Self-assembly and non-equilibrium phase coexistence in a binary granular mixture.

Area of Science:

• Granular physics
• Soft matter physics
• Non-equilibrium statistical mechanics

Background:

• Granular materials exhibit complex behaviors not fully explained by equilibrium thermodynamics.
• Spontaneous pattern formation in vibrated granular systems is a key area of research.
• Understanding phase transitions in non-equilibrium systems is crucial for materials science.

Purpose of the Study:

• To experimentally observe and characterize the formation of a square crystalline phase in a vibrated binary mixture of spherical grains.
• To investigate the coexistence of granular fluid and solid phases under varying area fractions.
• To elucidate the non-equilibrium mechanisms driving phase coexistence and temperature gradients.

Main Methods:

• Experimental observation of vibrated binary granular mixtures.
• Discrete Element Method (DEM) simulations for realistic modeling.
• Event-driven molecular dynamics for an idealized collisional model.
• Direct phase coexistence method to analyze phase transitions.

Main Results:

• Experimental observation of a spontaneously forming square crystalline phase from a disordered state.
• Stable coexistence between granular fluid and isolated square crystal observed by varying area fraction.
• Simulations successfully reproduced experimental findings and provided insights into non-equilibrium phase coexistence.
• System exhibited behavior analogous to a first-order phase transition, with a hotter crystal than fluid.

Conclusions:

• The study demonstrates a striking non-equilibrium effect where the crystal phase maintains a higher granular temperature than the fluid.
• Coupling between local structure and energy transfer mechanisms sustains kinetic temperature gradients across the fluid-solid interface.
• Findings provide valuable insights into the fundamental physics of non-equilibrium phase transitions in granular systems.