Formation, manipulation, and elasticity measurement of a nanometric column of water molecules

Direct 3D-printed CdSe quantum dots via scanning micropipette

The micropipette, pencil-shaped with an aperture diameter of a few micrometers, is a potentially promising tool for the three-dimensional (3D) printing of individual microstructures based on its capability to deliver low volumes of nanomaterial solution on a desired spot resulting in micro/nanoscale patterning. Here, we demonstrate a direct 3D printing technique in which a micropipette with a cadmium selenide (CdSe) quantum dot (QD) solution is guided by an atomic force microscope with no electric field and no piezo-pumping schemes. We define the printed CdSe QD wires, which are a composite material with a QD-liquid coexistence phase, by using photoluminescence and Raman spectroscopy to analyze their intrinsic properties and additionally demonstrate a means of directional falling.

Interfacial Liquid Water on Graphite, Graphene, and 2D Materials

The optical, electronic, and mechanical properties of graphite, few-layer, and two-dimensional (2D) materials have prompted a considerable number of applications. Biosensing, energy storage, and water desalination illustrate applications that require a molecular-scale understanding of the interfacial water structure on 2D materials. This review introduces the most recent experimental and theoretical advances on the structure of interfacial liquid water on graphite-like and 2D materials surfaces. On pristine conditions, atomic-scale resolution experiments revealed the existence of 1-3 hydration layers. Those layers were separated by ∼0.3 nm. The experimental data were supported by molecular dynamics simulations. However, under standard working conditions, atomic-scale resolution experiments revealed the presence of 2-3 hydrocarbon layers. Those layers were separated by ∼0.5 nm. Linear alkanes were the dominant molecular specie within the hydrocarbon layers. Paradoxically, the interface of an aged 2D material surface immersed in water does not have water molecules on its vicinity. Free-energy considerations favored the replacement of water by alkanes.

Giant fluidic impedance of nanometer-sized water bridges: Shear capillary force at the nanoscale

We analytically show that the interfacial fluid's molecular dynamics of capillary bridges induces both elastic and dissipative forces to the shearing plane. Surprisingly, the nanometer-sized, liquid-solid contact line of the bridges exerts a giant "shear" force on the solid surface, which is 10^{5} higher than the usual viscous interaction and comparable to that of solid-solid direct-contact friction. These results are consistent with previously reported experimental data and may provide clues to longstanding questions on the apparent viscosity of the nanoconfined fluids.

The limits of near field immersion microwave microscopy evaluated by imaging bilayer graphene moiré patterns

Near field scanning Microwave Impedance Microscopy can resolve structures as small as 1 nm using radiation with wavelengths of 0.1 m. Combining liquid immersion microscopy concepts with exquisite force control exerted on nanoscale water menisci, concentration of electromagnetic fields in nanometer-size regions was achieved. As a test material we use twisted bilayer graphene, because it provides a sample where the modulation of the moiré superstructure pattern can be systematically tuned from Ångstroms up to tens of nanometers. Here we demonstrate that a probe-to-pattern resolution of 10⁸ can be obtained by analyzing and adjusting the tip-sample distance influence on the dynamics of water meniscus formation and stability.

Here, the authors image twisted bilayer graphene using scanning microwave imaging microscopy, revealing structures with sizes down to 1 nm. They show that is possible by using spontaneously forming nanoscale water menisci that concentrates the microwave fields in small regions.

Direct measurement of curvature-dependent surface tension of an alcohol nanomeniscus

Surface tension is a key parameter for understanding nucleation in the very initial stage of phase transformation. Although surface tension has been predicted to vary with the curvature of the liquid-vapor interface, particularly at the large curvature of, e.g., the subnanometric critical nucleus, experimental study still remains challenging due to inaccessibility to such a small cluster. Here, by directly measuring the critical size of a single capillary-condensed nanomeniscus using atomic force microscopy, we address the curvature dependence of surface tension of alcohols and observe that the surface tension is doubled for ethanol and n-propanol with a radius-of-curvature of ∼-0.46 nm. We also find that the interface of larger negative (positive) curvature exhibits larger (smaller) surface tension, which evidently governs nucleation at the ∼1 nm scale and below, indicating more facilitated nucleation than normally expected. Such well characterized curvature effects contribute to better understanding and accurate analysis of nucleation occurring in various fields including materials science and atmospheric science.

