Modulation of the path instability of buoyancy-driven swimming sheet.

Recently, the buoyancy-driven, low-energy swimming sheet robot (artificial bubble) has been reported, and here we focus on its path instability analogous to that of rising bubbles, to improve its controllability. For such a sheet-shaped body, the stability of the motion is affected by not only the geometry but the formation of an unstable wake flow. We systematically varied key parameters governing this path instability, such as moment of inertia, Reynolds number, and thickness-to-width ratio. Using shadowgraphy and particle image velocimetry, we measured the three-dimensional trajectory, velocity, and wake flow of the buoyant sheets, and tried to establish the geometric conditions for stable swimming at the Reynolds numbers up to 3,000. We also analyze the hydrodynamic forces acting on the body in the generalized Kirchhoff equations to understand the mechanism of identified conditions.