Observation of a transition to a localized ultrasonic phase in soft matter

• Published in: Communications physics 2022-01, Vol.5 (1), p.1-10, Article 21
• Main Authors: Matis, Bernard R., Liskey, Steven W., Gangemi, Nicholas T., Edmunds, Aaron D., Wilson, William B., Wheeler, Virginia D., Houston, Brian H., Baldwin, Jeffrey W., Photiadis, Douglas M.
• Format: Article
• Language: English
• Subjects: 639/301/923/1027; 639/301/923/1028; 639/766/119/2795; Anderson localization; Broadband; Condensed matter physics; Elastic media; Encapsulation; Frequencies; Localization; Low temperature; Mean free path; Phase transitions; Physics; Physics and Astronomy; Scattering
• ISSN: 2399-3650; 2399-3650
• DOI: 10.1038/s42005-021-00795-x

Anderson localization arises from the interference of multiple scattering paths in a disordered medium, and applies to both quantum and classical waves. Soft matter provides a unique potential platform to observe localization of non-interacting classical waves because of the order of magnitude difference in speed between fast and slow waves in conjunction with the possibility to achieve strong scattering over broad frequency bands while minimizing dissipation. Here, we provide long sought evidence of a localized phase spanning up to 246 kHz for fast (sound) waves in a soft elastic medium doped with resonant encapsulated microbubbles. We find the transition into the localized phase is accompanied by an anomalous decrease of the mean free path, which provides an experimental signature of the phase transition. At the transition, the decrease in the mean free path with changing frequency (i.e., disorder strength) follows a power law with a critical exponent near unity. Within the localized phase the mean free path is in the range 0.4–1.0 times the wavelength, the transmitted intensity at late times is well-described by the self-consistent localization theory, and the localization length decreases with increasing microbubble volume fraction. Our work sets the foundation for broadband control of localization and the associated phase transition in soft matter, and affords a comparison of theory to experiment. Anderson localization is a disorder-driven phenomenon typically considered in the context of low-temperature condensed-matter systems. However, the phenomenon can also be relevant to soft matter and here the authors experimentally demonstrate it is possible to observe Anderson localization of ultrasonic waves in soft matter doped with resonant encapsulated microbubbles.