Liquid Nanofilms Research Articles

液体ナノ薄膜の変形流動特性

In the head-disk interface (HDI) of a hard disk drive (HDD), the effects of lubricant deformation on the flying characteristics of a head slider increase as the flying height decreases. Therefore, it is very important to investigate the deformation and flow characteristics of lubricant on a recording disk. In particular, in heat-assisted magnetic recording (HAMR), laser heating of the lubricant induces surface tension distribution and evaporation of lubricant, which may cause deformation of the lubricant film. In this paper, deformation and flow characteristics of liquid nano-film of perfluoropolyether (PFPE) film on solid substrate is investigated numerically and experimentally. Calculation results are compared with experimental results, and validity of the analytical model is investigated.

A-P-1 液体ナノ薄膜の流動特性に関する実験的研究

In the head-disk interface (HDI) of a hard disk drive (HDD), the effects of lubricant deformation on the flying characteristics of a head slider increase as the flying height decreases. Therefore, it is very important to investigate the deformation and flow characteristics of lubricant on a recording disk. In particular, in heat-assisted magnetic recording (HAMR), laser heating of the lubricant induces surface tension distribution and evaporation of lubricant, which may cause deformation of the lubricant film. In this paper, flow characteristics of liquid nano-film of perfluoropolyether (PFPE) film on solid substrate is investigated experimentally. Using the modified Boltzmann-Matano method, diffusion coefficients of liquid nano-step are calculated. In addition, thicknesses of mobile layer and bonded layer of PFPE are measured.

20pm3-PM009 液体ナノ薄膜の流動特性の加熱による影響

In the head-disk interface (HDI) of a hard disk drive (HDD), the effects of lubricant deformation on the flying characteristics of a head slider increase as the flying height decreases. Therefore, it is very important to investigate the deformation and flow characteristics of lubricant on a recording disk. In particular, in heat-assisted magnetic recording (HAMR), laser heating of the lubricant induces surface tension distribution and evaporation of lubricant, which may cause deformation of the lubricant film. In this paper, flow characteristics of liquid nano-film of perfluoropolyether (PFPE) on solid substrate is investigated experimentally. Using the modified Boltzmann-Matano method, diffusion coefficients of liquid nano-step, which shows the flow characteristics, are calculated. As a result, effects of initial film thickness and temperature on the flow characteristics were clarified.

Micro/nanotribological behaviors of ionic liquid nanofilms with different functional cations

AbstractFour kinds of molecularly thin films of room temperature ionic liquids (RTILs) with different functional cations were prepared on silicon substrates by dip‐coating method. Thermal stability of the RTILs was evaluated using Mettler thermal gravity analysis (TGA) in a nitrogen atmosphere. Chemical compositions of the RTIL nanofilms were examined by means of multifunctional XPS. Nanoscaled adhesion and friction forces between the films and AFM tip were measured by FFM whereas the morphologies of the films were also investigated. Microscaled friction and wear behaviors between the films and Si3N4 ball were further measured by the microtribometer. The micro/nanotribological behaviors of different RTIL films were comparatively investigated and discussed in terms of functional cations of the RTILs molecules. Results in this paper revealed that the functional cations of the RTIL films significantly affected their tribological behaviors both in micro‐ and nanoscales. The corresponding micro/nanotribological mechanism of the tested ultrathin RTIL films under the test conditions was consequently proposed based on the experimental results. Copyright © 2010 John Wiley & Sons, Ltd.

Liquid nanofilms. A mechanical model for the disjoining pressure

Liquids in contact with solids are submitted to intermolecular forces making liquids heterogeneous and, in a mechanical model, the stress tensor is not any more spherical as in homogeneous bulks. The aim of this article is to show that a square-gradient functional taking into account the volume liquid free energy corrected with two surface liquid density functionals is a mean field approximation allowing to study structures of very thin liquid nanofilms near plane solid walls. The model determines analytically the concept of disjoining pressure for liquid films of thicknesses of a very few number of nanometers and yields a behavior in good agreement with the shapes of experimental curves carried out by Derjaguin and his successors.

Fourier-transform resonance shear measurement for studying confined liquids

The resonance shear measurement we recently developed is an efficient tool for investigating the rheological and tribological properties of liquid nanofilms confined between solid surfaces with varying film thicknesses. However, the previously employed resonance shear measurement measured shear responses at various oscillation frequencies for one film thickness (the frequency scanning method), so it required at least several minutes to obtain a reliable resonance curve. The fast Fourier-transform resonance shear method was developed to rapidly acquire the resonance curves. The obtained curves were very similar to those obtained by the frequency scanning method. The time necessary for the FFT method was 2-10 s, much shorter than the time required for the scanning method. This technique is easy to use and extends the applicability of the method to volatile liquids and unstable samples.

