Emerging colloidal dynamics away from equilibrium. Chiral active systems.
CECAM-HQ-EPFL, Lausanne, Switzerland
Soft matter systems in Nature, at length-scales spanning the nano- and the microscale, exhibit self-assembly far from the thermal equilibrium. Modern self-assembly techniques aiming to produce complex structural order or functional diversity often rely on non-equilibrium conditions in the system. Light, electric, or magnetic fields are often used to induce complex out-of-equilibrium ordering. Such dissipative colloidal materials use energy to generate and maintain structural complexity. Nontrivial collective dynamics and emerging large-scale structures are often observed in experiments and numerical simulations [1-24].
Chirality is an intrinsic fundamental property of many natural and artificial systems. Understanding the role of chirality in dynamics of interacting many-body systems is a major challenge. There has been a surge of interest in collective phenomena that arise when chirality comes into play in both biological [2-4] or artificial [5-9] systems. Microsystems driven out-of-equilibrium by external torques [10-18] are ideal model systems to investigate these phenomena since they avoid the inherent complexity of biological active matter . Spinning particles dispersed in a fluid represent a special class of artificial active systems that inject vorticity at the microscopic level [20-23]. Dense collections of interacting spinning particles represent a chiral fluid , which breaks parity and time-reversal symmetries, and displays a novel viscosity feature called the odd viscosity [25, 26]. The odd viscosity has been identified in interacting chiral spinners , and it led to remarkable effects such as production of flow perpendicular to the pressure , topological waves , or the emergence of edge currents . Magnetic rollers dynamically assemble into a vortex under harmonic confinement, that spontaneously selects a sense of rotation and is capable of chirality switching [28, 29]. Multiple motile vortices unbound from any confinement have been revealed in ensembles of magnetic rollers powered by a uniaxial field . Oscillating chiral flows were generated when a roller liquid was coupled to certain obstacles . There has been an increasing effort to investigate collective phenomena in systems with chiral active units [8, 32-38]. Synchronized self-assembled magnetic spinners at the liquid interface revealed structural transitions from liquid to nearly crystalline states and demonstrated reconfigurability coupled to a self-healing behavior . Activity-induced synchronization leading to a mutual flocking, and chiral self-sorting has been observed in modeled ensembles of self-propelled circle swimmers . Shape anisotropic particles powered by the Quincke phenomenon led to the realization of chiral rollers (similar to circle swimmers) with spontaneously selected handedness of their motion and activity-dependent curvature of trajectories . Multiple unconfined vortices with either polar or nematic ordering of particles have been revealed .
Developing an understanding of complex dynamics in chiral systems driven out-of-equilibrium by external fields represents a significant theoretical and computational challenge. Some of the features may be understood using phenomenological continuum descriptions [43,44,45] Nevertheless, the microscopic mechanisms leading to the dynamic self-assembly and their relations to the emergent behavior in chiral fluids often remain unclear. Computer simulations are practically the only method to theoretically investigate such questions; however, modeling the nonequilibrium self-assembly presents a huge computational challenge due to the complex many-body interactions and collective dynamics on different time scales. One of the main challenges is to properly account for the particle-fluid coupling. On a coarse-grained level, the fluid flow around colloids is modeled by molecular dynamics methods like Lattice-Boltzmann  and Multi Particle Collision Dynamics [46, 47]. An alternative approach is to describe the colloidal dynamics by molecular dynamics simulations or an amplitude equation (Ginzburg-Landau type equation) coupled to the Navier-Stokes equations describing large-scale time-averaged hydrodynamic flows induced by the colloids [30, 48].
Sabine Klapp (Institute of Theoretical Physics, Technical University Berlin) - Organiser
Pietro Tierno (University of Barcelona) - Organiser
Alexey Snezhko (Argonne National Laboratory) - Organiser