Simulation of common features and differences of surfactant-based and solid-stabilized emulsions

Microscopic derivation of the thin film equation using the Mori-Zwanzig formalism

The hydrodynamics of thin films is typically described using macroscopic models whose connection to the microscopic particle dynamics is a subject of ongoing research. Existing methods based on density functional theory provide a good description of static thin films but are not sufficient for understanding nonequilibrium dynamics. In this work, we present a microscopic derivation of the thin film equation using the Mori-Zwanzig projection operator formalism. This method allows to directly obtain the correct gradient dynamics structure along with microscopic expressions for mobility and free energy. Our results are verified against molecular dynamics simulations for both simple fluids and polymers.

Computational investigation of a switchable emulsion stabilized by the mixture of a surfactant and tertiary amine

Molecular dynamics simulations were performed to investigate the CO2-responsiveness of an oil-in-water (O/W) emulsion stabilized by sodium oleate (NaOA) with a tertiary amine additive, named pentamethyl diethylenetriamine (PMA). The simulated results were in accordance with the experimental observations. That is, the surfactant NaOA itself can stabilize dodecane/water emulsions in aqueous solution, while the CO2-reponsiveness was strongly related to the added PMA. The electroneutral PMA molecules preferred to be located in the core region of the droplets. Thus, under the same conditions, the size of the droplet containing PMA is predictably larger than that without PMA. The increased extent of the charged surfactant headgroups distribution can increase the electrostatic repulsion between the droplets in the emulsion solution, which is the important reason why a much more stable emulsion is obtained by adding PMA. When PMA molecules were protonated to PMA2+ by bubbling CO2, they migrated from the interior to the surface of the droplets under electrostatic attraction, forming ion pairs with OA-. The binding between PMA2+ and OA- made the distribution of the surfactants very concentrated on the droplet surface, leading to large hydrophobic areas exposed to water. Besides, the hydration interactions of OA- headgroups decreased because they were covered by PMA2+. The calculated potential of mean force (PMF) confirmed that the electrostatic repulsion between droplets was crucial for the emulsion stabilization.

2D Stokesian Approach to Modeling Flow Induced Deformation of Particle Laden Interfaces

A Stokesian dynamics simulation of the effect of surface Couette flow on the microstructure of particles irreversibly adsorbed to an interface is presented. Rather than modeling both bulk phases, the interface, and particles in a full 3D simulation, known interfacial interactions between adsorbed particles are used to create a 2D model from a top down perspective. This novel methodology is easy to implement and computationally inexpensive, which makes it favorable to simulate behavior of particles under applied flow at fluid-fluid interfaces. The methodology is used to examine microstructure deformation of monodisperse, rigid spherical colloids with repulsive interactions when a surface Couette flow is imposed. Simulation results compare favorably to experimental results taken from literature, showing that interparticle forces must be 1 order of magnitude greater than viscous drag for microstructure to transition from aligned particle strings to rotation of local hexagonal domains. Additionally, it is demonstrated that hydrodynamic interactions between particles play a significant role in the magnitude of these microstructure deformations.

Mesoscale models of dispersions stabilized by surfactants and colloids

In this paper we discuss and give an outlook on numerical models describing dispersions, stabilized by surfactants and colloidal particles. Examples of these dispersions are foams and emulsions. In particular, we focus on the potential of the diffuse interface models based on a free energy approach, which describe dispersions with the surface-active agent soluble in one of the bulk phases. The free energy approach renders thermodynamic consistent models with realistic sorption isotherms and adsorption kinetics. The free energy approach is attractive because of its ability to describe highly complex dispersions, such as emulsions stabilized by ionic surfactants, or surfactant mixtures and dispersions with surfactant micelles. We have classified existing numerical methods into classes, using either a Eulerian or a Lagrangian representation for fluid and for the surfactant/colloid. A Eulerian representation gives a more coarse-grained, mean field description of the surface-active agent, while a Lagrangian representation can deal with steric effects and larger complexity concerning geometry and (amphiphilic) wetting properties of colloids and surfactants. However, the similarity between the description of wetting properties of both Eulerian and Lagrangian models allows for the development of hybrid Eulerian/Lagrangian models having advantages of both representations.

Crystalline order and topological charges on capillary bridges

We numerically investigate crystalline order on negative Gaussian curvature capillary bridges. In agreement with the experimental results in [W. Irvine et al., Nature, Pleats in crystals on curved surfaces, 2010, 468, 947] we observe for decreasing integrated Gaussian curvature, a sequence of transitions, from no defects to isolated dislocations, pleats, scars and isolated sevenfold disclinations. We especially focus on the dependency of topological charge on the integrated Gaussian curvature, for which we observe, again in agreement with the experimental results, no net disclination for an integrated curvature down to -10, and an approximately linear behavior from there on until the disclinations match the integrated curvature of -12. In contrast to previous studies in which ground states for each geometry are searched for, we here show that the experimental results, which are likely to be in a metastable state, can be best resembled by mimicking the experimental settings and continuously changing the geometry. The obtained configurations are only low energy local minima. The results are computed using a phase field crystal approach on catenoid-like surfaces and are highly sensitive to the initialization.