Hedong Zhang

Monolithically fabricated double-ended tuning-fork-based force sensor

A highly sensitive force sensor that is suitable for precise gap control is presented. It combines a double-ended tuning fork (DETF) resonator with a cantilever beam. When the force to be measured is applied to the cantilever beam, the resonant frequency of the DETF beam changes due to the change in axial force. The DETF-based sensor can detect the force as an amplitude change due to the resonant frequency change. The performance of the sensor was theoretically and experimentally investigated. The sensor is rigid and can measure the force with a change in the sample or probe height of less than about 1nm. The experimentally attained minimum detection limit of the force was 19μN and this can be improved by more than one order of magnitude according to theoretical estimation. The combination of the DETF-based sensor and the fixed probe or sample is expected to provide the measurement of the surface force with precise gap control. It can be useful for various fields such as interface science, probe microscop...

Mechanical design and force calibration of dual-axis micromechanical probe for friction force microscopy

A dual-axis micromechanical probe that combines a double cantilever and torsion beams is presented. This probe can reduce the mechanical cross-talk between the lateral and vertical force detections. In addition, dual-axis forces can be detected by measuring the dual-axis displacement of the probe end using the optical lever-based method used in conventional friction force microscopes (FFMs). In this paper, the mechanical design of the probe, the details of the fabrication method, FFM performance, and calibration of the friction force are discussed. The mechanical design and the microfabrication method for probes that can provide a force resolution of the order of 1nN without mechanical cross-talk are presented. Calibration of the lateral force signal is possible by using the relationship between the lateral force and the piezodisplacement at the onset of the probe scanning. The micromechanical probe enables simultaneous and independent detection of atomic and friction forces. This leads to accurate invest...

Spreading properties of monolayer lubricant films: Effect of bonded molecules

Monolayer lubricant films applied to head-disk interface of hard disk drives consist of bonded and mobile molecules. We measured the diffusion coefficient of mobile molecules that spread through the spaces not covered with bonded molecules and revealed its dependence on the coverage fraction. The diffusion coefficient gradually decreased with increasing coverage from 0.2 to 0.8, and its trend was well represented by reptation theory. However, the diffusion coefficient significantly decreased at a coverage higher than 0.8, in contrast to the prediction of reptation theory. The reason for this decrease is thought to be the disappearance of minimum spaces required for mobile molecules to spread through.

Dual-axis micromechanical probe for independent detection of lateral and vertical forces

A dual-axis micromechanical probe that combines a double cantilever and torsion beams is presented. This probe can reduce the mechanical cross-talk between the lateral and vertical forces and detect dual-axis forces by measuring the dual-axis displacement of the probe end using the optical lever-based method used in conventional friction force microscopes. By reducing the cross-talk, this new probe design yields improved frictional force measurements for the identification of materials.

Simple Subtraction Method for Determining the Mean Path Length Traveled by Photons in Turbid Media.

The mean path length is a key parameter in the study of light propagation in turbid media such as living tissues. In this paper, we propose a simple subtraction method for determining the mean path length traveled by photons in turbid media. The method is based on the fact that the mean time delay (i.e., the center of gravity) of the measured re-emission profile is the sum of the mean time delays of the instrumental function (i.e., the impulse response of the measuring system) and the impulse response of a highly scattering medium. Using this method, the mean path length can be calculated quickly and accurately without the need for performing deconvolution. The theory, computer simulation and experimental demonstration are described.

Quantitation of Absorbers in Turbid Media Using Time-Integrated Spectroscopy Based on Microscopic Beer-Lambert Law

Based on the microscopic Beer-Lambert law, two practical time-integrated spectroscopy (TIS) methods, called dual-wavelength spectroscopy method, and dual-wavelength and dual-site spectroscopy method, are described to determine the absolute concentration of an absorber in variously shaped turbid media. We demonstrate, for the first time, the validity of the TIS methods by means of experiments in which the absolute concentrations of an absorber in a tissue-like phantom were determined with errors less than several percent. The advantages and disadvantages of both methods are also discussed.