Comparison of the Physicochemical Properties of Carboxylic and Phosphonic Acid Self-Assembled Monolayers Created on a Ti-6Al-4V Substrate

This study compared the tribological properties in nano- and millinewton load ranges of Ti‑6Al-4V surfaces that were modified using self-assembled monolayers (SAMs) of carboxylic and phosphonic acids. The effectiveness of the creation of SAMs with the use of the liquid phase deposition (LPD) technique was monitored by the contact angle measurement, the surface free energy (SFE) calculation, X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared spectroscopy (FTIR) measurements. The obtained results indicated that more stable and well-ordered layers, which were characterized by the lowest values of the coefficient of friction, adhesion, and wear rate, were obtained using phosphonic acid as a surface modifier. Based on the obtained results, it was found that the Ti-6Al-4V alloy modified by phosphonic acid would be the most advantageous for practical applications, especially in micro- and nanoelectromechanical systems (MEMS/NEMS).

Formation of a Stable Bridge between Two Disjoint Nanotubes with Single-File Chains of Water

It was recently demonstrated that stable water bridges can form between two relatively large disjoint nanochannels, such as carbon nanotubes (CNTs), under an applied pressure drop. Such bridges are relevant to fabrication of nanostructured materials, drug delivery, water desalination devices, hydrogen fuel cells, dip-pen nanolithography, and several other applications. If the nanotubes are small enough, however, then one has only single-file hydrogen-bonded chains of water molecules. The distribution of water in such nanotubes manifests unusual physical properties that are attributed to the low number of hydrogen bonds (HBs) formed in the channel since, on average, each water molecule in a single-file chain forms only 1.7 HBs, almost half of the value for bulk water. Using extensive molecular dynamics simulations, we demonstrate that stable bridges can form even between two small disjoint CNTs that contain single-file chains of water. The structure, stability, and properties of such bridges and their dependence on the applied pressure drop and the length of the gap between the two CNTs are studied in detail, as is the distribution of the HBs. We demonstrate, in particular, that the efficiency of flow through the bridge is at maximum at a specific pressure difference.

Water distillation modeling by disjoint CNT-based channels under the influence of external electric fields

Using molecular dynamics method, the ion rejection and water flow inside flexible disjoint carbon-based channels were examined in the presence of electric fields. The effects of the carbon nanotube diameters and field magnitude on the nano-channel efficiency were investigated. It was observed that water flow through the filter was modified by increasing the radius of nanotubes, while the salt rejection was reduced. The particles' behaviors inside the channel were described in view of Van der Waals interactions between the water molecules, ions, and carbon atoms. Furthermore, the results indicated that the ion rejection and water flow were increased under the application of proper magnitude of electric fields. Graphical abstract Using MD simulation method, a disjoint CNT-based filter was designed to produce freshwater from a NaCl solution by the aid of external electric field. It was observed that the filter operation was significantly affected by channel structural parameters and amount of applied electric fields.

Efficient Transport Between Disjoint Nanochannels by a Water Bridge

Water channels are important to new purification systems, osmotic power harvesting in salinity gradients, hydroelectric voltage conversion, signal transmission, drug delivery, and many other applications. To be effective, water channels must have structures more complex than a single tube. One way of building such structures is through a water bridge between two disjoint channels that are not physically connected. We report on the results of extensive molecular dynamics simulation of water transport through such bridges between two carbon nanotubes separated by a nanogap. We show that not only can pressurized water be transported across a stable bridge, but also that (i) for a range of the gap's width l_{g} the bridge's hydraulic conductance G_{b} does not depend on l_{g}, (ii) the overall shape of the bridge is not cylindrical, and (iii) the dependence of G_{b} on the angle between the axes of two nonaligned nanochannels may be used to tune the flow rate between the two.

