Sounds and hydrodynamics of polar active fluids

TL;DR

This paper demonstrates the propagation of sound waves in colloidal active fluids with broken rotational symmetry, enabling measurement of hydrodynamic parameters through active-sound spectroscopy, which could extend to biological collectives.

Contribution

It introduces active-sound spectroscopy to measure hydrodynamic constants in active fluids, revealing coupled density and velocity wave modes.

Findings

Propagation of mixed density-velocity modes in active fluids.

Hydrodynamic constants can be extracted from fluctuation spectra.

Method applicable to both synthetic and biological active systems.

Abstract

Spontaneously flowing liquids have been successfully engineered from a variety of biological and synthetic self-propelled units. Together with their orientational order, wave propagation in such active fluids have remained a subject of intense theoretical studies. However, the experimental observation of this phenomenon has remained elusive. Here, we establish and exploit the propagation of sound waves in colloidal active materials with broken rotational symmetry. We demonstrate that two mixed modes coupling density and velocity fluctuations propagate along all directions in colloidal-roller fluids. We then show how the six materials constants defining the linear hydrodynamics of these active liquids can be measured from their spontaneous fluctuation spectrum, while being out of reach of conventional rheological methods. This active-sound spectroscopy is not specific to synthetic active materials and could provide a quantitative hydrodynamic description of herds, flocks and swarms from inspection of their large-scale fluctuations