Date of Award

Summer 2009

Document Type

Dissertation

Degree Name

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

Department

Mechanical Engineering

First Advisor

Pushpendra Singh

Second Advisor

N. Aubry

Third Advisor

Ian Sanford Fischer

Fourth Advisor

Anthony D. Rosato

Fifth Advisor

Zhiming Ji

Sixth Advisor

Denis L. Blackmore

Abstract

In microfluidic devices, the fluid can be manipulated either as continuous streams or droplets. The latter is particularly attractive as individual droplets can not only move but also split and fuse, thus offer a greater flexibility in applications such as a laboratory-on- a-chip. In this thesis, a technique is developed that uses an externally applied electric field for manipulating and removing particles trapped on the surface of a drop. The drop is assumed to be immersed in another liquid with which it is immiscible, and the electric field is generated by placing electrodes on the sides of the microdevice. Both experiments and direct numerical simulation (DNS) approaches are used to study these problems.

The DNS approach used in this thesis is based on a finite element scheme in which the fundamental equations of motion for the droplets and the surrounding fluid are solved exactly within numerical errors. The interface is tracked by the level set method and the electrostatic forces are computed using the Maxwell stress tensor approach. The distributed Lagrange multiplier method is used for tracking particles.

One of the main results of this work is that the distribution of particles on the surface of a drop can be manipulated by subjecting it to a uniform electric field and these concentrated particles can then be removed by further increasing the electric field intensity. Specifically, it is shown that particles can be concentrated into well-defined regions on the drop surface while leaving the rest of the surface particle free. Experiments show that when the dielectric constant of the drop is greater than that of the ambient liquid, particles for which the Clausius-Mossotti factor is positive move along to the two poles of the drop. Particles with a negative Clausius-Mossotti factor, on the other hand, form a ring near the drop equator. This motion is due to the dielectrophoretic force that acts upon particles because the electric field on the surface of the drop is nonuniform, despite the fact that the applied electric field away from the drop is uniform. Experiments also show that when small particles collect at the poles of a deformed drop, the electric field needed to break the drop is smaller than it is without the particles. Also, the experimental results for the dependence of the dielectrophoretic force on the parameters of the system such as the particles' and drop's radii and the dielectric properties of the fluids and particles are studied, and a dimensionless parameter regime for which the technique is guaranteed to work is defined. Also it is shown that this technique can be used to separate particles experiencing positive dielectrophoresis on the surface of a drop from those experiencing negative dielectrophoresis, and form a composite (Janus) drop by aggregating particles of one type near the poles and of another near the equator.

The DNS approach is used to study the transport of particles via the traveling wave dielectrophoretic (twDEP) forces. This technique offers a promising method for transporting particles along the length of a channel without having to pump the liquid itself Since the magnitudes of twDEP forces and torque vary with the frequency of the electric field, a variety of complex dynamical regimes are possible. The DNS approach is used to analyze the various dynamical regimes for yeast cells in terms of the forces that act on the cells, i.e., the conventional dielectrophoretic and traveling wave dielectrophoretic force and torque, the viscous drag exerted by the fluid on the particle, and the electrostatic and hydrodynamic particle-particle interactions.

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