Effect of field direction on electrowetting in a nanopore

Electrostatic-Charge- and Electric-Field-Induced Smart Gating for Water Transportation

Regulating and controlling the transport of water across nanochannels is of great importance in both fundamental research and practical applications because it is difficult to externally control water flow through nanochannels as in biological channels. To date, only a few hydrophobic nanochannels controlling the transport of water have been reported, all of which use exotic hydrophobic molecules. However, the effect of electrostatic charges, which plays an indispensable role in membrane proteins and dominates the energetics of water permeation across aquaporins, has not gained enough attention to control water transport through a solid-state nanochannel/nanopore. Here, we report electrostatic-charge-induced water gating of a single ion track-etched sub-10 nm channel. This system can directly realize the gating transition between an open, conductive state and a closed, nonconductive state by regulating the surface charge density through a process that involves alternating capillary evaporation and capillary condensation. Compared to the introduction of exotic hydrophobic molecules, water gating controlled by electrostatic charges is simple, convenient, and effective. Such a system anticipates potential applications including desalination, controllable valves, and drug delivery systems.

Dynamic Response in Nanoelectrowetting on a Dielectric

Droplet spreading at an applied voltage underlies the function of tunable optical devices including adjustable lenses and matrix display elements. Faster response and the enhanced resolution motivate research toward miniaturization of these devices to nanoscale dimensions. The response of an aqueous nanodroplet to an applied field can differ significantly from macroscopic predictions. Understanding these differences requires characterization at the molecular level. We describe the equilibrium and nonequilibrium molecular dynamics simulations of nanosized aqueous droplets on a hydrophobic surface with the embedded concentric electrodes. Constant electrode potential is enforced by a rigorous account of the metal polarization. We demonstrate that the reduction of the equilibrium contact angle is commensurate to, and adjusts reversibly with, the voltage change. For a droplet with O(10) nm diameter, a typical response time to the imposition of the field is of O(10(2)) ps. Drop relaxation is about twice as fast when the field is switched off. The friction coefficient obtained from the rate of the drop relaxation on the nonuniform surface, decreases when the droplet approaches equilibrium from either direction, that is, by spreading or receding. The strong dependence of the friction on the surface hydrophilicity points to the dominance of the liquid-surface friction at the drop's perimeter as described in the molecular kinetic theory. This approach enables correct predictions of trends in dynamic responses associated with varied voltage or substrate material.

Biomimetic solution against dewetting in a highly hydrophobic nanopore

A water molecule is the foundation of life and is the primary compound in every living system. While many of its properties are understood in a bulk solvent, its behavior in a small hydrophobic nanopore still raises fundamental questions. For instance, a wetting/dewetting transition in a hydrophobic solid-state or a polymer nanopore occurs stochastically and can only be prevented by external physical stimuli. Controlling these transitions would be a primary requirement to improve many applications. Some biological channels, such as gramicidin A (gA) proteins, show a high rate of water and ion diffusion in their central subnanochannel while their external surface is highly hydrophobic. The diameter of this channel is significantly smaller than the inner size of the lowest artificial nanopore in which water drying occurs (i.e. 1.4 nm). In this paper, we propose an innovative idea to generate nanopore wetting as a result of which the application of an external field is no longer required. In a nanopore, the drying or wetting of the inner walls occurs randomly (in experiments and in simulations). However, we have shown how the confinement of gA, in a dried hydrophobic nanopore, rapidly generates a stable wetting of the latter. We believe that this simple idea, based on biomimetism, could represent a real breakthrough that could help to improve and develop new nanoscale applications.

Wetting behaviour of a translating sessile nanodrop under electrostatic actuation

Translation of nano droplet by switching successive electrode in an array.

Water–methanol separation with carbon nanotubes and electric fields

Methanol is used in various applications, such as fuel for transportation vehicles, fuel cells, and in chemical industrial processes. Conventionally, separation of methanol from aqueous solution is by distillation. However, this method consumes a large amount of energy; hence development of a new method is needed. In this work, molecular dynamics simulations are performed to investigate the effect of an electric field on water–methanol separation by carbon nanotubes (CNTs) with diameters of 0.81 to 4.07 nm. Without an electric field, methanol molecules fill the CNTs in preference to water molecules. The preference of methanol to occupy the CNTs over water results in a separation effect. This separation effect is strong for small CNT diameters and significantly decreases with increasing diameter. In contrast, under an electric field, water molecules strongly prefer to occupy the CNTs over methanol molecules, resulting in a separation effect for water. More interestingly, the separation effect for water does not decrease with increasing CNT diameter. Formation of water structures in CNTs induced by an electric field has an important role in the separation of water from methanol.

