Surface dependent enhancement in water vapor permeation through nanochannels.

Selective permeation of water vapor over liquid phase water through hydrophobic conduits finds broad use in separation processes, including desalination and membrane distillation. The tangential momentum accommodation coefficient (TMAC), a fundamental parameter that dictates momentum changes to a molecule colliding with a wall remains unknown for water vapor at room temperature and pressure conditions. Here, a nanofluidic platform with tunable hydrophobic regions that selectively barricaded flow of liquid water was patterned within glass nanochannels. The surface functionalization with an alkyltrichlorosilane led to either a fluoride or a methyl terminal group generating partially hydrophobic regions along the length of the nanochannels. Differential osmotic pressure solutions on either side of the hydrophobic region cause an isothermal evaporation-condensation process, which drives net water vapor transport from higher to lower vapor pressure solution, similar to osmotic distillation. Water vapor transport under such conditions for the 80 nm deep nanochannels was in the transitional regime with the Knudsen number ∼O(1). The TMAC was estimated experimentally to be of the order of 10-4-10-3 for both the hydrophobic coatings leading to a near-elastic collision of H2O molecules with the nanochannel walls. Use of the low TMAC surfaces was evaluated in two proof-of-concept technology demonstrations: (1) osmotic distillation using hyper-saline (brine) 3 M Utica shale flowback water as both the feed and draw and (2) separation of trace amounts of toluene and chloroform from water at high flux and selectivity. The results reported here likely provide new insights in designing hydrophilic-hydrophobic junctions for nanoscale liquid/vapor fluid transport with enhanced flux and selectivity.

Cation Dependent Surface Charge Regulation in Gated Nanofluidic Devices.

Surface charge governs nanoscale aqueous electrolyte transport, both in engineered analytical systems and in biological entities such as ion channels and ion pumps as a function of ion type and concentration. Embedded electrodes in a nanofluidic channel, isolated from the fluid in the channel by a dielectric layer, act as active, tunable gates to systematically modify local surface charge density at the interface between the nanochannel surface and the aqueous electrolyte solution, causing significant changes in measured nanochannel conductance. A systematic comparison of transport of monovalent electrolytes [potassium chloride (KCl), sodium chloride (NaCl)], 2:1 electrolytes [magnesium chloride (MgCl2), calcium chloride (CaCl2)], and electrolyte mixtures (KCl + CaCl2) through a gated nanofluidic device was performed. Ion-surface interactions between divalent Ca2+ and Mg2+ ions and the nanochannel walls reduced the native surface charge density by up to ∼4-5 times compared to the monovalent cations. In electrolyte mixtures, Ca2+ was the dominating cation with nanochannel conductance independent of KCl concentration. Systematic changes in local electrostatic surface state induced by the gate electrode are impacted by the divalent cation-surface interactions, limiting modulation of the local surface potential by the gate electrode and resulting in cation dependent nanoscale ion transport as seen through conductance measurements and numerical models.

Field effect nanofluidics.

Nanoscale fluid transport through conduits in the 1-100 nm range is termed as nanofluidics. Over the past decade or so, significant scientific and technological advances have occurred in the domain of nanofluidics with a transverse external electrical signal through a dielectric layer permitting control over ionic and fluid flows in these nanoscale conduits. Consequently, this special class of nanofluidic devices is commonly referred to as field effect devices, analogous to the solid-state field effect transistors that form the basis for modern electronics. In this mini-review, we focus on summarizing the recent developments in field effect nanofluidics as a discipline and evaluate both tutorially and critically the scientific and technological advances that have been reported, including a discussion on the future outlook and identifying broad open questions which suggest that there are many breakthroughs still to come in field-effect nanofluidics.

Effect of surface modification on interfacial nanobubble morphology and contact line tension.

