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.     

阵列凹坑结构分布与水滴黏附性质的关系

以新鲜玫瑰花花瓣正面为模板, 采用模板印刷法制备具有微米级阵列凹坑和纳米级沟壑结构的聚二甲基硅氧烷(PDMS)薄膜, 通过对该薄膜逐级拉伸改变其微观结构的分布; 采用场发射扫描电子显微镜(SEM)和原子力显微镜(AFM)观察了不同拉伸程度下薄膜表面微观结构的变化, 采用高敏感性微电力学天平测试了样品表面微观结构变化过程中水滴的黏附力, 分析了其微观结构分布与水滴黏附性质的关系; 采用接触角测量仪表征不同拉伸条件下薄膜的浸润性. 结果表明, 随着PDMS薄膜被逐次拉伸, 单位面积内的凹坑结构数目减少, 且凹坑逐渐分离, 凹坑的深度逐渐降低, 水滴更容易浸入到凹坑结构中, 因此水滴与薄膜的黏附力急剧增大; 随着薄膜进一步拉伸, 纳米级沟壑结构会随着凹坑的拉伸而不断伸展, 纳米级沟壑结构的面积增加, 纳米沟壑结构诱捕的空气量逐渐上升, 导致水滴与薄膜表面的接触面积降低, 使得水滴与薄膜的黏附力下降; 继续拉伸PDMS薄膜, 纳米级沟壑结构进一步伸展, 水滴逐渐浸入纳米级沟壑结构中, 水滴与薄膜的黏附力缓慢增大, 当水滴完全进入到纳米级沟壑中时, 水滴与薄膜的黏附力达到极大值, 此时继续拉伸PDMS薄膜, 纳米级沟壑结构随着拉伸程度的增加继续伸展, 水滴与薄膜的接触面积稍有减少, 黏附力将有所下降, 直至薄膜被完全破坏. 由此可见, 微米级凹坑结构和纳米级褶皱结构的分布是影响PDMS薄膜对水滴黏附性质的主要因素.     

Effect of water chemistry and aging on iron-mica interaction forces: implications for iron particle transport

The transport of particles through groundwater systems is governed by a complex interplay of mechanical and chemical forces that are ultimately responsible for binding to geological substrates. To understand these forces in the context of zero valent iron particles used in the remediation of groundwater, atomic force microscopy (AFM)-based force spectroscopy was employed to characterize the interactions between AFM tips modified with either carbonyl iron particles (CIP) or electrodeposited Fe as a function of counterion valency, temperature, particle morphology, and age. The measured interaction forces were always attractive for both fresh and aged CIP and electrodeposited iron, except in 100 mM NaCl, as a consequence of electrostatic attraction between the negatively charged mica and positively charged iron. In 100 mM NaCl, repulsive hydration forces appeared to dominate. Good agreement was found between the experimental data and predictions based on the extended DLVO (XDLVO) theory. The effect of aging on iron particle composition and morphology was assessed by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) revealing that the aged particles comprising a zero valent iron core passivated by a mixture of iron oxides and hydroxides. Force spectroscopy showed that aging caused variations in the adhesive force due to the changes in particle morphology and contact area.     

Connect the drops: using solids as adhesives for liquids

Colloidal particles are shown to be capable of developing adhesion between liquid phases through a bridging mechanism by which intervening, micrometer-scaled, fluid films are stabilized. Particle dynamics leading to the assembly of the stabilizing structure are discussed. Models for the resulting adhesive force are developed from considerations of both interface shape perturbation and the force applied by surface tension on an individual particle. Finally, predictions from these models are compared to direct measurements of the forces that arise during the separation of adhering interfaces. Such comparisons lead to a novel method for determining the three-phase contact angle inherent to particles residing at fluid interfaces.     

