Drop-in additives for suspension manipulation: Colloidal motion induced by sedimenting soluto-inertial beacons

Soluto-inertial (SI) suspension interactions involve nonequilibrium colloidal migration over millimeters, driven by beacons that establish long-lasting chemical gradients. Previous demonstrations of the SI concept were restricted to fixed, cylindrical beacons that would sustain solute fluxes in controlled two-dimensional geometries. Here we examine soluto-inertial interactions established by a spherical beacon that sediments through a suspension. The sedimenting beacon leaves a trailing solute wake, the flux from which drives the motion of neighboring suspended objects. Experiments reveal wakes that can attract or repel particles, depending on the solute-colloid pair, creating local regions in the suspension that are enriched in or devoid of particles. Theoretical descriptions accurately capture experimental observations and provide predictive ability to gauge the strength of these interactions in terms of the design parameters. We anticipate that long-range, bulk suspension interactions driven by drop-in beacon additives will enable on-demand flocculation of dilute suspensions and separation of colloidal mixtures.