Mixing and unmixing induced by active camphor particles

TL;DR

This study investigates how self-propelled camphor particles at the air-water interface induce partial mixing and unmixing of colloidal floaters, revealing a complex interplay of hydrodynamics and chemical effects that lead to a stationary but not fully mixed state.

Contribution

It provides experimental evidence of the competing effects of mixing and unmixing caused by active camphor particles and characterizes the resulting concentration spectra at the interface.

Findings

Floaters remain only partially mixed despite active motion.

Unmixing is driven by Marangoni flows around camphor swimmers.

The system exhibits turbulent-like concentration spectra with distinct regimes.

Abstract

In this experimental study, we report on the mixing properties of interfacial colloidal floaters (glass bubbles) by chemical and hydrodynamical currents generated by self-propelled camphor disks swimming at the air-water interface. Despite reaching a statistically stationary state for the glass bubbles distribution, those floaters always remain only partially mixed. This intermediate state results from a competition between (i) the mixing induced by the disordered motion of many camphor swimmers and (ii) the unmixing promoted by the chemical cloud attached to each individual self-propelled disk. Mixing/unmixing is characterized globally using the standard deviation of concentration and spectra, but also more locally by averaging the concentration field around a swimmer. Besides the demixing process, the system develops a "turbulent-like" concentration spectra, with a large-scale region, an inertial regime and a Batchelor region. We show that unmixing is due to the Marangoni flow around the camphor swimmers, and is associated to compressible effects.