Robust perfluorosilanized copper surfaces

Polished copper (Cu) surfaces modified with 1H,1H,2H,2H‐perfluorodecyltrichlorosilane (PFTS) have been shown to be very hydrophobic and stable. Mechanically polished, oxidized, and PFTS‐reacted Cu surfaces were characterized by X‐ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy, which confirmed the presence of perfluorinated alkyl chains. For a PFTS‐modified Cu surface (PFTS/Cu), the sessile drop static contact angle of pure water at ambient temperature and high relative humidity (RH) was measured to be more than 125° and the Zisman critical surface energy to be typically less than 16 mN/m. Ellipsometry showed the thickness of the PFTS/Cu film to be typically less than 25 nm. Stability tests indicated that the PFTS/Cu film could survive in pure boiling water for one hour, boiling nitric acid (pH 1.5 or 1.8) for 30 min, sodium hydroxide solution (pH 12, 70 °C) for 30 min, and autoclave conditions (steam at 134 °C and 3 atm) for 15 min. The more commonly used self‐assembled monolayer (SAM) modifications of Cu surfaces, e.g. thiol compounds, are significantly less stable than PFTS/Cu. The extremely hydrophobic and stable PFTS/Cu could be a very good candidate for corrosion inhibition and/or heat exchangers exploiting condensation. Copyright © 2005 John Wiley & Sons, Ltd.     

Phosphonate self-assembled monolayers on aluminum surfaces

Substrates of aluminum (Al) deposited by physical vapor deposition onto Si substrates and then chemically reacted with perfluorodecylphosphonic acid (PFDPAlSi), decylphosphonic acid (DPAlSi), and octadecylphosphonic acid (ODPAlSi) were studied by x-ray photoelectron spectroscopy (XPS), contact angle measurements, atomic force microscopy (AFM), and friction force microscopy, a derivative of AFM, to characterize their surface chemical composition, roughness, and micro-/nanotribological properties. XPS analysis confirmed the presence of perfluorinated and nonperfluorinated alkylphosphonate molecules on the PFDPAlSi, DPAlSi, and ODPAlSi. The sessile drop static contact angle of pure water on PFDPAlSi was typically more than 130 degrees and on DPAlSi and ODPAlSi typically more than 125 degrees indicating that all phosphonic acid reacted AlSi samples were very hydrophobic. The surface roughness for PFDPAlSi, DPAlSi, ODPAlSi, and bare AlSi was approximately 35 nm as determined by AFM. The surface energy for PFDPAlSi was determined to be approximately 11 mNm by the Zisman plot method compared to 21 and 20 mNm for DPAlSi and ODPAlSi, respectively. Tribology involves the measure of lateral forces due to friction and adhesion between two surfaces. Friction, adhesion, and wear play important roles in the performance of micro-/nanoelectromechanical systems. PFDPAlSi gave the lowest adhesion and coefficient of friction values while bare AlSi gave the highest. The adhesion and coefficient of friction values for DPAlSi and ODPAlSi were comparable.     

Friction and adhesion between C60 single crystal surfaces and AFM tips: effects of the orientational phase transition

We have investigated the nanotribological properties of C60 single crystal (111) and (100) surfaces around its orientational order-disorder phase transition temperature, approximately 260 K, by atomic force microscopy and frictional force microscopy (AFM/FFM) in high vacuum. Results show that for both surfaces across the phase transition temperature, the friction force and the adhesive force between a C60 coated AFM tip and the C60 crystal surfaces exhibit discontinuous behavior. The friction force within the applied external load range in the low temperature phase is significantly larger than that in the high temperature phase, with no obvious change in the slope of the friction force curves (the friction coefficient) in the low and high temperature phases. The abrupt change in friction was found to be caused mainly by the abrupt change in adhesion, which, in turn, can be qualitatively understood through changes in the van der Waals interaction and the short-range Coulomb interaction associated with the structural changes across the phase transition. Compared to most other degrees of freedom, the rotation of C60 molecules was found to have little effect on friction and is an ineffective energy dissipation channel.     

