Thesis title: Spatiotemporal control of bacterial turbulence via light-driven activity
Active turbulence — the chaotic, collectively driven flow that spontaneously develops in dense suspensions of motile microorganisms — has emerged as a central phenomenon in active matter physics, connecting fundamental questions about self-organization far from equilibrium to the rich phenomenology of biological fluids. While the steady-state properties of active turbulence are increasingly well understood, a fundamental question remains open: how do the collective dynamics respond when the activity that sustains them is not constant, but is instead modulated in space and time? This thesis investigates this question experimentally, using dense suspensions of photokinetic Escherichia coli as a model system. We exploit this unique capability to explore two complementary control strategies — spatial and temporal — that probe the collective response of active turbulence to external modulation.By projecting spatially structured light onto the suspension, circular illuminated regions embedded in a dark, non-motile background generate coherent vortex dynamics analogous to those observed under physical geometric confinement, but in a fully reconfigurable geometry free of any fabricated boundary. An azimuthal decomposition of the vorticity field reveals a crossover radius of approximately 80 µm, below which the confinement selects a dominant coherent rotation and above which the bulk turbulent state is recovered. The escape dynamics at the virtual boundary exhibit strong radial alignment driven by steric interactions with the dense passive layer, highlighting the central role that bacterial packing plays in shaping the collective response.This role extends beyond spatial control. When the suspension is cycled between active and passive states through temporal modulation of light, the flow field after reactivation retains partial directional correlation with its pre-quench configuration, revealing a form of directional memory with no previously reported counterpart in active matter. The reactivation timescale also grows with the duration of the passive interval, although a quantitative test of time–waiting-time superposition shows that the recovery curves do not collapse onto a master curve in the explored density range, ruling out an aging interpretation of glassy type. The directional memory itself, however, emerges sharply above a threshold density of approximately 45 OD and is traced to structural arrest in the passive state: at sufficiently high packing fractions, bacterial orientations freeze on experimental timescales, providing a structural template that biases the collective flow upon reactivation. Two independent structural diagnostics — the intermediate scattering function and a nematic overlap analysis — identify the same density threshold for arrested dynamics, establishing a quantitative connection between active turbulence and glass-like physics.Beyond the specific experimental findings, the results demonstrate that dense active systems can encode and transmit directional information across activity cycles, opening avenues for the design of spatiotemporal protocols that exploit structural memory to steer collective motion.