Active spinner materials

Room Pere Pascual, Departament Física de la Matèria Condensada (Planta 5, Facultat de Física, Universitat de Barcelona, c/ Martí i Franqués 1) 2017-06-29 10:30:00

Abstract:
Strongly interacting colloids driven out-of-equilibrium by an external periodic forcing often develop nontrivial collective dynamics. Active magnetic colloids proved to be excellent model experimental systems to explore emergent behavior and active (out-of-equilibrium) self-assembly phenomena. While colloidal systems are relatively simple, understanding their collective response, especially in out of equilibrium conditions, remains elusive. 
Ferromagnetic micro-particles immersed in water and sediment on the bottom surface of the flat cell are energized by a single-axis homogeneous alternating magnetic field applied perpendicular to the surface supporting the particles. Upon application of the alternating magnetic field the magnetic torque on each particle is transferred to the mechanical torque giving rise to a rolling motion of the particle in a certain range of excitation parameters. 
Experiments reveal a rich collective dynamics of magnetic rollers. Flocking and spontaneous formation of steady vortex motion have been observed. The effects are fine-tuned and controlled by the parameters of the driving magnetic field. By combing experiments and discrete particle simulations, we have identified primary physical mechanisms leading to the emergence of largescale collective motion: spontaneous symmetry breaking of the clock/counterclockwise particle rotation, 
collisional alignment of particle velocities, and random particle re-orientations due to shape imperfections. 
Ferromagnetic micro-particles, suspended at a liquid interface and energized 
by a rotational homogeneous alternating magnetic field applied along the supporting interface, spontaneously form ensembles of synchronized self-assembled spinners with well-defined characteristic length. The size and the torque of an individual self-assembled spinner are controlled by the frequency of the driving magnetic field. Experiments reveal nontrivial collective dynamics in large ensembles of synchronized magnetic spinners that can spontaneously form dynamic spinner lattices at the interface in a certain range of the excitation parameters. Unusual dynamics inside of the formed spinner lattices is observed. Transport of passive cargo particles in a gas of spinners and structure of the underlying self-induced surface flows is analyzed. Active turbulent behavior of induced flows is reported.

Self-organization and criticality in martensite

Facultat de Física, Aula Pere Pascual, planta 6 2017-06-08 12:00:00

A martensitic phase-transformation is a first-order diffusionless transition occurring in elastic crystals and characterized by an abrupt change of shape of the underlying crystal lattice. It is the basic activation mechanism for the Shape-Memory effect. In this talk we present a probabilistic model for the description of martensitic microstructure as an avalanche process. Our approach to the analysis of the model is based on an associated general branching random walk process. Comparisons are reported for numerical and analytical solutions and experimental observations.

Deliberate exotic magnetism via frustration and topology

Aula Eduard Fontserè - Facultat de Física UB 2017-06-01 14:00:00

So called "Artificial Spin Ices" are two dimensional arrays of magnetic, interacting nanostructures whose geometry can be chosen at will, and whose elementary degrees of freedom can be characterized directly. They were introduced at first to study frustration in a controllable setting, to mimic the behavior of spin ice, rare heart pyrochlores, but at more useful temperature and field ranges and with direct characterization, and to provide practical implementation to celebrated, exactly solvable models of statistical mechanics previously devised to gain understanding of degenerate ensembles. With the evolution of nano fabrication and of experimental protocols it is now possible to characterize the material in real-time, real-space, and to realize virtually any geometry, for direct control over the collective dynamics. This has recently opened a path toward the deliberate design of novel, exotic states, not found in natural materials. We will provide an introduction to the material, the early works, and then, by reporting on more recent results, we will proceed to describe directions, which includes the design of desired topologically protected states and their implications to kinetics.