Development of contact sliders with nanotextures by femtosecond laser processing

The information on the frictional resistance of a self-propelled robotic capsule endoscope moving inside the body is very important for the design and the performance enhancement of such parameters of the capsule endoscope as power consumption, motion control and positioning accuracy. Based on this motivation, the ultimate goal of this research was to develop an analytical model that can predict the frictional resistance of the capsule endoscope moving inside the living body. In this work, experimental investigations of the fundamental frictional characteristics and the viscoelastic behaviors of the small intestine were performed by using custom-built testers and various capsule dummies. The small intestine of a pig was used for the experiments. Experimental results showed that the average frictional force was 10–50 mN and higher moving speed of the capsule dummy resulted in larger frictional resistance of the capsule. In addition, the friction coefficient did not change significantly with respect to the apparent area of contact between the capsule dummy and the intestine, and also the friction coefficients decreased with an increase in the normal load and varied from 0.08 to 0.2. Such frictional behaviors could be explained by the lubrication characteristics of the intestine surface and typical viscoelastic characteristics of the small intestine material. Also, based on the experimental results, a viscoelasticity model for the stress relaxation of the small intestine could be derived.     

Effects of ultra-thin liquid lubricant films on contact slider dynamics in hard-disk drives

This paper describes the effects of ultra-thin liquid lubricant films on contact slider dynamics in hard-disk drives. In the experiments, the contact slider dynamics as well as ultra-thin liquid lubricants behavior are investigated using three types of lubricants, which have different end-groups and molecular weight as a function of lubricant film thickness. The dynamics of a contact slider is mainly monitored using acoustic emission (AE). The disks are also examined with a scanning micro-ellipsometer before and after contact slider experiments. It is found that the lubricant film thickness instability occurs as a result of slider–disk contacts, when the lubricant film thickness is thicker than one monolayer. Their unstable lubricant behavior depends on the chemical structure of functional end-groups and molecular weight. In addition, it is also found that the AE RMS values, which indicate the contact slider dynamics, are almost same, independent of the end-groups and molecular weight for the lubricants, when the lubricant film thickness is approximately one monolayer. The molecular weight, however, affects the contact slider dynamics, when the lubricant film thickness is less than one monolayer. In other words, the AE RMS values increase remarkably as the molecular weight for the lubricant increases. When the lubricant film thickness is more than one monolayer, the AE RMS values decrease because of the effect of mobile lubricant layer, while the lubricant instability affects the contact slider dynamics. Therefore, it may be concluded that the lubricant film thickness should be designed to be approximately one monolayer thickness region in order to achieve contact recording for future head–disk interface.     

Visualization of lubricant pickup phenomena by lubricant thickness mapping on slider surface

A slider surface analyzing tester was developed to observe the lubricant thickness distribution on a slider surface by interferometry. We observed three phenomena related to lubricant pickup by the slider. The picked-up lubricant gathered around the boundary of the pad and recess area during the unloading interval, and the gathered lubricant flowed to the trailing edge of the pad after loading on the disk surface. There were two lubricant flows on the pad surface. The first was a circulation flow from the dynamic flying height protrusion area to the leading edge of the pad. The second was a circulation flow from the lubricant pool to the leading edge of the pad. Lubricant dewetting occurred on the slider pad surface when a thick layer of lubricant was adhered to the pad surface.     

MAGNETIC AND GRAVITATIONAL CONVECTION OF AIR WITH A COIL INCLINED AROUND THE X AXIS

Air is filled in a cubic enclosure whose one vertical wall is isothermally heated and the opposite one is cooled while the other four walls are thermally insulated. A large coil is placed outside of this enclosure with the coil center coinciding with the cube center. An electric current in the coil generates a magnetic field to affect the convection of air, because the air contains oxygen whose magnetic susceptibility is exceptionally large among gases. The coil is further inclined around the X axis, which is horizontal and perpendicular to the hot and cold walls through the wall center. The heat transfer rate changes depending on the inclination angle. This system is studied numerically for convection for the following combination of parameters: Ra = 1.51 × 10⁴, 9.06 × 10⁴; Pr = 0.71; γ = 0 ? 100; x_Euler = 0–π/2, where γ represents the strength of magnetic field and x_Euler is the angle of inclination of the coil. For example, at Ra = 1.51 × 10⁴ and γ = 30, the average Nusselt number 2.535 at x_Euler = 0 increased to 2.823 at x_Euler = π/2. This study suggests that the coil inclination affects the heat transfer rate extensively.     