Quantifying Density Fluctuations in Water at a Hydrophobic Surface: Evidence for Critical Drying

Employing smart Monte Carlo sampling techniques within the grand canonical ensemble, we investigate the properties of water at a model hydrophobic substrate. By reducing the strength of substrate-water attraction, we find that fluctuations in the local number density, quantified by a rigorous definition of the local compressibility χ(z), increase rapidly for distances z within one or two molecular diameters from the substrate as the degree of hydrophobicity, measured by the macroscopic contact angle θ, increases. Our simulations provide evidence for a continuous (critical) drying transition as the substrate-water interaction becomes very weak: cos(θ)→-1. We speculate that the existence of such a transition might account for earlier simulation observations of strongly enhanced density fluctuations.

The local compressibility of liquids near non-adsorbing substrates: a useful measure of solvophobicity and hydrophobicity?

We investigate the suitability of the local compressibility χ(z) as a measure of the solvophobicity or hydrophobicity of a substrate. Defining the local compressibility as the derivative of the local one-body density ρ(z) w.r.t. the chemical potential μ at fixed temperature T, we use density functional theory (DFT) to calculate χ(z) for a model fluid, close to bulk liquid-gas coexistence, at various planar substrates. These range from a 'neutral' substrate with a contact angle of θ≈90°, which favours neither the liquid nor the gas phase, to a very solvophobic, purely repulsive substrate which exhibits complete drying, i.e. θ = 180°. We find that the maximum in the local compressibility χ(z), which occurs within one-two molecular diameters of the substrate, and the integrated quantity χ(ex) (the surface excess compressibility, defined below) both increase rapidly as θ increases and the substrate becomes more solvophobic. χ(z) provides a more pronounced indicator of solvophobicity than the density depletion in the vicinity of the surface which increases only weakly with increasing θ. For the limiting case of drying, θ = 180°, we find lnχ(l) ∼ l, where l is the thickness of the intruding film of gas which diverges in the approach to bulk coexistence μ → μ(co). When the fluid is confined in a parallel slit with two identical solvophobic walls, or with competing solvophobic and solvophilic walls, χ(z) close to the solvophobic wall is altered little from that at the single substrate. We connect our results with simulation studies of water near to hydrophobic surfaces exploring the relationship between χ(z) and fluctuations in the local density and between χ(ex) and the mean-square fluctuation in the number of adsorbed molecules.

Presence of electrolyte promotes wetting and hydrophobic gating in nanopores with residual surface charges

Single nanopores containing hydrophobic and hydrophilic islands on the pore walls exhibit salt concentration modulated hydrophobic gating, with more concentrated solutions promoting wetting.

Electrostatic field-exposed water in nanotube at constant axial pressure

Water confined within nanoscale geometries under external field has many interesting properties which is very important for its application in biological processes and engineering. Using molecular dynamics simulations, we investigate the effect of external fields on polarization and structure as well as phase transformations of water confined within carbon nanotubes. We find that dipoles of water molecules tend to align along external field in nanoscale cylindrical confinement. Such alignment directly leads to the longitudinal electrostriction and cross-sectional dilation of water in nanotube. It also influences the stability of ice structures. As the electrostatic field strengthens, the confined water undergoes phase transitions from a prism structure to a helical one to a single chain as the electrostatic field strengthens. These results imply a rich phase diagram of the confined water due to the presence of external electriostatic field, which can be of importance for the industrial applications in nanopores.