Molecular tilting and its impact on frictional properties of n-alkane self-assembled monolayers

Hydrophobic, methyl-terminated self-assembled monolayer (SAM) surfaces can be used to reduce friction. Among methyl-terminated SAMs, the frictional properties of alkanethiol SAMs and silane SAMs have been well-studied. In this research, we investigated friction of methyl-terminated n-hexatriacontane (C36) SAM and compared its friction properties with the alkanethiol and silane SAMs. Alkane SAM does not have an anchoring group. The alkane molecules stand on the surface by physical adsorption, which leads to a higher surface mobility of alkane molecules. We found that C36 SAM has a higher coefficient of friction than that of octadecyltrichlorosilane (OTS) silane. When an atomic force microscope (AFM) tip was swiped across the alkane SAM with a loading force, we found that the alkane SAM can withstand the tip loading pressure up to 0.48 GPa. Between 0.48 and 0.49Ga, the AFM tip partially penetrated the SAM. When the tip moved away, the deformed SAM healed and maintained the structural integrity. When the loading pressure was higher than 0.49 GPa, the alkane SAM was shaved into small pieces by the tip. In addition, we found that the molecular tilting of C36 molecules interacted with the tribological properties of the alkane SAM surface. On one hand, a higher loading force can push the rod-like alkane molecules to a higher tilting angle; on the other hand, a higher molecular tilting leads to a lower friction surface.     

In situ adsorption studies of a 14-amino acid leucine-lysine peptide onto hydrophobic polystyrene and hydrophilic silica surfaces using quartz crystal microbalance, atomic force microscopy, and sum frequency generation vibrational spectroscopy

The adsorption of a 14-amino acid amphiphilic peptide, LK14, which is composed of leucine (L, nonpolar) and lysine (K, charged), on hydrophobic polystyrene (PS) and hydrophilic silica (SiO2) was investigated in situ by quartz crystal microbalance (QCM), atomic force microscopy (AFM), and sum frequency generation (SFG) vibrational spectroscopy. The LK14 peptide, adsorbed from a pH 7.4 phosphate-buffered saline (PBS) solution, displayed very different coverage, surface roughness and friction, topography, and surface-induced orientation when adsorbed onto PS versus SiO2 surfaces. Real-time QCM adsorption data revealed that the peptide adsorbed onto hydrophobic PS through a fast (t < 2 min) process, while a much slower (t > 30 min) multistep adsorption and rearrangement occurred on the hydrophilic SiO2. AFM measurements showed different surface morphologies and friction coefficients for LK14 adsorbed on the two surfaces. Surface-specific SFG spectra indicate very different ordering of the adsorbed peptide on hydrophobic PS as compared to hydrophilic SiO2. At the LK14 solution/PS interface, CH resonances corresponding to the hydrophobic leucine side chains are evident. Conversely, only NH modes are observed at the peptide solution/SiO2 interface, indicating a different average molecular orientation on this hydrophilic surface. The surface-dependent difference in the molecular-scale peptide interaction at the solution/hydrophobic solid versus solution/hydrophilic solid interfaces (measured by SFG) is manifested as significantly different macromolecular-level adsorption properties on the two surfaces (determined via AFM and QCM experiments).     

Chemical Force Microscopy Study of Adhesion and Friction between Surfaces Functionalized with Self-Assembled Monolayers and Immersed in Solvents

Adhesive and frictional forces between surfaces modified with self-assembled monolayers (SAMs) and immersed in solvents were measured with chemical force microscopy as functions of surface functionality and solvent. Si/SiO2 substrates were modified with SAMs of alkylsiloxanes (SiCl3(CH2)n-X), and gold-coated AFM tips were modified with SAMs of alkylthiolates (HS-(CH2)n-X). SAMs of alkylsiloxanes terminated in a methyl or oxidized vinyl group; SAMs of alkanethiolates terminated in a methyl or carboxyl group. Adhesive and frictional forces were measured in hexadecane, ethanol, 1,2-propanediol, 1,3-propanediol, and water. The work of adhesion (W) was calculated with the Johnson-Kendall-Roberts theory of adhesive contact. The JKR values agreed well with values derived from the Fowkes-van Oss-Chaudhury-Good surface tension model and from contact angle results. Calculated values of W for all combinations of contacting surfaces and solvents spanned two orders of magnitude. W correlated with the surface tension of the solvent for hydrophobic/hydrophobic interactions; hydrophilic/hydrophilic and hydrophobic/hydrophilic interactions were more complex. Friction forces were fit to a modified form of Amonton's law. For any solvent, friction coefficients were largest for the hydrophilic/hydrophilic contacting surfaces. The friction coefficient for any contacting pair was largest in hexadecane. In polar solvents, friction coefficients scaled with solvent polarity only for hydrophobic/hydrophobic contacting pairs. Copyright 1999 Academic Press.     

