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Coalescence kinetics in surfactant stabilized emulsions: evolution equations from direct numerical simulations
R Skartlien1, B Grimes, P Meakin
1FACE - the Multiphase Flow Assurance and Innovation Center, P.O. Box 40, N-2027 Kjeller, Norway. roar.skartlien@ife.no
The Journal of Chemical Physics
|December 13, 2012
Summary
Lattice Boltzmann simulations reveal that increased surfactant slows emulsion droplet coalescence. Coalescence kinetics are influenced by quasi-turbulent flow and interfacial energy conversion, challenging simple power-law scaling models.
Area of Science:
- Colloid and Surface Science
- Fluid Dynamics
- Computational Physics
Background:
- Emulsion stability is crucial in many industrial applications.
- Understanding droplet coalescence kinetics, especially in systems with viscosity contrast and surfactants, is essential.
- Previous models often assume simple scaling laws that may not capture complex dynamics.
Purpose of the Study:
- To investigate the coalescence kinetics of emulsions with amphiphilic surfactants using Lattice Boltzmann simulations.
- To explore the influence of surfactant concentration and interfacial effects on droplet coalescence.
- To develop and validate phenomenological models for emulsion droplet size evolution.
Main Methods:
- Utilized 3D Lattice Boltzmann simulations on a large grid (256^3) to capture droplet dynamics.
- Simulated neutrally buoyant emulsions with significant viscosity contrast, mimicking oil-in-water systems.
- Derived coupled differential equations from simulation data and film draining theories to model droplet size and number density.
Main Results:
- Increased surfactant content significantly slowed coalescence rates via the Gibbs-Marangoni effect.
- Coalescence was driven by a quasi-turbulent velocity field, with early kinetic energy decay attributed to interfacial energy conversion.
- Time-dependent exponents were derived, suggesting that simple power-law scaling for droplet growth may not be universally applicable.
Conclusions:
- The study challenges the assumption of simple power-law scaling for emulsion droplet size evolution under complex conditions.
- Phenomenological models incorporating time-dependent exponents provide a more accurate description of coalescence kinetics.
- No evidence for stretched logarithmic or exponential scaling associated with arrested growth was found, differing from some prior studies.
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