SUMMARY

Colloidal polystyrene particles in dilute suspensions flowing through fused quartz and polydimethyl siloxane(PDMS)-quartz microchannels are first attracted to the wall, then assemble into near-wall streamwise “bands” in combined Poiseuille and electroosmotic (EO) “counterflow” where the two flows are in opposite directions. These flows are driven by an applied pressure gradient and dc electric field, respectively. These bands, which have a cross-sectional dimension of less than 10 m and a length of cm, are roughly periodic with a cross-stream spacing of O(10) m, and occur over a range of particle sizes, particle zeta potentials, and particle concentrations. Although particles are always attracted to the wall, they only assemble into bands above a minimum electric field magnitude and near-wall flow shear rate which is determined by the Poiseuille flow.
Given that there is no theory to date that explains why these particles would be attracted to the wall, much less assemble into bands, this thesis proposes to experimentally characterize how different flow parameters, such as near-wall shear rate and applied electric field, and different particle properties such as particle size and zeta potential, affect steady-state band characteristics. These characteristics include the time for the first bands to form once the electric field is applied and the number of bands formed in steady-state. In addition, the initial stages of band formation will be characterized by examining the time evolution of near-wall particle concentration and subsequent band formation at different streamwise locations in the channel.
The experimental studies of band formation will then be extended to “heterogeneous assembly” where bands form from a mixture of particles of different size or different surface charge, characterized by the zeta-potential. This research will determine how different flow parameters affect the near-wall concentrations of the different particle species, the conditions under which the particles assemble into near-wall structures, and the relative concentrations of the particles within these structures. These results will provide insight into the mechanisms underlying particle attraction and assembly, and may lead to new microfluidics-based approaches for scalable continuous high-throughput fabrication of microscale “fibers” with an internal structure.