Surfactant boundary lubricant film modified by an amphiphilic diblock copolymer

The effect of the uptake of a low-molecular-weight amphiphilic diblock copolymer on the morphology of didodecyldimethylammonium bromide (DDAB) adsorbed layers on mica, the interactions between two coated surfaces, and the frictional properties of the boundary film have been studied using an atomic force microscope and a dynamic surface forces apparatus nanotribometer. When DDAB-coated surfaces in aqueous solution were compressed, hemifusion or removal of the adsorbed surfactant bilayers could not be induced, and no frictional force could be measured between the surfaces, which display superior lateral cohesion and lubricant properties. Coadsorbing octadecyl end modified poly(ethylene oxide) chains at low density facilitates hemifusion, generating significant shear stress and leading to stick-slip instabilities. The mixed films regain their lateral cohesion at higher adsorbed copolymer densities, but an extra short-range attraction brings the adsorbed layers into adhesive contact without causing bilayer hemifusion. Here, noticeable frictional forces are also measured.     

超疏水性聚二甲基硅氧烷膜的制备及其表面吸附性研究

采用简单的激光刻蚀方法制备了具有类“菜花”状多级结构的粗糙聚二甲基硅氧烷(PDMS)膜, 并用CCD与高敏感性微电力学天平观察和测量PDMS表面对水的吸附情况. 结果表明, 该膜表面具有超疏水性, 同时对水滴具有超低的吸附力. 还对其表面特殊多级结构产生的机理进行了分析, 并探讨了在化学组成和表面结构对超疏水性以及吸附性产生的影响.     

Experimental Study of Fibrin/Fibrin-Specific Molecular Interactions Using a Sphere/Plane Adhesion Model

Fibrin, the biopolymer produced in the final step of the coagulation cascade, is involved in the resistance of arterial thrombi to fragmentation under shear flow. However, the nature and strength of specific interactions between fibrin monomers are unknown. Thus, the shear-induced detachment of spherical monodispersed fibrin-coated latex particles in adhesive contact with a plane fibrin-coated glass surface has been experimentally studied, using an especially designed shear stress flow chamber. A complete series of experiments for measuring the shear stress necessary to release individual particles under various conditions (various number of fibrin layers involved in the adhesive contact, absence or presence of plasmin, the main physiological fibrinolytic enzyme) has been performed. The nonspecific DLVO interactions have been shown to be negligible compared to the interactions between fibrin monomers. A simple adhesion model based on the balance of forces and torque on particles, assuming an elastic behavior of the fibrin polymer bonds, to analyze the experimental data in terms of elastic force at rupture of an elementary intermonomeric fibrin bond has been used. The results suggested that this force (of order 400 pN) is an intrinsic quantity, independent of the number of fibrin layers involved in the adhesive contact. Copyright 2001 Academic Press.     

Pull-off force measurements between rough surfaces by atomic force microscopy

Direct measurements of the pull-off (adhesion) forces between pharmaceutical particles (beclomethasone dipropionate, a peptide-type material, and lactose) with irregular geometry and rough polymeric surfaces (series of polypropylene coatings, polycarbonate, and acrylonitrile-butadiene-styrene) were carried out using the atomic force microscope. These measurements showed that roughness of the interacting surfaces is the significant factor affecting experimentally measured pull-off forces. A broad distribution of pull-off force values was noted in the measurements, caused by a varying adhesive contact area for a particle located on rough substrate. The possibility of multiple points of contact between irregularly shaped pharmaceutical particles and substrate surfaces is demonstrated with nanoindentations of the particle in a fluoro-polymer film. Force-distance curves showing the "sawtooth" pattern are additional evidence that particles make contact with substrates at more than one point. Reduced adhesion of 10- to 14-microm-diameter lactose and peptide material particles to the polypropylene coatings with a roughness of 194 nm was found in this study. Similar pull-off force versus roughness relationships are also reported for the model spherical particles, silanized glass particle with a size of 10 microm and polystyrene particle with a diameter of 9 microm, in contact with polypropylene coatings of varying roughness characteristics. It was found that the model recently proposed by Rabinovich et al. (J. Colloid Interface Sci. 232, 1-16 (2000)) closely predicts the pull-off forces for glass and lactose particles. On the other hand, the adhesion of the peptide material and polystyrene particle to polypropylene is underestimated by about an order of magnitude with the theoretical model, in which the interacting substrates are treated as rigid materials. The underestimate is attributed to the deformation of the peptide material and polystyrene particles.     

