Fiber Wobbling Method for Dynamic Viscoelastic Measurement of Liquid Lubricant Confined in Molecularly Narrow Gaps

Abstract

An understanding of the viscoelastic properties of molecularly thin lubricant film is essential to clarify tribological issues of head-disk interface (HDI) in high-density recording hard disk drives. Characteristic conditions for the HDI occur when lubricant molecules are extremely confined in the gap between the head and the disk surfaces, and the surfaces slide at high speeds. The lower the flying height, the more this confinement affects the flying characteristics. However, a few attempts have been made at clarifying the dynamic viscoelastic properties of confined lubricant molecules. This is because a method of measuring the dynamic shear force has not yet been established. Fiber wobbling method enables us to measure the shear force with a detection limit of less than 1 nN. Additionally, frequency of shear can be set at several kHz. Further, the gap which confines the lubricant is controlled with a resolution of 0.1 nm. Using the FWM, we investigated the effect that confinement had on the dynamic viscoelastic properties of perfluoropolyether lubricants on a magnetic disk. We found that the viscosity started to increase at a gap width that was less than a few hundred nanometers, which is hundreds of times larger than the molecular size. On the other hand, elasticity suddenly appeared at a gap width that was less than a few nanometers, which is equivalent to a few molecular sizes. Both the viscosity and elasticity increased monotonically as the gap decreased.

Simultaneously Measuring Lateral and Vertical Forces with Accurate Gap Control for Clarifying Lubrication Phenomena at Nanometer Gap

“…The details are described in Refs. [4,5]. From the measured amplitude ratio a and phase shift of the optical fiber probe d, the complex viscosity g ef = g 0 -ig 00 can be obtained as…”

“…By synchronous detection of the difference signal with a lock-in amplifier (LI5640, NF Corp., Tokyo) using the piezo excitation signal as a reference signal, the amplitude and phase shift of the probe vibration were obtained. This method can provide the lateral displacement with accuracy on the order of 0.01 nm [4,5]. The spring constant of the probe, which was measured with a load cell (XS-1021, MAGNEBIT, San Jose, USA), was 165 N/m.…”

“…For the measurement of the lateral force, a method using an optical fiber ending with a micro glass ball was developed (Fig. 1a) [4,5]. An oscillating optical fiber probe is inserted into a lubricant droplet that has been deposited on a sample disk.…”

Controlling thin liquid film viscosity via modification of substrate surface chemistry

“…It is well-established that the properties of ultrathin films can vary as a function of the film thickness [1-4] with polymer chain segments at the film/substrate interface exhibiting decreased mobility compared with the bulk film, whereas segments at the free surface exhibit greater mobility than the bulk film. The effect of this anisotropy affects the properties of materials which include viscosity [1], glass transition temperatures [2-3], diffusion coefficients [4], and rheological properties [5]. For systems where there was a favourable interaction between the film and the surface chemical moieties of the supporting substrate, the properties of the film differed from those of the bulk.…”

Displacement Measurement for High Speed Tribological Measurement Using Oscillating Optical Fiber Probe

“…Equation (1) indicates that the probe amplitude was enhanced by the Q factor in the resonant oscillation. Usually, the optical fiber used in the FWM has a Q factor of around 1000 and a resonant frequency of 1-20 kHz.…”