Volume of a nanoscale water bridge

Water bridges formed through capillary condensation at nanoscale contacts first stretch and then break during contact rupture. Atomic force microscopy (AFM) pull-off experiments performed in air with hydrophilic tips and samples show that stretched nanoscopic water bridges are in mechanical equilibrium with the external pull-off force acting at the contact but not in thermodynamic equilibrium with the water vapor in air. The experimental findings are explained by a theoretical model that considers constant water volume and decrease of water meniscus curvature during meniscus stretching. The model predicts that the water bridge breakup distance will be roughly equal to the cubic root of the water bridge volume. A thermodynamic instability was noticed for large water bridges formed at the contact of a blunt AFM tip (curvature radius of 400 nm) with a flat sample. In this case, experiments showed rise and stabilization of the volume of the water at the contact in about 1 s. For sharp AFM tips (curvature radius below 50 nm), the experiments indicated that formation of stable water bridges occurs in a much shorter time (below 5 ms).     

Cleaning and hydrophilization of atomic force microscopy silicon probes

The silicon surface of commercial atomic force microscopy (AFM) probes loses its hydrophilicity by adsorption of airborne and package-released hydrophobic organic contaminants. Cleaning of the probes by acid piranha solution or discharge plasma removes the contaminants and renders very hydrophilic probe surfaces. Time-of-flight secondary-ion mass spectroscopy and X-ray photoelectron spectroscopy investigations showed that the native silicon oxide films on the AFM probe surfaces are completely covered by organic contaminants for the as-received AFM probes, while the cleaning methods effectively remove much of the hydrocarbons and silicon oils to reveal the underlying oxidized silicon of the probes. Cleaning procedures drastically affect the results of adhesive force measurements in water and air. Thus, cleaning of silicon surfaces of the AFM probe and sample cancelled the adhesive force in deionized water. The significant adhesive force values observed before cleaning can be attributed to formation of a bridge of hydrophobic material at the AFM tip-sample contact in water. On the other hand, cleaning of the AFM tip and sample surfaces results in a significant increase of the adhesive force in air. The presence of water soluble contaminants at the tip-sample contact lowers the capillary pressure in the water bridge formed by capillary condensation at the AFM tip-sample contact, and this consequently lowers the adhesive force.     

Effect of surface roughness and softness on water capillary adhesion in apolar media

The roughness and softness of interacting surfaces are both important parameters affecting the capillary condensation of water in apolar media, yet are poorly understood at present. We studied the water capillary adhesion between a cellulose surface and a silica colloidal probe in hexane by AFM force measurements. Nanomechanical measurements show that the Young's modulus of the cellulose layer in water is significantly less (~7 MPa) than in hexane (~7 GPa). In addition, the cellulose surface in both water and hexane is rather rough (6-10 nm) and the silica probe has a comparable roughness. The adhesion force between cellulose and silica in water-saturated hexane shows a time-dependent increase up to a waiting time of 200 s and is much (2 orders of magnitude) lower than that expected for a capillary bridge spanning the whole silica probe surface. This suggests the formation of one or more smaller bridges between asperities on both surfaces, which is confirmed by a theoretical analysis. The overall growth rate of the condensate cannot be explained from diffusion mediated capillary condensation alone; thin film flow due to the presence of a wetting layer of water at both the surfaces seems to be the dominant contribution. The logarithmic time dependence of the force can also be explained from the model of the formation of multiple capillary bridges with a distribution of activation times. Finally, the force-distance curves upon retraction show oscillations. Capillary condensation between an atomically smooth mica surface and the silica particle show less significant oscillations and the adhesion force is independent of waiting time. The oscillations in the force-distance curves between cellulose and silica may stem from multiple bridge formation between the asperities present on both surfaces. The softness of the cellulose surface can bring in additional complexities during retraction of the silica particle, also resulting in oscillations in the force-distance curves.     

