Mechanical Engineering ETDs

Jianwei Ju

Publication Date

6-24-2015

Abstract

The behavior of the suspended particles in a fluid is an important subject in fluid flow studies. Earlier research works mostly were performed using a continuum treatment on a macro scale level. Recent developments in the field of micro- and nanofluids have led to a renewed interest in molecular hydrodynamics phenomena. The micro hydrodynamic interactions between particles and solid surfaces have been shown to play important roles in the ordering of particles in vibrated fluids, self-organization of biological cells, and collective dynamics of swimming particles[Voth2002, Riedel2005, Hernandez2005], micro-electronic fluid behaviors and also have great prospects in industry applications. Though a lot of researches have been done in this area, some problems are still unclear. For example, the boundary conditions of the interaction surfaces. For hundreds of years it has relied on the no-slip boundary condition at the solid-liquid interface in the continuum theory, and it was also applied to model many macroscopic experiments[Batchelor00]. However, the no-slip boundary started to break down in molecular level, for certain scenarios such as in the near field between a colloidal particle and a solid surface, what kind of boundary condition may be applied to the interface are still not clear. Another problem is the microscopic particle size. There is no clear theory for the effective particle size. As a result, lot of studies all assumed an ad hoc position for the wall that is not been defined in terms of the actual interactions with the fluid. This introduced some ambiguity iv in the determination of the correct comparison to the continuum theory as well as to the general force versus distance results. Further more, in a number of more recent studies, researchers applied either the repulsive Weeks-Chandler-Andersen (WCA) potentials, or some cut off and shifted Lennard-Jones (LJ) potentials. The WCA potential is a pure repulsive potential, though the regular LJ potential with a finite cutoff distance, has both repulsive and attractive parts, LJ potential usually has a discontinuity where it is cut-off, this non-smoothness may affect the results. A new potential model is needed to conserve the simulation system energy better. In this research we focused on the hydrodynamic interactions experienced by colloidal particles in the vicinity of a solid surface. We developed a new potential model, which was smoothly cut at the finite cutoff distance with both the repulsive and attractive parts. Using this new potential model, we investigated the wall-particle interaction and the solvation forces for a single suspended particle in a LJ fluid. We also focused especially on the phenomena when the suspended sphere is positioned quite close to the wall. Additionally, we explored how moving a non-interacting sphere through the particle wall, can be used to determine the effective radius of the suspended particle. Next, we analized the dynamic drag force of spherical particles of different radii with various velocities to assess the validity of Brenners expression as the suspended particle approaches the wall. The study finally determined the slip or no-slip boundary conditions in the microscale hydrodynamics.'

Keywords

molecular dynamics, boundary conditon, Brenner's solution

Degree Name

Mechanical Engineering

Level of Degree

Doctoral

Department Name

Mechanical Engineering

First Committee Member (Chair)

Heinrich, Juan

Second Committee Member

Poroseva, Svetlana

Third Committee Member

Kober, Edward

Document Type

Dissertation

Language

English

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