Author: Cheng-Wei Tai (Advisor:  Prof. Vivek Narsimhan)

Title: Particulate motion in viscoelastic fluids – effect of particle shape

Abstract:  Particle migration in viscoelastic suspensions is vital in many applications in the biomedical community and the chemical/oil industries. Previous studies in viscoelastic media provided insight on the dynamics of spherical particles in simple shear viscoelastic flows, yet the combined effect of more complex flow profiles and particle shapes on the particle motion is underexplored. We study the dynamics of arbitrary-shaped particles in a second-order fluid, in a flow profile up to quadratic order. For the two model constants ψ1 and ψ2 (first and second normal stress coefficient) we assume the relationship ψ1 = -2 ψ2 so that the flow dynamics will behave as a Stokes flow with modified fluid pressure. The assumption greatly simplifies the mathematical procedure and allows us to qualitatively capture the fundamental physics of in any slow, viscoelastic flow. We apply multipole expansion approximation to derive the analytical expressions for the polymeric force and torque on an arbitrary-shaped particle. The solutions compared well to previous studies on spheres as well as to our boundary element method (BEM) simulation for arbitrary shaped particles. We apply the derived solutions to study the dynamics of sphere, spheroids (prolate & oblate) and ellipsoid in a quadratic flow of second-order fluid. We examine the particle migration behavior at different particle orientation and positions in a pressure driven flow, as well as the translational and rotational trajectories of the particles. We find that particles tend to migrate towards the flow center and the migration trajectory is affected by their shape, initial orientation and initial position. The effect of these factors is also addressed in this work. We extracted scaling information from the theory and find that the length that particle spans through the shear gradient direction is the dominant factor for the particle migration velocity. We summarize by discussing the future directions for experimental studies on particle dynamics as well as for extending current theory towards more complicated systems.

 

Author: Jaeyub Chung (Co-Advisors: Prof. Elias Franses and Prof. Bryan Boudouris)

Title: Interfacial Tension, and Phase Behavior of Oil/Aqueous Systems with Applications to Enhanced Oil Recovery

Abstract: The interfacial tension (IFT) between an aqueous surfactant solution and displaced crude oil in a tertiary oil recovery process is the most important parameter affecting the oil mobilization. The key question, which has not been addressed previously in a systematic way, is which IFT is the most relevant to the oil recovery: the un-pre-equilibrated IFT, or the IFT obtained after the aqueous solution has equilibrated, totally or partially, with the oil. The un-pre-equilibrated IFT values may be close to the IFT values of oil/water during the initial stages of the oil recovery process before phase equilibration. Conversely, the pre-equilibrated IFT values may be close to the IFTs during the later oil recovery stages, because then the phases are closer to equilibrium. Here, we compare the IFT behavior in the laboratory at 24 °C and at 1 atm of un-pre-equilibrated and pre-equilibrated systems of a crude oil, a brine containing eight salts, and a commercial surfactant. The synthetic brine used is similar to the one present in an actual oil reservoir. The pre-equilibrated equilibrium IFT (EIFT) values are quite different from the un-pre-equilibrated EIFT values. In addition, the effects of three mixing methods and the water-to-oil volume ratio (WOR) on the pre-equilibrated IFT were evaluated. Of the three methods examined here, (A) mild mixing, (B) magnetic stirring, and (C) shaking vigorously by hand, method C combined with centrifugation is the best method to evaluate the phase behavior and EIFT of premixed systems and produces mixtures that are the closest to the equilibrium state. For method C, the surfactant concentration in the aqueous layer after equilibration was the lowest due to surfactant partitioning into the oil phase. Moreover, the WOR affects the pre-equilibrated EIFT in brine systems significantly because of the different proportions of surfactant components that partition into the oil phase. For the surfactant at an initial concentration of 8,000 ppm in the aqueous phase, as the WOR decreases from 2.33 to 0.43, or as the oil volume fraction in the mixture, φ, increases from 0.3 to 0.7, the surfactant concentration in the aqueous layer drops to the range from 5,900 ppm to 2,200 ppm, and the EIFT increases by a factor of ~70. In addition, the pre-equilibrated EIFTs are different from the un-pre-equilibrated EIFTs at the same surfactant concentration in the aqueous layer evidently because of preferential partitioning of the various surfactant components. Therefore, the effects of the mixing method and centrifugation, the WOR, and the preferential partitioning of surfactant components into the oil phase should be evaluated for general surfactant screenings for uses in EOR applications.