Spreading Characteristics of Molecularly Thin Lubricant on Surfaces With Groove-Shaped Textures: Effects of Molecular Weight and End-Group Functionality

Effects of molecular weight and end-group functionality on spreading of molecularly thin perfluoropolyether (PFPE) film over solid surfaces with groove-shaped textures have been studied by experiments and Monte Carlo simulations. In the experiments, lubricant spreading on a surface with groove-shaped textures was measured by making use of the phenomenon in which diffracted light weakens in the lubricant-covered region. It is found that grooves serve to accelerate spreading and this effect increases for deeper grooves, and also the accelerating rate becomes larger for a lubricant having a larger molecular weight or functional end-groups. In the simulations, the Monte Carlo method based on the Ising model was extended to enable us to evaluate the effect of molecular weight on the spreading of non-functional lubricant inside a groove. The validity of the newly developed simulation method was well confirmed from the agreement between the simulation and experimental results.

Spreading Characteristics of Molecularly Thin Lubricant on Surfaces With Groove-Shaped Textures: Monte Carlo Simulation and Measurement Using PFPE Film

Spreading characteristics of molecularly thin lubricant on a grooved surface have been studied numerically by Monte Carlo simulations and compared with measurements obtained by perfluoropolyether (PFPE) thin film spreading on a solid surface with minute grooves. In the simulations, by incorporating the interactions between molecules and the side surfaces of a groove, the Monte Carlo method based on the Ising model was extended to the case of a surface with grooves and applied to simulate the spreading of non-polar lubricant inside a groove. Compared with the spreading on a smooth surface, lubricant spreads rapidly inside a groove, indicating an acceleration of the spreading along the groove. In the experiments, lubricant spreading on a surface with groove-shaped textures was measured by making use of the phenomenon in which diffracted light decreases or vanishes in the lubricant-covered region. Based on the results showing lubricant spreading predominantly along the groove, the accelerating effect obtained in the simulations is well confirmed by the measurements.

Quantification of the Surface Morphology of Lubricant Films With Polar End Groups Using Molecular Dynamics Simulation: Periodic Changes in Morphology Depending on Film Thickness

Using a coarse-grained molecular dynamics simulation based on the bead-spring polymer model, we reproduced the film distribution of molecularly thin lubricant films with polar end groups coated on the disk surface and quantified the film surface morphology using a molecular-probe scanning method. We found that the film surface morphology changed periodically with increasing the film thickness. The monolayer of a polar lubricant that entirely covers the solid surface provides a flat lubricant surface by exposing its nonpolar backbone outside of the monolayer. By increasing film thickness, the end beads aggregate to make clusters, and bulges form on the lubricant surface accompanying an increase in surface roughness. The bulges continue to grow up even though the averaged film thickness reaches or exceeds the bilayer thickness. With further increases in film thickness, the clusters start to be uniformly distributed in the lateral direction to clearly form a third layer. As for the formation of fourth-fifth layers, the process is basically the same as that for the second-third layers. Through our calculations of the intermolecular potential field and the intermolecular force field, these values are found to change periodically and are synchronized with the formation of molecule aggregations, which explains the mechanism of forming the layered structure that is inherent to a polar lubricant.Copyright

Direct Visualization of Molecularly Thin Lubricant Films on Magnetic Disks by Ellipsometric Microscopy With a White Light Source

We demonstrated the direct visualization of molecularly thin lubricant films on magnetic disks with a thickness resolution of 0.1 nm by using an ellipsometric microscope with a white light source. It was able to reduce the optical interference noise that arises in conventional laser-based ellipsometric microscopes and to provide a larger SNR by a factor of about 6 compared to a laser-based ellipsometric microscope. The wavelength width should be given first priority in designing a white-light-source ellipsometric microscope, and the width should be determined after considering the required coherence length and thickness resolution. Theoretical calculations indicate that a wavelength width of less than 10 nm can provide a thickness resolution of 0.1 nm. A white-light-source ellipsometric microscope can provide real-time visualisation of a molecularly thin lubricant film with a thickness resolution of 0.1 nm, which is useful in investigating the kinetic behavior of molecularly thin lubricant films on magnetic disks.