Direct observation of crystal nucleation and growth in a quasi-two-dimensional nonvibrating granular system

TL;DR

This study observes crystal nucleation and growth in a quasi-two-dimensional magnetic granular system, revealing a two-stage process involving initial amorphous aggregation followed by internal rearrangement into a crystalline structure, supporting non-classical nucleation theories.

Contribution

First direct observation of crystal nucleation and growth in a macroscopic granular system under controlled magnetic and thermal conditions, highlighting non-classical formation pathways.

Findings

Crystallization occurs at slow cooling rates with hexagonal lattice formation.

Initial dense amorphous aggregates form before internal rearrangement into crystals.

Crystal growth transitions from non-classical to classical behavior as the aggregate size increases.

Abstract

We study a quasi-two-dimensional macroscopic system of magnetic spherical particles settled on a shallow concave dish under a temporally oscillating magnetic field. The system reaches a stationary state where the energy losses from collisions and friction with the concave dish surface are compensated by the continuous energy input coming from the oscillating magnetic field. Random particle motions show some similarities with the motions of atoms and molecules in a glass or a crystal-forming fluid. Because of the curvature of the surface, particles experience an additional force toward the center of the concave dish. When decreasing the magnetic field, the effective temperature is decreased and diffusive particle motion slows. For slow cooling rates we observe crystallization, where the particles organize into a hexagonal lattice. We study the birth of the crystalline nucleus and the subsequent growth of the crystal. Our observations support non-classical theories of crystal formation. Initially a dense amorphous aggregate of particles forms, and then in a second stage this aggregate rearranges internally to form the crystalline nucleus. As the aggregate grows, the crystal grows in its interior. After a certain size, all the aggregated particles are part of the crystal and after that, crystal growth follows the classical theory for crystal growth.