Nanopipette/Nanorod-Combined Quartz Tuning Fork⁻Atomic Force Microscope

We introduce a nanopipette/quartz tuning fork (QTF)–atomic force microscope (AFM) for nanolithography and a nanorod/QTF–AFM for nanoscratching with in situ detection of shear dynamics during performance. Capillary-condensed nanoscale water meniscus-mediated and electric field-assisted small-volume liquid ejection and nanolithography in ambient conditions are performed at a low bias voltage (~10 V) via a nanopipette/QTF–AFM. We produce and analyze Au nanoparticle-aggregated nanowire by using nanomeniscus-based particle stacking via a nanopipette/QTF–AFM. In addition, we perform a nanoscratching technique using in situ detection of the mechanical interactions of shear dynamics via a nanorod/QTF–AFM with force sensor capability and high sensitivity.

Contact Dependence and Velocity Crossover in Friction between Microscopic Solid/Solid Contacts

Friction at the nanoscale differs markedly from that between surfaces of macroscopic extent. Characteristically, the velocity dependence of friction between apparent solid/solid contacts can strongly deviate from the classically assumed velocity independence. Here, we show that a nondestructive friction between solid tips with radius on the scale of hundreds of nanometers and solid hydrophobic self-assembled monolayers has a strong velocity dependence. Specifically, using laterally oscillating quartz tuning forks, we observe a linear scaling in the velocity at the lowest accessed velocities, typically hundreds of micrometers per second, crossing over into a logarithmic velocity dependence. This crossover is consistent with a general multicontact friction model that includes thermally activated breaking of the contacts at subnanometric elongation. We find as well a strong dependence of the friction on the dimensions of the frictional probe.

Contact Forces between Single Metal Oxide Nanoparticles in Gas-Phase Applications and Processes

In this work we present a comprehensive experimental study to determine the contact forces between individual metal oxide nanoparticles in the gas-phase using atomic force microscopy. In addition, we determined the amount of physisorbed water for each type of particle surface. By comparing our results with mathematical models of the interaction forces, we could demonstrate that classical continuum models of van der Waals and capillary forces alone cannot sufficiently describe the experimental findings. Rather, the discrete nature of the molecules has to be considered, which leads to ordering at the interface and the occurrence of solvation forces. We demonstrate that inclusion of solvation forces in the model leads to quantitative agreement with experimental data and that tuning of the molecular order by addition of isopropanol vapor allows us to control the interaction forces between the nanoparticles.

AFM investigation of effect of absorbed water layer structure on growth mechanism of octadecyltrichlorosilane self-assembled monolayer on oxidized silicon

The growth mechanism of an octadecyltrichlorosilane (OTS) self-assembled monolayer on a silicon oxide surface at various relative humidities has been investigated. Atomic force microscopy images show that excess water may actually hinder the nucleation and growth of OTS islands. A moderate amount of water is favorable for the nucleation and growth of OTS islands in the initial stage; however, the completion of the monolayer is very slow in the final stage. The growth of OTS islands on a low-water-content surface maintains a relatively constant speed and requires the least amount of time. The mobility of water molecules is thought to play an important role in the OTS monolayers, and a low-mobility water layer provides a steady condition for OTS monolayer growth.

Young's modulus of nanoconfined liquids?

