  Tuning the mass density and bulk modulus independently is the key to
manipulate the propagation of sound wave. Acoustic metamaterials provide a
feasible method to realize various acoustic parameters. However, the relevant
studies are mainly concentrated in air, and the huge impedance difference makes
it difficult to directly extend these airborne structures to underwater
application. Here, we propose a metafluid to realize independent manipulation
of the mass density and bulk modulus underwater. The metafluid is composed of
hollow regular polygons immersed in the water. By adjusting the side number of
the hollow regular polygons and choosing proper materials, the effective mass
density and bulk modulus of the metafluid could be modulated independently.
Based on the flexible adjustment method, metafluids with same impedance but
different sound velocities are designed and used to realize an underwater
impedance-matched gradient index lens. In addition, by combining the proposed
metafluid with other artificial structures, acoustic parameters with great
anisotropy can be achieved, which is exemplified by the design and
demonstration of an impedance-matched underwater acoustic carpet cloak. This
work can expand the practicability of underwater metamaterials and pave the way
for future potential engineering applications in the practical underwater
devices.