Effect of humidity and temperature on polymer electrolyte membrane (Nafion 117) studied by positron annihilation spectroscopy

Polymer electrolyte fuel cells (PEMFC) have emerged as portable power source for future transportation and home based systems. Over the past years perfluorosulfonated cation exchange membranes have been used as solid polymer electrolyte in fuel cells, however, humidity control and water management have remained major issues in the area of stack development. Local drying that results in the changes in the local conductivity behavior and physical failure of the membrane is one of the key issues to be addressed in detail. It is therefore necessary to estimate these changes precisely to evaluate the membrane behavior during the fuel cell operation. Positron annihilation spectroscopy offers a very sensitive probe to the determination of polymer free-volume as well as electronic state of the polymer. In the present study we studied Nafion 117 with varying temperature and humidity using positron annihilation spectroscopy. The ortho-positronium lifetime (o-Ps, i.e., τ₃) which gives the size of the free-volume holes inside the polymer was determined. It was found that the o-Ps lifetime increases for relative humidity between 40% and 60% and thereafter it decreases in the temperature range of 62-95 °C. When the inversion point at which the o-Ps lifetime is the maximum was plotted with actual vapor pressure, it was observed that it has a logarithmic behavior with the ordinate intercept of 1 ns. This indicates that at higher temperature the membrane has larger free-volume holes; and also at higher temperature a larger vapor pressure is essential to saturate the membrane. Above this point the clusters expand and the free-volume hole size decreases. This also indicates the optimum vapor in the cell to be maintained during the fuel cell operation. The studies also indicated that there is a correlation between o-Ps lifetime and the hydrogen crossover current density under the similar temperature and humidity condition.     

CONVECTIVE AND DIFFUSIVE PHENOMENA OF AIR IN A VERTICAL CYLINDER UNDER A STRONG MAGNETIC FIELD

Transient characteristics of air flow in a vertical pipe under a superconducting magnet were studied numerically with new mathematical modeling equations of magnetizing force. Air is a paramagnetic fluid and has an exceptionally large magnetic susceptibility. The distribution of the magnetic field in the bore of a superconducting magnet was calculated by Biot-Savart's law. When the vertical cylinder was filled initially with hot air, the hot air was repelled from the pipe upward immediately, due to the buoyancy and magnetizing forces. However, when the vertical pipe was filled initially with cold air, the cold air was sustained for a long time due to the magnetizing force overcoming the buoyancy force.     

CONJUGATE NUMERICAL COMPUTATION FOR THE NATURAL CONVECTION OF LIQUID METAL HEATED FROM BELOW

ABSTRACT

The oscillatory Rayleigh-Benard convection in a shallow layer of liquid metal (Pr = 0.023), sandwiched between two copper plates, was numerically computed for three computational domains simultaneously. The horizontal cross section is square and the aspect ratio (fluid layer width/height) is 10. This conjugate solution suggested that the hot plate temperature oscillates almost simultaneously with the fluid temperature and throughout the whole hot copper plate with almost uniform temperature. The oscillatory temperature amplitude provided the upper and lower limits in the Rayleigh number and the Nusselt number estimated from the relationship, constant = Ra* = Nu ? Ra, where Ra* is a modified Rayleigh number based on the uniform heat flux. This group of data provided quite similar characteristics to the previous experimental observation by Yamanaka et al. for the oscillatory change of the Nusselt number and to the one by Rossby for the time-averaged values.     

Demonstration test on purification systems of contaminated water using liquid–solid settling technology by flocculants (decontamination technology using high-pressure water)

For the purpose of decontaminating residential buildings, roads, rubble, and so on, the high-pressure water washing decontamination system had been developed, which are composed of a recovery system for contaminated washing water and mobile purification system of liquid–solid settling technology by flocculants. The high-pressure washing technology applied in this demonstration test can be used for both high- and ultrahigh-pressure washing decontamination and water surface chipping, by changing pumps and attachments. These systems were applied to decontaminate pools, concrete floor and side ditch around the pool in an elementary school located at Motomiya, Fukushima Prefecture as a demonstration test. Additionally, to compare decontamination effects, the tests, to chip and decontaminate surfaces of outer concrete and asphalt pavement had been conducted. From the results, high-pressure water jet decontamination systems have been practically proven.     

Tribological Properties of Ultrathin DLC-Coated Nanotextured Surface on a PET Film

The wear resistance of two types of nanotextured surfaces, fabricated on polyethylene terephthalate films and coated subsequently with a diamond-like carbon (DLC) layer and then a self-assembled monolayer (SAM) film with perfluoropolyether main chains, was compared. Nanotextures with hole and pillar patterns were fabricated by a nanoimprint process. The hole nanotexture with DLC and SAM films showed better wear resistance than the pillar nanotexture, and that film showed higher oil repellency than the pillar nanotexture with DLC and SAM films. The von Mises and shear stresses of the pillar nanotexture were concentrated on the center and at the bottom of the asperity surface, and it was estimated from finite element method calculations that these stresses accelerate the wear of asperities.