In material science, bioengineering, and biology, thin liquid films and soft matter membranes play an important role in micro-lubrication, ion transport, and fundamental biological processes. Various attempts have been made to characterize the elastic properties, such as Young's modulus, of such films using Hertz theory by incorporating convoluted mathematical corrections. We propose a simple way to extract tip-size independent elastic properties based on stiffness and force measurement through a spherical tip on a flat surface. Using our model, the Young's modulus of nanoconfined, molecularly-thin, layers of a model liquid TEHOS (tetrakis 2-ethylhexoxy silane) and water were determined using a small-amplitude AFM. This AFM can simultaneously measure the stiffness and forces of nanoscale films. While the stiffness scales linearly with the tip radius, the measured Young's modulus essentially remains constant over an order of magnitude variation in the tip radius. The values obtained for the elastic modulus of TEHOS and water films on the basis of our method are significantly lower than the confining surfaces' elastic moduli, in contrast with the uncorrected Hertz model, suggesting that our method can serve as a simple way to compare elastic properties of nanoscale thin films as well as to characterize a variety of soft films. In addition, our results show that the elastic properties (elastic modulus) of nanoconfined liquid films remain fairly independent of increasing confinement.

Force exerted by a nanoscale capillary water bridge between two planar substrates

Molecular dynamics simulation of a nanoscale capillary water bridge between two planar substrates is used to determine the resulting force between the substrates without arbitrariness regarding geometry and location of the free surface of the bridge. The substrates are moderately hydrophilic. The force changes continuously as the separation between the substrates changes except for small gaps where it becomes discontinuous because the bridge is unable to adopt stable configurations at any distance apart. Further exploration of the bridge and the force as the substrates approach each other reveals an underlying oscillatory force with an increasing repulsive component at separation distances equivalent to few water molecules. According to the average number of hydrogen bonds per water molecule (HBN), at very small gap sizes, water molecules which are very close to the surfaces are unable to maximize HBN thus contributing to the repulsive force. Our simulation results of force versus gap size agree with calculations based on other methods, some very different, and also reproduce the typical magnitude of the experimental force. Finally, a macroscopic force balance correctly describes the force-distance curve except for bridges constituted of water layers only.

3D flexible water channel: stretchability of nanoscale water bridge

Artificial water channels can contribute to a better understanding of natural water channels and offer a highly selective, advanced conductance system. Most studies use nanotubes, however it is difficult to fabricate a flexible structure, and the nanosized diameter brings nanoconfinement effects, and nanotube toxicity arouses biosafety concerns. In this paper, we use an electric field to restrain the water molecules to form a nanoscale water bridge as an artificial water channel to connect a separated solid plate by molecular dynamics simulations. We observe strong 3D flexible stretchability in the water bridge, maintaining a variable length and an arbitrary angle for a considerably long time. The stretching of the water bridge enables it to be polarized at an arbitrary angle and the stretchability is linearly dependent upon the polarization strength. More interestingly, we show the possibility of establishing complex water networks, e.g., triangle, rectangle, hexagon, and tetrahedron-tetrahedron water networks. Our results may help realize structurally flexible and environmentally friendly water channels for lab-on-a-chip applications in nanofluidics.

Noncontact friction via capillary shear interaction at nanoscale

Friction in an ambient condition involves highly nonlinear interactions of capillary force, induced by the capillary-condensed water nanobridges between contact or noncontact asperities of two sliding surfaces. Since the real contact area of sliding solids is much smaller than the apparent contact area, the nanobridges formed on the distant asperities can contribute significantly to the overall friction. Therefore, it is essential to understand how the water nanobridges mediate the ‘noncontact' friction, which helps narrow the gap between our knowledge of friction on the microscopic and macroscopic scales. Here we show, by using noncontact dynamic force spectroscopy, the single capillary bridge generates noncontact friction via its shear interaction. The pinning–depinning dynamics of the nanobridge's contact line produces nonviscous damping, which occurs even without normal load and dominates the capillary-induced hydrodynamic damping. The novel nanofriction mechanism may provide a deeper microscopic view of macroscopic friction in air where numerous asperities exist.

The contribution from water bridges at nanoscale between rough surfaces is important for macroscopic friction under ambient conditions. Here, Lee et al. show that water nanobridge produce noncontact friction originated from the pinning–depinning dynamics of the contact line at the interface.

