Analogue of surface melting in a macroscopic non-equilibrium system

TL;DR

This study demonstrates that a macroscopic wet granular crystal exhibits surface melting behavior similar to molecular systems, with a first-order transition triggered by capillary bridge rupture, highlighting potential universality in non-equilibrium phase transitions.

Contribution

It provides experimental evidence of surface melting in a macroscopic non-equilibrium system, linking microscopic rupture energy to macroscopic phase transition behavior.

Findings

Surface melting occurs from the free surface in wet granular crystals.

The transition behaves like a first-order phase transition.

The threshold is related to the rupture energy of capillary bridges.

Abstract

Agitated wet granular matter can be considered as a nonequilibrium model system for phase transitions, where the macroscopic particles replace the molecules and the capillary bridges replace molecular bonds. It is demonstrated experimentally that a two-dimensional wet granular crystal driven far from thermal equilibrium melts from its free surface, preceded by an amorphous state. The transition into the surface melting state, as revealed by the bond orientational order parameters, behaves like a first order phase transition, with a threshold being traceable to the rupture energy of a single capillary bridge. The observation of such a transition in the macroscopic nonequilibrium system triggers the question of the universality of surface melting.