Measuring the Stokes’ drag in a microtubule-kinesin active gel

Activity invalidates many of the traditional assumptions of hydrodynamics. So far, many studies have been limited to characterizing and simulating activity-driven flows: few have measured forces and torques exerted by an active fluid on an external body. Yet quantitative measurements of active stresses are required to test active hydrodynamics theories and conceive practical applications of active fluids. Here, we experimentally measure the drag force on a sphere sedimenting in a 3D active fluid powered by the continuous extension and buckling of kinesin-microtubules bundles. The sphere sediments under the combined effects of large-scale spontaneous coherent flows, mesoscopic turbulence and gravity. Its motion can be described by a nonlinear Stokes drag characterized by an effective viscosity and an effective diffusivity. Using a custom-built bright-field microscope, we track active sedimentation with micron resolution, even for large bead diameters up to 100 microns, in 3D microfluidic chambers, through continuous scanning of a moving volume with a piezo-driven objective. Combined with theoretical and numerical frameworks, we present the statistics of the bead trajectories and develop the form of the Stokes drag for active media.