Transversally stable acoustic pulling force produced by two crossed plane waves

In this letter, we show that a particle in liquid can be pulled backward by simply launching two crossed acoustic plane waves at it. It is demonstrated that the counterintuitive pulling force originates in essence from the interference effect: while the radiation force components due to the two waves themselves are always positive (pushing), the radiation force component due to the interference can turn negative (pulling); when it becomes dominant over the formers, a net pulling force results. For this type of simple acoustic sources, our numerical results show the existences of transversally stable pulling forces for particles of different size scales. The extension to the cases of multiple symmetrically incident plane waves is also discussed. This study could be very helpful for the experimental realization of pulling force by acoustic waves.