Roughness and ordering at the interface of oxidized polystyrene and water

For the first time, atomistically detailed molecular dynamics calculations revealed molecular ordering of the water-oxidized atactic polystyrene (aPS) interface. Both ordering of the water molecules and the phenyl rings occur. In addition, the natural roughness of the surface has been simulated and compared to experimental values. The composition of the simulated aPS films is based on spin-coated aPS films that have been oxidized and characterized experimentally. The aPS surfaces are oxidized with ultraviolet-ozone radiation and have been characterized by XPS, AFM, and water contact angle measurements. XPS measurements show that the oxygen content in the sample increases rapidly with exposure and reaches saturation near 24 at. % of oxygen. The molecular dynamics simulations show smoothening of an hydrophobic aPS surface upon transition from vacuum to water. The smoothening decreases with increasing hydrophilicity. The calculations reveal ordering of oxidized phenyl rings for aPS surfaces in water. The order increases with increasing hydrophilicity. Additionally, we investigated the water structure near the aPS-water interface as a function of the surface hydrophilicity. With increasing hydrophilicity, the density of water at the aPS-water interface increases. The water density profile is steeper in the presence of hydrophobic aPS. The water shows an ordered layer near both the hydrophobic and hydrophilic surfaces; the position of this layer shifts toward the interface with increasing hydrophilicity.     

Capillary force in atomic force microscopy

Under ambient conditions, a water meniscus generally forms between a nanoscale atomic force microscope tip and a hydrophilic surface. Using a lattice gas model for water and thermodynamic integration methods, we calculate the capillary force due to the water meniscus for both hydrophobic and hydrophilic tips at various humidities. As humidity rises, the pull-off force rapidly reaches a plateau value for a hydrophobic tip but monotonically increases for a weakly hydrophilic tip. For a strongly hydrophilic tip, the force increases at low humidities (<30%) and then decreases. We show that mean-field density functional theory reproduces the simulated pull-off force very well.     

Forces between surfaces in liquids

The results and implications of direct force measurements between molecularly smooth mica surfaces in liquids are reviewed. These discussions include four interactions fundamental to colloid science: van der Waals forces, double layer forces, adhesion forces and structural or solvation forces (e.g. hydration forces). Also considered are the effects of preferential surface adsorption of solute molecules on these interactions, e.g. surfactant adsorptions from aqueous solutions and water condensation from non-aqueous solvents.In aqueous media it is apparent that the DLVO theory is valid at all surface separations down to the “force barrier”, but that under certain conditions hydration forces can become significant at distances below 30 Å.The measured adhesion force between two solid surfaces can be simply related to their surface energies and where meniscus forces are also present due to “capillary condensation” from vapor solvent, their effect on adhesion can be understood in terms of straightforward bulk thermodynamic principles. Here, too, it is concluded that structural forces cannot be ignored.Our results suggest that structural forces may either very monotonically with distance or be oscillatory with a periodicity equal to the molecular size. Their origin, nature, mode of action and importance for particle interactions will no doubt take many years to sort out.     

In situ determination of the thermodynamic surface properties of chemically modified surfaces on a local scale: an attempt with the atomic force microscope

We have monitored deflection-distance curves with an atomic force microscope (AFM) in contact mode, with a silicon nitride tip, on chemically modified silicon wafers, in the air. The wafers were modified on their surface by grafting self-assembled monolayers (SAMs) of different functional groups such as methyl, ester, amine, or methyl fluoride. A chemically modified surface with a functionalized hydroxyl group was also considered. Qualitative analysis allowed us to compare adhesive forces versus chemical features and surface energy. The systematic calibration procedure of the AFM measurements was performed to produce quantitative data. Our results show that the experimentally determined adhesive force or thermodynamic work of adhesion increases linearly with the total surface energy determined with contact angles measured with different liquids. The influence of capillary condensation of atmospheric water vapor at the tip-sample interface on the measured forces is discussed. Quantitative assessment values were used to determine in situ the SAM-tip thermodynamic work of adhesion on a local scale, which have been found to be in good agreement with quoted values. Finally, the determination of the surface energy of the silicon wafer deduced from the thermodynamic work of adhesion is also proposed and compared with the theoretical value.     

Contact angles of surface nanobubbles on mixed self-assembled monolayers with systematically varied macroscopic wettability by atomic force microscopy

The dependence of the properties of so-called "surface nanobubbles" at the interface of binary self-assembled monolayers (SAMs) of octadecanethiol (ODT) and 16-mercaptohexadecanoic acid (MHDA) on ultraflat template-stripped gold and water on the surface composition was studied systematically by in situ atomic force microscopy (AFM). The macroscopic water contact angle (θ(macro)) of the SAMs spanned the range between 107° ± 1° and 15° ± 3°. Surface nanobubbles were observed on all SAMs by intermittent contact-mode AFM; their size and contact angle were found to depend on the composition of the SAM. In particular, nanoscopic contact angles θ(nano) < 86° were observed for the first time for hydrophilic surfaces. From fits of the top of the bubble profile to a spherical cap in three dimensions, quantitative estimates of nanobubble height, width, and radius of curvature were obtained. Values of θ(nano) calculated from these data were found to change from 167° ± 3° to 33° ± 58°, when θ(macro) decreased from 107° ± 1° to 37° ± 3°. While the values for θ(nano) significantly exceeded those of θ(macro) for hydrophobic SAMs, which is fully in line with previous reports, this discrepancy became less pronounced and finally vanished for more hydrophilic surfaces.     

