Measuring Force Fluctuations and Diffusion in Active Baths*

Actively driven systems such as suspensions of active colloids, bacteria, and enzymes have been observed to enhance diffusion at the microscopic scale. Active suspensions also exhibit anomalous mechanical properties that are not observed in typical equilibrium fluids. To investigate the interplay between enhanced diffusion and fluid response we measure fluctuations of forces, motion, and viscosity in active baths composed of actively swimming bacteria. To do so, we use two approaches: (1) Differential Dynamic Microscopy, to analyze motion in the form of image intensity fluctuations to quantify decorrelations of the active suspension. (2) Optical tweezers, to trap suspended colloids and measure the force fluctuation spectrum due to non-equilibrium fluctuations of the active bath. We quantify the space-time correlations and calculate the energy dissipated by non-equilibrium processes to extract information about the system.