Determination of useful parameter space for a double-walled carbon nanotube based motor subjected to a sinusoidally varying electric field

A molecular dynamics analysis has been performed for a double-walled carbon-nanotube based motor driven by an externally applied sinusoidally varying electric field, in the presence of a 'frozen' sleeve. It is shown that to produce unidirectional (motor-like) rotation, it is necessary to operate over a 'useful' region in the parameter space defined by the amplitude and frequency of the applied electric field. For a given frequency, electric field amplitudes below a threshold are not able to overcome the potential energy barriers due to interaction of the rotating shaft with the frozen sleeve. This is followed by a range of amplitudes where unidirectional motion is observed. At still higher amplitudes, distortion of the shaft increases the potential energy barriers to levels higher than those that can be overcome by the electric field. For a given amplitude, as the frequency is varied, more complex behavior is obtained, which can be broken up into four regions. At low frequencies, large distortion of the shaft leads to an increase in potential energy barriers, hindering rotation. Over an intermediate range, unidirectional motion is observed. This is followed by an anomalous region, where resonant excitation of a characteristic mode of the shaft leads to very large distortions, which greatly enhances the barrier. The distortion falls off with further rise in frequency. A detailed physical explanation has also been provided for the anomalous behavior in terms of resonant excitation of characteristic modes.