(421f) Nanoparticle and Surfactant Effects on Oil Drop Migration in Water

Janus particles (JPs) have a wide range of applications in the fields of biology, oil recovery^{1, 2} and drug delivery^{3, 4}. They have been found to stabilize emulsions of oil in water, as do surfactants. Surfactants reduce the interfacial tension (IFT)^5-7 at oil-water interfaces and alter the wettability of liquid/solid interfaces significantly.⁸ Mixtures of particles and surfactants are more realistic in applications and understanding the synergistic effects of these two stabilizers and the tunability of their behavior is of significant interest. While previous studies by both experiments⁹ and simulations^{10, 11} have been concerned with the effect of surfactants and JPs on a flat oil/water interface, rather few studies have considered the displacement of a drop of oil in water when both surfactant and JPs are present. In this work, we investigate oil drop migration on the wall of a nanoslit under an external force using the dissipative particle dynamics (DPD) method.

Octaethylene glycol monododecyl ether (C12E8) and hexadecane were used to represent surfactant and oil, respectively. The flow was set up in a plane Couette channel, where water was flowing in a nanoslit with one wall stationary and the other moving with controlled velocity. The Couette flow allowed for controlling the shear rate in the nanoslit. The oil drop was placed on the stationary wall, and its displacement as a function of the shear and the concentration of surfactant and JPs on the drop surface was recorded over time. Firstly, the concentration of surfactant on the drop interface was found to affect the drop displacement. The presence of surfactants reduced the IFT and the slip of the water over the oil drop, leading to larger displacement as the IFT was decreased. When the drop of oil moved in the Couette flow nanochannel, surfactant molecules tended to concentrate at the front of the oil drop rather than the back. Then, the effect of JPs on the system of water-surfactant-oil was investigated by changing the number of nanoparticles on the drop interface, while the surfactant concentration was kept constant. The JPs moved on the surface of the drop in a periodic manner, first moving to the front of the drop and then circulating around its base towards the back of the drop and repeating this motion. The results indicated that the oil drop migrated farther in presence of both surfactants and JPs at the interface compared to the case of bare oil drop under shear. However, the displacement was smaller when compared to the case of an oil drop covered with only surfactants. A combination of changes of the drag coefficient and of the slip velocity of the water as it flows above the drop seem to be responsible for the observed behavior. Based on this approach, we show how the DPD method can be applied to simulate surfactants and JPs that are present together on a drop of oil under water flow. Results from these simulations can contribute to enhanced oil recovery and environmental remediation of oil contamination in the subsurface.

ACKNOWLEDGEMENTS

Acknowledgment is made to the donors of The American Chemical Society Petroleum Research Fund for partial support of this research through grant PRF # 58518-ND9, and to NSF for grant CBET 1934513. The use of computing facilities at the University of Oklahoma Supercomputing Center for Education and Research (OSCER) and at XSEDE (under allocation CTS-090025) is gratefully acknowledged.

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