Effects of contact geometry on pull-off force measurements with a colloidal probe

This paper examines the effects of contact geometry on the pull-off (adhesion) force between a glass sphere (colloidal probe) and a silicon wafer in an environment with controlled relative humidity. An atomic force microscope is used to measure the pull-off force between the colloidal probe and the sample mounted at different tilt angles. The results show that the measured pull-off force is very sensitive to the tilt angle. Through the use of a newly developed direct scanning method, the exact contact geometry is determined for the zero-tilt angle case. The obtained digital image is then rotated to determine the contact geometry for the cases with other tilt angles. A detailed examination of the contact geometry, along with a magnitude analysis of the capillary force, suggests that the adhesion is most likely dominated by the capillary force from the meniscus formed between the probe and the sample. The strong dependence of the adhesion on the tilt angle may result from the change of meniscus dimensions associated with the probe-sample separation, which in turn is controlled by the highest peak on the probe sphere. Our observation emphasizes the combined role of microsurface shape near the contact and nanoroughness within the contact in determining the colloidal probe pull-off force and also microadhesion force in general.     

An improved meniscus surface model for contacting rough surfaces

Based on the Extended-Maugis-Dugdale (EMD) elastic theory, a single asperity capillary meniscus model considering asperity deformation due to both contact and adhesive forces was developed. Specifically included in the single asperity meniscus model was the solid surface interaction inside the contact area. Subsequently, the single asperity model was coupled with a statistical roughness surface model to develop an improved meniscus surface model applicable to a wide-range of humidity levels and adhesion parameter values. Simulations were performed using typical surfaces found in microelectromechanical systems (MEMS) and magnetic storage hard disk drives to examine the effects of surface roughness and relative humidity. It was found that smoother surfaces give rise to higher adhesive and pull-off forces, and at higher relative humidity levels, the capillary force governs the adhesive behavior. As the humidity decreases, the solid surface interaction increases and needs to be included in the total meniscus adhesion. By integrating the adhesive force-displacement curves, the adhesion energy per unit area was calculated for MEMS surfaces and favorably compared with reported experimental data.     

Correction of random surface roughness on colloidal probes in measuring adhesion

Atomic force microscopes (AFM) are commonly used to measure adhesion at nanoscale between two surfaces. To avoid uncertainties in the contact areas between the tip and the surface, colloidal probes have been used for adhesion measurements. We measured adhesion between glass spheres and silicon (100) surface using colloidal probes of different radii under controlled conditions (relative humidity of < 3%, temperature of 25 +/- 1 degrees C). Results showed that the adhesion forces did not correlate with the radii of the spheres as suggested by elastic contact mechanics theories. Surface roughness and random surface features were found on the surfaces of the colloidal probes. We evaluated various roughness parameters, Rumpf and Rabinovich models, and a load-bearing area correction model in an attempt to correct for the roughness effects on adhesion, but the results were unsatisfactory. We developed a new multiscale contact model taking into account elastic as well as plastic deformation in a successive contacting mode. The new model was able to correct for most of the surface roughness features except for surface ridges with sharp angular features, limited by the spherical asperity assumption made in the model.     

Effect of contact geometry on the pull-off force evaluated under high-vacuum and humid atmospheric conditions

The effect of condensed water on pull-off forces under high vacuum (HV) and 0 to 83% relative humidity (RH) N2 atmospheric conditions was evaluated for different contact geometries using atomic force microscopy (AFM). The pull-off force was measured using two types of contact geometry: contact between hemispherical asperities and a flat silicon probe on an AFM cantilever (called a spherical-flat contact) and between a flat silicon substrate and a flat nickel probe on an AFM cantilever (called a flat-flat contact). The hemispherical asperities were fabricated using a focused ion beam (FIB) system, and each peak had a radius of curvature of between 70 and 610 nm. The flat nickel probe was fabricated by friction-induced wear. Measurement results showed that for the spherical-flat contact the pull-off force was proportional to the radius of curvature of the asperity peak and was slightly lower in HV than in humid 14% RH N2. For the flat-flat contact in HV, with increasing contact time, the pull-off force increased in HV but decreased in humid 62 and 83% RH N2. The pull-off force in HV was lower than that in humid N2 when the contact time was less than 10 s but was higher when the contact time was longer than 30 s. The estimated adhesion force based on the Laplace pressure from the contact geometry agreed reasonably well with the measured pull-off force.     

