Spreading of perfluoropolyalkylether films on amorphous carbon surfaces

Molecularly thin fluoro-polymeric nanolubricant films: tribology, rheology, morphology, and applications

Molecularly thin perfluoropolyether (PFPE) has been used extensively as a high-performance lubricant in various applications and, more importantly, on carbon overcoats to enhance the reliability and lubrication of micro-/nanoelectro-mechanical systems, where the tribological performance caused by its molecular architecture is a critical issue, as are its physical properties and rheological characteristics. This Highlight addresses recent trends in the development of fluoro-polymeric lubricant films with regard to their tribology, rheology, and physio-chemical properties as they relate to heat-assisted magnetic recording. Nanorheology has been employed to examine the dynamic response of nonfunctional and functional PFPEs, while the viscoelastic properties of nanoscale PFPE films and the relaxation processes as a function of molecular structure and end-group functionality were analyzed experimentally; furthermore, the characteristics of binary blends were reported.

Hydrophobic but hygroscopic polymer films--identifying interfacial species and understanding water ingress behavior

The hydrophobic but hygroscopic nature of polydimethylsiloxane (PDMS) with quaternary ammonium cationic side chains adsorbed on a SiO(2) surface was investigated with sum frequency generation vibration spectroscopy (SFG) and attenuated total reflectance infrared spectroscopy (ATR-IR). PDMS with cationic side chains, named cationic polymer lubricant (CPL), forms a self-healing boundary lubrication film on SiO(2). It is interesting that CPL films are externally hydrophobic but internally hydrophilic. The comparison of SFG and ATR-IR data revealed that the methyl groups of the PDMS backbone are exposed at the film/air interface and the cationic side groups and counterions are embedded within the film. The hydrophobicity must originate from the surface CH(3) groups, while the ionic groups inside the film must be responsible for water uptake. The surface hydrophobicity can alleviate the capillary adhesion while the hygroscopic property enhances the mobility and self-healing capability of the CPL boundary lubrication film.

Identification of mobile species in cationic polymer lubricant layer on silicon oxide from AFM and XPS analyses

The nanoscale spreading of a cationic polymer lubricant (CPL) film consisting of polydimethylsiloxane with quaternary ammonium salt side chains on a SiO(2) surface was studied with the disjoining pressure measurements using atomic force microscopy. CPL shows a monotonic decrease in disjoining pressure as the film thickness increases from 1.3 to 4.5 nm, which suggests stable spreading in this thickness range. Comparing the spreading rates calculated from disjoining pressure and the viscosity of CLP to the self-healing time after tribo-contacts revealed that the ionic form may not be the main mobile species. The X-ray photoelectron spectroscopy analysis found that the CPL film on SiO(2) has about 30% of the quaternary ammonium salts (cationic groups) reduced to tertiary amines (neutral groups). The reduced CPL polymer has much lower viscosity than the original CPL polymer and yields a spreading rate consistent with that measured at the macroscale. Thus, the mobile component in the CPL/SiO(2) film responsible for self-healing is concluded to be the reduced tertiary amine components of CPL.

Motion picture imaging of a nanometer-thick liquid film dewetting by ellipsometric microscopy with a submicrometer lateral resolution

We visualized the detwetting of a nanometer-thick unstable liquid film on a nanotextured solid surface with a high lateral spatial resolution. The dewetting was imaged as a motion picture at a submicrometer spatial resolution and a frame rate of 4 frames/s, using ellipsometric microscopy in a vertical objective configuration. The observation revealed that the dewetting process significantly depends on the sign of the disjoining pressure Pi. When Pi is negative, the film rupture due to the spinodal dewetting proceeds to droplet formation in a single step, whereas, when Pi is positive, the film rupture due to the spinodal dewetting stops when the pressure of the thicker region balances with that of the thinner region, and then the heterogeneous grooves are nucleated and grow. The dewetting process dependence on the sign of Pi can be found in systems other than that reported here because the sign of Pi changes at the local maximum of the surface energy.

Spreading of droplets on lubricant-patterned substrates

Droplet spreading behaviors on lubricant-patterned substrates are investigated by using molecular dynamics simulations to explore application potentials in magnetic storage drive systems. Microscopic spreading processes are studied by both potential fields of lubricant-patterned substrates and single molecule movements in lubricant droplets. The potential fields indicate that the wall molecules patterned on the substrates attract the mobile ones in the lubricant droplets. Due to the attraction force, the mobile molecules experience difficulties in diffusing freely along the substrates. The single molecule movements in lubricant droplets demonstrate that during the diffusion process, the mobile molecules encounter, adsorb, encompass, and disengage the wall ones. The spreading behaviors are significantly impacted by the bonded ratio. The potential fields indicate that as the bonded ratio increases, the attractive regions of wall molecules merge to overlap, which indicate combined interactions formed by the adjacent wall molecules.

Multiscale analysis of liquid lubrication trends from industrial machines to micro-electrical-mechanical systems

An analytic multiscale expression is derived that yields conditions for effective liquid lubrication of oscillating contacts via surface flow over multiple time and length scales. The expression is a logistics function that depends on two quantities, the fraction of lubricant removed at each contact and a scaling parameter given by the logarithm of the ratio of the contact area to the product of the lubricant diffusion coefficient and the cycle time. For industrial machines the expression confirms the need for an oil mist. For magnetic disk drives, the expression predicts that existing lubricants are sufficient for next-generation data storage. For micro-electrical-mechanical systems, the expression predicts that a bound + mobile lubricant composed of tricresyl phosphate on an octadecyltrichlorosilane self-assembled monolayer will be effective only for temperatures greater than approximately 200 K and up to approximately MHz oscillation frequencies.

The Molecular Spreading of Nonpolar Perfluoropolyether Films on Amorphous Carbon Surfaces

The spreading mechanism of nonpolar perfluoropolyether films on carbon surfaces is examined in the mesoscopic regime, including both submonolayer and multilayer films. For the submonolayer film, adsorption-desorption is a main mechanism for spreading, and the surface diffusion coefficients increase as the film thickness increases. The driving force for the spreading in the submonolayer regime is the gradient of the disjoining pressure, which is described by the two-dimensional virial equation. For the multilayer film regime, the spreading characteristics are determined by the molecular weight and the disjoining pressure gradient, which is assumed to be purely van der Waals in nature. We adopt a partial slip boundary condition to analyze the multilayer film, which qualitatively explains the dependence of the surface diffusion coefficient on film thickness. Copyright 2000 Academic Press.

Monte Carlo simulations of liquid spreading on a solid surface: effect of end-group functionality

The spreading of liquid droplets composed of molecules with or without reactive end groups over a solid surface has been studied using Monte Carlo simulations. For molecules without reactive end groups, a molecular layering in the spreading profiles is predicted, depending on the ratio of the magnitude of intermolecular interactions to thermal energy. As intermolecular interactions become smaller than thermal energy, the layered structure vanishes. For molecules with reactive end groups, interactions between end groups and between end groups and the surface complicate the situation. By assuming an end-to-end interaction between molecules and the pinning of end groups to the surface, a complex layered structure is obtained. Our simulation predicts spreading profiles that accurately describe the broad spectrum of data obtained from scanning microellipsometry for perfluoropolyalkylethers with and without reactive end groups.