Measurement of spreading characteristics of molecularly thin lubricant films over grooved solid surfaces based on diffraction simulations

Abstract

Characteristics of molecularly thin lubricant films are basically determined by their interactions with solid surfaces. Since these interactions can be modified by engineered microscopic surface textures, it is expected that rational design of the textures will make it possible to attain desired tribological functions and performance. In this research, with the aim of applying it to head-disk interface of hard disk drives, we propose a method based on diffraction simulations that enables thickness measurement of molecularly thin films coated on grooved solid surfaces. Using this method, we experimentally investigate the spreading characteristics of nanometer-thick polymeric liquid lubricant films on grooved surfaces. The results revealed that the average thicknesses of the films dipcoated on the grooved and smooth surfaces under identical conditions were approximately the same, whereas lubricant spreading on grooved surfaces was significantly faster than that on smooth surfaces.

Polymer Spreading on Unidirectionally Nanotextured Substrates Using Molecular Dynamics

“…From Figure b, we observe that the polymer typically spreads much less perpendicular to the texture than parallel to the texture, in agreement with previous reports. ,,− Similar to the case of Figure a, we observe that textures with the same aspect ratio can yield different d ⊥ . Thus, the texture aspect ratio does not necessarily determine anisotropic spreading d || / d ⊥ , in opposition to previous reports. , We also observe that d ⊥ decreases with decreasing L and that this is more pronounced for textures with sharp grooves, especially grooves with θ = 60°.…”

“…The ability to create anisotropic spreading in particular, which enables liquid spreading to occur in a user-specified direction, finds application in micro/nanofluidics, flexible electronics, and drag reduction surfaces . Several research groups have shown both experimentally and numerically that unidirectionally textured substrates lead to anisotropic spreading, where liquids spread preferentially in the direction parallel to the texture. − Anisotropic spreading on unidirectionally textured substrates is most commonly explained by texture peaks, which impose a mechanical barrier that effectively “pins” the three-phase (solid/liquid/gas) contact line of the spreading liquid, inhibiting spreading in the direction perpendicular to the texture. − ,− The texture peaks are often described as energetic barriers that oppose wetting, which Yong and Zhang attribute to a high local atomic density at texture peaks, based on the results of molecular dynamics (MD) simulations (texture height of 1.85-23.77 Å). The liquid may be confined by texture peaks in the perpendicular direction and “squeezed” in the parallel direction.…”

“…The liquid may be confined by texture peaks in the perpendicular direction and “squeezed” in the parallel direction. Alternatively, the liquid may only be slowed by texture peaks, causing the contact line to advance in a stick-slip manner, pinning-depinning-repinning on subsequent texture peaks. ,,,, Conversely, others propose that anisotropic spreading is caused by texture grooves that accelerate spreading along the direction parallel to the texture, i.e., along the grooves. ,,− Fischer et al attribute this accelerated spreading in part to capillary action inside the grooves, based on experiments with sinusoidal textures (height of 8 or 16 μm). In addition, Zhang et al used Monte Carlo simulations and experiments (square-shaped textures with a height of 0.03-0.05 μm) to explain that an increased surface force exists inside a groove because it is surrounded by as many as three surfaces; i.e., a square groove is composed of a base and two side-walls.…”