Acoustic Probes of Soft Condensed Matter Systems

Soft matter systems display many of the phenomena also known in conventional condensed phase physics, while offering entirely new insights due to the accessibility and tunability of the effective material properties and system dynamics. The ‘soft’ of soft matter typically refers to the relative strength of material cohesion and fluctuating thermal forces – here we present soft matter where the cohesion and fluctuating forces are both derived from high-order scattering of sound by submillimeter objects. When subjected to intense sound fields, granular objects can be supported against gravity and interact strongly to form liquid-like and solid-like condensed materials with surprising effective material properties. Furthermore, coupling between particle configurations and the encompassing acoustic cavity drives instabilities which subject such materials to noisy fluctuations that may or may not resemble a thermal bath. When levitated in air, particles experience underdamped dynamics and so linear and angular momentum play important roles in configurational transitions, while size and shape provide additional axes along which interparticle forces can be tuned. By using a combination of high-speed videography, particle tracking, and fluid simulation, we explore acoustic levitation as a tool for preparing and probing exotic soft matter systems with strong classical wave-particle coupling.