Analysis of bidirectional vibrational transport of small objects by periodic wave trains of pulses

A new mechanism for pulsed acoustic transport of small particles or droplets, similar to the previously studied case of "traveling electrostatic waves", is described. For this mechanism, in contrast to the traditional notion of acoustic transport, the time-averaged force of the wave action upon the moving object is equal to zero. Effective transport of this kind occurs only when an optimal choice is made in terms of the carrier-wave phase with respect to the leading edge of the wave pulses. The direction of this transport may be either forward or backward with respect to the direction of wave propagation and can be controlled by switching the polarity of the driving pulses. The effect of friction as a deceleration factor leads to a gradual violation of the optimal phase relationship between the vibrational motion of the moving object and the action of radiofrequency wave pulses on it. This in turn restricts the length of the wave pulses that are effective in bringing about transport, making it necessary to use a periodic wave train of pulses. In media where friction cannot be neglected, undesirable negative displacement of the object with respect to its principal direction of transport may be reduced through the use of pulses with an optimal exponential decay. The value of the optimal phase of the carrier-wave is found to be the same for the various pulse shapes studied. A new kind of particle-wave interaction in the form of "dielectrophoresis with acoustic drive" is suggested.