Soft 3D acoustic metamaterial with negative index

Area of Science:

• Acoustics
• Materials Science
• Soft Matter Physics

Background:

• Metamaterials with negative refractive indices have been extensively studied in electromagnetism.
• Extending negative index properties to three-dimensional (3D) acoustic media remains a significant challenge.
• Soft-matter techniques offer new avenues for engineering advanced materials.

Purpose of the Study:

• To realize 3D acoustic metamaterials with negative acoustic indices.
• To investigate the acoustic properties of novel soft metafluids.
• To explore potential applications in acoustics, such as sub-wavelength imaging.

Main Methods:

• Fabrication of macroporous microbeads using soft-matter techniques.
• Creation of concentrated suspensions of these microbeads to form ultrasonic metafluids.
• In situ ultrasonic experiments to study Gaussian pulse propagation.
• Multiple-scattering calculations to predict acoustic behavior.

Main Results:

• Demonstrated a new class of locally resonant ultrasonic metafluids.
• Observed negative real parts of the acoustic index (up to -1) over broad frequency bandwidths.
• Experimental results align with predictions from multiple-scattering calculations.
• Acoustic index behavior is tunable by adjusting the microbead volume fraction.

Conclusions:

• Successfully engineered soft 3D acoustic metamaterials with negative acoustic indices.
• These metafluids offer a pathway for achieving negative or zero acoustic indices.
• Opens possibilities for transformative acoustic applications, including sub-wavelength imaging and transformation acoustics.