Self-assembled monolayers prepared from ω-thiophene-functionalized n-alkyltrichlorosilane on silicon substrates

Self-assembled monolayers (SAMs) of thienyl-functionalized n-alkyltrichlorosilane (11-(3-thienyl)undecyltrichlorosilane [TUTS]) have been prepared by adsorption from solution and characterized by using X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), contact angle measurements, ellipsometry, and scanning electron microscopy (SEM). Using contact angle and SEM measurements, the film preparation protocol was optimized, resulting in reproducible SAM formation with no adverse deposition of polysiloxane particles. XPS and ellipsometry studies confirmed the existence of SAM formation. AFM results show a smooth and homogeneous SAM, with surface roughness of Ra≤0.2 nm, which is slightly higher than the corresponding values for octadecyltrichlorosilane (OTS) SAMs. Such thiophene-based SAM surfaces can be used for surface-initiated polymerization of thiophene. The resulting formed polythiophene layers at non-compatible surfaces offer some practical applications in manufacturing [W. Plieth, A. Fikus, D. Appelhans, H.-J. Adler, German Patent Application No. 2661977 (1998); D. Appelhans, D. Ferse, H.-J. Adler, A. Fikus, W. Plieth, B. Aldolphi et al., J. Electrochem. Soc. (accepted)].     

Alkylphosphonate modified aluminum oxide surfaces

The surface properties of aluminum, such as chemical composition, roughness, friction, adhesion, and wear, can play an important role in the performance of micro-/nano-electromechanical systems, e.g., digital micromirror devices. Aluminum substrates chemically reacted with octadecylphosphonic acid (ODP/Al), decylphosphonic acid (DP/Al), and octylphosphonic acid (OP/Al) have been investigated and characterized by X-ray photoelectron spectroscopy (XPS), contact angle measurements, and atomic force microscopy (AFM). XPS analysis confirmed the presence of alkylphosphonate molecules on ODP/Al, DP/Al, and OP/Al. No phosphonates were found on bare Al as a control. The sessile drop static contact angle of pure water on ODP/Al and DP/Al was typically more than 115 degrees and on OP/Al typically less than 105 degrees indicating that all phosphonic acid reacted Al samples were highly hydrophobic. The root-mean-square surface roughness for ODP/Al, DP/Al, OP/Al, and bare Al was less than 15 nm as determined by AFM. The surface energy for ODP/Al and DP/Al was determined to be approximately 21 and 22 mJ/m2, respectively, by the Zisman plot method, compared to 25 mJ/m2 for OP/Al. ODP/Al and OP/Al were studied by friction force microscopy, a derivative of AFM, to better understand their micro-/nano-tribological properties. ODP/Al gave the lowest coefficient of friction values while bare Al gave the highest. The adhesion forces for ODP/Al and OP/Al were comparable.     

Forces between polyethylene surfaces in oxyethylene dodecyl ether solutions as influenced by the number of oxyethylene groups

