|dc.description.abstract||Dielectrophoresis (DEP) is the motion of a dielectric particle in an aqueous solution due to the polarization effects in a non-uniform electric field. Due to its property of label-free, scalable, and capable of generating both negative and positive forces to manipulate bio-particles, DEP shows crucial applications in various biological and clinical analysis such as trapping, sorting, separation and characterization of micro and nanoparticles, cells, viruses, bacteria, and DNA. However, to date, the traditional DEP techniques involve the issues of complex fabrication of arrays of the embedded microelectrodes, particles clogging, low sensitivity and resolution, as well as the Joule heating effect with high electric fields applied.
This thesis investigates and develops a novel dielectrophoretic platform for the asymmetric-orifices based manipulation and separation of nanoparticles and micron droplets, as well as characterization and identification of droplets and biological cells by the pressure-driven flow in the polydimethylsiloxane (PDMS) microchannels. At the beginning of this thesis, a nano-orifice based dielectrophoretic microfluidic chip is developed. In such a chip, the non-uniform electric field is generated by applying the electric field via a pair of asymmetric orifices, a small orifice on one side of the channel walls and a large orifice on the opposite side of the channel walls. In order to obtain a strong gradient of the non-uniform electric fields, i.e., a large width ratio between the small orifice and the large orifice, a small microchannel or a nanochannel fabricated by the solvent-induced cracking method is used to form the small orifice. The electric field and the flow field inside the channel are simulated and studied.
Then two fundamental research projects are conducted on the nano-orifice based direct current (DC) DEP microfluidic chips to investigate the separation of the nanoparticles and Janus particles in microchannels. In the first research project, the size-dependent separation of micro and nanoparticles and the separation of similar size nanoparticles by type are studied. The Clausius-Mossotti factors of the particles as a function of the electrical conductivity of the suspending medium are discussed, and the effects of the applied electric field, the flow rate as well as the width and length of the small orifice are investigated. The experimental results of the particle trajectory show good agreements with the numerical simulation results. Distinguishing of nanoparticles as small as 51 nm and 140 nm, as well as 140 nm polystyrene (PS) and 150 nm magnetic nanoparticles with high separation resolution, have been achieved. In the second research project, the dielectrophoretic manipulation and separation of the Janus particles are numerically investigated. Effects of the strength of the electric fields, as well as the coating coverage, thickness, and electrical conductivity of the Janus particles on their DEP behaviors and trajectories under DC electric field are systematically studied. The effect of the coating thickness of the Janus particles on their dielectrophoretic behaviors is negligible when using the DC-DEP method and the Janus particles with gold coating coverage over 50% will experience positive DEP effects.
Afterward, the manipulation and separation of the oil and ionic liquid (IL)-in-water emulsion droplets are investigated under DC electric field in the asymmetric orifice based microfluidic chips. The effects of the type and content of the oil droplets and the ionic concentration and types of the electrolyte solutions on the trajectories of the emulsion droplets are analyzed. By using the pressure-driven flow and a stream of sheath flow, the mixed emulsion droplets move closely to the vicinity of the nano-orifice and experience the stronger DEP effects. As the magnitude of DEP forces exerting on the droplets is determined by the size of the droplet, the separation of smaller silicone oil droplets with a small size difference of only 3.5 µm is demonstrated. By selecting the surrounding solution with a specific electrical conductivity, the separation of the emulsion droplets of similar size but different contents is achieved by opposite DEP effects, i.e., p-DEP and n-DEP, respectively, providing a platform to manipulate different kinds of emulsion droplets carrying different biomolecules or bioparticles.
Lastly, by using the alternating current (AC) DEP microfluidic chips, the tunable characterization and identification of droplets and biological cells are investigated. To generate DEP forces, two electrode-pads are embedded in a set of asymmetric orifices on the opposite sidewalls to produce the non-uniform electric fields. In the vicinity of a small orifice, the cells experience the strongest non-uniform gradient. The effects of the strength and frequency of the applied AC electric field, as well as the ionic concentrations, i.e., different electrical conductivities on their DEP behaviors, are investigated, respectively. By adjusting the frequency and strength of the AC electric field, the separation of live and dead yeast cells, as well as the cells with the targeted diameter and dielectric property, are achieved. To evaluate the critical frequency of the specific droplets and cells and manipulate the targeted cells, a microfluidic system is developed to measure the lateral distance between the cells center and the centerline of the main channel as a function of the AC frequency. The trends of measured lateral migrations of yeast cells are similar to the corresponding Clausius−Mossotti (CM) factors. This system provides a method to characterize the crossover frequency of the specific cells and manipulate the targeted cells.
This thesis provides the microfluidic research platform with a comprehensive working procedure for the asymmetric orifice based DEP microfluidic applications. The fundamental studies in this thesis expand our understanding of the dielectrophoretic behaviors of the nanoparticles, micron droplets, Janus particles, and the biological cells and overcome the shortcomings of the conventional DEP methods, and the microfluidic systems developed on the asymmetric orifice based dielectrophoretic chips open a new avenue to nanoparticles separation as well as biological cells characterization.||en