Quantifying Activity and Response of an Active Bath

Active systems such as suspensions of active colloids, bacteria, and enzymes have been observed to enhance diffusion at the microscopic scale. These suspensions also exhibit anomalous mechanical properties not observed in typical equilibrium fluids. To investigate the interplay between enhanced diffusion and fluid response, we study a suspended colloidal particle in an active bath using optical tweezers to conduct force measurements. We quantify the space-time correlations and energy dissipated by non-equilibrium processes to extract the activity of the system. A simple theoretical model of a particle in an active bath is used to compare experimental results to predictions. Microrheological approaches are used to quantify the mechanics of the active bath. Lastly, the relaxation time of the system due to both activity and material response is quantified. This approach quantitatively characterizes the activity and mechanical response of an active bath. Preliminary results suggest this approach can extract the timescale of the active process driving non-equilibrium fluctuations.