Date of Award

Fall 2008

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering - (Ph.D.)

Department

Mechanical Engineering

First Advisor

Ian Sanford Fischer

Second Advisor

Pushpendra Singh

Third Advisor

N. Aubry

Fourth Advisor

David James Horntrop

Fifth Advisor

I. Joga Rao

Sixth Advisor

Anthony D. Rosato

Abstract

This dissertation is divided into two parts, first deals with the numerical study of motion of dielectric particles subjected to nonuniform electric fields and second deals with the process of self-assembly of particles at fluid-fluid interfaces and subjected to uniform electric field normal to the interface. In the numerical study, the particles are moved using a direct simulation scheme (DNS) in which the fundamental equations of motion of fluid and solid particles are solved without the use of models. The motion of particles is tracked using a distributed Lagrange multiplier method (DLM) and the electric force acting on the particles is calculated by integrating the Maxwell stress tensor (MST) over the particle surfaces. One of the key features of the DLM method is that the fluid-particle system is treated implicitly by using a combined weak formulation where the forces and moments between the particles and fluid cancel, as they are internal to the combined system. The MST is obtained from the electric potential, which, in turn, is obtained by solving the electrostatic problem. A comparison of the DNS results with those from the point-dipole approximation shows that the accuracy of the latter diminishes when the distance between the particles becomes comparable to the particle diameter, the domain size is comparable to the diameter, and also when the dielectric mismatch between the fluid and particles is relatively large.

The second part of the dissertation deals with the process of self assembly of particles at fluid fluid interface. One of the most popular techniques for two-dimensional assembly (self-assembled monolayers) is based on capillary forces acting on particles placed at a liquid interface. Capillarity-induced clustering, however, has several limitations: it applies to relatively large (radius greater than -10 um) particles only, the clustering is usually non-defect free and lacks long range order, and the lattice spacing cannot be adjusted. The goal of this thesis is to show that these shortcomings can be addressed by utilizing an external electric field normal to the interface. The resulting self- assembly is capable of controlling the lattice spacing statically or dynamically, forming virtually defect-free monolayers, and manipulating a broad range of particle sizes and types including nano-particles and electrically neutral particles. It is also demonstrated that technique also works for rod-like, ellipsoidal and cubical particles floating on fluid- fluid interfaces. The method consists of sprinkling particles at a liquid interface and applying an electric field normal to the interface, thus resulting in a combination of hydrodynamic (capillary) and electrostatic forces acting on the particles. It is shown that the relative orientation of two rod-like particles can be controlled by applying an electric field normal to the interface. The spacing between monolayer of ellipsoids is also controlled. The spacing between two cubes, as well as the spacing of a monolayer of cubes, can be adjusted by controlling the electric field strength. Similarly, the lattice spacing of the self-assembled monolayer of rods increases with increasing the electric field strength. Furthermore, there is a tendency for the rods to align so that they are parallel to each other.

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