics. Nano sized Ag particles in the range from 1 to 20 nm are formed on a silicon substrate with a de-wetting method. Then the silicon nanopores with an approximate 15 nm average diameter and 200 μm height are successfully produced by the metal-assisted chemical etching method. In addition, electrically driven ion transport in the nanopores is demonstrated for nanofluidic applications. Ion transport through the nanopores is observed and could be controlled by an application of a gating voltage to the nanopores.

Fast water flow through graphene nanocapillaries: a continuum model approach involving the microscopic structure of confined water

Abstract: Water inside a nanocapillary becomes ordered, resulting in unconventional behavior. A profound enhancement of water flow inside nanometer thin capillaries made of graphene has been observed [B. Radha this http URL., Nature (London) 538, 222 (2016)]. Here we explain this enhancement as due to the large density and the extraordinary viscosity of water inside the graphene nanocapillaries. Using the Hagen-Poiseuille theory with slippage-boundary condition and incorporating disjoining pressure term in combination with results from molecular dynamics (MD) simulations, we present an analytical theory that elucidates the origin of the enhancement of water flow inside hydrophobic nanocapillaries. Our work reveals a distinctive dependence of water flow in a nanocapillary on the structural properties of nanoconfined water in agreement with experiment, which opens a new avenue in nanofluidics.

Photoacoustic Sensing of Trapped Fluids in Nanoporous Thin Films: Device Engineering and Sensing Scheme.

Abstract: Accessing fluid infiltration in nanogranular coatings is an outstanding challenge, of relevance for applications ranging from nanomedicine to catalysis. A sensing platform, allowing quantifying the amount of fluid infiltrated in a nanogranular ultrathin coating, with thickness in the 10–40 nm range, is here proposed and theoretically investigated by multiscale modeling. The scheme relies on impulsive photoacoustic excitation of hypersonic mechanical breathing modes in engineered gas-phase-synthesized nanogranular metallic ultrathin films and time-resolved acousto-optical read-out of the breathing modes frequency shift upon liquid infiltration. A superior sensitivity, exceeding 26 × 103 cm2/g, is predicted upon equivalent areal mass loading of a few ng/mm2. The capability of the present scheme to discriminate among different infiltration patterns is discussed. The platform is an ideal tool to investigate nanofluidics in granular materials and naturally serves as a distributed nanogetter coating, integratin...

Thermoelectricity in Heterogeneous Nanofluidic Channels.

Abstract: Ionic fluids are essential to energy conversion, water desalination, drug delivery, and lab-on-a-chip devices. Ionic transport in nanoscale confinements and complex physical fields still remain elusive. Here, a nanofluidic system is developed using nanochannels of heterogeneous surface properties to investigate transport properties of ions under different temperatures. Steady ionic currents are observed under symmetric temperature gradients, which is equivalent to generating electricity using waste heat (e.g., electronic chips and solar panels). The currents increase linearly with temperature gradient and nonlinearly with channel size. Contributions to ion motion from temperatures and channel properties are evaluated for this phenomenon. The findings provide insights into the study of confined ionic fluids in multiphysical fields, and suggest applications in thermal energy conversion, temperature sensors, and chip-level thermal management.

Nano X-ray diffractometry device for nanofluidics

Abstract: Nanofluidics is gaining attention because it has unique liquid and fluidic properties that are not observed in microfluidics. It has been reported that many liquid properties change when the size of a fluidic channel is reduced below 500–800 nm. To discuss the underlying mechanism, information on the microscopic liquid structure must be obtained (e.g., by X-ray diffractometry). However, the very small volume (attoliters to femtoliters) of a nanochannel and the large volume of its glass substrate prevent measurement of signals from the nanochannel liquid. In this study, we report a novel nanofluidic device that can be used in conjunction with X-ray diffractometry to analyze the structure of water confined in nanochannels. Top-down and bottom-up micro- and nano-fabrication processes were established, and the substrate thickness of the measurement area was reduced to only 2.7 μm, which was almost 1000 times smaller than that of conventional substrates (millimeter scale). With this new device, X-ray diffraction signals were clearly observed in nanochannels 500 nm wide and deep. Based on the X-ray diffraction pattern, the radial distribution function was calculated, which showed a structure nearly similar to that of a bulk sample. Therefore, X-ray diffractometry in nanochannels was realized. This method will provide important information on how a liquid behaves when confined in a nanospace and contribute to chemistry and biology on scales of 10–100 nm (e.g., inter- and intra-cellular spaces). It is also important for designing chemical reactions and fluidic circuits in nanochannels for realizing highly functional devices.

