Miniaturized analytical systems have been widely investigated over the past decade due to their tremendous advantages over conventional systems and their potential applications to many areas. In general, a chemical or biochemical analysis requires multiple steps, of which separation is an essential one. Electrical field flow fractionation (EFFF) is a flow-based separation technique that combines an electric field and a perpendicular hydrodynamic flow with a parabolic profile. It gains several advantages by miniaturization due to a stronger electric field and a reduced channel thickness. Moreover, its simple device structure and its wide applicability make EFFF possible to be integrated with other micro modules. However, the major problem of EFFF is the weak effective field for separatio...[ Read more ]

Miniaturized analytical systems have been widely investigated over the past decade due to their tremendous advantages over conventional systems and their potential applications to many areas. In general, a chemical or biochemical analysis requires multiple steps, of which separation is an essential one. Electrical field flow fractionation (EFFF) is a flow-based separation technique that combines an electric field and a perpendicular hydrodynamic flow with a parabolic profile. It gains several advantages by miniaturization due to a stronger electric field and a reduced channel thickness. Moreover, its simple device structure and its wide applicability make EFFF possible to be integrated with other micro modules. However, the major problem of EFFF is the weak effective field for separation across its flow channel due to the double layers on the electrode surfaces. In this thesis, we firstly overcome the problem of the weak effective field by applying a pulsed voltage on the EFFF, and advance this technique and the corresponding micro device for the separation of nanoparticles and biomolecules.

The dynamics of the electrical double layer in a micro EFFF (μ-EFFF) device is analyzed numerically and characterized experimentally. The device consists of two indium tin oxide (ITO) electrodes with a flow channel, and it is fabricated using micromachining technology. In comparison to the constant voltage operation, the μ-EFFF device operated with pulsed voltage obtains a stronger electric field. The practical use of pulsed voltage in the micro device is explored by investigating the retention and separation of polystyrene nanoparticles. A longer retention time was measured for higher pulse frequencies. The improved separation of nanoparticles with different surface charges and sizes was demonstrated. Pulsed voltage operation does not only overcome the weak effective field, but also offers additional parameters, such as the pulse frequency and waveform to optimize its separation performance.

The mechanisms of EFFF operated with pulsed voltage (pulsed EFFF) were studied by in-situ visualization and theoretical calculation of the particle motion in the μ-EFFF devices with different electrode designs. Charged nanoparticles can be manipulated by either electrophoretic or dielectrophoretic forces depending on the electrode designs. Two mechanistic models of pulsed EFFF were postulated for planer and segmented μ-EFFF devices. The applicability of this EFFF based micro device for DNA separation was discussed.