Measurement of polyamide and polystyrene adhesion with coated-tip atomic force microscopy

This work presents atomic force microscopy (AFM) measurements of adhesion forces between polyamides, polystyrene and AFM tips coated with the same materials. The polymers employed were polyamide 6 (PA6), PA66, PA12 and polystyrene (PS). All adhesion forces between the various unmodified or modified AFM tips and the polymer surfaces were in the range -1.5 to -8 nN. The weakest force was observed for an unmodified AFM tip with a PS surface and the strongest was between a PS-coated tip and PS surface. The results point to both the benefits and drawbacks of coated-tip AFM force-distance measurements. Adhesion forces between the two most dissimilar (PA6-PS and PA66-PS) materials were significantly asymmetric, e.g., the forces were different depending on the relative placement of each polymer on the AFM tip or substrate. Materials with similar chemistry and intermolecular interactions yielded forces in close agreement regardless of placement on tip or substrate. Using experimental forces, we calculated the contact radii via four models: Derjaguin, Muller, and Toporov; Johnson, Kendall, and Roberts; parametric tip-force-distance relation; and a square pyramid-flat surface (SPFS) model developed herein. The SPFS model gave the most reasonable contact tip radius estimate. Hamaker constants calculated from the SPFS model using this radius agreed in both magnitude and trends with experiment and Lifshitz theory.     

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.     

Transport mechanisms in capillary condensation of water at a single-asperity nanoscopic contact

Transport mechanisms involved in capillary condensation of water menisci in nanoscopic gaps between hydrophilic surfaces are investigated theoretically and experimentally by atomic force microscopy (AFM) measurements of capillary force. The measurements showed an instantaneous formation of a water meniscus by coalescence of the water layers adsorbed on the AFM tip and sample surfaces, followed by a time evolution of meniscus toward a stationary state corresponding to thermodynamic equilibrium. This dynamics of the water meniscus is indicated by time evolution of the meniscus force, which increases with the contact time toward its equilibrium value. Two water transport mechanisms competing in this meniscus dynamics are considered: (1) Knudsen diffusion and condensation of water molecules in the nanoscopic gap and (2) adsorption of water molecules on the surface region around the contact and flow of the surface water toward the meniscus. For the case of very hydrophilic surfaces, the dominant role of surface water transportation on the meniscus dynamics is supported by the results of the AFM measurements of capillary force of water menisci formed at sliding tip-sample contacts. These measurements revealed that fast movement of the contact impedes on the formation of menisci at thermodynamic equilibrium because the flow of the surface water is too slow to reach the moving meniscus.     

运用功能化修饰的碳纳米管针尖测量配体-受体的作用力

电弧法自制碳纳米管原子力显微镜针尖,对其末端进行功能化修饰,然后测量配体-受体之间的作用力。运用没有功能化修饰的碳纳米管针尖与修饰有亲和素分子的基底进行接触测量时,没有粘滞力出现;而运用末端修饰生物素分子的碳纳米管针尖测量时,有粘滞力产生。功能化的碳纳米管针尖直接测得的粘滞力均大约200pN,此值符合一对配体生物素和受体亲和素之间的作用力。这一结果很难用传统的针尖获得,功能化修饰的碳纳米管针尖能够克服传统针尖在力测量中的局限,在生物学和化学领域有着广泛的应用前景。     

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.     

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.     

Comparison of the adhesion forces in single and double-layer coatings on the MEMS surfaces by JKR and DMT models

In many medical and industrial applications, some strategies are needed to control the adhesion forces between the materials, because surface forces can activate or hinder the function of the device. All actual surfaces present some levels of roughness and the contact between two surfaces is transferred by the asperities on the surfaces. The force of the adhesion, which depends on the operating situations, can be influenced by the contact region. The aim of the present study is to predict the adhesion force in MEMS surfaces using the JKR and DMT models. The surfaces of the coating material in this research consisted of the single-layer coating of Gold and Silver, and the double-layer coating of TiO₂/Gold and TiO₂/Silver on the silicon (100) substrates. The depositing was done by the thermal evaporation method. The results showed that the double-layer coating developed by the new deposition method helped the reduction of the adhesion forces between the probe tip and the specimen surface. The predicted adhesion forces between the probe and the specimens with DMT and JKR models were compared with the experimental results. For all specimens, the simulated data by applying the JKR theory were in a good agreement with the adhesion force experimental values.     