Dynamics of liquid nanofilms

The van der Waals forces across a very thin liquid layer (nanofilm) in contact with a plane solid wall make the liquid nonhomogeneous. The dynamics of such flat liquid nanofilms is studied in isothermal case.The Navier–Stokes equations are unable to describe fluid motions in very thin films. The notion of surface free energy of a sharp interface separating gas and liquid layer is disqualified. The concept of disjoining pressure replaces the model of surface energy. In the nanofilm a supplementary free energy must be considered as a functional of the density.The equation of fluid motions along the nanofilm is obtained through the Hamilton variational principle by adding, to the conservative forces, the forces of viscosity in lubrication approximation. The evolution equation of the film thickness is deduced and takes into account the variation of the disjoining pressure along the layer.

Effect of the Anion on the Tribological Properties of Ionic Liquid Nano-Films on Surface-Modified Silicon Wafers

The films of three kinds of 3-butyl-1-methylimidazolium base ionic liquids with thickness of 2 nm were prepared on hydroxyl-terminated and amino-terminated Si substrates by dip-coating method. As anions, tetrafluoroborate, hexafluorophosphate, and adipate, respectively, were chosen. The tribological performances of these thin films were examined by the determination of the film durability and friction coefficient by means of a UMT-2MT tribometer using a steel ball as counterpart. The morphologies of worn surfaces were investigated by a non-contact interferometric microscope. The findings showed that 3-butyl-1-methyl-imidazolium hexafluorophosphate having the poorest hydrophilicity of the ionic liquids exhibited the best tribological properties on aminated Si surface at 0.4 N and 4 Hz.

Surface excitations in liquid helium nanofilms

The structural dynamic factor S(Q, ω) of liquid 4He has been measured by inelastic neutron scattering in films of different thickness at temperatures from 0.6 to 0.05 K. The measurements were performed on an IN6 spectrometer (Institut Laue-Langevin, France). Analysis of the obtained experimental data on S(Q, ω) made it possible to establish the main parameters of surface excitations arising in a helium film and their dependences on the film temperature and thickness. The measurements showed that surface excitations arise in a helium film at a thickness of about two to three atomic layers, and the film thickness plays a decisive role in the formation of surface excitations.

Model of a liquid nanofilm on a solid substrate based on the van der Waals concept of capillarity

Van der Waals attractive forces drastically change the material properties of thin liquid layers several nanometers when in contact with a solid. At this scale, the fluid is no longer homogeneous. Moreover, it has properties which are analogous to those of solids. In particular, in equilibrium the stress tensor is no longer spherical. For such fluids, we use a long-wave approximation to derive the evolution of a liquid nanofilm on a substrate. We establish that the driving pressure in the nanofilm should be associated with the mean value of the component of the pressure tensor tangential to the liquid interface (along the substrate). Finally, we derive the equation for nanofilm dynamics by using a mass conservation formulation. This is not a conventional, conservative equation for the position of the free surface normally used in the theory of thick films where the density is homogeneous, but rather a conservative equation for the liquid mass. The equation turns out to be a nonlinear parabolic equation with a diffusion coefficient of a "good" sign.

Can Contact-Angle Measurements Determine the Disjoining Pressure in Liquid Nanofilms on Rigid Substrates?

The disjoining pressure of lubricant nanofilms used in the magnetic recording industry controls the equilibrium wetting, the dynamics of film restoration, and the evaporation kinetics of the film. It has been claimed that by measuring the contact angle of nonpolar and polar liquids on lubricant films, the disjoining pressure can be extracted using the method of Girafalco and Good, and such analyses have appeared in the literature. The approximations underlying the method have been discussed before in the literature. In view of the importance of measuring disjoining pressure in nanofilms of lubricants, it is timely to revisit these assumptions to understand the validity of the contact-angle method presently in use. We re-derive the relevant equations using a thermodynamic-interaction-energy approach which is free of the problems inherent in the original derivation and make explicit the assumptions which must be made in the derivation. General interaction energy arguments are then invoked to demonstrate that it does not appear possible to obtain the disjoining pressure in the film from contact-angle measurements in an unambiguous manner.