2025/06/03 by Yamil Abraham, B. A. van Tiggelen, Nicolás Benech +3 · 1 voice
Engineering · #Acoustic Wave Phenomena Research #Granular flow and fluidized beds #Ultrasonics and Acoustic Wave Propagation
paper · doi:10.1103/47wj-s8lj
openalex publication_date 2025/06/03 · openalex created_date 2025/06/04 · openalex updated_date 2026/07/22
We demonstrate that ultrasound diffusion—typically associated with the transport of average wave energy and the breaking of time-reversal symmetry—can nonetheless be revealed through a time-reversal experiment. This is achieved using an unprecedented configuration: A single piezoelectric transducer, acting as a time-reversal mirror (TRM), is buried deep inside a strongly scattering medium (a dense granular suspension), while an array of transducers is positioned at a distance, outside the scattering region. A short pulse is emitted by a single array element and the TRM records the resulting ultrasonic field, composed of a coherent ballistic wave followed by a diffuse coda wave. When the entire coda is time-reversed and re-emitted from the TRM, the wave refocuses at the original source with a focal spot size that decreases with the inverse of the TRM depth, consistent with diffusive transport. By time-reversing short coda segments at increasing times <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mi>t</a:mi> </a:math> , we observe a focal spot size scaling as <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:mrow> <b:mn>1</b:mn> <b:mo>/</b:mo> <b:msqrt> <b:mrow> <b:mi>D</b:mi> <b:mi>t</b:mi> </b:mrow> </b:msqrt> </b:mrow> </b:math> , where <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:mi>D</c:mi> </c:math> is the ultrasound diffusion coefficient. Fitting this evolution with a microscopic diffusion model allows us to extract <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"> <d:mi>D</d:mi> </d:math> . Remarkably, this measurement does not require ensemble averaging, because of the inherent stability of time-reversal against statistical fluctuations.