Ken Nakano

Control of friction coefficient by applying electric fields across liquid crystal boundary films

Abstract Experiments are described in which friction coefficients under boundary lubrication regime are measured with a nematic liquid crystal as the lubricant. Using steel-to-steel contacts, with one of the mating surfaces coated with a thin insulating film, electric fields are applied across the liquid crystal film. A substantial decrease in the friction coefficient is observed with both d.c. and low-frequency a.c. voltages, suggesting the possibility of controlling friction by external fields.

Electroviscous effect of nematic liquid crystals

Abstract Liquid crystals are characterized by the orientation of their molecules, the major axis of whose direction can be controlled by an applied electric or magnetic field. In the present paper the viscosity variation of three thermotropic liquid crystals in the nematic phase having different dielectric constants is studied at varying electric field strength, shear field strength and temperature. The results show that the viscosity of liquid crystals can be varied by the applied electric field strength and that a lower temperature reduces the threshold electric field strength for the electroviscous effect to appear as well as decreasing the attainable values at higher electric field strength. In addition, the frequency of the applied electric field has little effect on electroviscosity.

Map of low-frequency stick–slip of a creep groan

Abstract A map that shows the necessary condition for avoiding the generation of low-frequency stick–slip of a creep groan is introduced. The map is obtained as a result of comprehensive investigation employing a novel caliper–slider experimental model. According to the map, creep groan generation is controlled by two dimensionless parameters, designated as stiffness ratio, Sr, and low-frequency stick–slip index, Ls. Stiffness ratio, Sr, is the ratio of structure stiffness to the stiffness at contact interface and Ls is an index formed by various parameters such as normal force, sliding velocity, and difference between static and kinetic coefficients of friction. On the experimental model, creep groan generation can be avoided if one or both of the following conditions is fulfilled: (a) stiffness ratio, Sr, larger than 40 (Sr>40) and/or (b) low-frequency stick–slip, Ls, index larger than 400 (Ls index>400). It is expected that the map can be used as a useful guideline for avoiding the generation of similar phenomenon on a real brake system.

Novel Friction Law for the Static Friction Force based on Local Precursor Slipping

The sliding of a solid object on a solid substrate requires a shear force that is larger than the maximum static friction force. It is commonly believed that the maximum static friction force is proportional to the loading force and does not depend on the apparent contact area. The ratio of the maximum static friction force to the loading force is called the static friction coefficient µM, which is considered to be a constant. Here, we conduct experiments demonstrating that the static friction force of a slider on a substrate follows a novel friction law under certain conditions. The magnitude of µM decreases as the loading force increases or as the apparent contact area decreases. This behavior is caused by the slip of local precursors before the onset of bulk sliding and is consistent with recent theory. The results of this study will develop novel methods for static friction control.

Magnetic near-field measurements over LSI package pins by fiber-edge magnetooptic probe

To establish a method for investigating hidden radiation sources and their mechanisms in a printed circuit board, we performed preliminary measurements of one-dimensional magnetic near-field distribution over pins of a large-scale integrated circuit (LSI) package by means of an optical method: the fiber-edge magnetooptic (FEMO) probing technique. The FEMO probe consists of fiber optics and a magnetooptic crystal glued at a fiber edge. Its planar spatial resolution is approximately 100 /spl mu/m. It was found that a magnetic field generated from each LSI pin could be distinguished and some radiation was generated from ground and power supply lines. We compared the measured results with corresponding radiated electric field strength that was separately measured. The frequency of interest was the tenth harmonic of the output signal. We observed a strong correlation between those two experimental results, which suggests the effectiveness of our proposed method for near-field investigation. One of the beneficial features of the FEMO probe is its small probe head, due to which one can perform detailed near-field evaluations in a microscopic region. Furthermore, we tried to specify a major electromagnetic interference source by additional measurements of near-field distributions and frequency dependence of magnetooptic signals. It was suggested that the short-through current flowing in the power-supply system of the input/output circuits caused high-level radiated emission.

Minimization of Friction at Various Speeds Using Autonomous Viscosity Control of Nematic Liquid Crystal

To demonstrate the concept of the smart lubrication system using liquid crystal (LC) lubricant proposed by Nakano (Tribol Lett 14:17–24, 2003), the following three types of tests were performed: (1) film thickness measurements in pure rolling contacts, (2) friction coefficient measurements in rolling–sliding contacts, and (3) molecular orientation measurements in stationary cells. In all of these types of tests, a nematic LC [4-pentyl-4′-cyanobiphenyl (5CB)] and a carboxylic acid [hexadecanoic acid (HDA)] were used as the base fluid and the additive, respectively. The results of these tests confirm the following mechanism. First, surface films of the HDA additive that spontaneously adsorb onto contact surfaces induce surface anchoring, which has the effect of making 5CB molecules align themselves perpendicular to the surfaces, competing with the flow alignment of 5CB molecules and inducing an increase in the apparent viscosity of 5CB with decreasing entrainment speed. This increase in the apparent viscosity generates a constant friction coefficient region in the Stribeck curve on the left side of the minimum friction coefficient point, resulting in the minimization of friction at various speeds.