A modeling approach to describe the adhesion of rough, asymmetric particles to surfaces

A combined theoretical and experimental study of the adhesion of alumina particles and polystyrene latex spheres to silicon dioxide surfaces was performed. A boundary element technique was used to model electrostatic interactions between micron-scale particles and planar surfaces when the particles and surfaces were in contact. This method allows quantitative evaluation of the effects of particle geometry and surface roughness on the electrostatic interaction. The electrostatic interactions are combined with a previously developed model for van der Waals forces in particle adhesion. The combined model accounts for the effects of particle and substrate geometry, surface roughness and asperity deformation on the adhesion force. Predictions from the combined model are compared with experimental measurements made with an atomic force microscope. Measurements are made in aqueous solutions of varying ionic strength and solution pH. While van der Waals forces are generally dominant when particles are in contact with surfaces, results obtained here indicate that electrostatic interactions contribute to the overall adhesion force in certain cases. Specifically, alumina particles with complex geometries were found to adhere to surfaces due to both electrostatic and van der Waals interactions, while polystyrene latex spheres were not affected by electrostatic forces when in contact with various surfaces.     

Kinetics of liquid annulus formation and capillary forces

The dependence of the capillary adhesion force F(cap) between a silica microsphere and a flat silica surface versus a time period t of the samples' contact (i.e., dwell-in time) is experimentally investigated using atomic force microscopy (AFM). F(cap) was found to be dependent on t if the humidity was >30-35%. This dependence is exponential, with decay (characteristic) times of ～10 s. It is suggested that the kinetics of the adhesion process are related to the growth of the water annulus between surfaces. Furthermore, we propose that the growth kinetics has two components: (1) water vapor diffusion from the surrounding humid media into the gap between samples and (2) water drainage from the gap. The theory of diffusion through thin pores (i.e., gaps) is developed, and analytical formulas are obtained for the dependence of the meniscus radius r versus time t. However, the experimental dependence of F(cap) versus t and, consequently, r versus t obtained in this article disagrees with the theoretical prediction by several orders of magnitude. Similar results were obtained from the literature data for capillary forces between an AFM cantilever tip and a flat surface. Possible reasons for the deviation from diffusion theory are suggested: surface and Knudsen regimes of vapor diffusion, nonsteady state vapor flow, and tortuosity. Taking into account the viscous drainage of water from the multilayer gap can explain the experimental kinetics of bridge formation, but only if the viscosity of the adjacent multilayer of water is several orders of magnitude larger than the bulk viscosity.     

Locally enhanced relative humidity for scanning probe nanolithography

The formation of a water meniscus between a sharp tip and a solid surface is one of the prevailing requirements for scanning probe microscope (SPM)-based lithographies, such as dip-pen nanolithography (DPN) and conductive tip induced oxidation. The water meniscus functions as a medium for the oxidation of or mass transfer to the solid surface. Here we report a simple, efficient, and effective approach to enhance the local relative humidity and thus increase the size of the water meniscus by bringing a water-containing capillary tube to the proximity of the tip-surface contact area. The enhancement in local relative humidity is confirmed via an increase in the measured tip-surface adhesion forces and the widths of DPN generated parallel lines. Compared to the global control of relative humidity for the whole lithography system, the short distance between the "water reservoir" and the tip-surface contact area enables rapid increase in the local vapor pressure of water, less perturbation, and minimal erosion to the state-of-the-art electronics. As a result, most scanning probe lithography experiments at high relative humidity can now be performed in a reasonable time frame.     

Effects of adsorbed water layer structure on adhesion force of silicon oxide nanoasperity contact in humid ambient

The origin of the large relative-humidity (RH) dependence of the adhesion force in the single-asperity contact between silicon oxide surfaces is elucidated. As RH increases, the adhesion force measured with an atomic force microscopy (AFM) initially increases, reaches a maximum, and then decreases at high RH. The capillary force alone cannot explain the observed magnitude of the RH dependence. The origin of the large RH dependence is due to the presence of an icelike structured water adsorbed at the silicon oxide surface at room temperature. A solid-adsorbate-solid model is developed calculating the contributions from capillary forces, van der Waals interactions, and the rupture of an ice-ice bridge at the center of the contact region. This model illustrates how the structure, thickness, and viscoelastic behavior of the adsorbed water layer influence the adhesion force of the silicon oxide nanoasperity contact.     

