INTERFACIAL STABILITY WITH SURFACE-ACTIVE AGENTS USING COARSE-GRAINED MODELING

Abstract

The stability of interfaces in the presence of surface-active agents such as surfactants and nanoparticles (NPs) is imperative in various industrial and environmental contexts. These include separation processes, enhanced oil recovery, and environmental remediation, highlighting the significance of interface stability in diverse scenarios. In stabilizing oil-water interfaces, surfactants and NPs play a vital role, but with different mechanisms in each case. NPs affect interfacial behavior by reducing the interfacial area, whereas surfactants lead to a reduction in interfacial tension of the oil-water system. Although many studies have concentrated on equilibrium conditions without considering fluid flow, industrial applications typically involve displacing immiscible fluids. Therefore, there is a need to explore the impact of flow conditions on the properties of the oil-water interface, especially in the context of the presence of surfactants and NPs. In this thesis, we investigated the effect of anionic surfactant (sodium dodecylsulfate, SDS) and nonionic surfactant (octaethylene glycol monododecyl ether, C12E8) on the oil-water interface by using Dissipative Particle Dynamics (DPD) simulation methods. In particular, SDS represented the case of a short surfactant molecule on the interface, while C12E8 was a longer molecule. Surfactants are in conjunction with NPs (homogeneous and anisotropic particles) either on the flow of oil and water in a narrow channel under Poiseuille or Couette flow conditions. It is important first to verify the integrity of the computations by comparing the velocity profile resulting from simulations of Poiseuille and Couette flows to theoretical expectations. Based on this approach, satisfying the no-slip boundary conditions required the following: (1) creating frozen solid walls, (2) applying the bounce-back boundary condition, and (3) determining the DPD parameters necessary for ensuring that the boundary conditions were obeyed, that the oil and water viscosities were represented correctly. Moreover, the effects of surfactants at the oil-water interface and the interfacial instability under shear were investigated. A critical shear rate was found for Poiseuille flow, beyond which the surfactants desorb from the interface, forming micelles and destabilizing the interface. The surfactant-covered interface remained stable under Couette flow even at high shear rates. Besides the flat interface of oil and water, the motion of an oil droplet with and without the presence of surfactants and Janus particles (JPs) under shear was studied. In certain cases, oil droplets are observed to be mobilized along a solid surface. To describe this phenomenon, simulations were conducted that modeled the movement of oil droplets along solid walls. The presence of surfactants at the oil droplet surface led to the most considerable migration velocity and displacement observed under Couette flow. The surfactants affect the interfacial tension, enhancing the momentum transfer from the water to the oil and reducing the oil−water slip. When the surfactant concentration is relatively low, the effects of Marangoni stress are overwhelmed by the flow field. Concerning Janus particles (JPs), they impede the migration of droplets. This is attributed to a weaker interaction between Janus particles and the solid wall in the presence of surfactants. Consequently, the coexistence of surfactants and JPs results in a more significant displacement than in bare oil because of the smaller interfacial tension. Additionally, the anticipation of fingering is crucial when addressing the displacement of immiscible fluids, as viscous fingering is prevalent in the oil and gas industry. This phenomenon occurs when oil and water move together in porous media close to reservoir wells, but water moves more readily than oil, leading to instabilities and the development of water fingers. While the viscous fingering phenomenon has been investigated for bare oil-water systems, the effects of surfactants or particles at the oil-water interface are yet to receive detailed investigation. Our simulations involve injecting water into an oil-filled channel to displace the oil with different types of NPs (including homogeneous particles and amphiphilic JPs), surfactants, or both JPs and surfactants before pushing the oil forward. JPs exhibiting a dual chemistry, with a polar face featuring a contact angle θ_P and an apolar compartment characterized by a contact angle θ_A, inherently possess an amphiphilic nature. The degree of amphiphilicity of JPs, denoted by Δθ=(θ_A-θ_P)/2, is controlled during the computations so that the range is from zero to 47.8°. The computation results suggest that an oil droplet attached to a channel wall exhibits the greatest migration distance when only surfactants are presented at the interface. The droplet demonstrates the least migration in the presence of particles alone, regardless of their wetting characteristics. The re-arrangement of the surfactant molecules and the NPs at the interface, generating weak Marangoni stresses, plays a role in the motion of the drop. The instability of the oil-water meniscus arises from water flow, resulting in fingering phenomena before the eventual full detachment of the oil drop from the channel wall. The appearance of fingers depends on the presence of NPs or surfactants, with the particles leading to longer fingers at the same fluid velocity. A critical contact angle appears to dictate the detachment of the oil droplet from the channel wall, and this remains constant whether particles are present or not. When the oil drop detaches fully from the channel walls, the particles and the surfactant molecules reorient on the interface, modifying the shape of the droplet. Last but not least, investigating the desorption energy of NPs at the interface of oil and water phases in the presence of surfactants and/or NPs is vital for numerous applications across various industries. The presence of surfactants alters the interfacial properties, potentially influencing the desorption behavior of NPs. Both surfactants and NP have the potential to influence the behavior of interfacial properties, which lead to variations in the desorption energy required for NPs to detach from the interface. In addition, while many studies overlook specific interactions between particles and surfactants, it is notable that surfactants can indeed interact with the surface of NPs, potentially influencing desorption energy. Hence, careful consideration of the interplay between surfactants and NPs is essential in the study of desorption energy. The thesis seeks to gain insights into the energy needed to detach a NP (including the homogeneous particle and amphiphilic JP) from the oil-water interface with the presence of NPs, and surfactants, as well as their coexistence, into one of the homogeneous phases. When examining the desorption energies of individual particles with the center of NPs as they traverse the oil-water interface, it is observed that the presence of surfactants can reduce the energy requirement, whereas the presence of NPs may lead to a challenging trend. Regarding the effect of NP type, hydrophilic NPs are favored in the water phase due to their lower desorption energy compared to the significantly higher energy required in the oil phase. Conversely, hydrophobic NPs exhibit an opposite trend. In the case of Janus NPs, the desorption energy level lies between those of homogeneous NPs. These findings will be focused on exploring the mechanism of stabilizing the interface between oil and water using NPs and surfactants, as well as the potential implications for tuning oil and water interfacial behavior.