Capillary forces: influence of roughness and heterogeneity

A formalism is described to calculate capillary forces between solid surfaces analytically. Assumptions are that the liquid menisci (1) have a much larger extension parallel to the gap than normal and (2) are formed by capillary condensation and are in equilibrium with the vapor. To calculate capillary forces, first the gap between the two surfaces is described by a height distribution function. Roughness is considered with an asperity distribution function. Both distributions can at least in principal be measured by light, electron, or atomic force microscopy or grazing incidence X-ray reflectivity. The total capillary force versus distance or vapor pressure is obtained by a convolution of both distributions and an integration. The formalism is applied to calculate the capillary force between rough spherical particles. In addition, a method to consider surface heterogeneity is suggested.     

Adhesion Mechanisms of the Contact Interface of TiO(2) Nanoparticles in Films and Aggregates

Fundamental knowledge about the mechanisms of adhesion between oxide particles with diameters of few nanometers is impeded by the difficulties associated with direct measurements of contact forces at such a small size scale. Here we develop a strategy based on AFM force spectroscopy combined with all-atom molecular dynamics simulations to quantify and explain the nature of the contact forces between 10 nm small TiO(2) nanoparticles. The method is based on the statistical analysis of the force peaks measured in repeated approaching/retracting loops of an AFM cantilever into a film of nanoparticle agglomerates and relies on the in-situ imaging of the film stretching behavior in an AFM/TEM setup. Sliding and rolling events first lead to local rearrangements in the film structure when subjected to tensile load, prior to its final rupture caused by the reversible detaching of individual nanoparticles. The associated contact force of about 2.5 nN is in quantitative agreement with the results of molecular dynamics simulations of the particle-particle detachment. We reveal that the contact forces are dominated by the structure of water layers adsorbed on the particles' surfaces at ambient conditions. This leads to nonmonotonous force-displacement curves that can be explained only in part by classical capillary effects and highlights the importance of considering explicitly the molecular nature of the adsorbates.     

Atomic force microscopy measurement of heterogeneity in bacterial surface hydrophobicity

The structure and physicochemical properties of microbial surfaces at the molecular level determine their adhesion to surfaces and interfaces. Here, we report the use of atomic force microscopy (AFM) to explore the morphology of soft, living cells in aqueous buffer, to map bacterial surface heterogeneities, and to directly correlate the results in the AFM force-distance curves to the macroscopic properties of the microbial surfaces. The surfaces of two bacterial species, Acinetobacter venetianus RAG-1 and Rhodococcus erythropolis 20S-E1-c, showing different macroscopic surface hydrophobicity were probed with chemically functionalized AFM tips, terminating in hydrophobic and hydrophilic groups. All force measurements were obtained in contact mode and made on a location of the bacterium selected from the alternating current mode image. AFM imaging revealed morphological details of the microbial-surface ultrastructures with about 20 nm resolution. The heterogeneous surface morphology was directly correlated with differences in adhesion forces as revealed by retraction force curves and also with the presence of external structures, either pili or capsules, as confirmed by transmission electron microscopy. The AFM force curves for both bacterial species showed differences in the interactions of extracellular structures with hydrophilic and hydrophobic tips. A. venetianus RAG-1 showed an irregular pattern with multiple adhesion peaks suggesting the presence of biopolymers with different lengths on its surface. R. erythropolis 20S-E1-c exhibited long-range attraction forces and single rupture events suggesting a more hydrophobic and smoother surface. The adhesion force measurements indicated a patchy surface distribution of interaction forces for both bacterial species, with the highest forces grouped at one pole of the cell for R. erythropolis 20S-E1-c and a random distribution of adhesion forces in the case of A. venetianus RAG-1. The magnitude of the adhesion forces was proportional to the three-phase contact angle between hexadecane and water on the bacterial surfaces.     