Finite Element Analysis of Precursors to Macroscopic Stick–Slip Motion in Elastic Materials: Analysis of Friction Test as a Boundary Value Problem

AbstractIn this study, we apply the finite element method to investigate precursor to frictional sliding phenomena arising immediately prior to macroscopic stick–slip transitions in elastic bodies within the framework of a continuum theory. Using a numerical model that mimics an actual experimental system, we study the behavior of contact surface nodes to assess the influence of stiffness, driving velocity, initial conditions, and discretization conditions on the propagation characteristics of microscopic slips. In particular, we show that the initial distribution of frictional stress arising due to the Poisson effect has a significant effect on the propagation characteristics in slip regions. Next, based on the results of a finite element analysis of precursor phenomena that accounts for the influence of bulk compliance, we consider the determination of parameters in rate-dependent friction models. With regard to the behavior of sliding friction, we show that the relationship between friction tests and friction models is fundamentally different from the relationship between materialtests and constitutive models for material deformation. We conclude that a proper understanding and classification of friction tests, friction models, and the relationship between these tests and boundary value problems are crucial ingredients in the application of computer-aided engineering techniques to sliding-friction phenomena; indeed, friction tests must ultimately be treated as boundary value problems.

New Technique for Evaluating Adhesion Properties between Soft Materials

A new, simple apparatus for measuring the surface adhesion properties of soft materials was designed, where the adhesion force of a point contact between soft materials and the total energy required to separate the contact can be measured using the springs of phosphor–bronze thin plates with strain gauges. The adhesion between swollen hydrogels was studied here by this simple technique in air at room temperature. The gels used in the present preliminary experiments were poly(sodium acrylate) hydrogels physically cross-linked by aluminum ions. The adhesion force and the separation energy showed a power-law increase with separation velocity. The apparatus was applied to evaluate the adhesion properties of seven anti-inflammatory analgesic cataplasms on the market. It was found that the easiness to separate (rank of adhesion force and the separation energy) was consistent with the results of those obtained by organoleptic evaluations.

Yawing Angular Misalignment Provides Positive Damping to Suppress Frictional Vibration: Basic Applicability to Disc Brake Systems

A novel method for suppressing frictional vibration in sliding systems without using additional devices (e.g., dampers or actuators) is theoretically proposed. This method is based on the principle that yawing angular misalignment provides positive damping to sliding systems. Even if frictional vibration occurs due to a negative slope of the friction versus velocity characteristic, it can be suppressed by applying a misalignment angle φ that is larger than the critical misalignment angle φcr. To examine the basic applicability of this method to disc brake systems, a braking test at a constant normal load was simulated numerically. It was found that yawing angular misalignment can not only suppress frictional vibration but also shorten the braking time as a secondary effect of suppressing frictional vibration.

Polarization Observations of Adsorption Behavior of Fatty Acids Using Optical Anisotropy of Liquid Crystal

Chemical interactions and formation of adsorbed films of additives strongly affect the tribological behavior in boundary lubrication. In order to examine the relationship between the adsorption behavior and friction properties of fatty acids, polarization observations using a liquid crystal and friction measurements were performed. A nematic liquid crystal [4-pentyl-4′-cyanobiphenyl (5CB)] and normal fatty acids with 16–22 C chains were base fluid and additive, respectively. The adsorption behavior of the fatty acids was observed using polarization microscopy and analyzing the optical anisotropy of 5CB. The adsorbed film of fatty acids induced a change in the orientation of 5CB from planar to homeotropic. The expansion speed of the homeotropic domain increased as the chain length of the fatty acid decreased. The friction coefficient of the test lubricants was measured in a steel–steel sliding contact using a low-speed reciprocating tribometer. At the first cycle of reciprocating sliding motion, the lower friction coefficient was measured using fatty acids with the shorter alkyl chain length. The friction coefficient at the first cycle showed a strong correlation with the expansion speed of the homeotropic domain. These results show that the shorter-chain fatty acids form an adsorbed film highly oriented perpendicular to the solid surface, and the adsorbed film provides a lower friction coefficient at the first cycle.