Electrocapillarity of an electrolyte solution in a nanoslit with overlapped electric double layer: continuum approach

A nanoslit is a long narrow opening between two parallel plates that are nanometers apart from each other. When an electrolyte solution is present inside a nanoslit, an overlapped electrical double layer (EDL) is formed and there exist distributions of the osmotic pressure and the Maxwell stress across the nanoslit. It is well known that the total normal stress (osmotic pressure contribution + Maxwell stress contribution) in the direction normal to the nanoslit surface is uniform and the value is the same as the osmotic pressure at the centerline. On the other hand, it is not well known that the total normal stress in the direction parallel to the slit surface is not uniform. When there is an electrolyte-gas interface inside a nanoslit, this total normal stress in the direction parallel to the slit surface generates the electrocapillarity effect. In the present work, the electromechanical approach is adopted to estimate the electrocapillarity effect in terms of the slit surface potential (or the surface charge density), the gap size, and the bulk ion concentrations. In order to handle the problem analyically, it is assumed that the nanoslit problem is in the continuum range and the interface is initially flat. The deformation of the interface due to the nonuniform total normal stress along the interface is also obtained by using the first order perturbation method. The significance of the present work can be manifested by the fact that external voltage is frequently used in nanoscaled systems and the electrocapillarity effect should be considered in addition to the intrinsic capillarity due to surface tension.

Electrowetting -- from statics to dynamics

More than one century ago, Lippmann found that capillary forces can be effectively controlled by external electrostatic forces. As a simple example, by applying a voltage between a conducting liquid droplet and the surface it is sitting on we are able to adjust the wetting angle of the drop. Since Lippmann's findings, electrocapillary phenomena - or electrowetting - have developed into a series of tools for manipulating microdroplets on solid surfaces, or small amounts of liquids in capillaries for microfluidic applications. In this article, we briefly review some recent progress of fundamental understanding of electrowetting and address some still unsolved issues. Specifically, we focus on static and dynamic electrowetting. In static electrowetting, we discuss some basic phenomena found in DC and AC electrowetting, and some theories about the origin of contact angle saturation. In dynamic electrowetting, we introduce some studies about this rather recent area. At last, we address some other capillary phenomena governed by electrostatics and we give an outlook that might stimulate further investigations on electrowetting.

Superwetting of TiO2 by light-induced water-layer growth via delocalized surface electrons

Titania, which exhibits superwetting under light illumination, has been widely used as an ideal material for environmental solution such as self-cleaning, water-air purification, and antifogging. There have been various studies to understand such superhydrophilic conversion. The origin of superwetting has not been clarified in a unified mechanism yet, which requires direct experimental investigation of the dynamic processes of water-layer growth. We report in situ measurements of the growth rate and height of the photo-adsorbed water layers by tip-based dynamic force microscopy. For nanocrystalline anatase and rutile TiO2 we observe light-induced enhancement of the rate and height, which decrease after O2 annealing. The results lead us to confirm that the long-range attraction between water molecules and TiO2, which is mediated by delocalized electrons in the shallow traps associated with O2 vacancies, produces photo-adsorption of water on the surface. In addition, molecular dynamics simulations clearly show that such photo-adsorbed water is critical to the zero contact angle of a water droplet spreading on it. Therefore, we conclude that this "water wets water" mechanism acting on the photo-adsorbed water layers is responsible for the light-induced superwetting of TiO2. Similar mechanism may be applied for better understanding of the hydrophilic conversion of doped TiO2 or other photo-catalytic oxides.

Self-assembly of proteins into a three-dimensional multilayer system: investigation of the surface of the human fungal pathogen Aspergillus fumigatus

Hydrophobins are small surface active proteins that fulfil a wide spectrum of functions in fungal growth and development. The human fungal pathogen Aspergillus fumigatus expresses RodA hydrophobins that self-assemble on the outer conidial surface into tightly organized nanorods known as rodlets. AFM investigation of the conidial surface allows us to evidence that RodA hydrophobins self-assemble into rodlets through bilayers. Within bilayers, hydrophilic domains of hydrophobins point inward, thus making a hydrophilic core, while hydrophobic domains point outward. AFM measurements reveal that several rodlet bilayers are present on the conidial surface thus showing that proteins self-assemble into a complex three-dimensional multilayer system. The self-assembly of RodA hydrophobins into rodlets results from attractive interactions between stacked β-sheets, which conduct to a final linear cross-β spine structure. A Monte Carlo simulation shows that anisotropic interactions are the main driving forces leading the hydrophobins to self-assemble into parallel rodlets, which are further structured in nanodomains. Taken together, these findings allow us to propose a mechanism, which conducts RodA hydrophobins to a highly ordered rodlet structure. The mechanism of hydrophobin assembly into rodlets offers new prospects for the development of more efficient strategies leading to disruption of rodlet formation allowing a rapid detection of the fungus by the immune system.

