Self-propulsive swimmers: Two linked acoustic radiating spheres

Abstract

We propose a simple, practical, and versatile acoustic-driven swimmer, composed of two spherical bodies that may radiate the sound field, monochromatically, at monopole state (breathing mode of vibration) and are linked by a rigid rod. Considering the nonlinear acoustic effects, the net acoustic radiation force exerted on the device is analytically derived and it is shown that the resultant radiation force exerted on the swimmer may be nonzero. Two different configurations are considered: In the first, both spheres radiate, and in the second, one of the spheres is off. In both cases, the full manipulability conditions of swimmers are obtained and the effects of size factors, frequency of radiation, etc., are discussed. Assuming low Reynolds number swimming condition, the frequency-dependent swimming velocity is obtained via the so-called reflection method and the optimal radiating states are discussed. Finally, the challenge of random walk due to host medium fluctuations is discussed and it is shown that the Brownian noise is negligible. Our methodology will open a path toward self-propulsive controllable devices, which may play the role of carriers, machines, or mechanisms at small scales.