Band Propagation, Scaling Laws, and Phase Transition in a Precipitate System. 2. Computational Study

Area of Science:

• Chemical kinetics
• Materials science
• Pattern formation

Background:

• Liesegang patterns are complex spatiotemporal structures formed by precipitation reactions.
• The Ni(2+)/NH3-OH(-) system exhibits unique nickel hydroxide polymorph band formation.
• Understanding the dynamics of these patterns requires sophisticated modeling.

Purpose of the Study:

• To develop a chemical kinetic model for the Ni(2+)/NH3-OH(-) Liesegang system.
• To investigate the dynamics of nickel hydroxide polymorph band formation.
• To validate the model against experimental observations.

Main Methods:

• A reaction-diffusion model incorporating precipitation, dissolution, nucleation, and polymorphic transition.
• Coupling of precipitation-diffusion equations with nucleation, polymorphic transition, and growth rate equations.
• Numerical simulations to confirm model validity and derived constraints.

Main Results:

• The model accurately describes the dynamics of β-nickel hydroxide bands led by α-nickel hydroxide pulses.
• Pulse position and width scale as t(α) with α ≃ 0.5, matching experimental data.
• Simulations show oscillating band mass, indicating competition between growth, transition, and dissolution.

Conclusions:

• The proposed model successfully captures the complex dynamics of the Ni(2+)/NH3-OH(-) Liesegang system.
• A novel constraint on critical constants is identified, differing from classical Lifshitz-Slyozov instability.
• The findings provide insights into the interplay of precipitation, polymorphic transition, and dissolution in pattern formation.