Molecular dynamics simulation of the electrically induced spreading of an ionically conducting water droplet

Molecular dynamics simulations are applied to study the spreading behavior of a nanosized water droplet that contains freely moving Na(+)/Cl(-) ions subject to an imposed electric field parallel to a solid surface. Results show that the positive and negative ions move relatively freely in response to an applied electric field, whereas polar water molecules realign themselves. These localized behaviors of the ions and the polar molecules are affected by both the applied electric field strength and the ion concentration, which in turn determine the deformation and spreading of the droplet on a solid substrate. The presence of the freely moving ions causes the ion-containing droplet to spread differently from a droplet of pure water. In a weak electric field of 0.05 V/Å, a droplet of a lower ion concentration spreads asymmetrically and the spreading asymmetry is considerably smaller than that associated with a pure water droplet of the same size. In a stronger field of 0.1 V/Å, a droplet of a higher ion concentration spreads symmetrically and completely wets the solid surface whereas a less ionically conducting droplet undergoes an asymmetric-to-symmetric transition in spreading until it reaches equilibrium.

The tunable hydrophobic effect on electrically doped graphene

Using molecular dynamics simulations, we study the hydrophobic effect on electrically doped single layer graphene. With doping levels measured in volts, large changes in contact angle occur for modest voltages applied to the sheet. The effect can be understood as a renormalization of the surface tension between graphene and water in the presence of an electric field generated by the dopant charge, an entirely collective effect termed electrowetting. Because the electronic density of states scales linearly in the vicinity of the Fermi energy, the cosine of the contact angle scales quartically with the applied voltage rather than quadratically, as it would for a two-dimensional metal or in multiple layer graphene. While electrowetting explains the phenomenon, it does not account for the slight asymmetry observed in the hydrophobic response between n- and p-doping.

Vapor-liquid coexistence of the Stockmayer fluid in nonuniform external fields

We investigate the structure and phase behavior of the Stockmayer fluid in the presence of nonuniform electric fields using molecular simulation. We find that an initially homogeneous vapor phase undergoes a local phase separation in a nonuniform field due to the combined effect of the field gradient and the fluid vapor-liquid equilibrium. This results in a high-density fluid condensing in the strong field region. The system polarization exhibits a strong field dependence due to the fluid condensation.

Electrospray ionization on porous spraying tips for direct sample analysis by mass spectrometry: enhanced detection sensitivity and selectivity using hydrophobic/hydrophilic materials as spraying tips

RATIONALE

Despite various porous materials having been widely adopted as spraying tips for direct sample analysis using electrospray ionization mass spectrometry (ESI-MS), the effect of surface property and porosity of spraying tip materials on their analytical performances is not clear. Investigation of their relationships could provide insight into the proper choice and/or design of spraying tip materials for direct sample analysis.

METHODS

The effect of spraying tip materials with different polarities, including polyester and polyethylene (hydrophobic) and wood (hydrophilic), on the detection sensitivity for a variety of compounds, and on the ESI onset voltage, were studied using ESI-MS. The porosity of each type of spraying tip was characterized by scanning electron microscopy (SEM). Factors governing the detection sensitivity were determined based on the correlation of the detection sensitivity to the ESI onset voltage, the polarity, and the porosity of the spraying tip materials.

RESULTS

Hydrophobic tips (i.e., polyester and polyethylene) show better detection sensitivity for polar compounds but not for non-polar compounds, while hydrophilic tips (wooden tips) show the opposite effect. This phenomenon could be due to the difference in interaction between the analytes and the tips, causing the analytes to adsorb on the tip to different extents. In addition, the micro-porous nature of the tips could facilitate solvent diffusion for transporting analytes to the tip and maintain a stable spray for recording MS data. With the proper choice of spraying tip materials, trace amount of analytes at the picomole level can be detected with minimal sample pretreatment.

CONCLUSIONS

Both the polarity and the porosity of the spraying tip materials could significantly affect detection sensitivity for a wide variety of analytes. With proper choice of spraying tip material, ESI on a porous spraying tip could be a sensitive method for the direct analysis of daily life samples.

