Wave-based liquid-interface metamaterials

Abstract

The control of matter motion at liquid–gas interfaces opens an opportunity to create two-dimensional materials with remotely tunable properties. In analogy with optical lattices used in ultra-cold atom physics, such materials can be created by a wave field capable of dynamically guiding matter into periodic spatial structures. Here we show experimentally that such structures can be realized at the macroscopic scale on a liquid surface by using rotating waves. The wave angular momentum is transferred to floating micro-particles, guiding them along closed trajectories. These orbits form stable spatially periodic patterns, the unit cells of a two-dimensional wave-based material. Such dynamic patterns, a mirror image of the concept of metamaterials, are scalable and biocompatible. They can be used in assembly applications, conversion of wave energy into mean two-dimensional flows and for organising motion of active swimmers.

Acoustic versus electromagnetic field theory: scalar, vector, spinor representations and the emergence of acoustic spin

“…Finally, the vector-potential representation produces non-zero spin angular momentum density (30) in generic sound wave fields. This quantity, surprising for purely longitudinal (curl-less) fields associated with spin-0 phonons, was introduced only recently [5,6,7,8,9], but it has already found direct experimental and numerical confirmations [6,9,27].…”

“…First, the Lagrangian takes the expected form of the difference between the kinetic and potential energies of the medium particles (molecules). Second, the canonical momentum density (30) can be directly associated with the Stokes drift of the molecules [27]. Finally, the vector-potential representation produces non-zero spin angular momentum density (30) in generic sound wave fields.…”

“…In the case of monochromatic fields, we have iωφ = c √ βp and iωā = c √ ρv, and the time-averaged momentum and spin densities in the two representations, Eqs. (27) and (28) become, respectively:…”

“…In the source-free case, where q = 0 and u = 0, the canonical momentum and spin densities become the average of the scalar-representation and vector-representation expressions (27) and (28):…”

“…Moreover, the spin density (2) has a clear physical interpretation: microscopic particles (molecules) constituting the medium move along small elliptical trajectories, so thatS originates from their mechanical angular momentum [6,7,27]. In turn, the velocity-related contribution to the canonical momentum density (2) can be directly associated with the Stokes drift of molecules in the medium [27]. Note that the momentum and spin densities (1) and (2) consist of contributions from the vector electric and vector magnetic fields in the case of electromagnetism and from the vector velocity and scalar pressure fields in the case of acoustics.…”