SUMMARY

Recent studies of electroosmotic (EO) flows driven by an electric field with magnitude E along the flow direction have shown that neutrally buoyant radii a=O(0.1-1 um) particles experience a "dielectrophoretic-like' wall normal repulsive force whose magnitude increases with a^2 [Langmuir 27:11481]; hence, different size tracers will have different average wall-normal positions and velocities in the same nonuniform flow. Evanescent-wave particle velocimetry was therefore used to study a=125 nm and 245 nm polystyrene tracers in combined EO and Poiseuille flow. For "coflow" where the EO and Poiseuille flows are in the same direction, the larger particles are strongly repelled from the wall; surprisingly, the magnitude of the repulsive force exceeds the sum of the dielectrophoretic-like force and the shear induced electrokinetic lift force [J. Colloid Interf. Sci, 175:411]. For "counterflow", where the EO and Poiseuille flows are in opposite directions, particles are instead attracted to the wall. In "counterflow" if electric field magnitude is large enough so that flow reversal occurs within 1 um of the wall, the particles reversibly self assemble into concentrated bright bands consistent along the streamwise direction with a cross-stream frequency of O(10 um).

The objective of this study is to: 1) further clarify the apparently nonlinear interaction between the electric field and flow shear; 2) determine why the direction of electric field changes the direction of the wall-normal force, and 3) clarify what appears to be a novel electrokinetic instability that causes self-assembly of microparticles into streamwise bands near the wall. A better understanding of these unexpected observations could lead to new methods to manipulate near-wall microparticles, which should be useful in microfluidics-based immunoassays and drug delivery applications as well as fabricating colloidal crystals. Evanescent-wave particle velocimetry will be used to perform experiments over a range of parameters, varying E and pressure gradient; particle size, density, zeta-potential and volume fraction; ionic strength, permittivity and wall zeta-potential.