Ion transport through a graphene nanopore

Molecular dynamics simulation is utilized to investigate the ionic transport of NaCl in solution through a graphene nanopore under an applied electric field. Results show the formation of concentration polarization layers in the vicinity of the graphene sheet. The nonuniformity of the ion distribution gives rise to an electric pressure which drives vortical motions in the fluid if the electric field is sufficiently strong to overcome the influence of viscosity and thermal fluctuations. The relative importance of hydrodynamic transport and thermal fluctuations in determining the pore conductivity is investigated. A second important effect that is observed is the mass transport of water through the nanopore, with an average velocity proportional to the applied voltage and independent of the pore diameter. The flux arises as a consequence of the asymmetry in the ion distribution which can be attributed to differing mobilities of the sodium and chlorine ions and to the polarity of water molecules. The accumulation of liquid molecules in the vicinity of the nanopore due to re-orientation of the water dipoles by the local electric field is seen to result in a local increase in the liquid density. Results confirm that the electric conductance is proportional to the nanopore diameter for the parameter regimes that we simulated. The occurrence of fluid vortices is found to result in an increase in the effective electrical conductance.

Uncovering molecular mechanisms of electrowetting and saturation with simulations

Molecular dynamics simulations are used to explore the physical mechanisms of electrowetting and the limits of continuum theories. Nanoscale drops exhibit the same behavior seen in macroscopic experiments: The contact angle θ follows continuum theory at low voltages and then saturates. Saturation limits applications of electrowetting and its origin is of great interest. In the simulations, saturation occurs when ions are pulled from the drop by large local fields. Saturation can be controlled by changing temperature, screening, or the energy binding ions to the fluid. We show a local force balance equation for θ remains valid even after saturation and that the interface approaches the equilibrium contact angle within a few nanometers of the solid.

Electric-field-induced wetting and dewetting in single hydrophobic nanopores

The behaviour of water in nanopores is very different from that of bulk water. Close to hydrophobic surfaces, the water density has been found to be lower than in the bulk, and if confined in a sufficiently narrow hydrophobic nanopore, water can spontaneously evaporate. Molecular dynamics simulations have suggested that a nanopore can be switched between dry and wet states by applying an electric potential across the nanopore membrane. Nanopores with hydrophobic walls could therefore create a gate system for water, and also for ionic and neutral species. Here, we show that single hydrophobic nanopores can undergo reversible wetting and dewetting due to condensation and evaporation of water inside the pores. The reversible process is observed as fluctuations between conducting and non-conducting ionic states and can be regulated by a transmembrane electric potential.

Nanoscale electrowetting effects observed by using friction force microscopy

We report the study of electrowetting (EW) effects under strong electric field on poly(methyl methacrylate) (PMMA) surface by using friction force microscopy (FFM). The friction force dependence on the electric field at nanometer scale can be closely related to electrowetting process based on the fact that at this scale frictional behavior is highly affected by capillary phenomena. By measuring the frictional signal between a conductive atomic force microscopy (AFM) tip and the PMMA surface, the ideal EW region (Young-Lippmann equation) and the EW saturation were identified. The change in the interfacial contact between the tip and the PMMA surface with the electric field strength is closely associated with the transition from the ideal EW region to the EW saturation. In addition, a reduction of the friction coefficient was observed when increasing the applied electric field in the ideal EW region.

Effect of electric field on liquid infiltration into hydrophobic nanopores

Understanding the variation of nanofluidic behavior in the presence of an external electric field is critical for controlling and designing nanofluidic devices. By studying the critical infiltration pressure of liquids into hydrophobic nanopores using molecular dynamics (MD) simulations and experiments, important insights can be gained on the variation of the effective liquid-solid interfacial tension with the magnitude and sign of electric field, as well as its coupling with the pore size and the solid and liquid species. It is found that the effective hydrophobicity reduces with the increase of electric intensity and/or pore size, and the behavior is asymmetric with respect to the direction of the electric field. The underlying molecular mechanisms are revealed via the study of the density profile, contact angle, and surface tension of confined liquid molecules.

