A molecular dynamics study on the boundary between homogeneous and heterogeneous nucleation

Area of Science:

• Materials Science
• Physical Chemistry
• Computational Physics

Background:

• Classical nucleation theory (CNT) predictions often diverge from experimental and simulation data for nucleation kinetics.
• This discrepancy raises questions about the fundamental validity of CNT.
• Understanding nucleation mechanisms is critical for materials synthesis and processing.

Purpose of the Study:

• To investigate the atomistic mechanisms of nucleation in liquid aluminum (Al) interacting with amorphous substrates.
• To elucidate the role of substrate surface topography (smooth vs. rough) in determining nucleation pathways.
• To explain the observed discrepancies in nucleation kinetics between theory, simulations, and experiments.

Main Methods:

• Molecular dynamics (MD) simulations were employed to model liquid Al in contact with amorphous substrates.
• Simulations focused on analyzing the atomic structures and nucleation events at different surface conditions.
• Comparison of nucleation kinetics under homogeneous and heterogeneous conditions.

Main Results:

• Smooth amorphous surfaces promote heterogeneous nucleation via a structural templating mechanism.
• Rough amorphous surfaces lead to homogeneous nucleation at higher undercooling through a fluctuation mechanism.
• Initial nucleus growth on smooth surfaces is significantly faster than in homogeneous nucleation.
• The findings reconcile discrepancies between CNT predictions and simulation/experimental results.

Conclusions:

• A clear boundary exists between homogeneous and heterogeneous nucleation mechanisms.
• Structural templating is a general mechanism for heterogeneous nucleation.
• In the absence of templating, homogeneous nucleation occurs via fluctuations, potentially explaining the role of impurities.
• This study provides a deeper understanding of nucleation phenomena, bridging theory and experiment.