Motohisa Hirano

Simulating molecular shuttle movements: Towards computer-aided design of nanoscale transport systems

Molecular shuttles based on the motor protein kinesin and microtubule filaments have the potential to extend the lab-on-a-chip paradigm to nanofluidics by enabling the active, directed and selective transport of molecules and nanoparticles. Based on experimentally determined parameters, in particular the trajectory persistence length of a microtubule gliding on surface-adhered kinesin motors, we developed a Monte-Carlo simulation, which models the transport properties of guiding structures, such as channels, rectifiers and concentrators, and reproduces the properties of several experimentally realized systems. Our tool facilitates the rational design of individual guiding structures as well as whole networks, and can be adapted to the simulation of other nanoscale transport systems.

Comparing guiding track requirements for myosin- and kinesin-powered molecular shuttles.

The design of nanoscale transport systems utilizing motor proteins as engines has advanced rapidly. Here, actin/myosin- and microtubule/kinesin-based molecular shuttles are compared with respect to their requirements for track designs. To this end, the trajectory persistence length of actin filaments gliding on myosin-coated surfaces has been experimentally determined to be equal to 8.8 +/- 2 microm. This measurement complements an earlier determination of the trajectory persistence length of microtubules gliding on kinesin-coated surfaces and enables a comparison of the accessible track designs for kinesin and myosin motor-powered systems. Despite the 200-fold smaller stiffness of actin filaments compared to that of microtubules, the dimensions of myosin tracks for actin filaments have to be quite similar to the dimensions of kinesin tracks for microtubules (radii larger than 200 nm and widths smaller than 0.9 microm compared to 600 nm and 19 microm). The difference in gliding speed is shown to require additional consideration in the design of track modules.

Friction of elastomer-on-glass system and direct observation of its frictional interface

We performed a study on the static friction of PDMS elastomers with well-defined surface topography sliding over glass. An experimental setup for simultaneous measurements of friction force and direct observations of frictional interface has been developed. The static friction force was nearly proportional to normal load. The static friction force was independent of stick time. The simultaneous measurements revealed that the static friction force was proportional to the total area of contact. The coefficient was nearly independent of the surface topography of PDMS elastomers.

Evaluation of friction transition for metal-semiconductor interfaces using model potential comprising three-body contributions

Whether or not the friction transition [1, 2] occurs in the frictional systems of W(011) and Si(001) atomically clean surfaces has been examined in relation to a previous ultrahigh vacuum scanning tunneling microscopy experiment investigating friction transition. This examination takes into account of empirical inter-atomic potentials with three-body interactions. To obtain equilibrium atomic arrangements of the frictional system for evaluating friction transition, the parameters of the interatomic potential applicable for tetrahedrally bonded materials were examined. From studying the criterion of the friction transition for the frictional systems of W(011) and Si(001), it has been concluded that friction transition does not occur for the real systems, which supports the experimental results of a previous ultrahigh vacuum scanning tunneling microscopy experiment.

2 – Superlubricity of Clean Surfaces

Publisher Summary This chapter describes the atomistics of friction—explaining the atomistic origin of the friction forces—and discusses the mechanisms of superlubricity based on a model from the atomistic theory. In Tomlinsons mechanism, the atoms change their equilibrium positions nonadiabatically, which leads to the energy transfer of the elastic energy into the kinetic energy of the atoms. Tomlinsons mechanism explains the energy dissipation; however, it is shown that Tomlinsons mechanism is unlikely to occur in the realistic frictional systems. The origin of the dynamic friction force explains the irreversible energy transfer of the translational kinetic energy into the internal kinetic energies. The translational kinetic energy is a constant for motion, and the frictional system is in a state of superlubricity. The superlubricity can appear when the sum of the forces of each atom vanishes. When the kinetic energy is given to a solid so that it is made to slide at a finite speed, it would come to a stop in a short time. In this case, the energy of the translational motion dissipates because of friction. This energy dissipation does not originate from the nonadiabatic motion of atoms.

Atomistics of Friction

The atomistic mechanisms are proposed for the origin of the static and the dynamic friction forces. The mechanism for the origin of the static friction force resembles the mechanical locking mechanism in a surface roughness model. The origin of the dynamic friction force is formulated as a problem of how the given translational kinetic energy dissipates into the internal relative motions of constituent atoms of bodies during sliding. From studying that the available phase space volume of the translational motion becomes negligible small for a large system size, compared with that of the internal motions, it is concluded that the energy dissipation occurs irreversibly from the translational motion to the internal motions. A phenomenon of superlubricity, where two solid bodies move relatively with no resistance, is discussed.Copyright