Dynamics of a disturbed sessile drop measured by atomic force microscopy (AFM)

A new method for studying the dynamics of a sessile drop by atomic force microscopy (AFM) is demonstrated. A hydrophobic microsphere (radius, r ～ 20-30 μm) is brought into contact with a small sessile water drop resting on a polytetrafluoroethylene (PTFE) surface. When the microsphere touches the liquid surface, the meniscus rises onto it because of capillary forces. Although the microsphere volume is 6 orders of magnitude smaller than the drop, it excites the normal resonance modes of the liquid interface. The sphere is pinned at the interface, whose small (<100 nm) oscillations are readily measured with AFM. Resonance oscillation frequencies were measured for drop volumes between 5 and 200 μL. The results for the two lowest normal modes are quantitatively consistent with continuum calculations for the natural frequency of hemispherical drops with no adjustable parameters. The method may enable sensitive measurements of volume, surface tension, and viscosity of small drops.     

Simulations of novel nanostructures formed by capillary effects in lithography

High aspect ratio three-dimensional nanostructures are of tremendous interest to a wide range of fields such as photonics, plasmonics, fluid mechanics, and biology. Recent developments in capillary force lithography (CFL) have focused on taking advantage of the formation of menisci to enhance the functionality of small size-scale structures. In this study, simulations of the three-dimensional shapes of equilibrium menisci formed in capillaries with various cross-section geometries are studied. The capillary cross sections include regular polygons and equilateral star-shapes with sharp and rounded corners. The characteristic dimension of the physical lithography systems which are simulated is on the order of 100nm. At such size-scale, surface-tension-effects are predominant, and as a consequence, our simulations demonstrate that nanometer-sized structures with great application potentials can be fabricated. Specifically, this study demonstrates that surfaces with three-dimensional nanoscale structures can be fabricated from templates with micron or sub-micron features through the development of cusps in the corners of the polygonal capillaries. Quantitatively, the effects of contact angle, corner angle, meniscus confinement, and corner rounding radius are examined and scaling analyses are presented to describe the dependencies of the height variation across the meniscus on these parameters. These simulations serve as useful guides for extending the development and implementation of capillary force lithography.     

Surface Energy and Wettability of Plasma-treated Polyacrylonitrile Fibers

Polyacrylonitrile fibers were treated with a nitrogen glow-discharge plasma. The surfaces of untreated and treated fibers were examined with contact angle measurements, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Surface energy calculations of the fibers were carried out from contact angle measurements using the relationships developed by Fowkes. It is found that plasma treatment causes a reduction in water contact angle on the fiber surfaces. The dispersion component of surface energy changes slightly, while the polar component is increased significantly from 14.6 mN/m to 58.7 mN/m and the total surface energy increase is 139%. The increase of surface energy is mainly caused by the introduction of hydrophilic groups on the fiber surfaces after plasma treatment.     

Single DNA molecules as probes for interrogating silica surfaces after various chemical treatments

We examined the adsorption of single YOYO-1-labeled λ-DNA molecules at glass surfaces after treatment with various chemical cleaning methods by using total internal reflection fluorescence microscopy (TIRFM). The characteristics of these surfaces were further assessed using contact angle (CA) measurements and atomic force microscopy (AFM). By recording the real-time dynamic motion of DNA molecules at the liquid/solid interface, subtle differences in adsorption affinities were revealed. The results indicate that the driving force for adsorption of DNA molecules on glass surfaces is mainly hydrophobic interaction. We also found that surface topography plays a role in the adsorption dynamics.     

Atomic-scale roughness effect on capillary force in atomic force microscopy

We study the capillary force in atomic force microscopy by using Monte Carlo simulations. Adopting a lattice gas model for water, we simulated water menisci that form between a rough silicon-nitride tip and a mica surface. Unlike its macroscopic counterpart, the water meniscus at the nanoscale gives rise to a capillary force that responds sensitively to the tip roughness. With only a slight change in tip shape, the pull-off force significantly changes its qualitative variation with humidity.     

Surface restructuring of polymers

Time-dependent contact angle measurements are employed to follow the dynamics of surface modifications of various polymeric surfaces of different hydrophilicities. The equilibration of a hydrophilic polymer in a strong polar environment (such as water) induces an increase in the polarity of the surface; the subsequent exposure of the restructured solid to a nonpolar environment decreases the polarity of the surface. The dynamics of these processes depends on the history of the specimen. Various phenomena, such as surface restructuring by the reorientation of the buried polar or nonpolar moieties, water penetration into the polymer, and the reorganization of water in the neighborhood of the surface, are suggested to be responsible for the time evolution of the dynamic contact angles.     

