Session 2

Nonvibrating granular systems, excellent tool to study glass transition, crystallization, and active matter.

Fernando Donado Pérez

Universidad Autónoma del Estado de Hidalgo

México

A nonvibrating granular system is composed of magnetic beads under an alternating magnetic field. Each particle independently takes energy from the field and converts it into kinetic energy; thus, this system is an example of active matter. The system is out of equilibrium because of highly dissipative processes produced by friction between particles and the container and particle collisions. However, the continuous energy input prevents particles from stopping. The system is not in thermodynamic equilibrium but is in a stationary state. In these conditions, the system fits the conditions to be an Ornstein-Uhlenbeck process. Thus, this system can display behavior similar to that of systems in equilibrium, therefore can be described using the same tools of statistical mechanics used in equilibrium systems. The slow dynamics and the high spatial resolution allow us to describe the system at the particle level. One can describe the motion of each particle from a short time regime, in an almost ballistic motion, to a long time regime, where particles display diffusive motion. The system is cooled down by decreasing the magnetic field; this allows us to study the phase transition from a fluid-like state to a solid-like state. We have studied a nonclassical nucleation crystallization composed of at least two steps. This system can be easily modified to study complex systems such as magnetorheological fluids, vortices, and confined Brownian particles in complex geometries.

Theoretical approaches for the understanding of non-equilibrium states in colloidal mixtures.

Luis Fernando Elizondo-Aguilera

Universidad Autónoma de San Luis Potosí

Instituto de Física

México

Providing a physically sound explanation of the non-equilibrium behavior experimentally found in various colloidal mixtures poses a number of challenging problems for statistical thermodynamics. In the case of binary mixtures composed of largely asymmetric particles, for instance, one is required to comprehend how various time and length scales influence the out-of-equilibrium (structural and dynamical) behavior of the whole mixture. In turn, this requires the resolution of multiple relaxation modes and other physical observables which, from an experimental point of view, can be extremely complicated (or even impossible). In this contribution I will review some recent advances in the theoretical study of glassy states in highly asymmetric binary mixtures of hard spheres, one of the simplest representations of a many body system with competing time and length scales. In addition, I will discuss some progress in the description of the process of formation of amorphous states by arrested spinodal decomposition in liquids with repulsive plus attractive interactions, and its possible connection with the non-equilibrium behavior observed in colloid-polymer mixtures at sufficiently small concentration of colloids, where descriptions based on integrating out the dynamics of the polymers seem to fail to predict the dynamics of the whole mixture.

Sinergy of experimental and theoretical approaches for the understanding of non-equilibrium states of colloidal mixtures.

Marco Laurati

Università di Firenze and CSGI

Dipartimento di Chimica “Ugo Schiff”

Italy

The experimental study of complex non-equilibrium states formed by colloidal mixtures requires complementary experimental techniques, that allow selectivity with respect to the different species involved, and that give access to average as well as local structure and dynamics. In this talk, I will review results of applying this approach to unravel the existence and properties of different glass states formed by binary colloidal mixtures. I will show in addition that the rich set of experimental observables provides unique opportunities to test the predictions of non-equilibrium statistical thermodynamic theories. Similar approaches can be also adopted for the study of non-equilibrium gel states formed by colloid-polymer mixtures. I will present recent results of confocal microscopy and differential dynamic microscopy experiments applied to the investigation of the gelation process in soft colloid-polymer mixtures. The results show that these mixtures present slow and unusual gelation kinetics, and aging effects that need a detailed theoretical modeling.