Particle imaging diagnostics and stacking dynamics in microfluidic systems

In recent times considerable interest has been focused on the design of miniaturized fluidic devices for performing various chemical analysis functions including fluid transport, mixing, and separation. The development of diagnostics to study fluid flow in microfluidic channels is of prime importance. In this dissertation, the application of particle-based diagnostics to microfluidic systems is described. Micron-resolution particle image velocimetry (muPIV) is validated as a quantitative diagnostic tool for flow fields in microfluidic systems by conducting full-field velocity measurements in pressure-driven flows. There are no deviations from conventional theory for flows in channels of order 50 mum dimensions. Velocity fields from the application of both muPIV and particle tracking velocimetry (PTV) to flow at a cross-channel intersection are presented and compared with results from numerical simulations performed with CFDRC-ACE+. A measurement depth for PTV, based on the decay of the cross-correlation coefficient between a Gaussian mask and reference particle images is proposed.;The design and validation of a particle imaging system for quantifying velocity fields in electrokinetic microflows has been accomplished. Individual particle displacements were measured with PTV to establish the particle electrophoretic mobility distributions. Desorption of coatings from channel walls is quantitatively demonstrated through particle mobility distributions. Spheres, whose electrophoretic mobilities have been calibrated, are introduced as tracer particles into electrolyte systems to make quantitative measurements of the velocity field. These measurements enable the demonstration of similarity between the electric and velocity fields for electrokinetic flows. The depth-resolved measurement capability of muPIV is exploited to measure distinct electroosmotic mobilities in a hybrid microchannel system. An experimental investigation of field-amplified sample stacking by introducing microspheres into the flow field is reported. A one-dimensional lubrication model for the temporal development of the internal pressure gradient generated is presented and good agreement with measurements is shown. As a second application of muPIV in heterogeneous electrolyte systems, polystyrene spheres are introduced in two co-flowing streams in a T-channel system with transverse conductivity gradients to demonstrate a novel separation of charged particles in solution. Numerical simulations with FEMLAB confirm the generation of a transverse electric field in electrokinetic flows with transverse conductivity gradients.