Yuya Hamamoto

Shear thinning behavior of monolayer liquid lubricant films measured by fiber wobbling method

It is essential to clarify mechanical properties of monolayer lubricant films coated on magnetic disks under shearing motion for designing future hard disk drives with ultra-low flying height. Many of previous researchers reported that strong shear rate dependence of viscoelasticity was one of the typical phenomena observed with molecularly thin liquid films. However, it has not been clarified whether or not perfluoropolyether (PFPE) lubricant films, which are used for the head-disk interface (HDI) lubrication, show shear thinning behavior under actual HDI conditions. In this study, we used the fiber wobbling method that can achieve both highly-sensitive shear force measurement and precise gap control and measured shear rate dependence of viscoelastic properties of monolayer PFPE films coated on the magnetic disk. Our experimental results showed that shear thinning does occur at high shear rate ranged from 102 to 106 s−1.

Temperature Dependence of the Viscoelastic Properties of a Confined Liquid Polymer Measured by Using an Oscillating Optical Fiber Probe

We measured the temperature dependence of the viscoelastic properties of a liquid polymer confined and sheared within a nanometer-sized gap. In the viscoelastic measurements, we used the fiber wobbling method, a highly sensitive method that we have developed for measuring shear forces. As a liquid sample, we used the fluoropolyether lubricant Fomblin Zdol4000. Our experimental results showed that the temperature dependence of the viscosity was well expressed by the well-known Andrade equation, even in the confined state. The activation enthalpy was calculated by assuming that Eyrings theory of viscosity holds for gaps of a width ranging from 100 nm down to a few nanometers. We observed a significant decrease in the activation enthalpy for gaps smaller than 10 nm. Elasticity, which only appeared for confinement in gaps smaller than 10 nm, roughly decreased with increasing temperature.

Detection of the Asperity Contact Between Sliding Surfaces by Monitoring the Excitation of Resonant Oscillation Using the Fiber Wobbling Method

In the head disk interface (HDI) of a magnetic recording system, lubrication caused by a monolayer thick lubricant film is necessary to achieve stable relative motion between a magnetic disk and a magnetic head. Viscoelastic properties of lubricant films should be clarified for improvement of lubrication performance, however, measurement methods have not been established yet. In this study, we present a new method precisely detecting the starting point of asperity contact of sliding solid surfaces in order to measure viscoelastic properties of the molecularly thin lubricant film on the magnetic disk.Copyright

Nanorheological Measurement of Monolayer Lubricant Films Using the Oscillating Optical Fiber Probe

We achieved a viscoelastic measurement of monolayer lubricant films using the fiber wobbling method, which is the highly sensitive shear force measuring method that we have developed. In order to make the measurement possible, we attained the accurate determination of nanometer-sized gap widths between sliding surfaces. In addition, we developed a new method to align the parallelism between the sliding surfaces. We observed the differences in the mechanical responses of sheared lubricant films due to their adsorptivity to the substrate surfaces.Copyright

Nanorheometry of Molecularly Thin Liquid Lubricant Films Coated on Magnetic Disks

Molecularly thin lubricant films are used for the lubrication of head disk interfaces in hard disk drives. The film thickness is reduced to 1-2 nm to minimize the magnetic spacing, and optimal, precise design is required to obtain sufficient lubrication. However, until now, there was no generally applicable method for investigating such thin films. Therefore, we developed a highly sensitive shear force measuring method and have applied it to the viscoelastic measurement of lubricant films coated on magnetic disk surfaces. In this paper, we review the method and summarize the useful findings we have demonstrated so far.