Direct force measurement of the stability of poly(ethylene glycol)-polyethylenimine graft films

The stability and passivity of poly(ethylene glycol)-polyethylenimine (PEG-PEI) graft films are important for their use as antifouling coatings in a variety of biotechnology applications. We have used AFM colloidal-probe force measurements combined with optical reflectometry to characterize the surface properties and stability of PEI and dense PEG-PEI graft films on silica. Initial contact between bare silica probes and PEI-modified surfaces yields force curves that exhibit a long-range electrostatic repulsion and short-range attraction between the surfaces, indicating spontaneous desorption of PEI in the aqueous medium. Further transfer of PEI molecules to the probe occurs with subsequent application of forces between FR = 300 and 500 microN/m. The presence of PEG reduces the adhesive properties of the PEI surface and prevents transfer of PEI molecules to the probe with continuous contact, though an initial desorption of PEI still occurs. Glutaraldehyde crosslinking of the graft films prevents both the initial desorption and subsequent transfer of the PEI, resulting in sustained attractive interaction forces of electrostatic origin between the negatively charged probe and the positively charged copolymer graft films.     

The hydrophilic nature of gold and platinum

Clean gold and platinum surfaces have been shown to be hydrophilic in borate and sulphate media at all potentials between hydrogen and oxygen evolution. Finite contact angles were observed at the nitrogen/metal/solution interface immediately after polishing with diamond paste, but electrochemical evidence demonstrated that such surfaces were contaminated. After cleaning the surface chemically or electrochemically, zero contact angles were observed.     

Observation of adhesive force and energy of adsorbents on rubber substrates by atomic force microscopy

We observed the surface morphology and adhesive interaction of adsorbents on rubber substrates by atomic force microscopy (AFM). The detachment of adsorbents from rubber substrates is an important issue for various machines like home appliances and laundry machine. Since a clean surface of the functioning parts is required, a frequent cleaning process must be developed. In particular, dust and lint have a tendency to bind to the rubber surface of a laundry machine. Several practical methods have been attempted to remove these particles from the surface. Pure water, detergent, sodium hypochlorite (65 °C), and cold water (18 °C) are treated onto artificial dust and lint mixtures on rubber with water fluid by rapid rpm. The dust‐and‐lint adsorbents are investigated by AFM after the treatment, and topographic images and force–distance (F–D) curves were generated for the samples. The roughness, measured as the root mean square, is a key factor to judge the cleaning quality. From the F–D curves, we are able to obtain adhesive energy in addition to adhesive force which will yield qualitative measures of the interactions between the remaining adsorbents and the rubber surface. Considering the values that were measured, hot water with water fluid by rapid rpm offers the best performance when cleaning the surface. The chemical like sodium hypochlorite is good for thinning the materials, but it solidifies them, which is eventually detrimental to proper functioning. Copyright © 2014 John Wiley & Sons, Ltd.     

Interfacial properties for real rough MEMS/NEMS surfaces by incorporating the electrostatic and Casimir effects–a theoretical study

This paper studied the adhesive properties of real rough micro/nano‐electromechanical systems (MEMS/NEMS) surfaces by considering the electrostatic force and the Casimir force theoretically, and an improved model has been proposed. A statistical approach for characterizing surface topography was used by taking the surface standard deviation, the asperity density and the radius of curvature into account. The effects of surface roughness on the electrostatic force and the Casimir force were analysed individually, and a comparison between the proposed model and existing models has been conducted. The whole adhesive force increases with the surface standard deviation, and the prediction by the proposed model becomes more in agreement with the one by existing models when the surface standard deviation is increased. The contribution of the Casimir force to the total adhesive force tends to vanish when the surface standard deviation is relatively large. The electrostatic force and the Casimir force contribute more to the total adhesive forces calculated based on the proposed model with the increase of the asperity density and the radius of curvature. Copyright © 2009 John Wiley & Sons, Ltd.     