The atomic force microscopy (AFM) colloidal probe technique was used to study the effect of oxyethylene dodecyl ethers, C12En (n = 1-7), on interactions between hydrophobic polyethylene (PE) surfaces in aqueous solutions. Long-range (colloidal) and contact (pull-off) forces were measured between 10 to 20 microm PE spheres and a flat PE surface at concentrations of surfactant of 1 x 10(-6) and 1 x 10(-4) M. The surface tension of the surfactant solutions and contact angles at PE surfaces were also studied. The influence of the number of oxyethylene groups in the surfactant molecule was examined. Initially, long-range attractive (hydrophobic) forces between the PE surfaces were observed that decreased in range and magnitude with an increase in the number of oxyethylene groups in 1 x 10(-4) M solutions. Above four oxyethylene groups per molecule, repulsive forces were observed. The measured pull-off force between PE surfaces decreased monotonically from approximately 500 mJ/m2 for C12E1 to 150 mJ/m2 for C12E7. The interfacial energy was calculated on the basis of the JKR model, taking into account long-range forces operating outside the contact area. The interfacial energies decreased from 43-47 mJ/m2 for PE-water and PE-C12E1 (1 x 10(-4) M) interfaces to approximately 18 mJ/m2 for PE-C12E7 (1 x 10(-4) M). The interfacial energy was also calculated from measured contact angles and surface tensions using Neumann's equation of state and Young's equation. A similar relationship between interfacial energy and the number of oxyethylene groups was observed on the basis of contact and surface tension measurements. However, interfacial energy values were smaller, within 15-20 mJ/m2, than those calculated from AFM pull-off force measurements.     

Effect of surface wettability on the adhesion of proteins

Besides significantly broadening the scope of available data on adhesion of proteins on solid substrates, we demonstrate for the first time that all seven proteins (tested here) behave similarly with respect to adhesion exhibiting a step increase in adhesion as wettability of the solid substrate decreases. Also, quantitative measures of like-protein-protein and like-self-assembled-monolayer (SAM)-SAM adhesive energies are provided. New correlations, not previously reported, suggest that the helix and random content (as measures of secondary structure) normalized by the molecular weight of a protein are significant for predicting protein adhesion and are likely related to protein stability at interfaces. Atomic force microscopy (AFM) was used to directly measure the normalized adhesion or pull-off forces between a set of seven globular proteins and a series of eight well-defined model surfaces (SAMs), between like-SAM-immobilized surfaces and between like-protein-immobilized surfaces in phosphate buffer solution (pH 7.4). Normalized force-distance curves between SAMs (alkanethiolates deposited on gold terminated with functional uncharged groups -CH3, -OPh, -CF3, -CN, -OCH3, -OH, -CONH2, and -EG3OH) covalently attached to an AFM cantilever tip modified with a sphere and covalently immobilized proteins (ribonuclease A, lysozyme, bovine serum albumin, immunoglobulin, gamma-globulins, pyruvate kinase, and fibrinogen) clearly illustrate the differences in adhesion between these surfaces and proteins. The adhesion of proteins with uncharged SAMs showed a general "step" dependence on the wettability of the surface as determined by the water contact angle under cyclooctane (thetaco). Thus, for SAMs with thetaco < approximately 66 degrees, (-OH, -CONH2, and -EG3OH), weak adhesion was observed (>-4 +/- 1 mN/m), while for approximately 66 < thetaco < approximately 104 degrees, (-CH3, -OPh, -CF3, -CN, -OCH3), strong adhesion was observed (< or =8 +/- 3 mN/m) that increases (more negative) with the molecular weight of the protein. Large proteins (170-340 kDa), in contrast to small proteins (14 kDa), exhibit characteristic stepwise decompression curves extending to large separation distances (hundreds of nanometers). With respect to like-SAM surfaces, there exists a very strong adhesive (attractive) interaction between the apolar SAM surfaces and weak interactive energy between the polar SAM surfaces. Because the polar surfaces can form hydrogen bonds with water molecules and the apolar surfaces cannot, these measurements provide a quantitative measure of the so-called mean hydrophobic interaction (approximately -206 +/- 8 mN/m) in phosphate-buffered saline at 296 +/- 1 K. Regarding protein-protein interactions, small globular proteins (lysozyme and ribonuclease A) have the least self-adhesion force, indicating robust conformation of the proteins on the surface. Intermediate to large proteins (BSA and pyruvate kinase-tetramer) show measurable adhesion and suggest unfolding (mechanical denaturation) during retraction of the protein-covered substrate from the protein-covered AFM tip. Fibrinogen shows the greatest adhesion of 20.4 +/- 2 mN/m. Unexpectedly, immunoglobulin G (IgG) and gamma-globulins exhibited very little adhesion for intermediate size proteins. However, using a new composite index, n (the product of the percent helix plus random content times relative molecular weight as a fraction of the largest protein in the set, Fib), to correlate the normalized adhesion force, IgG and gamma-globulins do not behave abnormally as a result of their relatively low helix and random (or high sheet) content.     

