| If a dilute solution of a polyelectrolyte such as DNA is forced through a microcapillary by an electric field, while simultaneously driven by a pressure gradient, then the polymer will migrate in directions transverse to the field lines. This work introduces experimental methods to investigate the migration in a microfluidic channel. Furthermore the dynamics of the polyelectrolytes is studied with the introduced experimental methods.;The first part of the dissertation includes a lithography-free method to produce microfluidic channels for imaging applications using microcapillaries is described. Silica microcapillaries, with either circular or square cross-sections, are embedded in the surface of a polydimethylsiloxane (PDMS) base to facilitate their use for imaging studies of flow and electrokinetics within a straight channel. Successful production of the devices relies on precisely timing the assembly process. This timing is identified by an in-situ elasto-capillary thinning experiment on PDMS; a rheological study supports the use of the in-situ measurements. The assembled devices are characterized using atomic force microscopy (AFM) and bright field microscopy.;A sharp increase in concentration at the center of the channel that arises when the flow and electric field drive the polymer in the same direction is investigated in the second part of this dissertation. The first systematic investigation of the effects of flow velocity, electric field, and ionic strength on the degree of migration is reported. It is find that migration increases with increasing shear and electric field as predicted by kinetic theory [Butler et. al Phys. Fluids, 2007, 19 113101], but eventually saturates as suggested by computer simulations [Kekre et. al Phys. Rev. E, 2010, 82 050803(R)]. The addition of salt reduces the strength of the migration, consistent with a screening of long-range hydrodynamic flow fields by added salt. However, increasing the ionic strength of a tris-acetate-EDTA buffer solution has much less effect on the degree of migration.;Lastly, a novel trapping method for polyelectrolytes in solution by applying opposing flow and electric fields to a microfluidic device is introduced. Modest pressure drops (less than 1cm) and voltages (less than 100V), when acting in opposition, will drive a long polyelectrolyte such as lambda-DNA to the walls of a microcapillary (0.1mm thick) in distances of the order of 1cm. The DNA becomes focused in a narrow sheet, about 1 micron thick, next to the wall of the capillary. It is further concentrated by a treadmilling effect, which drives the molecules into a small volume (about 100 pL in our device) at the inlet of the capillary. DNA is convected into the capillary by the flow, but then it rapidly migrates to the walls; since there is no fluid flow at the walls, electrophoresis returns the DNA to the inlet. Tuning the flow and electric field produces a stagnation point at the inlet to the channel where a very large concentration of DNA builds up. In this paper we report amplifications of femto-molar concentrations of T4 DNA by factors of 100,000 over times of a few hours. Furthermore, the trapping is to some extent length dependent, suggesting that long molecules can be separated from short ones by an appropriate choice of fields. The highly concentrated DNA is trapped very close to the walls of the device, and in a predictable location in the plane, making it readily available for detection or extraction. |
