Sedimentation and Effective Temperature of Active Colloidal Suspensions

TLDR

Active colloids are chemically powered particles that self‑propel in hydrogen peroxide. The study experimentally probes the nonequilibrium steady state of an active colloidal suspension under gravity. Using a microfluidic device with permeable gel microstructures, the authors measured both individual colloid dynamics and the global stationary state by optical microscopy. They directly measured the effective temperature as a function of particle activity via the fluctuation‑dissipation relation, marking a first step toward exploring out‑of‑equilibrium properties of active colloids.

Abstract

In this Letter, we investigate experimentally the nonequilibrium steady state of an active colloidal suspension under gravity field. The active particles are made of chemically powered colloids, showing self propulsion in the presence of an added fuel, here hydrogen peroxide. The active suspension is studied in a dedicated microfluidic device, made of permeable gel microstructures. Both the microdynamics of individual colloids and the global stationary state of the suspension under gravity are measured with optical microscopy. This yields a direct measurement of the effective temperature of the active system as a function of the particle activity, on the basis of the fluctuation-dissipation relationship. Our work is a first step in the experimental exploration of the out-of-equilibrium properties of active colloidal systems.