Past research has confirmed the existence of surface nanobubbles on various hydrophobic substrates (static contact angle >90°) when imaged in air-equilibrated water. Additionally, the use of solvent exchange techniques (based on the difference in saturation levels of air in various solvents) also introduced surface nanobubbles on hydrophilic substrates (static contact angle <90°). In this work, tapping mode atomic force microscopy was used to image interfacial nanobubbles formed on bulk polycarbonate (static contact angle of 81.1°), bromo-terminated silica (BTS; static contact angle of 85.5°), and fluoro-terminated silica (FTS; static contact angle of 105.3°) surfaces when immersed in air-equilibrated water without solvent exchange. Nanobubbles formed on the above three substrates were characterized on the basis of Laplace pressure, bubble density, and contact line tension. Results reported here show that (1) the Laplace pressures of all nanobubbles formed on both BTS and polycarbonate were an order of magnitude higher than those of FTS, (2) the nanobubble number density per unit area decreased with an increase in substrate contact angle, and (3) the contact line tension of the nanobubbles was calculated to be positive for both BTS and polycarbonate (lateral radius, Rs < 50 nm for all nanobubbles), and negative for FTS (Rs > 50 nm for all nanobubbles). The nanobubble morphology and distribution before and after using the solvent exchange method (ethanol-water), on the bulk polycarbonate substrate was also characterized. Analysis for these polycarbonate surface nanobubbles showed that both the Laplace pressure and nanobubble density reduced by ≈98% after ethanol-water exchange, accompanied by a flip in the magnitude of contact line tension from positive (0.19 nN) to negative (-0.11 nN).

Unsteady separation in vortex-induced boundary layers.

This paper provides a brief review of the analytical and numerical developments related to unsteady boundary-layer separation, in particular as it relates to vortex-induced flows, leading up to our present understanding of this important feature in high-Reynolds-number, surface-bounded flows in the presence of an adverse pressure gradient. In large part, vortex-induced separation has been the catalyst for pulling together the theory, numerics and applications of unsteady separation. Particular attention is given to the role that Prof. Frank T. Smith, FRS, has played in these developments over the course of the past 35 years. The following points will be emphasized: (i) unsteady separation plays a pivotal role in a wide variety of high-Reynolds-number flows, (ii) asymptotic methods have been instrumental in elucidating the physics of both steady and unsteady separation, (iii) Frank T. Smith has served as a catalyst in the application of asymptotic methods to high-Reynolds-number flows, and (iv) there is still much work to do in articulating a complete theoretical understanding of unsteady boundary-layer separation.

Forces affecting double-stranded DNA translocation through synthetic nanopores.

One of the recent applications of nanopores is to use them as detectors/analyzers for bio-molecules and nanopore based sequencing has been studied to quickly sequence DNA. In this paper, three categories of forces proposed in the literature to oppose the electrical driving forces in the DNA translocation process are analyzed, (1) the entropic forces of DNA uncoiling/recoiling at the pore entrance/exits, (2) the viscous drag acting on the blob like DNA outside the nanopore, and (3) the viscous drag acting on the linear DNA inside the nanopore. The magnitudes of these forces are calculated based on the parameters used in experiments and it is shown that the first two of the aforementioned categories of forces are usually small compared to the electrical driving force, while the last one is of the same order as the electrical driving force. To evaluate the viscous drag force acting on the linear DNA inside the nanopore, a hydrodynamic model based on the lubrication approximation is used to calculate the flow field and the viscous drag force acting on a DNA immobilized in a nanopore. This model is validated by good agreement with the experimental data for the tethering force used to immobilize a DNA inside the nanopore.

Characterizing the surface charge of synthetic nanomembranes by the streaming potential method.