On Developing Field-Effect-Tunable Nanofluidic Ion Diodes with Bipolar, Induced-Charge Electrokinetics.

Abstract: We introduce herein the induced-charge electrokinetic phenomenon to nanometer fluidic systems; the design of the nanofluidic ion diode for field-effect ionic current control of the nanometer dimension is developed by enhancing internal ion concentration polarization through electrochemical transport of inhomogeneous inducing-counterions resulting from double gate terminals mounted on top of a thin dielectric layer, which covers the nanochannel connected to microfluidic reservoirs on both sides. A mathematical model based on the fully-coupled Poisson-Nernst-Plank-Navier-Stokes equations is developed to study the feasibility of this structural configuration causing effective ionic current rectification. The effect of various physiochemical and geometrical parameters, such as the native surface charge density on the nanochannel sidewalls, the number of gate electrodes (GE), the gate voltage magnitude, and the solution conductivity, permittivity, and thickness of the dielectric coating, as well as the size and position of the GE pair of opposite gate polarity, on the resulted rectification performance of the presented nanoscale ionic device is numerically analyzed by using a commercial software package, COMSOL Multiphysics (version 5.2). Three types of electrohydrodynamic flow, including electroosmosis of 1st kind, induced-charge electroosmosis, and electroosmosis of 2nd kind that were originated by the Coulomb force within three distinct charge layers coexist in the micro/nanofluidic hybrid network and are shown to simultaneously influence the output current flux in a complex manner. The rectification factor of a contrast between the ‘on’ and ‘off’ working states can even exceed one thousand-fold in the case of choosing a suitable combination of several key parameters. Our demonstration of field-effect-tunable nanofluidic ion diodes of double external gate electrodes proves invaluable for the construction of a flexible electrokinetic platform for ionic current control and may help transform the field of smart, on-chip, integrated circuits.

Molecular dynamics simulation of nanofluidics

Abstract: Abstract This review reports the progress on the recent development of molecular dynamics simulation of nanofluidics. Molecular dynamics simulations of nanofluidics in nanochannel structure, surface roughness of nanochannel, carbon nanotubes, electrically charged, thermal transport in nanochannels and gases in nanochannels are illustrated and discussed. This paper will provide an expedient and valuable reference to designers who intend to research molecular dynamics simulation of nanofluidic devices.

Colloidal lithography-based fabrication of highly-ordered nanofluidic channels with an ultra-high surface-to-volume ratio

Abstract: This article shows a new strategy for the fabrication of nanofluidics based on nanoscale gaps in nanopillar arrays. Silicon nanopillar arrays are prepared in a designed position by combining conventional photolithography with colloidal lithography. The nanogaps between the pillars are used as nanochannels for the connection of two polydimethylsiloxane-based microchannels in microfluidics. The gap between neighbouring nanopillars can be accurately controlled by changing the size of initial colloidal spheres and by an etching process, which further determines the dimensions of the nanochannels. At a low ionic strength, the surface charge-governed ion transportation shows that the nanochannels possess the same electrokinetic properties as typical nanofluidics. Benefiting from the advantage of photolithography, large-area nanochannel arrays can be prepared in a parallel manner. Due to the perm-selectivity of the nanochannels, the nanofluidic chips can be used to preconcentrate low concentration samples. The large-area ordered nanostructures preserve their high-throughput property and large surface-to-volume ratio, which shows their great potential in the development of nanofluidics and their applications, such as in the separation of small molecules, energy conversion, etc.

Mechano-nanofluidics: water transport through CNTs by mechanical actuation

Abstract: Mechano-nanofluidics, defined as the study of mechanical actuation effects on the properties of nanofluidics, have received broad interest recently in the field of nanofluidics. The coupling between phonons in carbon nanostructures and fluids under confinement is verified to enhance the diffusion of fluids. Especially, carbon nanotubes (CNTs) are applied as perfect nanochannels with fast water mass transport, making them to be one of the next generation of membranes. Here, we investigated water permeation through CNT membranes with the mechanical vibrations using non-equilibrium molecular dynamics simulations. The simulation results reveal that the water flux is highly promoted by a travelling surface waves at 1 THz. The water flux is enhanced by as large as 20 times for the single-file structured water at 20 MPa. The vibration effect is verified to be equivalent to a pressure drop with $${{{\Delta}}}P$$ up to 295 MPa. We show that the role of vibration diminishes for water in a larger CNTs. The water structure and hydrogen bond network are analyzed to understand the phenomena. The results of the present work are applied to provide guidance for the development of high-performance membranes.

