The Woodruff School

SUMMARY

Over the last twenty years, methods that use external forces to manipulate and arrange, or assemble, colloidal particles have become a research topic of interest with applications in microfluidics and nanotechnology. This research studies a novel method that uses a combination of Poiseuille flow driven by a pressure gradient, and electroosmotic (EO) flow driven by a voltage gradient, or electric field, to manipulate radius a < 500 nm polystyrene particles suspended at very low concentrations (<0.5 vol%) flowing through a microchannel. When the particles lead the flow due to electrophoresis, they are attracted to, and accumulate near, the wall. Above a minimum electric field, these concentrated particles ultimately assemble into “bands”: streamwise structures a few microns in diameter and a few cm long with a consistent transverse spacing. These structures are unique because they form in a flowing (vs. quiescent) suspension, which could lead to a method for continuous particle assembly. This thesis is an experimental study based on evanescent-wave visualizations to characterize how, and under what flow and conditions, these bands form. Bands form over a range of particle radii and zeta-potentials, near-wall shear rates, where the parabolic velocity profile for Poiseuille flow near the wall can be approximated by a constant shear rate, and electric fields. The bands are characterized in terms of the time for band formation, which appears to be scaled by the near-wall shear rate, and the average transverse period of bands over a ~200 micron square field of view. Estimates based on tracking individual tracer particles imply that near-wall particle concentrations increase 100- to 200-fold (compared with the bulk concentration) as the particles are attracted to the wall. Estimates of the wall-normal “lift” force that attracts particles from the bulk to the channel wall suggest that this force is comparable to the force predicted by models recently proposed by other researchers. Measurements of the streamwise near-wall particle velocities are significantly different from what would be expected, namely the superposition of the flow and particle electrophoretic velocities. If the flow velocity is, as expected, the superposition of Poiseuille and EO flow, these results suggest that wall effects greatly suppress particle electrophoresis.

SUBJECT:	Ph.D. Dissertation Defense

BY:	Andrew Yee

TIME:	Thursday, August 12, 2021, 1:00 p.m.

PLACE:	Love Building, 210

TITLE:	Experimental Studies of Particle Assembly into Streamwise Bands

COMMITTEE:	Dr. Minami Yoda, Chair (ME) Dr. Alexander Alexeev (ME) Dr. Todd Sulchek (ME) Dr. Sven Behrens (ChBE) Dr. Shaurya Prakash (M&AE)

SUMMARY Over the last twenty years, methods that use external forces to manipulate and arrange, or assemble, colloidal particles have become a research topic of interest with applications in microfluidics and nanotechnology. This research studies a novel method that uses a combination of Poiseuille flow driven by a pressure gradient, and electroosmotic (EO) flow driven by a voltage gradient, or electric field, to manipulate radius a < 500 nm polystyrene particles suspended at very low concentrations (<0.5 vol%) flowing through a microchannel. When the particles lead the flow due to electrophoresis, they are attracted to, and accumulate near, the wall. Above a minimum electric field, these concentrated particles ultimately assemble into “bands”: streamwise structures a few microns in diameter and a few cm long with a consistent transverse spacing. These structures are unique because they form in a flowing (vs. quiescent) suspension, which could lead to a method for continuous particle assembly. This thesis is an experimental study based on evanescent-wave visualizations to characterize how, and under what flow and conditions, these bands form. Bands form over a range of particle radii and zeta-potentials, near-wall shear rates, where the parabolic velocity profile for Poiseuille flow near the wall can be approximated by a constant shear rate, and electric fields. The bands are characterized in terms of the time for band formation, which appears to be scaled by the near-wall shear rate, and the average transverse period of bands over a ~200 micron square field of view. Estimates based on tracking individual tracer particles imply that near-wall particle concentrations increase 100- to 200-fold (compared with the bulk concentration) as the particles are attracted to the wall. Estimates of the wall-normal “lift” force that attracts particles from the bulk to the channel wall suggest that this force is comparable to the force predicted by models recently proposed by other researchers. Measurements of the streamwise near-wall particle velocities are significantly different from what would be expected, namely the superposition of the flow and particle electrophoretic velocities. If the flow velocity is, as expected, the superposition of Poiseuille and EO flow, these results suggest that wall effects greatly suppress particle electrophoresis.