Frequency Dependence of Viscoelasticity of Liquid Lubricant Confined in Nanometer-Scale Gaps

When a liquid is confined in nanometer-scale gaps, it exhibits characteristic viscoelastic properties that are different from those properties measured in a bulk state. In hard disk drives, a liquid lubricant film whose thickness is around 1-2 nm is used to lubricate the interface between the magnetic head and the magnetic disk surface. When the magnetic head touches the disk surface, the lubricant film is confined in the nanometer-scale gaps and sheared. In order to attain the proper lubrication, it is essential to investigate the dynamic viscoelasticity of the confined liquid lubricant under shearing motion. In this study, we focused on one of the typical phenomenon of the confined liquid which is referred to as the shear thinning. The shear thinning is the property whereby the viscosity of the liquid decreases when the shear rate or shear frequency increases. This can be observed in many lubricants in a bulk state. The characteristic behavior of the confined lubricant is that the shear thinning can be observed at much lower shear rates or shear frequencies compared to the shear thinning occurs in a bulk state. The mechanism of the shear thinning in the confined state must be different from that of the bulk state. In order to clarify the shear thinning mechanism of the confined lubricant, we measured the gap dependence of the viscoelasticity at different shear frequencies in a range of 100 Hz to 2 kHz. The experimental results showed that the shear thinning behavior suddenly appeared at gap width of approximately 15 nm or less. Further, the shear thinning synchronized with the enhancement of the elasticity which means the solidification of the liquid lubricant.

Temperature Dependence of Viscoelasticity of PFPE Lubricant Confined in Nanometer-Sized Gaps

In this study, we measured temperature dependence of viscoelastic properties of PFPE lubricant that was confined in nanometer-sized gap widths. In the viscoelastic measurement, we used the fiber wobbling method, which is the highly sensitive shear force measuring method that we have originally developed. Our experimental results showed that the temperature dependence of the lubricant’s viscosity was declined at the gap width of less than 5 nm. Since the elasticity also appeared at the gap width of 5 nm, the reduced temperature dependence is considered to be caused by the repression of molecular mobility due to the solidification of the confined lubricant.Copyright

Dynamic Viscoelastic Properties of Confined Polymer Liquids Under Oscillatory Shear Flow

When liquids are confined in nanometer-scale gaps, they have characteristic viscoelastic properties that are greatly different from those of the liquids in the bulk state. In order to clarify the mechanism of this phenomenon, we have developed a new shear force measuring method. Our method can measure the viscoelasticity of the liquids that are confined and sheared in nanometer-scale gaps. By using the method, we investigated the gap dependence of the polymer liquid and found that both viscosity and elasticity increased drastically when the gap was decreased to a molecularly narrow width. In addition, we measured the shear rate dependence of the viscosity and found that the dependence to the shear rate increased as a result of the confinement.

Measurement of dynamic viscoelasticity of confined lubricant by using oscillating optical fiber probe

When a liquid is confined in molecularly narrow gaps, it shows characteristic viscoelasticity such as enhanced viscosity or prolonged relaxation time. In order to investigate the dynamic viscoelasticity of the confined liquid, we developed a new shear force measuring method that uses a ball-ended optical fiber as a shearing probe. Our method can measure the shear force of 0.1 nN order with the oscillation frequency of up to 10 kHz. In addition, the gap that confines the liquid can be set at any constant value ranging from 10 µm to 0.1 nm. In this study, we measured the gap dependence of viscoelasticity of confined liquid lubricants. The gap ranged from 200 nm to a few nm. The tested lubricant was Fomblin Z03 and Zdo14000. A magnetic disk was used as the solid substrate. Oscillation frequency was set at 800 Hz. The experiment showed the viscosity of both Z03 and Zdo14000 gradually increased as the confining gap decreased. The gap width where the viscosity increase started was wider than 100 nm, which is dozens of times larger than the gyration diameter of lubricant molecules. Although Z03 and Zdo14000 have negligibly small elasticity in a bulk state, elasticity suddenly appeared at gaps less than about 8 nm with Zdo14000, and at gaps less than about 4 nm with Z03. Stronger affinity of Zdo14000 molecules to the solid substrate could cause the wider gap width of elasticity appearance.