Crossover dynamics at large metastability in gas-liquid nucleation

Area of Science:

• Physical Chemistry
• Materials Science
• Statistical Mechanics

Background:

• Classical nucleation theory (CNT) describes phase transitions but struggles with complex dynamics.
• Observed changes in nucleation mechanisms and dynamical crossovers at high metastability require a refined theoretical framework.

Purpose of the Study:

• To develop an extended theoretical description of nucleation dynamics using an ordered set of kth largest clusters as order parameters.
• To explain the observed crossover from activated to barrierless nucleation and associated dynamical phenomena.
• To provide an analytic expression for free energy and nucleation rates in the barrierless regime.

Main Methods:

• Developed an alternate description of nucleation dynamics based on an extended set of order parameters (kth largest clusters).
• Derived an analytic expression for the free energy of the kth largest cluster.
• Modeled cluster growth as diffusion on the free energy surface in the barrierless regime.

Main Results:

• The derived free energy expression shows excellent agreement with simulation results.
• The theory explains the disappearance of the free energy barrier at large supersaturation, leading to barrierless nucleation.
• Successfully explains the observed change in nucleation mechanism and dynamical crossover at high metastability, which CNT cannot.
• Identified the crossover to barrierless nucleation at supersaturation where multiple clusters cross the critical size, termed the kinetic spinodal.
• Derived a nucleation rate expression that reproduces the experimentally observed slower increase with supersaturation in the barrierless regime.

Conclusions:

• The extended nucleation theory provides a more comprehensive description of nucleation dynamics, particularly at high metastability.
• The concept of kth largest clusters as order parameters successfully explains complex nucleation phenomena and dynamical crossovers.
• The derived models accurately predict free energy barriers and nucleation rates, aligning with experimental observations.