This dissertation deals with the numerical and experimental studies of the phenomenon of dielectrophoresis, i.e., the motion of neutral particles in nonuniform electric fields. Dielectrophoresis is the translatory motion of neutral particles suspended in a dielectric medium when they are subjected to an external nonuniform electric field. The translatory motion occurs because a force called the dielectrophoretic force, which depends on the spatial variation of the electric field, acts on the particles. As the generation of force involves no moving parts and the particles can be moved without touching them, dielectrophoresis can be used in many applications, including manipulation and separation of biological particles, manipulation of nanoparticles, etc.
In the present study, the numerical simulations of the fluid-particle system are performed using a direct numerical simulation scheme based on the distributed Lagrange multiplier method. In this scheme, the fluid flow equations are solved both inside and outside the particle boundaries and flow inside the particle boundary is forced to be a rigid body motion by using the distributed Lagrange multiplier. The electrostatic force acting on the particles is computed using the point dipole method. The scheme is used to study the behavior of particles in the suspension under the influence of a nonuniform electric field.
The numerical scheme is used to study the influence of a dimensionless parameter, which is the ratio of electrostatic particle-particle interactions and dielectrophoretic force, in the dynamics of particle structure formation and the eventual particle collection. When this parameter is of order one or greater, which corresponds to the regime where particle-particle interactions are comparable in magnitude to the dielectrophoretic force, simulations reveal that the particles form interparticle chains and the chains then move to the electrode edges in the case of positive dielectrophoresis. When this parameter is of order ten the particles collect in the form of chains extending from one electrode to the opposite one clogging the entire domain. On the other hand, when this parameter is less than order one, particles move to the electrode edges individually and agglomerate at the edges of the electrodes.
The results of numerical simulations are verified experimentally using a suspension of viable yeast cells subjected to dielectrophoresis using microelectrodes. The experiments show that at frequencies much smaller than the crossover frequency where the value of the above parameter is greater than order one, the yeast particles form chains and then move and collect at the electrode edges. Where as, at frequencies closer to the crossover frequency where the value of the parameter is less than order one, particles move individually without forming chains and agglomerate at the electrode edges.
The numerical simulation scheme is also used to study the dielectrophoresis of nanoparticles. Simulations show that in a uniform electric field the Brownian force is dominant and results in the random scattering of the particles. In the case of nonuniform electric field, it is possible to overcome the Brownian force and collect the particles at pre-determined locations, even though the trajectories of the particles are influenced by Brownian motion.
Finally, the method of images is used to improve the electric field solution when the particles are close to the domain walls. Simulations performed for uniform electric fields with the method of images shows that when the distance between the particle and domain boundary is of the order of particle diameter the influence of the particles on the electric field boundary conditions is significant.
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