(125c) Viscosity Considerations for Accurate Coarse-Grained Simulations of the Interface between Immiscible Fluids in a Nano-Slit

The dissipative particle dynamics (DPD) method for coarse-grained molecular computations has the advantage of allowing modeling of systems with large molecules and large time and length scales, when compared to atomistic simulations. In this way, systems of complex fluids with particles, surfactants, polymers and multiple phases of fluids can be simulated [1-4]. In DPD, groups of atoms or molecules are consolidated in “beads” that interact with each other based on Newton’s equations of motion, and based on forces that depend on the type of molecules of the system. The parameters used in the model equations should be validated with either theory, molecular computations or experiments [5, 6]. For flowing systems, basic macroscopic behavior, like no-slip boundary conditions and velocity profiles that follow the Hagen-Poiseuille equations, should be recovered. No-slip boundary conditions have been addressed in the DPD literature. However, when two or more fluid phases are involved, accurate representation of the viscosity of each fluid is important and systematic determination of the DPD model parameters that play vital role in investigating the flow properties for such systems is needed [7]. So far, previous research has been concerned about the movement of fluids with the same viscosity, while it was unclear how to model multiphase flows of immiscible fluids with different viscosity [8]. Such multiphase systems are often found in chemical engineering applications, such as oil-water systems. The differences in viscosity exert an impact on the dynamic behavior [8].

In this study, we focus on the relation of the DPD dissipative coefficient with fluid viscosity as well as the velocity distribution of heptadecane-water flow systems. The dissipative parameter determines the friction between interacting particles, between beads of the same fluid, between beads of different fluids, and between the fluids and the solid walls. Firstly, the DPD model parameters for the laminar flow of a fluid between two parallel planes in Hagen-Poiseuille conditions and in plane Couette flow are determined. Secondly, the effect of the dissipative parameter on the velocity profile of two adjacent immiscible fluids with various ratios of oil and water are adjusted. Finally, these simulations are validated with Navier–Stokes solutions [9]. The effect of a third phase, i.e., surfactants on the flow field is also considered.

The results indicate that the best-fit dissipative parameters of oil and water should be different because of their different viscosity. A linear relationship between the dissipative coefficient and the fluid viscosity is generated. The dissipative parameter between the oil and water is also determined based on the validation of results with theory. Taking all of this into account, the velocity profile of Hagen-Poiseuille flow with various ratios of oil and water in the flow field is found to be in good agreement with the theoretical results. Based on this approach, we show how the DPD method can be applied for other coarse-grained methods [10], and how it can be applied to any multiphase flow systems, when the dynamic properties of the system need to be modeled accurately.

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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