Substrate Morphology and Particle Adhesion in Reacting Systems

This paper describes an effort to measure and model changes in the adhesion of micron-scale particles to substrates in systems in which chemical reactions are occurring. Contact interactions between polystyrene latex spheres and silicon substrates (with surface oxide) immersed in aqueous KNO(3) solutions were studied. Two important results were obtained. First, it was shown that the AFM can be employed to monitor, in situ, changes in adhesive interactions induced by surface chemical reactions in this system. Second, the morphology of the interacting surfaces plays a controlling role in particle adhesion. In particular, for this system, changes in roughness of the substrate changed the interaction force by nearly 90%. Copyright 2000 Academic Press.     

Effect of humidity and temperature on nano/microtribolgical properties of perfluorinated carboxylic acid and hydrogenated carboxylic acid self‐assembled monolayers deposited on aluminum‐coated silicon substrate

Two series of perfluorinated carboxylic acid (FC) and hydrogenated carboxylic acid (HC) self‐assembled monolayer (SAM) films were prepared on aluminum surfaces separately by chemical vapor deposition. The formation, structure and morphology of these films were characterized by measuring contact angle with ellipsometric method, x‐ray photoelectron spectrometry, and atomic force microscopy, respectively. FC and HC SAMs with long chains formed more densely packed films than those with short chains did. The comparative micro/nanoscale friction and adhesive properties of FC and HC SAMs, with various chain lengths on aluminum‐coated silicon substrate, were investigated. The influence of environmental conditions, such as relative humidity (RH) and temperature, on the friction and adhesion behavior was studied. Micro/nanotribological properties of the films were greatly influenced by their backbones and terminal groups. FC SAMs with long chain exhibited adhesion‐resistance, friction reduction, and environmental independence. Copyright © 2009 John Wiley & Sons, Ltd.     

静电纺丝制备具有浸润性梯度的聚酰亚胺纳米纤维膜

采用高压静电纺丝技术, 在非对称异型电极上制备得到放射状聚酰亚胺(PI)纳米纤维膜. 采用环境扫描电子显微镜(ESEM)观察了PI膜的微观形貌以及纳米纤维的排列状态; 采用接触角测量仪研究了水滴浸润性的变化; 采用高敏感性力学微电力学天平测量了水滴的黏附力, 分析了微观形貌变化与水滴浸润性质和黏附性质的关系. 结果表明, 该PI纳米纤维膜沿着非对称异型电极三角电极至弧型电极方向纤维排列由密到疏, 呈放射状, 具有独特的微结构梯度; 整个纤维膜上的PI纳米纤维直径均一且具有光滑均匀表面, 纤维与纤维之间的距离约为几微米到几十微米. 由于PI纳米纤维膜所具有的独特的微结构梯度, 致使沿着微结构梯度方向水滴的接触角(从超疏水到疏水)和黏附力(从低黏附到高黏附)均表现出梯度变化的特征.     

Colloidal interactions between Langmuir-Blodgett bitumen films and fine solid particles

In oil sand processing, accumulation of surface-active compounds at various interfaces imposes a significant impact on bitumen recovery and bitumen froth cleaning (i.e., froth treatment) by altering the interfacial properties and colloidal interactions among various oil sand components. In the present study, bitumen films were prepared at toluene/water interfaces using a Langmuir-Blodgett (LB) upstroke deposition technique. The surface of the prepared LB bitumen films was found to be hydrophobic, comprised of wormlike aggregates containing a relatively high content of oxygen, sulfur, and nitrogen, indicating an accumulation of surface-active compounds in the films. Using an atomic force microscope, colloidal interactions between the LB bitumen films and fine solids (model silica particles and clay particles chosen directly from an oil sand tailing stream) were measured in industrial plant process water and compared with those measured in simple electrolyte solutions of controlled pH and divalent cation concentrations. The results show a stronger long-range repulsive force and weaker adhesion force in solutions of higher pH and lower divalent cation concentration. In plant process water, a moderate long-range repulsive force and weak adhesion were measured despite its high electrolyte content. These findings provide more insight into the mechanisms of bitumen extraction and froth treatment.     

