Hovering Microswimmers Exhibit Ultrafast Motion to Navigate under Acoustic Forces

The goal of this study is to engineer 3D-microswimmers containing a bubble that can be stimulated and guided with acoustic waves emitted by transducers. By using 3D-microfabrication techniques, 20 x 20 x 26 mu m swimmers are designed with a trapped air bubble pointing toward the substrate, thus mimicking a hovercraft. Acoustic vibrations are then remotely applied to the bubble, which generates a strong steady flow (0.1-2 mm s(-1)), an effect referred as acoustic streaming, resulting in a jet below the hovercraft. It is found that the motion of the swimmer relies on two parameters, namely the frequency and amplitude of the acoustic wave. The swimmer velocities are measured and a very wide distribution from 0.05 to 350 mm s(-1) or 17 500 body lengths is observed. Such a high velocity in terms of body length makes this swimmer one of the fastest among the different microswimmers reported in the literature. The motion of the swimmer is found to be a combination of two forces orientated in different directions: the streaming force and the radiation force. While the first one is reducing adhesion, the second one is helping the motion. Using different transducers orientated toward different directions, the swimmer is enabled to be navigated into different directions as well.