%'hen solid bodies contact and slide against each other, the frictional phenomenon occurs. The origin of the related frictional force is studied by assuming the existence of two clean crystal surfaces, which follows the current experimental trends. This study theoretically clarifies the atomistic origin of the frictional force intrinsically generated by the molecular interactions between the constituent atoms of solids, but not the force extrinsically generated by surface asperities, the existence of foreign atoms, etc. Furthermore, this study assumes that the constituent atoms of the two contacting surfaces interact with each other due to the interaction potential. This study found that there are two origins: atomistic locking and dynamic locking. Atomistic locking occurs when the configuration of the atoms on a contact surface continuously changes with the sliding distance and when the interatomic potentials have an arbitrary strength. In contrast, dynamic locking occurs when the configuration discontinuously changes due to the dynamic process and if the interatomic potential is stronger than a specific given value. A criterion is derived for the occurrence of dynamic locking. From studying various systems, it can be seen that dynamic locking is unlikely to occur in realistic systems. The frictional forces due to atomistic locking are calculated for a-iron. One other important finding prior to the experiments is that certain unique cases exist where the frictional force exactly vanishes if completely clean solid surfaces are prepared.

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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.

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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.

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