Motors on the molecular scale

An approach which has been recently introduced to construct microscopic engines is investigated. The main characteristic of the approach is the possibility to determine dynamically the direction of motion of the engines. The engines, which are moving objects on a substrate, are able to move translationally or rotationally and simultaneously perform useful functions such as pulling of a cargo. We discuss an example in which the energy is fed into the engine by changing locally the intrinsic lengths of the moving object. The local changes might be obtained by externally exciting the system. The transformation of the supplied energy into directed motion is through dynamical competition between the intrinsic lengths given by the moving object and the characteristic lengths of the substrate.