Ostwald-like ripening in the two-dimensional clustering of passive particles induced by swimming bacteria

TL;DR

This study demonstrates that passive particles in bacterial suspensions undergo Ostwald-like ripening, with clustering dynamics driven by bacteria-induced attraction, revealing insights into active matter and colloidal assembly.

Contribution

It reports the first observation of Ostwald-like ripening in passive particles driven by motile bacteria, including characterization of clustering dynamics and forces involved.

Findings

Clustering time scale depends on initial encounter of beads.

Cluster growth follows a t^{1/3} law similar to Ostwald ripening.

Bacteria induce short-range attractive forces on passive particles.

Abstract

Clustering passive particles by active agents is a promising route for fabrication of colloidal structures. Here, we report the dynamic clustering of micrometric beads in a suspension of motile bacteria. We characterize the coarsening dynamics for various bead sizes, surface fractions and bacterial concentrations. We show that the time scale $\tau$ for the onset of clustering is governed by the time of first encounter of diffusing beads. At large time ($t \gg \tau$), we observe a robust cluster growth as $t^{1/3}$, similar to the Ostwald ripening mechanism. From bead tracking measurements, we extract the short-range bacteria-induced attractive force at the origin of this clustering.