Fabrication and Characterization of Au Nanoparticle-aggregated Nanowires by Using Nanomeniscus-induced Colloidal Stacking Method

We fabricate and characterize Au nanoparticle-aggregated nanowires by using the nano meniscus-induced colloidal stacking method. The Au nanoparticle solution ejects with guidance of nanopipette/quartz tuning fork-based atomic force microscope in ambient conditions, and the stacking particles form Au nanoparticle-aggregated nanowire while the nozzle retracts from the surface. Their mechanical properties with relatively low elastic modulus are in situ investigated by using the same apparatus.

Dynamic and static measurement of interfacial capillary forces by a hybrid nanomechanical system

The forces resulting from the presence of interfacial liquids have mechanical importance under ambient conditions. For holistic understanding of the liquid-mediated interactions, we combine the force-gradient sensitivity of an atomic force microscope (AFM) with the force measuring capability of a micro-electromechanical force sensor. Simultaneous measurement of the viscoelasticity of the water nanomeniscus and the absolute capillary force shows excellent agreement in its entire length, which justifies the validity of the widely used AFM results. We apply the hybrid system to measure the stress and strain, whose hysteretic response provides the intrinsic quantities of the liquid nanocluster.

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.

Effective stiffness of qPlus sensor and quartz tuning fork

Quartz tuning forks (QTFs) have been extensively employed in scanning probe microscopy. For quantitative measurement of the interaction in nanoscale using QTF as a force sensor, we first measured the effective stiffness of qPlus sensors as well as QTFs and then compared the results with the cantilever beam theory that has been widely used to estimate the stiffness. Comparing with the stiffness and the resonance frequency in our measurement, we found that those calculated based on the beam theory are considerably overestimated. For consistent analysis of experimental and theoretical results, we present the formula to calculate the stiffness of qPlus sensor or QTF, based on the resonance frequency. We also demonstrated that the effective stiffness of QTF is twice that of qPlus sensor, which agrees with the recently suggested model. Our study demonstrates the use of QTF for quantitative measurement of interaction force at the nanoscale in scanning probe microscopy.

Nanopipette combined with quartz tuning fork-atomic force microscope for force spectroscopy/microscopy and liquid delivery-based nanofabrication

This paper introduces a nanopipette combined with a quartz tuning fork-atomic force microscope system (nanopipette/QTF-AFM), and describes experimental and theoretical investigations of the nanoscale materials used. The system offers several advantages over conventional cantilever-based AFM and QTF-AFM systems, including simple control of the quality factor based on the contact position of the QTF, easy variation of the effective tip diameter, electrical detection, on-demand delivery and patterning of various solutions, and in situ surface characterization after patterning. This tool enables nanoscale liquid delivery and nanofabrication processes without damaging the apex of the tip in various environments, and also offers force spectroscopy and microscopy capabilities.

Observation of Universal Solidification in the Elongated Water Nanomeniscus

The ubiquitous capillary water bridge in nature plays an important role in interfacial phenomena under ambient conditions such as adhesion and friction. We present experimental measurements of the mechanical properties of the nanometric water column by using noncontact atomic force microscopy. We observe the universal behaviors that the relaxation time (RT) associated with the meniscus increases with its elongation and ruptures at the same value of RT, independent of the meniscus volume. In particular, the enhancement of RT between formation and rupture of the meniscus is indicative of the increased solid-like response, similar to that observed in nanoconfined water layers. Our results that the longer water column is more solid-like and less stable suggest (i) water at the vapor/liquid interface is more solid-like than that inside the meniscus and (ii) the associated smaller mobility of the interfacial water molecules is responsible for the structural stability of the water meniscus.

