Speaker
Description
Brownian colloids can acquire effective activity when immersed in a dense, out-of-equilibrium medium. In these environments, the surrounding particles continuously transfer energy to the colloids, generating persistent, non-equilibrium fluctuations that can drive ballistic motion at short times and enhanced diffusion at long times. In this talk I will present two distinct scenarios in which activity is introduced through a surrounding, externally controlled medium, allowing us to tune fluctuations and probe their effects in different regimes.
First, I will show a two-dimensional colloidal crystal activated by a bath of light-driven bacteria. In this system, thermal and active fluctuations coexist and can be independently controlled via magnetic interactions and bacterial activity. By analyzing the crystal’s relaxation modes, we show that for short persistence times, the system is well described by a single effective temperature, while for more persistent activity the equipartition of energy among modes is broken and multiple mode-dependent temperatures emerge.
Finally, I will present a second system where activity is generated by a driven medium: a chiral fluid of rotating magnetic microparticles. Depending on the driving conditions, this system forms either phase-separated rotating clusters or a homogeneous chiral fluid. Passive particles in these environments acquire effective active motion from the hydrodynamic flows generated by the rotors, with long persistence times and strongly enhanced diffusion at long times.
Together, these results illustrate how distinct mechanisms of energy injection, from living bacterial baths to driven chiral flows, can be used to control active fluctuations and study their emergent phenomena in both solid and fluid colloidal systems.
