Hydrodynamic and Mass Transport Properties of Microfluidic Geometries.

摘要

Microfluidic geometries allow direct observation of microscale phenomena while conserving liquid volumes. They also enable modeling of experimental data using simplified transport equations and static force balances. This is possible because the length scales of these geometries ensure low Re conditions approaching the Stokesian limit, where the fluid flow profile is laminar, viscous forces are dominant and inertial forces are negligible. This work presents results on two transport problems in microfluidic geometries. The first examines heterogeneous binding kinetics in a microbead array, where beads with different chemical functionalities are sequentially captured in a well geometry over which analyte solution is owed. Finite element simulations identified the flow rates and microbead surface receptor densities at which the binding rate approaches the kinetic limit, validating the results for the prototype NeutrAvidin-biotin assay. The second part of this work discusses the dielectrophoretic motion of surfactant-stabilized water droplet pairs in a microchannel as they approach and coalesce under a uniform electric field. Experimental data measuring droplet-droplet separation distance versus time were fitted to a model using the quasi-static force balance between the attractive electrostatic force and the resistive hydrodynamic force with a single adjustable parameter representing the drag force coefficient between each droplet and the adjacent microchannel walls. For glass microchannels, the drag force coefficient values demonstrate no-slip. However, PDMS microchannels have significantly lower coefficient values corresponding to hydrodynamic slip lengths of 1-2 micrometer. These large slip lengths demonstrate that nanoporosity plays an important role in the hydrodynamics of PDMS microchannels.