Fabrication of flexible gold films with periodic sub-micrometer roughness and their wettability control by modification of SAM

Flexible honeycomb gold films supported by polymer sheets are fabricated by using polystyrene particle monolayers. The surfaces of the flexible gold films are covered with self-assembled monolayers (SAMs) of hydrophobic or hydrophilic thiol compounds, and the wettability of the modified surface is evaluated by measurements of the contact angles of water droplets. The contact angle of the film covered with hydrophobic SAM is ca. 150 degrees, which is greater than the value of 112 degrees for a flat gold surface, while the values for hydrophilic SAM are below 10 degrees.     

Chemical force titrations of functionalized Si(111) surfaces

Chemical force titrations-plots of the adhesive force between an atomic force microscope tip and sample as a function of pH-were acquired on alkyl monolayer-derivatized Si(111) surfaces. Gold-coated AFM tips modified with thioalkanoic acid self-assembled monolayers (SAM) were employed. Alkyl monolayer-derivatized Si(111) surfaces terminated with methyl, carboxyl, and amine groups were produced via hydrosilylation reactions between 1-alkene reagents and H-terminated silicon. The functionalized surfaces were characterized using standard surface science techniques (AFM, FTIR, and XPS). Titration of the methyl-terminated surface using the modified (carboxyl-terminated) atomic force microscope tip resulted in a small pH-independent hydrophobic interaction. Titration of the amine-terminated surface using the same tip resulted in the determination of a surface pKa of 5.8 for the amine from the pH value from the maximum in the force titration curve. A pK(1/2) of 4.3 was determined for the carboxyl-terminated Si(111) in a similar way. These results will be discussed in relation to the modified Si(111) surface chemistry and organic layer structure, as well as with respect to existing results on Au surfaces modified with SAMs bearing the same functional groups.     

辛基三乙氧基硅烷/十八烷基三氯硅烷均相混合自组装单分子膜的表征及其形成机制

使用接触角、原子力显微镜(AFM)、静电力显微镜(EFM)和傅里叶变换红外(FTIR)光谱对辛基三乙氧基硅烷(C8TES)/十八烷基三氯硅烷(OTS)均相混合自组装单分子膜(SAM)及其形成过程中样品表面的润湿性、表面形貌、表面电势和膜内分子的有序度进行了表征,对采用分步法利用C8TES分子空间位阻制备C8TES/OTS均相混合SAM的反应机制进行了研究.结果表明,C8TES/OTS均相混合SAM表面接触角为105°,样品表面平整、光滑;对样品表面电势进行分析后发现,混合SAM表面电势分布均匀,电势频率分布为典型的正态分布;在均相混合SAM的形成过程中,样品表面电势的分布始终十分均匀,电势频率分布均为典型的正态分布;C8TES/OTS均相混合SAM是一种具有上下两层分子排列密度不同的膜结构的单分子膜,其内部结构至少在500 nm×500 nm到20μm×20μm尺度上是高度均匀一致的,膜内没有明显的特征结构,具有典型的均相混合SAM特征.     

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.     

Nanotribological properties of alkanephosphonic acid self-assembled monolayers on aluminum oxide: effects of fluorination and substrate crystallinity

Two phosphonic acid (PA) self-assembled monolayers (SAMs) are studied on three aluminum oxide surfaces: the C and R crystallographic planes of single crystal alpha-alumina (sapphire) and an amorphous vapor-deposited alumina thin film. SAMs are either fully hydrogenated CH3(CH2)17PO3H2 or semifluorinated CF3(CF2)7(CH2)11PO3H2. Atomic force microscope (AFM) topographic imaging reveals that the deposited films are homogeneous, atomically smooth, and stable for months in the laboratory environment. Static and advancing contact angle measurements agree with previous work on identical or similar films, but receding measurements suggest reduced coverage here. To enable reproducible nanotribology measurements with the AFM, a scanning protocol is developed that leads to a stable configuration of the silicon tip. Adhesion for the semifluorinated films is either comparable to or lower than that for the hydrogenated films, with a dependence on contact history observed. Friction between each film and the tips depends strongly upon the type of molecule, with the fluorinated species exhibiting substantially higher friction. Subtle but reproducible differences in friction are observed for a given SAM depending on the substrate, revealing differences in packing density for the SAMs on the different substrates. Friction is seen to increase linearly with load, a consequence of the tip's penetration into the monolayer.     