Direct observation of self-assembled chain-like water structures in a nanoscopic water meniscus

Sawtooth-like oscillatory forces generated by water molecules confined between two oxidized silicon surfaces were observed using a cantilever-based optical interfacial force microscope when the two surfaces approached each other in ambient environments. The humidity-dependent oscillatory amplitude and periodicity were 3-12 nN and 3-4 water diameters, respectively. Half of each period was matched with a freely jointed chain model, possibly suggesting that the confined water behaved like a bundle of water chains. The analysis also indicated that water molecules self-assembled to form chain-like structures in a nanoscopic meniscus between two hydrophilic surfaces in air. From the friction force data measured simultaneously, the viscosity of the chain-like water was estimated to be between 10(8) and 10(10) times greater than that of bulk water. The suggested chain-like structure resolves many unexplained properties of confined water at the nanometer scale, thus dramatically improving the understanding of a variety of water systems in nature.

Low-volume liquid delivery and nanolithography using a nanopipette combined with a quartz tuning fork-atomic force microscope

Electric-field-induced low-volume liquid ejection under ambient conditions was realized at a low bias potential of 12 V via a nanopipette (aperture diameter of 30 nm) combined with a non-contact, distance-regulated (within 10 nm) quartz tuning fork-atomic force microscope. A capillary-condensed water meniscus, spontaneously formed in the tip-substrate nanogap, reduces the ejection barrier by four orders of magnitude, facilitating nanoliquid ejection and subsequent liquid transport/dispersion onto the substrate without contact damage from the pipette. A study of nanofluidics through a free-standing liquid nanochannel and nanolithography was performed with this technique. This is an important breakthrough for various applications in controlled nanomaterial-delivery and selective deposition, such as multicolor nanopatterning and nano-inkjet devices.

Investigation of Nanoscale Interactions by Means of Subharmonic Excitation

Multifrequency atomic force microscopy holds promise as a method to provide qualitative and quantitative information about samples with high spatial resolution. Here, we provide experimental evidence of the excitation of subharmonics in ambient conditions in the regions where capillary interactions are predicted to be the mechanism of excitation. We also experimentally decouple a second mechanism for subharmonic excitation that is highly independent of environmental conditions such as relative humidity. This implies that material properties could be mapped. Subharmonic excitation could lead to experimental determination of surface water affinity in the nanoscale whenever water interactions are the mechanism of excitation.

Measuring the true height of water films on surfaces

Measuring the level of hydrophilicity of heterogeneous surfaces and the true height of water layers that form on them in hydrated conditions has a myriad of applications in a wide range of scientific and technological fields. Here, we describe a true non-contact mode of operation of atomic force microscopy in ambient conditions and a method to establish the source of apparent height. A dependency of the measured water height on operational parameters is identified with water perturbations due to uncontrolled modes of imaging where intermittent contact with the water layer, or even the surface, might occur. In this paper we show how to (1) determine when the water is being perturbed and (2) distinguish between four different interaction regimes. Each of the four types of interaction produces measurements ranging from fractions of the true height in one extreme to values which are as large as four times the real height in the other. We show the dependence of apparent height on the interaction regime both theoretically and empirically. The agreement between theory and experiment on a BaF2(111) sample displaying wet and un-wet regions validates our results.

Use of scanning probe microscopy to study the evolution of nanometer sized liquid structures

The evolution of the profile of nanometer sized water drops on a mica surface has been studied through hydration scanning probe microscopy. A time range from a few seconds down to a fraction of millisecond after the formation of the drop has been explored. This high time resolution has been obtained by sampling a series of statistically equivalent drops. This approach also avoids any probe interference during the drop evolution process.

Simultaneous measurement of normal and friction forces using a cantilever-based optical interfacial force microscope

We measured normal and friction forces simultaneously using a recently developed cantilever-based optical interfacial force microscope technique for studies of interfacial structures and mechanical properties of nanoscale materials. We derived how the forces can be incorporated into the detection signal using the classical Euler equation for beams. A lateral modulation with the amplitude of nanometers was applied to create the friction forces between tip and sample. We demonstrated its capability by measuring normal and friction forces of interfacial water at the molecular scale over all distance ranges.

