Hydrodynamic Coupling Melts Acoustically Levitated Crystalline Rafts

TL;DR

This study demonstrates how tuning particle size in acoustic levitation creates a hydrodynamically coupled system that transitions from a crystalline raft to a liquid-like state due to fluid-driven excitations.

Contribution

It introduces a novel method to control particle interactions in acoustic levitation by balancing attractive and repulsive hydrodynamic forces through particle size tuning.

Findings

Particles form open, tunable lattices in levitation.

Hydrodynamic coupling induces spontaneous lattice excitations.

Raft transitions from crystalline to liquid-like state with fluctuations.

Abstract

The acoustic levitation of small particles provides a versatile platform to investigate the collective dynamical properties of self-assembled many-body systems in the presence of hydrodynamic coupling. However, acoustic scattering forces can only generate attractive interactions at close range in the levitation plane, limiting self-assembly to rafts where particles come into direct, dissipative, contact. Here, we overcome this limitation by using particles small enough that the viscosity of air establishes a repulsive streaming flow at close range. By tuning the size of particles relative to the characteristic length scale of the viscous flow, we control the interplay between attractive and repulsive forces. In this novel granular raft, particles form an open lattice with tunable spacing. Hydrodynamic coupling between particles gives rise to spontaneous excitations in the lattice, in turn driving intermittent particle rearrangements. Under the action of these fluctuations, the raft transitions from a predominantly quiescent, crystalline structure, to a two-dimensional liquid-like state. We show that this transition is characterized by dynamic heterogeneity and intermittency, as well as cooperative particle movements, that produce an effectively `cageless' crystal. These findings shed light on fluid-coupling driven excitations that are difficult to isolate and control in many other hydrodynamic systems.