辛基三乙氧基硅烷/十八烷基三氯硅烷均相混合自组装单分子膜的表征及其形成机制

使用接触角、原子力显微镜（AFM）、静电力显微镜（EFM）和傅里叶变换红外（FTIR）光谱对辛基三乙氧基硅烷（C8TES）/十八烷基三氯硅烷（OTS）均相混合自组装单分子膜（SAM）及其形成过程中样品表面的润湿性、表面形貌、表面电势和膜内分子的有序度进行了表征,对采用分步法利用C8TES分子空间位阻制备C8TES/OTS均相混合SAM的反应机制进行了研究. 结果表明,C8TES/OTS均相混合SAM表面接触角为105°,样品表面平整、光滑;对样品表面电势进行分析后发现,混合SAM表面电势分布均匀,电势频率分布为典型的正态分布;在均相混合SAM的形成过程中,样品表面电势的分布始终十分均匀,电势频率分布均为典型的正态分布;C8TES/OTS均相混合SAM是一种具有上下两层分子排列密度不同的膜结构的单分子膜,其内部结构至少在500 nm×500 nm到20 μm×20 μm尺度上是高度均匀一致的,膜内没有明显的特征结构,具有典型的均相混合SAM特征.     

Cholesterol biosensor based on amino-undecanethiol self-assembled monolayer using surface plasmon resonance technique

Cholesterol oxidase has been covalently immobilized onto 11-amino-1-undecanethiol hydrochloride (AUT) self-assembled monolayer (SAM) fabricated on gold (Au) substrates using glutaraldehyde as a cross-linker. These ChOx/AUT/Au bioelectrodes characterized using contact angle (CA) measurements; electrochemical technique and atomic force microscopy (AFM) have been utilized for the estimation of cholesterol in solution using the surface plasmon resonance (SPR) technique. These biosensing electrodes exhibiting linearity from 50 to 500 mg/dL of cholesterol in solution and sensitivity of 1.23 m0/(mg dL), can be used more than 20 times and have a shelf life of about 10 weeks when stored at 4 degrees C.     

Preparation and tribological properties of self‐assembled poly(amide amine)‐Cu nanofilm on silicon

Self‐assembled poly(amide amine)‐copper (PAMAM/Cu) film on silicon was prepared and investigated by means of contact angle measurement, XPS and atomic force microscopy (AFM). The tribological properties were evaluated using a reciprocal ball‐on‐disc test rig and a lateral force microscope. Results of XPS show the existence of Cu(0) and PAMAM molecule on the surface of the film. Compared with the self‐assembled monolayer of the poly(amide amine) generation 4.0 dendrimer, the friction force of PAMAM/Cu film is lower and the friction coefficient is smaller which can be attributed to the existence of Cu nanoparticles. The PAMAM/Cu film shows a good lubrication effect. The stability of friction and wear resistance of film is improved. Copyright © 2011 John Wiley & Sons, Ltd.     

Nanoscale electrowetting effects observed by using friction force microscopy

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

Friction and nanowear of polystyrene against hydrophobic and hydrophilic substrates

The ambition of this study is to analyze the role of interfacial interactions in friction and nanowear of polystyrene, by comparing friction against hydrophobic wafers (methyl‐terminated) and hydrophilic wafers (hydroxyl‐terminated) as a function of sliding velocity and normal force. Friction experiments are performed with a translation tribometer and nanowear investigation is achieved by using atomic force microscopy (AFM) analysis of the wafer surfaces after friction. Experimental results show that the friction coefficients measured on hydrophilic surfaces are always larger than those obtained with hydrophobic surfaces, indicating a relationship between friction and interfacial interactions. Elsewhere, AFM analysis shows that polystyrene transfer appears for a higher normal force in the case of hydrophobic substrates compared to hydrophilic one. However, the corresponding tangential (or friction) force necessary to detect transfer is quite similar for both types of substrates, indicating that the initial wear of polystyrene occurs for a similar threshold interfacial shear. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2449–2454, 2006