Analog of surface melting in a macroscopic nonequilibrium system

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.

Order-disorder transition in swirled granular disks

“…Furthermore, we point out that in our simulations of two dimensional disks we do not find the type of surface melting found in the experiments of May et al [16]. This result could be an indirect confirmation that the surface melting is due to the rolling friction of granular spheres with the bottom container as proposed in Ref.…”

“…Work has been done on the dynamical motion of the granular matter [12,13], as well as the formation of solid-like clusters [14,15]. Recently, May et al [16] investigated in more detail the formation of solid clusters and found that the melting of the solid clusters occurs first at their surface.…”

Clustering and melting in a wet granular monolayer

“…Further increase of Γ leads to a continuous melting predominately from the defects inside the crystal, reminiscent to a second-order phase transition. The difference to surface melting observed in a horizontally driven system [11] can be attributed to the additional degree of freedom of the particles along the direction of vibrations. The defects mediated melting transition observed in this nonequilibrium system triggers the question whether this transition falls into the scenario of KTHNY theory or not.…”

“…The nonequilibrium stationary states (NESS) depend on the balance between energy injection through, e.g., mechanical agitations, and dissipation through, e.g., inelastic collisions or viscous forces from wetting liquid [9,10]. In a previous investigation, it was shown that a 2D wet granular crystal under horizontal swirling motion (i.e., horizontal vibrations in two orthogonal directions with a phase shift of π/2 in a plane perpendicular to gravity) melts from its free surface [11]. In a following investigation, the fluctuations of the internal structures quantified by the bond-orientational order parameters (BOOP) show different types of noises e-mail: kai.huang@uni-bayreuth.de for different NESS, and surface melting was found to be associated with the emerging 1/ f noise [12].…”

Dynamics of wetting explored with inkjet printing

“…The enhancement of rigidity leads to an easily moldable material such as wet sand on the beach for sculpturing [7,8]. In the pendular regime [3] with capillary bridges formed between adjacent particles, previous investigations showed that the collective behaviors of partially wet granular materials, such as clustering, phase transitions and pattern formation, are related to the capillary force induced by the wetting liquid [9][10][11][12][13]. However, as more liquid is added, it is still challenging to understand how a wetting liquid film advances inside a geometrically heterogeneous granular packing, as well as how the particles respond to the capillary interactions induced by the additional wetting liquid.…”