The inference of the surface charge of polyethylene glycol (PEG)-coated and uncoated silicon membranes with nanoscale pore sizes from streaming potential measurements in the presence of finite electric double layer (EDL) effects is studied theoretically and experimentally. The developed theoretical model for inferring the pore wall surface charge density from streaming potential measurements is applicable to arbitrary pore cross-sectional shapes and accounts for the effect of finite salt concentration on the ionic mobilities and the thickness of the deposited layer of PEG. Theoretical interpretation of the streaming potential data collected from silicon membranes having nanoscale pore sizes, with/without pore wall surface modification with PEG, indicates that finite electric double layer (EDL) effects in the pore-confined electrolyte significantly affect the interpretation of the membrane charge and that surface modification with PEG leads to a reduction in the pore wall surface charge density. The theoretical model is also used to study the relative significance of the following uniquely nanoscale factors affecting the interpretation of streaming potential in moderate to strongly charged pores: altered net charge convection by applied pressure differentials, surface-charge effects on ionic conduction, and electroosmotic convection of charges.

DNA nanowire translocation phenomena in nanopores.

One recent application of nanopores is to use them as detectors and analyzers for fast DNA sequencing. To better understand the DNA electrokinetic transport through a nanopore, a hydrodynamic model is developed to investigate the flow field, the resistive forces acting on the DNA, the DNA velocity and the ionic current through the nanopore. The numerical results reveal the relation between the DNA velocity and various parameters such as nanopore surface charge and solution concentration. The model is validated by comparing the numerical results with the experimental data for both DNA velocity and ionic current through the nanopore.

Biomolecular transport through hemofiltration membranes.

A theoretical model for filtration of large solutes through a pore in the presence of transmembrane pressures, applied/induced electric fields, and dissimilar interactions at the pore entrance and exit is developed to characterize and predict the experimental performance of a hemofiltration membrane with nanometer scale pores designed for a proposed implantable Renal Assist Device (RAD). The model reveals that the sieving characteristics of the membrane can be improved by applying an external electric field, and ensuring a smaller ratio of the pore-feed and pore-permeate equilibrium partitioning coefficients when diffusion is present. The model is then customized to study the sieving characteristics for both charged and uncharged solutes in the slit-shaped nanopores of the hemofiltration device for the RAD. The effect of streaming potential or induced fields are found to be negligible under representative operating conditions. Experimental data on the sieving coefficient of bovine serum albumin, carbonic anhydrase and thyroglobulin are reported and compared with the theoretical predictions. Both steric and electrostatic partitioning are considered and the comparison suggests that in general electrostatic effects are present in the filtration of proteins though some data, particularly those recorded in a strongly hypertonic solution (10x PBS), show better agreement with the steric partitioning theory.

Effect of Divalent Ions on Electroosmotic Flow in Microchannels.

The electroosmotic flow (EOF) rate in fused silica microchannels is experimentally found to decrease when trace quantities of salts containing the divalent cations Ca(2+) and Mg(2+) are added to a constant ionic strength background electrolytic solution (BGE) flowing in a channel having negatively charged walls. Moreover, the observed effect is quantitatively different for the two ions Ca(2+) and Mg(2+). Since electrostatic interactions are identical for ions of the same valence modeled as point charges, a description of the electric double layer (EDL) based on the Poisson-Boltzmann equation alone cannot account for these experimental observations. New experimental observations on electroosmotic flow in presence of Ca(2+) and Mg(2+) are reported in this work. A site binding model that accounts for the chemical interactions of the BGE ions with the silica surface is developed. The model predictions are in good agreement with the experimental observations on divalent cations as well as data from the literature on how properties such as pH and ionic strength affect electroosmotic flow rates for a BGE with monovalent cations.

Effect of nonuniform surface potential on electroosmotic flow at large applied electric field strength.

Electroosmotic flow in nanochannels with non-uniform wall potential is investigated. While several researchers have presented results for the case of periodic potential and sudden change in potential, most of the previous work in this area is based on the Debye-Hückel approximation and the validity of Boltzmann distribution for ionic species. In this paper, the nonlinear distributions of potential, velocity and mole fractions are calculated numerically based on the Poisson-Nernst-Planck model including convective effects. The distribution of potential and species concentration are shown to be different from the Boltzmann distribution at large applied electric field strength. The convective effect is also investigated and found to be negligible.

