Nanoparticles (NPs) are widely used in environmental applications,
1 drug delivery
2, 3 and enhanced oil recovery.
4, 5 They can stabilize emulsions of oil in water, as do surfactants, but while surfactants lead to a reduction in interfacial tension (IFT) of the oil-water system, NPs affect interfacial behavior by reducing the interfacial area.
6 By using dissipative particle dynamics (DPD) simulation methods, we present a comprehensive comparison between the effects of two different types of NPs (homogeneous and anisotropic particles) in conjunction with surfactants on the interface of oil and water under flow conditions. The homogeneous NPs were either hydrophobic or hydrophilic, and the anisotropic NPs were amphiphilic Janus particles (JPs). JPs are representative of anisotropic particles, since their surface had two sides with distinct wettability.
7, 8 The hydrophobic and hydrophilic NPs used in our simulation represented amidine polystyrene latex and gold, respectively. The nonionic surfactant octaethylene glycol monododecyl ether (C12E8) and dodecane were used to represent surfactant and oil, respectively. The water flow was a Poiseuille flow in a nanochannel driven by an applied external body force. The interface was covered at 0-68% area by nanoparticles and contained surfactants at 0-83% of the interfacial surfactant concentration corresponding to the critical micelle concentration (CMC). An oil droplet would move when water was injected into the channel, causing finger-like instabilities. The instability was characterized by the critical external force (i.e., critical pressure drop), the three-phase contact angle (at the oil-water-solid wall interface) and the deformation of the oil droplet. It was found that the oil droplet could pinch off the channel wall and transition to a stratified two-phase flow of oil and water, or it could extend from one wall to the other in a nanochannel, in the form of a slag, depending on the driving force for the flow. NPs provided a more stable interface than bare oil. The pinch-off occurred when the three-phase contact angle was equal to a critical value, regardless of the number of particles, and the properties of the particles. The instability was found to develop at smaller pressure drops than the bare oil case when surfactants were present. When both surfactants and NPs were present, the shape of the oil-water interface was found to be different, as the NPs distributed more evenly at the interface, and the pinch off became more difficult than the case of bare oil or oil and surfactant. The presentation will discuss the mechanism of stabilizing the oil-water interface with nanoparticles and surfactants, and the implications for tuning oil-water interfacial behavior.
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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