This chapter highlights a variety of techniques that are commonly used to measure contact angles, including the conventional telescope-goniometer method, the Wilhelmy balance method, and the more recently developed drop-shape analysis methods. The various applications and limitations of these techniques are described. Notably, studies of ultrasmall droplets on solid surfaces allow wetting theories to be tested down to the nanometer scale, bringing new insight to contact angle phenomena and wetting behavior.

https://www.springer.com/cda/content/document/cda_downloaddocument/9783642342424-c1.pdf?SGWID=0-0-45-1368817-p174704744

Contact Angle and Wetting Properties

Common industrial lubricants include natural and synthetic hydrocarbons and perfluoropolyethers (PFPEs), where the latter is widely used in commercial applications requiring extreme operating conditions due to their high temperature stability and extremely low vapor pressure. However, PFPEs exhibit low electrical conductivity, making them undesirable in some nanotechnology applications. Ionic liquids (ILs) have been explored as lubricants for various device applications due to their excellent electrical conductivity as well as good thermal conductivity, where the latter allows frictional heating dissipation. Since they do not emit volatile organic compounds, they are regarded as “green” lubricants. In this article, we review the different types of ILs and their physical properties responsible for lubrication. We also discuss their suitability as lubricants, since the long-term performance of ILs as lubricants may be affected by issues such as corrosion, oxidation, tribochemical reactions, and toxicity. We present nanotribological, electrical, and spectroscopic studies of IL films along with conventional tribological investigations, recognizing that understanding the tribological performance at various length scales is a crucial step in selecting and designing effective lubricants.

A Review of Ionic Liquids for Green Molecular Lubrication in Nanotechnology

Nanotribology and nanomechanics of MEMS/NEMS and BioMEMS/BioNEMS materials and devices

Enhancing lubricant properties by nanoparticle additives

This paper reviews recent progress in molecular dynamics simulation of atomic-scale friction measured by an atomic force microscopy. Each section of the review focuses on an individual condition or parameter that affects atomic friction including materials, surfaces, compliance, contact area, normal load, temperature, and velocity. The role each parameter plays is described in the context of both experimental measurements and simulation predictions. In addition, the discussion includes an overview of the research community's current understanding of observed effects, guidelines for implementation of those effects in an atomistic simulation, and suggestions for future research to address open questions. Taken together, this review conveys the message that friction at the atomic scale is affected by many interrelated parameters and that the use of molecular dynamics simulation as a predictive tool can be accomplished only through careful model design.

Molecular dynamics simulation of atomic friction: A review and guide

Bonding, degradation, and environmental effects on novel perfluoropolyether lubricants

An atomic force microscopy (AFM) based technique was developed to measure the wetting properties of probe tips. By advancing and receding the AFM tip across the water surface, the meniscus force between the tip and the liquid was measured at the tip?water separation. The water contact angle was determined from the meniscus force. The obtained contact angle results were compared with that by the sessile drop method. It was found that the AFM based technique provided higher contact angle values than the sessile drop method. The mechanisms responsible for the difference are discussed.

https://iopscience.iop.org/article/10.1088/0022-3727/39/17/023/pdf

Wetting properties of AFM probes by means of contact angle measurement

Micro-/nanoelectromechanical devices and components operate at high sliding velocities. Studies of tribological properties of materials by atomic force microscope (AFM) are limited to low velocity regime (<100–250μm∕s) due to inherent instrument limitation of AFM. To overcome the limitation of AFM working velocity, a custom-calibrated ultrahigh velocity stage has been incorporated for tribological investigation up to 200mm∕s. The stage is driven by a piezoactuator which is controlled using proportional-integral-derivative algorithm. Friction data are obtained by processing the AFM laser photodiode signals using a high sampling rate data acquisition system. The friction data obtained from the modified setup at high sliding velocities are compared with the results from conventional AFM friction measurement. The effects of scan size, rest time, and velocity on the friction force and adhesive force for single crystal silicon (100) with native oxide have been studied.

New technique for studying nanoscale friction at sliding velocities up to 200mm∕s using atomic force microscope

Si 3 N 4 probes used for atomic force microscope exhibit large adhesion and friction resulting in artifacts of scanned image. In order to reduce adhesion and friction so as to reduce tip related artifacts, liquid lubricant (Z-TETRAOL) and fluorocarbon polymer (Fluorinert™) were applied on the Si 3 N 4 probe. A comprehensive investigation of adhesion, friction, and wear of the uncoated/coated tips in both ambient air and various humidity levels as well as the influence of the coatings on the image resolution was performed. Experiments show that the coatings reduce the adhesion and friction of the Si 3 N 4 tip, improve the initial image resolution, and exhibit less deterioration as compared to that ofuncoated tip after scanning. The image degradation of an uncoated Si 3 N 4 probe is also compared with that of an uncoated silicon probe. A probe cantilever deflection model was proposed to correlate the influence of the adhesion and friction with the image distortion.

Surface Modification of AFM Si3N4 Probes for Adhesion/Friction Reduction and Imaging Improvement

Micro∕nanoelectromechanical system devices and components often operate at high sliding velocities, and it requires the investigation of friction at high velocities. In this study, the velocity dependence of friction and the rest time effect on friction of hard diamondlike carbon films, soft perfluorodecyltrichlorosilane, and perfluoropolyether films were investigated up to 2×105μm∕s using an ultrahigh velocity stage and a high velocity stage. The velocity dependence of friction was found to vary with films and involve different mechanisms including adhesion due to solid-solid interaction, adhesion due to formation of meniscus bridges, atomic-scale stick slip, high velocity impact of the contacting asperities∕molecules, phase transformation∕tip jump, and viscoelastic shear. The increase in friction as a function of rest time was caused by continued growth and formation of water menisci bridges for the hydrophilic surfaces and continued deformation of soft films.

Zhenhua Tao

Papers

Bonding, degradation, and environmental effects on novel perfluoropolyether lubricants

Wetting properties of AFM probes by means of contact angle measurement

New technique for studying nanoscale friction at sliding velocities up to 200mm∕s using atomic force microscope

Surface Modification of AFM Si3N4 Probes for Adhesion/Friction Reduction and Imaging Improvement

Velocity dependence and rest time effect on nanoscale friction of ultrathin films at high sliding velocities