Hiroshi Matsukawa

Systematic Breakdown of Amontons' Law of Friction for an Elastic Object Locally Obeying Amontons' Law

In many sliding systems consisting of solid object on a solid substrate under dry condition, the friction force does not depend on the apparent contact area and is proportional to the loading force. This behaviour is called Amontons law and indicates that the friction coefficient, or the ratio of the friction force to the loading force, is constant. Here, however, using numerical and analytical methods, we show that Amontons law breaks down systematically under certain conditions for an elastic object experiencing a friction force that locally obeys Amontons law. The macroscopic static friction coefficient, which corresponds to the onset of bulk sliding of the object, decreases as pressure or system length increases. This decrease results from precursor slips before the onset of bulk sliding, and is consistent with the results of certain previous experiments. The mechanisms for these behaviours are clarified. These results will provide new insight into controlling friction.

Deconfinement of vortices with continuously variable fractions of the unit flux quanta in two-gap superconductors

We propose a new stage of confinement-deconfinement transition, which can be observed in laboratory. In two-gap superconductors (SCs), two kinds of vortex exist and each of them carries a continuously variable fraction of the unit flux quanta Φ0=hc/2e. The confined state of these two is a usual vortex and stable in the low-temperature region of the system under a certain magnetic field above Hc1. We see an analogy to quarks in a charged pion. An entropy gain causes two fractional vortices to be deconfined above a certain temperature.

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.

Flux Flow Resistivity in Two-Gap Superconductor

We investigate the flux flow state in a two-gap superconductor in which two s -wave gaps with different amplitudes exist on two separate Fermi surfaces. The flux flow resistivity is obtained on the...

Complex dynamics and directed transport of particles at a nanoscale interface

Novel directed motion of a one-dimensional particle array at nanointerface are studied by numerical simulation based on the extended Frenkel-Kontorova model. Even for a symmetric periodic potential with no direct external driving force, the directed motion of the particle array is induced by introducing spatiotemporally periodic modulation of natural length between adjacent particles with the assistance of the periodic potential. Time-averaged velocities of the directed motion can be locked to multiple values of a unit velocity. The locking state appears even for vanishingly small potential amplitude. The mechanism and peculiar slow dynamics of the directed motion of the particle array are discussed.

NANOSCALE EFFECTS OF SURFACES AND LUBRICANTS ON FRICTION

Nanoscale frictional phenomena at solid–solid surfaces with lubricants are studied numerically using a lattice model which consists of two rigid substrates and a monolayer of lubricant molecules. The maximum static frictional force, which works on the driven upper solid, is always finite and obeys a certain scaling relation. The lubricant layer, however, shows a kind of phase transition from a pinned state to free sliding state when the strength of the interaction potential with the substrates decreases. We discuss the peculiar pinning mechanism of the upper substrate in the presence of a lubricant monolayer.

Directed Motion and Novel Dynamics of Interfacial Systems

Using numerical simulations based on a lattice model of a nanoscale interface, we focus on novel dynamics concerning directed motion of a particle array. As a method to realize directed motion, we introduce spatiotemporal modulation into the natural length between nearest neighbor particles in the array. Under certain conditions, it is found that the directed motion appears in the particle array. The magnitude of the velocity is locked to certain values. The values to which the velocity locks are understood by assuming adiabatic motion of particles. It is found, however, that the velocity locking occurs even in a nonadiabatic regime.

Flux flow resistivity in the system with two superconducting gaps

Abstract We calculate the flux flow resistivity in the system with two s-wave superconducting gaps. Our result agrees well with anomalous field dependence of the resistivity recently observed in the vortex state of MgB 2 .

Friction of Macroscopic Systems

The frictional phenomena appear in various systems. Their scales spread to extraordinary wide range. The frictional systems of large scale extend to landslide, glacier and earthquake, while those of small scale reach to sub-nanometer. There are universal behaviors in friction which are independent from the scale of the system. Typical examples are the maximum static frictional force, which is the threshold strength of the external force to cause the translational motion, and the kinetic frictional force, which results from energy dissipation accompanied by the caused motion. Besides these, stick-slip phenomena, memory effect of static frictional force, velocity dependence of kinetic frictional force and so on appear in various systems with wide range of scale. There are common mechanisms in these universal phenomena. Here we review frictional phenomena of various systems from a unified point of view.

Numerical study of friction of flake and adsorbed monolayer on atomically clean substrate

Frictional behaviors of flake and adsorbed monolayer on substrate can be observed by Frictional force microscope and Quartz crystal microbalance experiments and are typical problems in nano-friction. Computer simulations had been played important roles in understanding those behaviors, but in most of them the driving direction coincides with one of the crystal axes of the substrate. Here we report our numerical results of direction dependence of friction of flake and adsorbed monolayer. We found a new kind of dynamical phase transition in which flake and adsorbed monlayer change their structure relative to the substrate and make incommensurae structure to reduce kinetic frictional force after certain transition time. When the driving velocity is decreased the transition time tends to diverge at certain critical velocity for the flake. For the adsorbed monlayer the transition time tends to diverge at certain critical magnitudes of the external force or the interaction strength between adsorbates when they are decreased.