Optical vortex beam direct-writing photolithography

Abstract: By using a focused unit-order vortex beam generated by a spiral phase plate, it is possible to acquire ~100 nm structures in direct-writing photolithography. This novel method is simple and inexpensive and can quickly fabricate nanometer-size dams or trenches that may have applications in nanoscale waveguides or micro- and nanofluidics. To overcome the defect of a small exposure dose tolerance in this method, which may lead to poor quality of the direct-writing lines, an alternative solution using a fractional-order vortex beam is proposed.

Multilevel Molecular Modeling Approach for a Rational Design of Ionic Current Sensors for Nanofluidics.

Abstract: The complexity displayed by nanofluidic-based systems involves electronic and dynamic aspects occurring across different size and time scales. To properly model such kind of system, we introduced a top-down multilevel approach, combining molecular dynamics simulations (MD) with first-principles electronic transport calculations. The potential of this technique was demonstrated by investigating how the water and ionic flow through a (6,6) carbon nanotube (CNT) influences its electronic transport properties. We showed that the confinement on the CNT favors the partially hydrated Na, Cl, and Li ions to exchange charge with the nanotube. This leads to a change in the electronic transmittance, allowing for the distinguishing of cations from anions. Such an ionic trace may handle an indirect measurement of the ionic current that is recorded as a sensing output. With this case study, we are able to show the potential of this top-down multilevel approach, to be applied on the design of novel nanofluidic devices.

Visualization of Fluid Dynamics in Nanoporous Media for Unconventional Hydrocarbon Recovery

Abstract: Evaporation at the nanoscale is critical to hydrocarbon production from nanoporous shale, a process that has reshaped global energy supply. However, there is a lack of understanding pertaining to phase change of hydrocarbons trapped in nanopores. This thesis develops and applies nanofluidics for the direct study of evaporation as relevant to shale oil and gas. Onset and dynamics of propane evaporation are studied in two-dimensional nanomodel. With sub-10 nm confinement, evaporation is vapor transport dominated with the Knudsen flow effect twice the viscous flow effect. A nanomodel is also developed that couples the inherent heterogeneity in shale pore sizes (100 nm pores gated by 5 nm-pores) to study vaporization of ternary hydrocarbons. Distinct spatiotemporal dynamics of vaporization are observed as a function of superheat. Results are compared to a vapor transport model. The differences highlight the inherent complexity of multi-component fluids in multi-scale geometries at the heart of unconventional resources.

Fluid Phase Separation via Nanochannel Array

Abstract: Microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS) generate ideas and techniques for creating new devices at the micro/nano scale. This dissertation study designed a gas generator system utilizing nanochannels for phase separation that is useful for micro-pneumatic actuators, micro-valves, and micro-pumps. The new gas generator has the potential to be an integral part of a propulsion system for small-scale satellites. Nano/picosatellites have limited orientation capability partly due to the current limitations of microthruster devices. Development of a self-contained micro propulsion system enables dynamic orbital maneuvering of picoand nano-class satellites. Additionally, the new gas generator utilizes a high efficiency, green propellant that is less harmful to the environment. This dissertation study tested aqueous antifreeze solutions to verify vapor pressures and establish previously unknown kinematic viscosities. A viscometer, developed expressly for this study, measured kinematic viscosity values between 1E-2 and 1E-4 m/s for water solutions mixed with propylene glycol, ethylene glycol, methanol, and glycerol. CubeSats, 10-centimeter cube satellites, are volume limited, and high strain expansion of water during crystallization could destroy the structure. Validation of the freezing point depression and measurement of previously unknown percent expansion with increasing concentration are valuable for setting safe design specifications. A 7.5 %w/w propylene glycol-water solution reduces the overall expansion by 2% while 20 %w/w PG reduces the expansion 4%. Potassium hydroxide etched silicon micro/nanochannels regulated vaporization of aqueous propylene glycol to a vacuum environment. Sequential still images captured with a Basler Scout camera were used to measure mass flow rates, representative of Washburn capillary flow. Magnitude of single channel flow rates ranged from 1E-10 mol/s to 1E-8 mol/s for 600nm channels up to 12μm channels, respectively. Although the flow rate increased using nanochannel arrays, it was 35% slower than expected based on single nanochannel measurements. A system utilizing nanochannel arrays for fluid phase separation, with propylene glycol as the propellant, is feasible in a low-cost, green, non-toxic, and non-pressurized CubeSat propulsion system. Depressing the freezing point of water by adding antifreeze creates a wider liquid working range, decreases power requirements, but also maintains appreciable flow rates for thrust generation (micro-millinewton). Successful nanofluidic research on an aqueous antifreeze solution is foundational for future propulsion system research and pushes CubeSats in the right direction.