On the adhesion between fine particles and nanocontacts: an atomic force microscope study

In this study we measured the adhesion forces between atomic force microscope (AFM) tips or particles attached to AFM cantilevers and different solid samples. Smooth and homogeneous surfaces such as mica, silicon wafers, or highly oriented pyrolytic graphite, and more rough and heterogeneous surfaces such as iron particles or patterns of TiO2 nanoparticles on silicon were used. In the first part, we addressed the well-known issue that AFM adhesion experiments show wide distributions of adhesion forces rather than a single value. Our experiments show that variations in adhesion forces comprise fast (i.e., from one force curve to the next) random fluctuations and slower fluctuations, which occur over tens or hundreds of consecutive measurements. Slow fluctuations are not likely to be the result of variations in external factors such as lateral position, temperature, humidity, and so forth because those were kept constant. Even if two solid bodies are brought into contact under precisely the same conditions (same place, load, direction, etc.) the result of such a measurement will often not be the same as that of the previous contact. The measurement itself will induce structural changes in the contact region, which can change the value for the next adhesion force measurement. In the second part, we studied the influence of humidity on the adhesion of nanocontacts. Humidity was adjusted relatively fast to minimize tip wear during one experiment. For hydrophobic surfaces, no signification change in adhesion force with humidity was observed. Adhesion force versus humidity curves recorded with hydrophilic surfaces either showed a maximum or continuously increased. We demonstrate that the results can be interpreted with simple continuum theory of the meniscus force. The meniscus force is calculated based on a model that includes surface roughness and takes into account different AFM tip (or particle) shapes by a two-sphere model. Experimental and theoretical results show that the precise contact geometry has a critical influence on the humidity dependence of the adhesion force. Changes in tip geometry on the sub-10-nm length scale can completely change adhesion force versus humidity curves. Our model can also explain the differences between earlier AFM studies, where different dependencies of the adhesion force on humidity were observed.     

Non-DLVO silica interaction forces in NMP-water mixtures. II. An asymmetric system

The interaction between energetically asymmetric hydrophilic and hydrophobic surfaces has fundamental and practical importance in both industrial and natural colloidal systems. The interaction forces between a hydrophilic silica sphere and a silanated, hydrophobic glass plate in N-methyl-2-pyrrolidone (NMP)-water binary mixtures were measured using atomic force microscopy (AFM). A strong and long-range attractive force was observed in pure water and was attributed to the formation of capillary bridges associated with nanoscale bubbles initially present on the hydrophobic surface. When NMP was added, the capillary force and corresponding pull-off force became less attractive, which was explained readily in terms of the surface wettability by the binary solvent mixture. Similar to the case of symmetric (two hydrophilic) surfaces, the range of attraction between the asymmetric surfaces was maximized at around 30 vol % NMP, which is consistent with the formation of a thick adsorbed macrocluster layer on the hydrophilic silica surface.     

The hydrophobic effect: molecular dynamics simulations of water confined between extended hydrophobic and hydrophilic surfaces

Structural and dynamic properties of water confined between two parallel, extended, either hydrophobic or hydrophilic crystalline surfaces of n-alkane C(36)H(74) or n-alcohol C(35)H(71)OH, are studied by molecular dynamics simulations. Electron density profiles, directly compared with corresponding experimental data from x-ray reflectivity measurements, reveal a uniform weak de-wetting characteristic for the extended hydrophobic surface, while the hydrophilic surface is weakly wetted. These microscopic data are consistent with macroscopic contact angle measurements. Specific water orientation is present at both surfaces. The ordering is characteristically different between the surfaces and of longer range at the hydrophilic surface. Furthermore, the dynamic properties of water are different at the two surfaces and different from the bulk behavior. In particular, at the hydrophobic surface, time-correlation functions reveal that water molecules have characteristic diffusive behavior and orientational ordering due to the lack of hydrogen bonding interactions with the surface. These observations suggest that the altered dynamical properties of water in contact with extended hydrophobic surfaces together with a partial drying of the surfaces are more indicative of the hydrophobic effect than structural ordering, which we suggest to be independent of surface topology.