The velocity dependence of nanoscale friction is studied for the first time over a wide range of velocities between 1 microm s(-1) and 10 mm s(-1) on large scan lengths of 2 and 25 microm. High sliding velocities are achieved by modifying an existing commercial atomic force microscope (AFM) setup with a custom calibrated nanopositioning piezo stage. The friction and adhesive force dependences on velocity are studied on four different sample surfaces, namely dry (unlubricated), hydrophilic Si(100); dry, partially hydrophobic diamond-like carbon (DLC); a partially hydrophobic self-assembled monolayer (SAM) of hexadecanethiol (HDT); and liquid perfluoropolyether lubricant, Z-15. The friction force values are seen to reverse beyond a certain critical velocity for all the sample surfaces studied. A comprehensive friction model is developed to explain the velocity dependence of nanoscale friction, taking into consideration the contributions of adhesion at the tip-sample interface, high impact velocity-related deformation at the contacting asperities and atomic scale stick-slip. A molecular spring model is used for explaining the velocity dependence of friction force for HDT.

Scale dependence of micro/nanotribological properties is studied for various materials, coatings and lubricants used in micro/nanoelectromechanical systems (MEMS/NEMS). The adhesive force and friction force dependence on rest time and sliding velocity and the effect of relative humidity and temperature on the scale dependence of these properties is studied. The scale dependence of the coefficient of friction is attributable to the sample surface roughness and the scan size. For larger scan sizes the sliding interface encounters larger asperities and so friction force is higher. The adhesive force is higher on the microscale although on the nanoscale surface forces such as electrostatic attraction that are generally negligible on the microscale can become dominant. The difference in the adhesive force on the micro- and nanoscale for different rest times, relative humidities and temperatures is due to the meniscus force dependence on the sample surface roughness. The velocity dependence of the friction force shows significant scale dependence due to the scale dependent roughness and the higher contact pressures that are encountered on the nanoscale.

Nanolatex Synthesis and CharacterizationOur nanolatexes were prepared via microemulsion polymerization, a special type of dispersion polymerization, from aqueous acrylate/methacrylate microemulsions stabilized by reactive ionic liquid surfactants. To exemplify our process we used methylmethacrylate (MMA) as a comonomer along with the ionic liquid reactive acrylate surfactant 1-(11acryloyloxyundecyl)-3-methylimidazolium bromide (ILBr) to compose the nanolatex precursor solution (microemulsion). ILBr was prepared as described in Refs. 11, 17, and 26 of the main text. This monomer contains an approximately 1% cross-linker, 1,3-bis(11acryloyloxyundecyl)imidazolium bromide as a preparative impurity from alkyl group scrambling in the quaternization step. Ionic liquids are defined as salts, mostly organic, that melt below 100°C; ILBr melts at about 50°C. Microemulsion PolymerizationA partial ternary phase diagram of this system is illustrated in Fig. S1, where one of the compositions for microemulsion polymerization we utilized is indicated by the "✕" in the lower left corner. The microemulsion domain is a thermodynamically stable and somewhat exotic single phase solution. In the illustrated domain, MMA swollen micelles of ILBr are the most prevalent complexes, although irregular bicontinuous microstructure exists at higher surfactant levels.The latexes were prepared by diluting a stock solution of 60% ILBr (w/w) in MMA with the appropriate amounts of water to reach a total volume of 25 ml for each composition. AIBN initiator was present in the MMA stock solution at 0.5% (w/w) with respect to total monomer weight. The clear microemulsion solutions were then heated overnight at 60 °C in a temperature controlled bath.The compositions ranging from 2 -4% ILBr content showed an increasing presence of a light blue Tyndall haze. This increasing blue haze indicated particles were present and that a dispersion had formed. Another indicator of particle formation was the increasing suspension viscosity with increasing ILBr/MMA content.Portions of each sample were dialyzed against daily changes of deionized water for three days in order to remove unreacted monomer. Dialysis was performed by placing approximately 1.5 ml of latex sample inside a 7 cm piece of regenerated cellulose dialysis

Silicon and aluminium are the substrates of choice for various micro/nanoelectromechanical systems (MEMS/NEMS) including digital micromirror devices (DMD ® ). For efficient and failure-proof operation of these devices, ultrathin lubricant films of self-assembled monolayers (SAMs) are increasingly being employed. In this study, we investigate friction, adhesion and wear properties of various SAMs. Surface properties such as contact angle, adhesive force, friction force and coefficient of friction are compared for SAMs with hydrocarbon and fluorocarbon backbone chains with different chemical structures, chain lengths and end groups. The influence of relative humidity, temperature and sliding velocity on the friction and adhesion behaviour is studied for various SAMs. Failure mechanisms of SAMs are investigated by wear tests and the potential mechanisms involved are discussed. These studies are expected to aid the design and selection of proper lubricants for MEMS/NEMS.

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