Rupture kinetics of liquid bridges during a pulling process: a kinetic density functional theory study

Capillary bridge is a common phenomenon in nature and can significantly contribute to the adhesion of biological and artificial micro- and nanoscale objects. Especially, it plays a crucial role in the operation of atomic force microscopy (AFM) and influences in the measured force. In the present work, we study the rupture kinetics and transition pathways of liquid bridges connecting an AFM tip and a flat substrate during a process of pulling the tip off. Depending on thermodynamic conditions and the tip velocity, two regimes corresponding to different transition pathways are identified. In the single-bridge regime, the initial equilibrium bridge persists as a single one during the pulling process until the liquid bridge breaks. While, in the multibridge regime the stretched liquid bridge transforms into an intermediate state with a collection of slender liquid bridges, which then break gradually during the pulling process. Moreover, the critical rupture distance at which the bridges break changes with the tip velocity and thermodynamic conditions, and its maximum value occurs near the boundary between the single-bridge regime and the multibridge regime, where the longest range capillary force is produced. In this work, the effects of tip velocity, tip size, tip-fluid interaction, and humidity on rupture kinetics and transition pathways are also systematically studied.

Universal pattern for the distribution of relaxation times in the isotropic phase of liquid crystalline n-cyanobiphenyls

A universal pattern emerging from the analysis of the distribution of relaxation times in the isotropic phase of liquid crystalline n-alkylcyanobiphenyls (nCB) from 4CB to 14CB is presented. The increase of the length of nCB molecules causes the high-frequency (short-time) branch of the primary relaxation loss curve to approach the form epsilon(f>fpeak) approximately omega(-n), with n-->1/2 in frequency (omega=2pif) or approximately sqrt[t] in time on cooling toward the isotropic-mesophase "clearing" phase-transition temperature (TC). Recently, such behavior was suggested as a hypothetical universal pattern for diverse glass forming organic liquids on approaching the glass temperature [A. I. Nielsen, T. Christensen, B. Jakobsen, K. Niss, N. B. Olsen, R. Richert, and J. C. Dyre, J. Chem. Phys. 130, 154508 (2009)]. The isotropic phase of rodlike liquid crystalline compounds is considered to be an important experimental model system for studying glassy dynamics since it constitutes a link with the model fluid of hard ellipsoids of revolution.

Molecular mechanism of water bridge buildup: field-induced formation of nanoscale menisci

We perform molecular dynamics calculations to describe, at the molecular level, the formation of a water bridge induced by an electric field. Restriction of orientational degrees of freedom (confinement) of water dipoles at the interfaces leads to a polarizability that depends on the shape of the water system, that is, droplet versus pillar. Above a threshold field of 1.2 V nm(-1), the competition between orientational confinement and electric field leads to the sudden formation of a water pillar. The formation of a water bridge is marked by a first order discontinuity in the total energy of the system. The simulations offer a molecular explanation for the threshold voltage and hysteresis behavior observed in the formation of nanoscale liquid bridges with a force microscope.

Origin of adhesion in humid air

The origin of adhesion in humid air is investigated by pull-off force measurements between nanoscale contacts using atomic force microscopes in controlled environments from ultrahigh vacuum through various humidity conditions to water. An equivalent work of adhesion (WOA) model with a simplified interface stress distribution is developed, combining the effects of screened van der Waals and meniscus forces, which describes adhesion in humid air and which self-consistently treats the contact stress and deformation. Although the pull-off force is found to vary significantly with humidity, the equivalent WOA is found to be invariant. Increasing humidity alters the nature of the surface adhesion from a compliant contact with a localized, intense meniscus force to a stiff contact with an extended, weak meniscus force.