Electroosmotic flow and particle transport in micro/nano nozzles and diffusers.

Micro/nano nozzles and diffusers have been used for ionic transport, drug and gene delivery. In this paper, a mathematical model is developed to simulate the electroosmotic flow (EOF) and particle transport in micro/nano nozzles/diffusers. The electrical potential and the flow field are investigated using the lubrication and the Debye-Huckel approximations specially for nanonozzles (overlapped electric double layers) and microdiffusers (thin EDLs) for which experimental results exist. The results show that a pressure field is induced by the presence of EDLs and the magnitude of this induced pressure is proportional to the ratio of the Debye length to the channel half-height. Embedded particles are often employed to illustrate the flow field and thus measure the local fluid velocity. The direction of particle motion is found to be dependent primarily on the particle charge and the wall charge. The calculated particle velocities compare well with experimental data.

Formation of vortices near abrupt nano-channel height changes in electro-osmotic flow of aqueous solutions.

We perform computational fluid dynamics simulations of electro-osmotic flow in nano-channels and examine the effects of sudden changes in channel cross-section area (e.g., channels with a backward-facing step) which may be a simple model of one type of fabrication flaw. When the width of the electric double layers is large enough, we observe the formation of vortices or recirculation regions near the step face. On the other hand, flows with thin electric double layers are less likely to exhibit recirculation regions. We also simulate the formation of vortices and recirculation regions for flows with asymmetrical boundary conditions for charge and concentrations.

Electro-osmotic flow of a model electrolyte.

Electro-osmotic flow is studied by nonequilibrium molecular dynamics simulations in a model system chosen to elucidate various factors affecting the velocity profile and facilitate comparison with existing continuum theories. The model system consists of spherical ions and solvent, with stationary, uniformly charged walls that make a channel with a height of 20 particle diameters. We find that hydrodynamic theory adequately describes simple pressure-driven (Poiseuille) flow in this model. However, Poisson-Boltzmann theory fails to describe the ion distribution in important situations, and therefore continuum fluid dynamics based on the Poisson-Boltzmann ion distribution disagrees with simulation results in those situations. The failure of Poisson-Boltzmann theory is traced to the exclusion of ions near the channel walls resulting from reduced solvation of the ions in that region. When a corrected ion distribution is used as input for hydrodynamic theory, agreement with numerical simulations is restored. An analytic theory is presented that demonstrates that repulsion of the ions from the channel walls increases the flow rate, and attraction to the walls has the opposite effect. A recent numerical study of electro-osmotic flow is reanalyzed in the light of our findings, and the results conform well to our conclusions for the model system.

Mass transfer and flow in electrically charged micro- and nanochannels.

In this work, the fluid flow and mass transfer due to the presence of an electric field in a rectangular channel is examined. We consider a mixture of water or another neutral solvent and a salt compound, such as sodium chloride, for which the ionic species are entirely dissociated. Results are produced for the case in which the channel height is much greater than the electric double layer (EDL) (microchannel) and for the case in which the channel height is of the order of the width of the EDL (nanochannel). Both symmetric and nonsymmetric velocity, potential, and mole fraction distributions are considered, unlike previous work on this problem. At small electrolyte concentrations, the Debeye-Huckel picture of the electric double layer is recovered; at larger concentrations, the Gouy-Chapman picture of the electric double emerges naturally. The numerical results presented here agree with analytical solutions of a singular perturbation analysis, which is valid as the channel height increases. In the symmetric case for the electroosmotic flow so induced, the velocity field and the potential are similar. In the asymmetric case corresponding to different wall potentials, the velocity and potential can be vastly different. The fluid is assumed to behave as a continuum, and the volume flow rate is observed to vary linearly with channel height for electrically driven flow, in contrast to pressure-driven flow, which varies as height cubed. This means that very large pressure drops are required to drive flows in small channels. However, useful volume flow rates may be obtained at a very low driving voltage.