LQ13873 - A shaken confined granular system presents a very unusual
solid-liquid-like phase transition; it occurs above a certain heating
threshold and, as this paper shows, it can be either abrupt or
continuous. For equilibrium systems it is believed that a liquid-solid
transition is always abrupt: a disordered liquid state cannot change
smoothly to an ordered solid state because of their different
symmetries. However, recent studies show that confined equilibrium
systems, like water nano-films, can behave differently. In this paper,
we show that a shaken non-equilibrium confined granular system also
differs from the classical image. The transition can be either abrupt or
smooth depending on the vertical height and filling density, meaning
that one can go smoothly from one state to the other by changing a
control parameter. In the experiments, density fluctuations do not
present strong variations at the transition, whereas the way grains are
locally ordered varies strongly, either abruptly or continuously. In the
second case, grains smoothly order in a square symmetry in highly
fluctuating domains. The associated fluctuations, characteristic size
and relaxation time seem to diverge at the transition in a way described
by the non-equilibrium theory of phase transitions.
