Numerical simulations of multiphase flows with various complexities

Surfactants are essential chemicals that interact with interfaces, characterized by their unique molecular structure consisting of a hydrophilic head and a hydrophobic tail. The head typically prefers aqueous environments, allowing surfactants to position themselves at the interface between different phases, such as water and air. In situations where surfactants are present at low concentrations, they predominantly reside at the interface, while at higher concentrations, they exist as monomers in the bulk solution, exchanging with the interface through defined kinetics.

The concentration of surfactants at the interface, denoted as γ, influences interfacial tension. When surfactants adsorb onto the interface, they not only reduce interfacial tension but also generate additional stresses known as Marangoni stresses. As the concentration increases beyond the critical micelle concentration (CMC), surfactants aggregate to form micelles, where the hydrophobic tails cluster together and are shielded by the hydrophilic heads. This process leads to a distribution of micelle sizes, although for the sake of simplicity, we will consider a preferred micellar size.

The behavior of surfactants can become complex, particularly in relation to their interactions with the substrate and the contact line. For this presentation, we will focus on scenarios where surfactants are either insoluble, soluble without micelles, or present above the CMC.

Additionally, it is important to recognize that the interface may exhibit surface rheological effects, which can introduce resistance to deformation akin to two-dimensional viscosity. This topic will be explored in future discussions, as our research continues to investigate these surface rheological properties. Surfactants play a significant role in various natural processes, often appearing as contaminants, highlighting their importance in both industrial applications and environmental contexts.