Experimental demonstration of a negative phase velocity in soft 3D metafluids

We report a new class of soft 3D locally resonant ultrasonic fluid metamaterials, engineered using soft-matter techniques coupled with microfluidics. The propagation of Gaussian pulses within random suspensions of “ultra-slow” porous micro-beads is investigated through in situ acoustical experiments. Via angular-phase measurements performed on signals propagating within these acoustic metafluids, the phase velocity (or refractive index) is shown to be negative over broad ultrasonic frequency bands in good agreement with multiple-scattering calculations.