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.     

Approximate scheme for calculating van der Waals interactions between finite cylindrical volume elements

A successful approach to calculating van der Waals (vdW) forces between irregular bodies is to divide the bodies into small cylindrical volume elements and integrate the vdW interactions between opposing elements. In this context it has been common to use Hamaker's expression for parallel plates to approximate the vdW interactions between the opposing elements. This present study shows that Hamaker's vdW expression for parallel plates does not accurately describe the vdW interactions for co-axial cylinders having a ratio of cylinder radius to separation distance (R/D) of 10 or less. This restricts the systems that can be simulated using this technique and explicitly excludes consideration of topographical or compositional variations at the nanoscale for surfaces that are in contact or within a few nm of contact. To address this limitation, approximate analytical expressions for nonretarded vdW forces between finite cylinders in different orientations are derived and are shown to produce a high level of agreement with forces calculated using full numerical solutions of the corresponding Hamaker's equations. The expressions developed here allow accurate calculation of vdW forces in systems where particles are in contact or within a few nm of contact with surfaces and the particles and/or surfaces have heterogeneous nanoscale morphology or composition. These calculations can be performed at comparatively low computational cost compared to the full numerical solution of Hamaker's equations.     

Adhesive contact based on the Lennard-Jones potential: a correction to the value of the equilibrium distance as used in the potential

A Lennard-Jones type surface law is commonly used in adhesive contact modeling; however, one of its parameters, namely the equilibrium distance z0, is not well defined. In this paper, a self-consistent method is used to derive the Lennard-Jones surface law from the interatomic Lennard-Jones potential. The parameters of the surface law are directly related to the material lattice parameter and surface energy, and the equilibrium distance z0 values are obtained for various materials. The effect of using the z0 proposed in the present work is demonstrated via the study of adhesive contact behavior for a single sphere and a flat surface, as well as the contact between planar rough surfaces. For pull-off force prediction of the contact between a single sphere and a flat surface, the error of using the z0 suggested in previous studies could be as large as 10% at intermediate ranges of a dimensionless adhesion parameter. For the contact between planar rough surfaces, the error of using the previously proposed z0 is larger for smoother cases, and the prediction of pull-off force could be different by as much as a factor of 5.     

Si/SiO2-templated formation of ultraflat metal surfaces on glass, polymer, and solder supports: their use as substrates for self-assembled monolayers

This paper describes the use of several methods of template stripping (TS) to produce ultraflat films of silver, gold, palladium, and platinum on both rigid and polymeric mechanical supports: a composite of glass and ultraviolet (UV)-curable adhesive (optical adhesive, OA), solder, a composite of poly(dimethyl siloxane) (PDMS) and OA, and bare OA. Silicon supporting its native oxide layer (Si/SiO2) serves as a template for both mechanical template stripping (mTS), in which the metal film is mechanically cleaved from the template, and chemical template stripping (cTS), in which the film-template composite is immersed in a solution of thiols, and the formation of the SAM on the metal film causes the film to separate from the template. Films formed on all supports have lower root-mean-square (rms) roughness (as measured by atomic force microscopy, AFM) than films used as-deposited (AS-DEP) by electron-beam evaporation. Monolayers of n-dodecanethiolate formed by the mTS and cTS methods are effectively indistinguishable by scanning tunneling microscopy (STM); molecularly resolved images could be obtained using both types of surfaces. The metal surfaces, before being cleaved, are completely protected from contact with the atmosphere. This protection allows metal surfaces intended to support SAMs to be prepared in large batch lots, stored, and then used as needed. Template stripping thus eliminates the requirement for evaporation of the film immediately before use and is a significant extension and simplification of the technology of SAMs and other areas of materials science requiring clean metal surfaces.