Microscopic origin of the humidity dependence of the adhesion force in atomic force microscopy

Water condenses between an atomic force microscope (AFM) tip and a surface to form a nanoscale bridge that produces a significant adhesion force on the tip. As humidity increases, the water bridge always becomes wider but the adhesion force sometimes decreases. The authors show that the humidity dependence of the adhesion force is intimately related to the structural properties of the underlying water bridge. A wide bridge whose width does not vary much with tip-surface distance can increase its volume as distance is increased. In this case, the adhesion force decreases as humidity rises. Narrow bridges whose width decreases rapidly with increasing tip-surface distance give the opposite result. This connection between humidity dependence of the adhesion force and the structural susceptibility of the water bridge is illustrated by performing Monte Carlo simulations for AFM tips with various hydrophilicities.

Wetting an oscillating nanoneedle to image an air-liquid interface at the nanometer scale: dynamical behavior of a nanomeniscus

The dynamical behavior of a nanomeniscus is investigated with a oscillating nanoneedle recording information on the change of the shape and viscous contribution. At the air-glycerol interface, the dynamical properties exhibit a nonlinear behavior making the nanomeniscus evolution similar to a first-order phase transition. Also shown is the capability to record height images of the liquid interface with resolutions at nanometer scale. At the air-water interface, evaporation leads to more complex dynamical properties. The viscous damping first increases as a consequence of a thinning effect, then, when the contact angle reaches zero, the nanomeniscus is unable to sustain the dissipation.

Ion permeation dynamics in carbon nanotubes

Molecular dynamics simulations are carried out to investigate the permeation of ions and water in a membrane consisting of single wall carbon nanotubes possessing no surface charges connecting two reservoirs. Our simulations reveal that there are changes in the first hydration shell of the ions upon confinement in tubes of 0.82 or 0.90 nm effective internal diameter. Although the first minimum in the g(r) is barely changed in the nanotube compared to in the bulk solution, the hydration number of Na(+) ion is reduced by 1.0 (from 4.5 in bulk to 3.5 in the 0.90 nm tube) and the hydration number is reduced further in the 0.82 nm tube. The changes in the hydration shell of Cl(-) ion are negligible, within statistical errors. The water molecules of the first hydration shell of both ions exchange less frequently inside the tube than in the bulk solution. We compare ion trajectories for ions in the same tube under identical reservoir conditions but with different numbers of ions in the tubes. This permits investigation of changes in structure and dynamics which arise from multiple ion occupancy in a carbon nanotube possessing no surface charges. We also investigated the effects of tube flexibility. Ions enter the tubes so as to form a train of ion pairs. We find that the radial distribution profiles of Na(+) ions broaden significantly systematically with increasing number of ion pairs in the tube. The radial distribution profiles of Cl(-) ions change only slightly with increasing number of ions in the tube. Trajectories reveal that Na(+) ions do not pass each other in 0.90 nm tubes, while Cl(-) ions pass each other, as do ions of opposite charge. An ion entering the tube causes the like-charged ions preceding it in the tube to be displaced along the tube axis and positive or negative ions will exit the tube only when one or two other ions of the same charge are present in the tube. Thus, the permeation mechanism involves multiple ions and Coulomb repulsion among the ions plays an essential role.

General theory of amplitude-modulation atomic force microscopy

We present a general analytical theory that enables one to determine accurately the unknown tip-sample interactions from the experimental measurement of the amplitude and phase of the oscillating tip in amplitude-modulation atomic force microscopy (AM-AFM). We apply the method to the known Lennard-Jones-type forces and find excellent agreement with the reconstructed results. AM-AFM, widely used in air and liquid, is now not only an imaging tool but also a quantitative force measurement tool.

Formation of capillary bridges in two-dimensional atomic force microscope-like geometry

We discuss the phase diagram of a fluid confined in a two-dimensional atomic force microscope-like geometry which allows the formation of liquid bridges connecting the opposite walls. The corresponding phase behavior of the fluid is influenced by the phenomenon of complete filling of a wedge.