Periodic Reporting for period 1 - StaMACS (Statistical Mechanics and transport phenomena in Active Colloidal Solids)
Reporting period: 2021-10-01 to 2023-09-30
Active solids, or self-propelling units elastically coupled on a lattice, are recently of growing interest and are predicted to show emerging out-of-equilibrium behaviour, while they can inspire the design of numerous applications. However, experimental studies have been few and limited due to the difficulty of practically realising systems of hundreds of confined active particles. The results of StaMACS show for the first time an experimental realisation of a large ordered active solid with activity and confinement tuneable in-situ and on- command. This two-dimensional active solid is composed of repulsive magnetic particles activated by a photokinetic bacterial bath. In this way, our crystal is in contact with two effective temperature reservoirs: 1) the thermal or magnetic temperature, related to the strength of the inter-particle repulsion and controlled by the magnetic field, and the active temperature, related to the power of the light-driven bacterial bath and controlled by green light. We tuned both independently and show that there is a regime in which these two temperatures are not equivalent. We drew an off-equilibrium phase diagram depending on the two temperatures of the system and discovered some unexpected differences between melting in the passive system and active melting. Furthermore, we studied this active system when the active temperature changes in space. The results of StaMACS provide a novel testbed for active solids with an unprecedented degree of tunability. By revealing new complex dynamics, the project expands the physics of active matter to the realm of non-equilibrium colloidal solids, bringing the field closer to the practical realization of new soft materials with distinctive properties and high tunability.
We built a new experimental set-up in the host group’s lab at the Physics Department of Sapienza University. We established new protocol that enables the assembly of a magnetic crystal activated by light-driven bacteria and its observation under an optical microscope. Specifically, we built a custom-made inverted microscope and integrated a magnetic coil to apply a controlled and constant magnetic field and a computer-controlled illumination that can be structured in space and time to control the crystal’s activity.
Thanks to these new methodologies, we studied the properties of a magnetic active crystal: its particle vibrations, the phase diagram, and the melting transition and compared it to the equivalent passive equilibrium system. The results revealed emerging out-of-equilibrium behaviour. The dissemination of the scientific results of StaMACS has been and will be public and open-access in the form of research articles, reports and oral presentations in conferences and workshops.
Thanks to these new methodologies, we studied the properties of a magnetic active crystal: its particle vibrations, the phase diagram, and the melting transition and compared it to the equivalent passive equilibrium system. The results revealed emerging out-of-equilibrium behaviour. The dissemination of the scientific results of StaMACS has been and will be public and open-access in the form of research articles, reports and oral presentations in conferences and workshops.
During this project we successfully assembled a large and ordered active crystal powered by bacteria that can be adjusted for activity and confinement in real-time, representing a significant step forward in the field of active matter. We identified new behavior signature of the out-of-equilibrium nature of the system. The project's outcomes may pave the way for innovative technologies and materials that could impact various industries, from materials science to biotechnology. Beyond practical applications, the results involve a significant advance in the physical understanding of the statistical mechanics of these far from equilibrium systems.