Combining Nanoplasmonics and Nanofluidics for Single Particle Catalysis

Abstract: Nanoparticles are, due to their large exposed surface area, widely used in the field of heterogeneous catalysis where they accelerate and steer chemical reactions. Although catalysis has been known about for centuries, the scrutiny of catalysts under realistic application conditions is still a major challenge. This difficulty originates from the fact that real catalyst materials are very complex, often consisting of large ensembles of nanoparticles that all are unique. Furthermore, the typically used macroscopic reactors in catalysis studies gives rise to locally, at the level of the active site, ill-defined reactant concentrations and diffusion limitations. To overcome these limitations, on one hand, techniques are being developed that are sensitive enough to probe individual catalytic particles and that at the same time can operate under realistic reaction conditions. On the other hand, strategies to more carefully control the amount and structure of catalyst material, as well as to precisely control mass transport to and from the active catalyst, are being investigated by scaling down the size of the used chemical reactor. To further push the limit of downsizing, in this thesis, I present a miniaturized reactor platform based on nanofluidic channels that have been carefully decorated with catalytic nanoparticles, and that is integrated with plasmonic nanospectroscopy readout. This optical technique relies on the nanoscale phenomenon known as the Localized Surface Plasmon Resonance (LSPR) and enables the study of individual metal nanoparticles in operando by means of dark-field scattering spectroscopy. As the first step in this development, we constructed a nanofluidic device with integrated plasmonic nanoparticles to detect minute changes in the liquid flowing through the channels, as well as molecules binding to the nanoparticles. As the second step, we developed the nanofluidic system with an integrated heater and to facilitate gas flow through the nanochannels with the possibility to connect to a mass spectrometer for on-line product analysis. This system was then successfully used to correlate activity with surface and bulk oxidation state changes taking place on individual catalytic Cu and Pt nanoparticles during CO oxidation, measured by means of plasmonic nanospectroscopy. To this end, in a separate study, I also employed the plasmonic approach to study the oxidation process of Cu nanoparticles both experimentally and by electrodynamics simulations.

Wafer-scale fabrication of CMOS-compatible, high aspect ratio encapsulated nanochannels

Abstract: Nanochannels are key structures in nanofluidics for a variety of different applications. However, typical nanochannel fabrication methods are ill-suited for full integration with other microfabricated components or devices. Here, nanochannels with an aspect ratio (length to cross-sectional dimension) of greater than 400 000 were demonstrated—where the width (35–40 nm) and height (140–150 nm) of the channels are sufficiently small to elongate macromolecules—at channel lengths on the order of millimeters. These channels were fabricated with a CMOS-compatible toolset, allowing for the batch fabrication of a multitude of channels and with the further potential of full integration with solid-state electronic and photonic devices on the same wafer. Finally, the versatility of the nanochannel fabrication platform was demonstrated by loading the channels with six different liquids, and it was verified that the fluid flow dynamics for each liquid can be well estimated with Washburn's equation.

A High Pressure Nanofluidic Micro-Pump Based on H 2 O Electrolysis

Abstract: Nanofluidic devices have many potential applications in biomedical field. One of the technical barriers of nanofluidics is to drive nanofluids in nanochannels with super-high hydraulic resistance (MPa scale) and super-small volume (fL scale). Electric field driving method (eg. electroosmotic flow) is commonly used in nanofluidics, since the conventional pumps applied in microfluidics are limited by their low pressure and low control precision. However, the electric field driving method is not suitable for all kinds of the nanofluids, which could affect the biochemical reactions, and lead to electrolytic reactions. We have developed a new type of high pressure nanofluidic micro-pump based on electrolysis. The pump system consists of electrolytic chamber, pressure sensor, control circuit, electrolytic electrodes and sample chamber that connects to nanofluidic chip. In our nanofluidic micro-pump system, high pressure gas generated from the electrolytic chamber pushes the liquid sample into the nanochannel, and the driving pressure to the fluidic sample can stably reach to 20MPa. Our high pressure micro-pump is suitable for both microfluidic and nanofluidic applications due to its very high output pressure, high control precision, fast response and wide output range.

Nanofluidic behavior at the interface of sectionalized hydrophobic/hydrophilic patterns in nanochannel

Abstract: We report a novel method to fabricate sectionalized hydrophobic/hydrophilic nanochannel. Photolithography, dry/wet etching, aligned anodic bonding and selective modification were utilized to fabric...