Control of unidirectional transport of single-file water molecules through carbon nanotubes in an electric field

The transport of water molecules through nanopores is not only crucial to biological activities but also useful for designing novel nanofluidic devices. Despite considerable effort and progress that has been made, a controllable and unidirectional water flow is still difficult to achieve and the underlying mechanism is far from being understood. In this paper, using molecular dynamics simulations, we systematically investigate the effects of an external electric field on the transport of single-file water molecules through a carbon nanotube (CNT). We find that the orientation of water molecules inside the CNT can be well-tuned by the electric field and is strongly coupled to the water flux. This orientation-induced water flux is energetically due to the asymmetrical water-water interaction along the CNT axis. The wavelike water density profiles are disturbed under strong field strengths. The frequency of flipping for the water dipoles will decrease as the field strength is increased, and the flipping events vanish completely for the relatively large field strengths. Most importantly, a critical field strength E(c) related to the water flux is found. The water flux is increased as E is increased for E ≤ E(c), while it is almost unchanged for E > E(c). Thus, the electric field offers a level of governing for unidirectional water flow, which may have some biological applications and provides a route for designing efficient nanopumps.

The influence of molecular-scale roughness on the surface spreading of an aqueous nanodrop

We examine the effect of nanoscale roughness on spreading and surface mobility of water nanodroplets. Using molecular dynamics, we consider model surfaces with sub-nanoscale asperities at varied surface coverage and with different distribution patterns. We test materials that are hydrophobic, and those that are hydrophilic in the absence of surface corrugations. Interestingly, on both types of surfaces, the introduction of surface asperities gives rise to a sharp increase in the apparent contact angle. The Cassie-Baxter equation is obeyed approximately on hydrophobic substrates, however, the increase in the contact angle on a hydrophilic surface differs qualitatively from the behavior on macroscopically rough surfaces described by the Wenzel equation. On the hydrophobic substrate, the superhydrophobic state with the maximal contact angle of 180 degrees is reached when the asperity coverage falls below 25%, suggesting that superhydrophobicity can also be achieved by the nanoscale roughness of a macroscopically smooth material. We further examine the effect of surface roughness on droplet mobility on the substrate. The apparent diffusion constant shows a dramatic slow down of the nanodroplet translation even for asperity coverage in the range of 1% for a hydrophilic surface, while droplets on corrugated hydrophobic surfaces retain the ability to flow around the asperities. In contrast, for smooth surfaces we find that the drop mobility on the hydrophilic surface exceeds that on the hydrophobic one.

Binary mixture adsorbed in a slit pore: Field-induced population inversion near the bulk instability

The recently proposed method for modulating through an external field the composition of a binary fluid mixture adsorbed in a slit pore is discussed. The population inversion near the bulk (demixing) instability is first analyzed in the case of a symmetric mixture of nonadditive hard spheres, without field. It is next investigated for a mixture comprising dipolar particles subject to an external field. The influence of several factors on the adsorption curves including bulk composition, pore width, field direction, polarizability versus permanent dipoles, and temperature on this field induced population inversion is shown by Monte Carlo simulation.

Charge, hydrophobicity, and confined water: putting past simulations into a simple theoretical framework

Water permeates all life, and mediates forces that are essential to the process of macromolecular self-assembly. Predicting these forces in a given biological context is challenging, since water organizes itself differently next to charged and hydrophobic surfaces, both of which are typically at play on the nanoscale in vivo. In this work, we present a simple statistical mechanical model for the forces water mediates between different confining surfaces, and demonstrate that the model qualitatively unifies a wide range of phenomena known in the simulation literature, including several cases of protein folding under confinement.

Microscopic dynamics of the orientation of a hydrated nanoparticle in an electric field

We use atomistic simulations to study the orientational dynamics of a nonpolar nanoparticle suspended in water and subject to an electric field. Because of the molecular-level effects we describe, the torque exerted on the nanoparticle exceeds continuum-electrostatics-based estimates by about a factor of 2. The reorientation time of a 16.2 x 16.2 x 3.35 A(3) nanoparticle in a field |E| > 0.015 V/A is an order of magnitude less than the field-free orientational time (approximately 1 ns). Surprisingly, the alignment speed is nearly independent of the nanoparticle size in this regime. These findings are relevant for design of novel nanostructures and sensors and development of nanoengineering methods.