Direct imaging of meniscus formation in atomic force microscopy using environmental scanning electron microscopy

Environmental scanning electron microscopy was used to image meniscus formation between an AFM tip and a surface. At high relative humidity, 70%-99%, the meniscus formed is 100 to 1200 nm in height, orders of magnitude larger than predicted by the Kelvin equation using spherical geometry. The height of the meniscus also demonstrates hysteresis associated with increasing or decreasing relative humidity.     

Probing colloidal forces between a Si3N4 AFM tip and single nanoparticles of silica and alumina

The atomic force microscope (AFM) has been used to measure surface forces between silicon nitride AFM tips and individual nanoparticles deposited on substrates in 10(-4) and 10(-2) M KCl solutions. Silica nanoparticles (10 nm diameter) were deposited on an alumina substrate and alumina particles (5 to 80 nm diameter) were deposited on a mica substrate using aqueous suspensions. Ionic concentrations and pH were used to manage attractive substrate-particle electrostatic forces. The AFM tip was located on deposited nanoparticles using an operator controlled offset to achieve stepwise tip movements. Nanoparticles were found to have a negligible effect on long-range tip-substrate interactions, however, the forces between the tip and nanoparticle were detectable at small separations. Exponentially increasing short-range repulsive forces, attributed to the hydration forces, were observed for silica nanoparticles. The effective range of hydration forces was found to be 2-3 nm with the decay length of 0.8-1.3 nm. These parameters are in a good agreement with the results reported for macroscopic surfaces of silica obtained using the surface force apparatus suggesting that hydration forces for the silica nanoparticles are similar to those for flat silica surfaces. Hydration forces were not observed for either alumina substrates or alumina nanoparticles in both 10(-4) M KCl solution at pH 6.5 and 10(-2) M KCl at pH 10.2. Instead, strong attractive forces between the silicon nitride tip and the alumina (nanoparticles and substrate) were observed.     

An atomic force microscopic investigation of electro-sensitive polymer surface

An electro-sensitive poly(2-acrylamide-2-methylpropane sulfonic acid) (PAMPS) film was fabricated by surface-initiated atom transfer radical polymerization (ATRP) method on silicon substrate. Atomic force microscopy (AFM) experiments in contact mode show that friction force and the adhesion force between the AFM tip and the film may change regularly with the alteration of the applied negative bias voltage between them, indicating that the microscopic wettability of the film can be adjusted by external electric field. On the other hand, the AFM experiments in tapping mode reveal that the film may take corresponding phase change under the electric field. These effects were considered to result from the conformational overturn of the sulfonic groups and the adjacent alkyl chains in the electric field.     

Adhesion models: from single to multiple asperity contacts

This review presents a summary of the current adhesion models available to date, between real rough surfaces, starting from single asperity models and expanding to multiple asperity contacts. The focus is made on multi-asperity contact interactions. Both van der Waals and contact mechanics approaches have been considered and relevant adhesion models are reviewed and discussed. The influence of the meniscus forces on adhesion has been considered, along with a summary of the various meniscus models. The effect of surface geometry, its topography and environmental conditions on meniscus action are also discussed along with its integration into multi-asperity adhesion models.     

Mechanistic aspects of protein/material interactions probed by atomic force microscopy

Physicochemical studies on the mechanisms of protein adsorption onto solid material surfaces have been extensively performed so far, mainly based on the analysis of factors such as the equilibrium adsorbed amount (adsorption isotherms), time-dependent change of adsorbed amount (adsorption kinetics), and conformational change of adsorbed protein. However, direct understanding of the strength of the molecular interaction between protein and the material surface has not been established yet. For this issue, the force measurement techniques of an atomic force microscope (AFM) using a protein-modified probe tip are recently becoming powerful tools to analyze the actual interaction forces between protein and material surfaces. In this mini review, we discuss the characteristics and interpretation of the AFM force-versus-distance curves (f–d curves) obtained with the protein-modified probe tip, and the relationship between the forces measured from the f–d curves and the driving forces in the natural process of protein adsorption. Relative degrees of each of the following contributions which determine the character of protein adsorption are clarified: (1) the intrinsic protein/material forces mediated by solvent, (2) the thermodynamic stability of protein/material adhesion interface, and (3) diffusion force of protein molecules. Within these driving forces, the latter two in particular are confirmed to play essential roles in determining the character of protein adsorption, based on the profiles of f–d curves.